Article

# Enhancement of the Dzyaloshinskii-Moriya interaction and domain wall velocity through interface intermixing in Ta/CoFeB/MgO

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## Abstract

The interfacial Dzyaloshinskii-Moriya interaction (DMI) plays a crucial role in chiral domain wall (DW) motion, favoring fast DW velocities. We explore the effect of interface disorder on DMI and DW dynamics in perpendicular magnetized Ta/CoFeB/MgO thin films. Light He+ irradiation has been used to gently engineer interface intermixing on a scale of 0.1 nm. We demonstrate that a slight modification of the Ta/CoFeB interface leads to an increase of the DMI value accompanied by an enhancement of DW velocity in the flow regime. Using micromagnetic simulations based on granular structures, we show that the enhancement of DW velocity is mainly related to an increase in the distribution of magnetic parameters related to the interface. We further infer that the DMI modulation is related to the asymmetric disorder induced by irradiation leading to alloying with the Ta buffer layer. Understanding the role of disorder is therefore crucial for the design of future devices where post-growth interface alloying can be used to finely tune the DMI.

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... A more practical approach to tailor DW chirality would require some form of post-growth treatment. So far, only a few studies have explored post-growth strategies to tune the DMI strength: these have involved either annealing the stack at different temperatures [25], or using ion irradiation to engineer interface intermixing [26,27]. Yet, while these studies clearly indicate that post-growth tuning of DMI is attainable, they do not address the correlation between DMI strength and internal DW structure, which is particularly important in the transition from a Bloch to a Néel DW. ...
... But rather than inspecting DW velocities along the direction of the in-plane field only, as it is most commonly done, we analyse how the overall morphology of the bubble domains changes during expansion. In this way we are able to clearly observe the tuning of DW chirality towards a fully Néel texture induced by an increasing irradiation dose, which in turns corresponds to both an increased intermixing at the Ta/CoFeB interface [27] and a decreased Fe content at the CoFeB/MgO interface [28]. The threshold irradiation dose for which the bubble shape evolution corresponds to DWs in the Néel state is found to be between 12 × 10 18 He + /m 2 and 16 × 10 18 He + /m 2 , which matches with predictions from the one dimensional DW model [11] based on the measured materials parameters. ...
... The exchange stiffness constant (A) for the pristine sample was estimated from that of pure Fe and Co samples, as well as from values reported in the literature [29]. As previously reported [27], A for the irradiated samples was evaluated assuming a weak dependence on M 2 S , in agreement with predictions and experimental observations [30,31]. The values of M S , K eff , and A for each of the five samples investigated are listed in Tab. ...
Preprint
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Ta/CoFeB/MgO trilayers with perpendicular magnetic anisotropy are often characterised by vanishing or modest values of interfacial Dzyaloshinskii-Moriya interaction (DMI), which results in purely Bloch or mixed Bloch-N\'eel domain walls (DWs). Here we investigate the creep evolution of the overall magnetic bubble morphology in these systems under the combined presence of in-plane and out-of-plane magnetic fields and we show that He$^+$ ion irradiation induces a transition of the internal DW structure towards a fully N\'eel spin texture. This transition can be correlated to a simultaneous increase in DMI strength and reduction in saturation magnetisation -- which are a direct consequence of the effects of ion irradiation on the bottom and top CoFeB interfaces, respectively. The threshold irradiation dose above which DWs acquire a pure N\'eel character is experimentally found to be between 12 $\times$ 10$^{18}$ He$^+$/m$^2$ and 16 $\times$ 10$^{18}$ He$^+$/m$^2$, matching estimations from the one dimensional DW model based on material parameters. Our results indicate that evaluating the global bubble shape during its expansion can be an effective tool to sense the internal bubble DW structure. Furthermore, we show that ion irradiation can be used to achieve post-growth engineering of a desired DW spin texture.
... However, this is expected only if the crystalline structure remains the same and film growth direction often breaks this symmetry. For nearly zero initial DMI values, non-equivalent intermixing of top and bottom interfaces due to ion irradiation may result in non-zero DMI [28]. Different material variations have been used to achieve a modulation of the strength and in some cases of the sign of effective DMI constant D: variation of the FM thickness [29], modification of the adjacent heavy-metal underlayer in HM/CoFeB/MgO heterostructure [30] and insertion of a Pt wedge layer in Ta/FeCoB/Pt(wedge)/MgO thin films [31]. ...
... As explained previously, the DMI of a trilayer with a ferromagnet inserted between two non-magnetic layers is usually considered to be the sum of the DMI from the two interfaces. In the present study, the bottom Ta/FeCoB interface gives a small, Fert-Levy type DMI [17], which surface value Ds=Dt, with t the ferromagnet thickness, is of the order of 20-30 fJ/m [28,39]. By contrast, the top oxide interface, FeCoB/TaOx, is expected to possess Rashba type DMI [19,20,26,34]. ...
... To understand this sign change of Δf and effective DMI for both FeCoB and top Ta thickness, we have to keep in mind that the measured (effective) DMI results from the contribution of the bottom Ta/FeCoB interface (which is the same in all samples) and the top interface that may change when oxidation and/or the FeCoB thickness are different. From the literature [28,39], we have taken a bottom surface DMI contribution around Ds=+30 fJ/m and deduced the contribution from top interface in our system and its variation along the two wedges ( Fig. 2c and d). For measurements as a function of Ta thickness, this contribution from the bottom interface is small with respect to the contribution from the top interface, despite having taken an upper limit for the bottom interface contribution. ...
Preprint
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Skyrmions are nontrivial spiral spin textures considered as potential building blocks for ultrafast and power efficient spintronic memory and logic devices. Controlling their chirality would provide an additional degree of freedom and enable new functionalities in these devices. Achieving such control requires adjusting the interfacial Dzyaloshinskii-Moriya interaction (DMI). Thanks to Brillouin Light scattering measurements in Ta/FeCoB/TaOx trilayer, we have evidenced a DMI sign crossover when tuning TaOx oxidation and suspected another DMI sign crossover when tuning FeCoB thickness. Moreover, using polar magneto-optical Kerr effect microscopy, we demonstrate skyrmion chirality inversion through their opposite current induced motion direction either by changing FeCoB thickness or TaOx oxidation rate. This chirality inversion enables a more versatile manipulation of skyrmions, paving the way towards multidirectional devices.
... This could be explained by the interface disorder induced by irradiation, which leads to a reduction of effective magnetic anisotropy and an extension of creep region of DW motion (corresponding to an increase of ). Later on, this group presented that the DW velocity was increased upon irradiation on the contrary in higher field region, namely the flow region, in the same film system [75] . This is an interesting result because it seems to withstand the common perception that the flow area is not sensitive to disorders. ...
... CoFeB/MgO interface [73] . Recent studies have also shown that using He+ ion irradiation induced intermixing in Ta-CoFeB-MgO films could also lead to a slight increase on DMI [75] , which is attributed to the irradiation induced asymmetric disorder on a scale of 0.1nm between the bottom and top ...
... The DMI of irradiated samples with 0.6 nm thick CoFeB has been checked by both BLS and magnetic bubble expansion under both perpendicular and in-plane magnetic fields. However, as shown in Figure 3- [75] . Then we determine the DMI by using magnetic bubble expansion in the creep regime under both perpendicular and in-plane magnetic fields. ...
Thesis
Magnetic Random Access Memory (MRAM), as one of the emerging technologies, aims to be a “universal” memory device for a wide variety of applications. The combination of the spin orbit torque (SOT) resulting from the spin Hall effect (SHE) and the Dzyaloshinskii–Moriya interaction (DMI) at interfaces between heavy metals and ferromagnetic layers has been demonstrated to be a powerful mean to drive efficiently domain-wall (DW) motion, which are expected to be the promising next generation of information carriers owing to ultra-low driving currents and ultra fast DW motion. However, the crucial limitation of SOT induced domain wall motion results from the presence of pinning defects that can induce large threshold currents and stochastic behaviors. Such pinning defects are strongly related to structural inhomogeneities at the interfaces between the ultra-thin ferromagnetic layer and the other materials (insulator and/or heavy metals) that induce a spatial distribution of magnetic properties such as perpendicular magnetic anisotropy (PMA) or DMI. Therefore, understanding the role of the interface structure on DW motion and DMI is crucial for the design of future low power devices.It is under this innovative context that my Ph.D. research has focused on the manipulation of interface structure in ultra-thin magnetic films with perpendicular magnetic anisotropy. CoFeB-MgO structures have been used in order to understand the impact of interface structure on anisotropy, DMI, domain wall motion and SOT phenomena. The innovative approach we have used in this PhD research is based on light ion irradiation to control the degree of intermixing at interfaces. In W-CoFeB-MgO structures with high DMI, we have observed a large increase of the DW velocity in the creep regime upon He⁺ irradiation, which is attributed to the reduction of pinning centres induced by interface intermixing. Asymmetric in-plane field-driven domain expansion experiments show that the DMI value is slightly reduced upon irradiation, and a direct relationship between DMI and interface anisotropy is demonstrated. Using local irradiated Hall bars in SOT devices, we further demonstrate that the current density for SOT induced magnetization switching through DW motion can be significantly reduced by irradiation. Our finding provides novel insights into the development of low power spintronic-memory, logic as well as neuromorphic devices.
... It is important to mention that the values of α eff obtained from the slopes of the data points in Fig. 5(b) contain not only the intrinsic contribution from the Gilbert damping parameter but also an extrinsic contribution. This extrinsic contribution can be associated with a variety of sources including spin pumping [34][35][36], due to the proximity with a heavy metal with high spin-orbit coupling, two-magnon scattering [37] or the existence of sample inhomogeneities such as a distribution of anisotropy values across the sample [38,39]. The extrinsic contribution can add a relatively small factor to the value of the Gilbert damping or in more extreme cases it can dominate the ∆F vs H dependence resulting in the loss of linearity [38,39]. ...
... This extrinsic contribution can be associated with a variety of sources including spin pumping [34][35][36], due to the proximity with a heavy metal with high spin-orbit coupling, two-magnon scattering [37] or the existence of sample inhomogeneities such as a distribution of anisotropy values across the sample [38,39]. The extrinsic contribution can add a relatively small factor to the value of the Gilbert damping or in more extreme cases it can dominate the ∆F vs H dependence resulting in the loss of linearity [38,39]. The damping parameter in CoFeB has been shown to depend on the oxidation level at the interface. ...
... Indeed, if the W were actively contributing to the interfacial DMI, a stronger dependence of the DMI energy with the alloy composition would have been expected. Additionally, the quality of the interfaces, which has a great impact on the DMI 39,40 , was assessed by cross-sectional TEM. Figure 2(b) shows a cross-section of the asymmetric Pt/CoGd(5 nm)/W film, while a closer view of the top and bottom interfaces of the CoGd layer is displayed in Fig. 2(c). The CoGd alloy and the W layers are amorphous and the Pt is polycrystalline. ...
... This is consistent with previous reports in the literature where the effect of interface roughness and intermixing on the inversion symmetry breaking has been extensively studied. It was reported that the DMI and the domain wall velocity increased with the difference of roughness and intermixing between the top and bottom interface of a magnetic layer 39,40,44,45 . Finally, as the skyrmion size depends on the magnetic film thickness 22,34 , it is thus important to understand how the interfacial DMI scales with the thickness. ...
Article
Full-text available
Skyrmions can be stabilized in magnetic systems with broken inversion symmetry and chiral interactions, such as Dzyaloshinskii-Moriya interactions (DMI). Further, compensation of magnetic moments in ferrimagnetic materials can significantly reduce magnetic dipolar interactions, which tend to favor large skyrmions. Tuning DMI is essential to control skyrmion properties, with symmetry breaking at interfaces offering the greatest flexibility. However, in contrast to the ferromagnet case, few studies have investigated interfacial DMI in ferrimagnets. Here we present a systematic study of DMI in ferrimagnetic CoGd films by Brillouin light scattering. We demonstrate the ability to control DMI by the CoGd cap layer composition, the stack symmetry and the ferrimagnetic layer thickness. The DMI thickness dependence confirms its interfacial nature. In addition, magnetic force microscopy reveals the ability to tune DMI in a range that stabilizes sub-100 nm skyrmions at room temperature in zero field. Our work opens new paths for controlling interfacial DMI in ferrimagnets to nucleate and manipulate skyrmions.
... ity is found at high magnetic fields instead of a linear dependence. This feature, which has already been observed in a number of materials including Pt/Co [21,25,26], has been attributed to instabilities of the internal structure of the DW above the Walker field, where the precession of the spins inside the DW limits its velocity. Micromagnetic simulations have been conducted in order to better understand the mechanism behind the observed velocity reduction and are also shown in Fig.5 (a) (full symbols). ...
... A higher value of D lifts the plateaux to higher velocity values for all curves and allows for a better correspondence with the experimental data confirming the high sensitivity of the velocity plateau to the value of D and that the E-field induced decrease observed can be directly linked to the reported reduction in D. The simulations presented in Fig. 5 (b) have been obtained with values of D of 1.3 mJ/m 2 , 1.05 mJ/m 2 and 0.75 mJ/m 2 for the samples biased for 1, 2 and 4 minutes, respectively, which are significantly higher than the D values obtained by either BLS or DW motion. Other studies in the literature [26] have shown that intermixing and disorder at the ferromagnetic/heavy metal interface can not only lead to changes in D but can also to have a large impact on the position of the DW velocity plateau. A careful analysis of the impact of E-field induced ionic motion on disorder is needed to complete the picture and better understand the discrepancies between the values of D obtained by different methods. ...
Conference Paper
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... Furthermore, Soucaille et al. (2016) reported large discrepancies between measurements of DMI performed through DW motion and Brillouin Light Scattering (BLS), particularly for systems with low DMI (D 0.2 mJ/m 2 ). Also worth mentioning is a recent study by Diez et al. (2019b) which showed that the DMI can be tuned in a Ta/CoFeB/MgO system through light He + irradiation, due to an increasing interface intermixing mostly between Ta and CoFeB layers. ...
... In Chaurasiya et al. (2016), asymmetry in the peak frequency, peak intensity and magnon lifetime were observed in W/CoFeB/SiO 2 . Also in this case the linewidth for spin-wave propagating in +k direction is smaller than the same for spin-wave propagating in −k di- (Diez et al., 2019b); Ta/Pt/CoFeB/MgO/Ta (Zhang and Li, 2004). ...
Preprint
Full-text available
The Dzyaloshinskii-Moriya interaction (DMI), being one of the origins for chiral magnetism, is currently attracting huge attention in the research community focusing on applied magnetism and spintronics. For future applications an accurate measurement of its strength is indispensable. In this work, we present a review of the state of the art of measuring the coefficient $D$ of the Dzyaloshinskii-Moriya interaction, the DMI constant, focusing on systems where the interaction arises from the interface between two materials. The measurement techniques are divided into three categories: a) domain wall based measurements, b) spin wave based measurements and c) spin orbit torque based measurements. We give an overview of the experimental techniques as well as their theoretical background and models for the quantification of the DMI constant $D$. We analyze the advantages and disadvantages of each method and compare $D$ values in different stacks. The review aims to obtain a better understanding of the applicability of the different techniques to different stacks and of the origin of apparent disagreement of literature values.
... 41,46−48 More specifically, the PMA and DMI, of interfacial origin in ultrathin films, can be finely tuned by playing on the irradiation fluence. [45][46][47]49,50 Spatial modulation of the magnetic properties can be achieved by irradiating through a mask 41 or by using a focused ion beam (FIB). 42 It can thus be employed as a method to pattern the magnetic properties without physical etching that would introduce defects at the edges of the nanostructures. ...
... 41,43 In particular, it was found that the Pt/Co interface roughness, in other words intermixing, increases linearly with irradiation fluence, resulting in a continuous reduction of interfacial PMA. 57 Regarding the DMI, a similar decrease with the He + -irradiation fluence was observed in W/CoFeB/MgO ultrathin films, 48 although an increase was reported in Ta/CoFeB/MgO 47,58 and Ta/ CoFeB/Pt. 50 Ab initio calculations predict that intermixing at the Pt/Co interface results in a slight diminution of the DMI, 59 around 20%, in good agreement with our experimental findings ( Figure 1b). ...
Article
Magnetic skyrmions are deemed to be the forerunners of novel spintronic memory and logic devices. While their observation and their current-driven motion at room temperature have been demonstrated, certain issues regarding their nucleation, stability, pinning, and skyrmion Hall effect still need to be overcome to realize functional devices. Here, we demonstrate that focused He+ ion irradiation can be used to create and guide skyrmions in racetracks. We show that the reduction of the perpendicular magnetic anisotropy and Dzyaloshinskii-Moriya interaction in the track defined by ion-irradiation leads to the formation of stable isolated skyrmions. Current-driven skyrmion motion experiments and simulations reveal that the skyrmions move along the irradiated track, resulting in the suppression of the skyrmion Hall effect, and that the maximum skyrmion velocity can be enhanced by tuning the magnetic properties. These results open up a new path to nucleate and guide magnetic skyrmions in racetrack devices.
... † fabrizio.riente@polito.it restricted to film-level investigations and light (He + ) ions [9][10][11]. In this work, we investigate the usage of heavier Ga + ions in an attempt to create artificial nucleation centers (ANCs) in Ta/Co-Fe-B/MgO nanomagnets with PMA, employing localized ion irradiation (not implantation), thus controlling domain-wall (DW) nucleation. ...
... An interesting observation related to the anisotropy decrease is the formation of highly ordered stripe domains at high ion doses, indicating changes in more then the primary material parameter. This is in line with reports on the increase of the interfacial Dzyaloshinskii-Moriya interaction upon the irradiation of Ta/Co-Fe-B/MgO films [11]. The 1000 superimposed differential WMOKE images of the nucleation events inside a 2.5-μm-wide nanodot. ...
Article
Comprehensive control of the domain-wall nucleation process is crucial for spin-based emerging technologies ranging from random-access and storage-class memories through domain-wall logic concepts to nanomagnetic logic. In this work, focused Ga+ ion irradiation is investigated as an effective means to control domain-wall nucleation in Ta/Co-Fe-B/MgO nanostructures. We show that, analogously to He+ irradiation, it is not only possible to reduce the perpendicular magnetic anisotropy but also to increase it significantly, enabling bidirectional manipulation schemes. First, the irradiation effects are assessed at the film level, sketching an overview of the dose-dependent changes in the magnetic energy landscape. Subsequent time-domain nucleation characteristics of irradiated nanostructures reveal substantial increases in the anisotropy fields but surprisingly small effects on the measured energy barriers, indicating shrinking nucleation volumes. Spatial control of the domain-wall nucleation point is achieved by employing focused irradiation of preirradiated magnets, with the diameter of the introduced circular defect controlling the coercivity. Special attention is given to the nucleation mechanisms, changing from the coherent radiation of a Stoner-Wohlfarth particle to depinning from an anisotropy gradient. Dynamic micromagnetic simulations and related measurements are used in addition to model and analyze this depinning-dominated magnetization reversal.
... It offers a realistic perspective to modify magnetic properties with nanometer precision. So far, studies on the irradiation effects on CoFeB/MgO have mainly been restricted to film level investigations and light (He + ) ions [9][10][11]. In this work, we investigate the usage of heavier Ga + ions in an attempt to create artificial nucleation centers (ANC) in Ta/CoFeB/MgO nanomagnets with PMA, employing localized ion irradiation (not implantation), thus controlling domain wall (DW) nucleation. ...
... An interesting observation related to the anisotropy decrease is the formation of highly ordered stripe domains at high ion doses, indicating changes in more than the primary material parameter. This is in line with reports on the increase of the interfacial Dzyaloshinskii-Moriya interaction upon the irradiation of Ta/CoFeB/Mg films [11]. ...
Preprint
Full-text available
Comprehensive control of the domain wall nucleation process is crucial for spin-based emerging technologies ranging from random-access and storage-class memories over domain-wall logic concepts to nanomagnetic logic. In this work, focused Ga+ ion-irradiation is investigated as an effective means to control domain-wall nucleation in Ta/CoFeB/MgO nanostructures. We show that analogously to He+ irradiation, it is not only possible to reduce the perpendicular magnetic anisotropy but also to increase it significantly, enabling new, bidirectional manipulation schemes. First, the irradiation effects are assessed on film level, sketching an overview of the dose-dependent changes in the magnetic energy landscape. Subsequent time-domain nucleation characteristics of irradiated nanostructures reveal substantial increases in the anisotropy fields but surprisingly small effects on the measured energy barriers, indicating shrinking nucleation volumes. Spatial control of the domain wall nucleation point is achieved by employing focused irradiation of pre-irradiated magnets, with the diameter of the introduced circular defect controlling the coercivity. Special attention is given to the nucleation mechanisms, changing from a Stoner-Wohlfarth particle's coherent rotation to depinning from an anisotropy gradient. Dynamic micromagnetic simulations and related measurements are used in addition to model and analyze this depinning-dominated magnetization reversal.
... One possible tool is the irradiation of magnetic multilayers by light irradiation using helium (He + ) ions 24 . In particular, it is possible to fine tune the PMA and DMI in ultrathin films and multilayers, which are dominated by interfacial effects, by appropriately adjusting the irradiation fluence [25][26][27][28][29] . Light He + ions, with energy of approximately 10 keV, are able to penetrate the stack, disturbing the atomic arrangement in their path and causing atoms to be slightly displaced, as He + ions end up deep inside the substrate 30 . ...
... Such a behavior could be ascribed to the intermixing caused by IR, consistently with the results of recent ab initio calculations that predict a reduction of the DMI strength exactly because of intermixing at the Pt/Co interface 48 . A similar decrease with the He + irradiation was experimentally observed in Pt/Co/MgO 24 and W/CoFeB/MgO 32 ultrathin films, although an increase was reported in Ta/CoFeB/Pt 29 and Ta/CoFeB/MgO 27,49 . Different materials used can lead to very different interfacial structures due to robustness to intermixing, spin-orbit coupling and therefore different DMI behaviour. ...
Article
Full-text available
We show a method to control magnetic interfacial effects in multilayers with Dzyaloshinskii–Moriya interaction (DMI) using helium (He $$^{+}$$ + ) ion irradiation. We report results from SQUID magnetometry, ferromagnetic resonance as well as Brillouin light scattering results on multilayers with DMI as a function of irradiation fluence to study the effect of irradiation on the magnetic properties of the multilayers. Our results show clear evidence of the He $$^{+}$$ + irradiation effects on the magnetic properties which is consistent with interface modification due to the effects of the He $$^{+}$$ + irradiation. This external degree of freedom offers promising perspectives to further improve the control of magnetic skyrmions in multilayers, that could push them towards integration in future technologies.
... Indeed, if the W were actively contributing to the interfacial DMI, a stronger dependence of the DMI energy with the alloy composition would have been expected. Additionally, the quality of the interfaces, which has a great impact on the DMI 38,39 , was assessed by cross-sectional TEM. Figure 2(b) shows a crosssection of the asymmetric Pt/CoGd(5 nm)/W film, while a closer view of the top and bottom interfaces of the CoGd layer is displayed in Fig. 2(c). The CoGd alloy and the W layers are amorphous and the Pt is polycrystalline. ...
Preprint
Skyrmions can be stabilized in magnetic systems with broken inversion symmetry and chiral interactions, such as Dzyaloshinskii-Moriya interactions (DMI). Further, compensation of magnetic moments in ferrimagnetic materials can significantly reduce magnetic dipolar interactions, which tend to favor large skyrmions. Tuning DMI is essential to control skyrmion properties, with symmetry breaking at interfaces offering the greatest flexibility. However, in contrast to the ferromagnet case, few studies have investigated interfacial DMI in ferrimagnets. Here we present a systematic study of DMI in ferrimagnetic CoGd films by Brillouin light scattering. We demonstrate the ability to control DMI by the CoGd cap layer composition, the stack symmetry and the ferrimagnetic layer thickness. The DMI thickness dependence confirms its interfacial nature. In addition, magnetic force microscopy reveals the ability to tune DMI in a range that stabilizes sub-100 nm skyrmions at room temperature in zero field. Our work opens new paths for controlling interfacial DMI in ferrimagnets to nucleate and manipulate skyrmions.
... [6][7][8] In Pt/Co/Pt films, for example, the values of H dep can be more than one order of magnitude higher. 2,4,5 Due to this low bulk pinning potential, the DW dynamics can be easily controlled even in full films by artificial pinning imposed through homogeneous material engineering processes, like light ion irradiation [9][10][11] or pre-patterned substrates. 12 Defects generated through micro/nanostructuring can also have a great impact on pristine materials. ...
Article
In this study, we report on the analysis of the magnetic domain wall (DW) curvature due to magnetic field induced motion in Ta/CoFeB/MgO and Pt/Co/Pt wires with perpendicular magnetic anisotropy. In wires of 20 μm and 25 μm, a large edge pinning potential produces the anchoring of the DW ends to the wire edges, which is evidenced as a significant curvature of the DW front as it propagates. As the driving magnetic field is increased, the curvature reduces as a result of the system moving away from the creep regime of DW motion, which implies a weaker dependence of the DW dynamics on the interaction between the DW and the wire edge defects. A simple model is derived to describe the dependence of the DW curvature on the driving magnetic field and allows us to extract the parameter σE, which accounts for the strength of the edge pinning potential. The model describes well the systems with both weak and strong bulk pinning potentials like Ta/CoFeB/MgO and Pt/Co/Pt, respectively. This provides a means to quantify the effect of edge pinning induced DW curvature on magnetic DW dynamics.
... As a new route to spintronic devices, chiral solitons may have advantages over skyrmions, whose motion is girotropic and therefore difficult to control [12][13][14], and over domain walls, since chiral solitons may provide a different route to avoid pinning effects hindering domain wall motion [15][16][17]. As we will show here, chiral solitons in monoaxial helimagnets move steadily under the application of a polarized current, reaching velocities of the order of 100 m/s for currents around 100 GA/m 2 . ...
Preprint
Chiral solitons are one dimensional localized magnetic structures that are metastable in some ferromagnetic systems with Dzyaloshinskii-Moriya interactions and/or uniaxial magnetic anisotropy. Though topological textures in general provide a very interesting playground for new spintronics phenomena, how to properly create and control single chiral solitons is still unclear. We show here that chiral solitons in monoaxial helimagnets, characterized by a uniaxial Dzyaloshinskii-Moriya interaction, can be stabilized with external magnetic fields. Once created, the soliton moves steadily in response to a polarized electric current, provided the induced spin-transfer torque has a dissipative (nonadiabatic) component. The structure of the soliton depends on the applied current density in such a way that steady motion exists only if the applied current density is lower than a critical value, beyond which the soliton is no longer stable.
... It was observed that ionic gating not only modifies the DMI but also the anisotropy and the saturation magnetization [89]. Interestingly, ion (He + ) irradiation was also shown to lead to an increase of the interfacial DMI and domain wall velocity in Ta/CoFeB/MgO due to disorder introduced at the interface [90] Computational methods, such as density functional theory (DFT) [91] and atomistic simulations [92], can play an important role in parameter engineering and tun- Tungsten composition (x) ing. These methods can not only be used to rationalize an observation and uncover insights, but also have predictive capabilities. ...
Preprint
Full-text available
Solitonic magnetic excitations such as domain walls and, specifically, skyrmionics enable the possibility of compact, high density, ultrafast, all-electronic, low-energy devices, which is the basis for the emerging area of skyrmionics. The topological winding of skyrmion spins affects their overall lifetime, energetics and dynamical behavior. In this review, we discuss skyrmionics in the context of the present day solid state memory landscape, and show how their size, stability and mobility can be controlled by material engineering, as well as how they can be nucleated and detected. Ferrimagnets near their compensation points are important candidates for this application, leading to detailed exploration of amorphous CoGd as well as the study of emergent materials such as Mn$_4$N and Inverse Heusler alloys. Along with material properties, geometrical parameters such as film thickness, defect density and notches can be used to tune skyrmion properties, such as their size and stability. Topology, however, can be a double-edged sword, especially for isolated metastable skyrmions, as it brings stability at the cost of additional damping and deflective Magnus forces compared to domain walls. Skyrmion deformation in response to forces also makes them intrinsically slower than domain walls. We explore potential analog applications of skyrmions, including temporal memory at low density that capitalizes on their near ballistic current-velocity relation, and decorrelators for stochastic computing at higher density that capitalizes on their interactions. We summarize the main challenges to achieve a skyrmionics technology, including maintaining positional stability with very high accuracy, electrical readout, especially for small ferrimagnetic skyrmions, deterministic nucleation and annihilation and overall integration with digital circuits with the associated circuit overhead.
... In this case, we observed a maximum frequency of 4.97 GHz. From experimental realization point of view, ion implantation of non-magnetic species or thermal annealing is a potential technique to alter the magnetic properties such as iDMI, M s , A, K u and α of a magnetic material [47][48][49][50][51][52][53][54][55]. ...
... Third, the buffer Ta layer reduces inhomogeneity of the bottom GdFeCo surface. Additionally, these heavy metals enhance other effects related to the spin-orbit coupling like the Dzyaloshinskii-Moriya interaction [55][56][57]177]. However, studying the influence of these effects on domain wall motion is beyond the scope of this thesis. ...
Thesis
Studying magnetic domain walls (DWs) in thin films is of great interest for the understanding of magnetization inversion mechanisms and for the development of spintronics devices. As DWs have an associated energy and lie in a material with intrinsic inhomogeneities, they can be studied within the theory of elastic interfaces in disordered media. In this thesis, we investigate the dynamic and morphological properties of DWs in thin films from this viewpoint. Our main experimental tool is the polar magneto-optical Kerr-effect (PMOKE) microscopy, which permits the direct observation of DWs. The studied samples are a ferrimagnetic 10nm-thick film of GdFeCo, and a ferromagnetic 4nm-thick semiconducting bilayer of (Ga,Mn)(As,P)/(Ga,Mn)As, both of them presenting perpendicular magnetic anisotropy.For the GdFeCo sample, we have studied the dynamics of field-driven DWs in a wide temperature range, from 10K to 353K, in the creep and depinning regimes. We have found that the depinning field Hd diverges at the magnetic compensation temperature TM, and that the characteristic pinning energy barrier kBTd grows in magnitude for decreasing temperature, what results in exceptionally low thermal effects below 100 K. This has allowed for the direct observation of the depinning transition at low temperatures and the subsequent determination of associated critical exponents. We have independently determined values of the order-parameter exponent β=0.30±0.03 and the correlation length exponent ν=1.3±0.3, both of them being consistent only with the quenched Edwards-Wilkinson universality class.Another investigation of this thesis concerns the statistical analysis of DW morphology in the GdFeCo sample. For different temperatures and applied fields, we have obtained representative values for the roughness exponent ς and the roughness amplitude B₀. We have found that the obtained zeta values cannot be directly identified with any of the theoretically predicted roughness exponents. In order to explain this disagreement, we propose a quantitative interpretation based on previous theoretical studies. We consider that the predicted exponents govern DW roughness at different length scales separated by two crossover lengths: the correlation length lopt associated to jumps over characteristic energy barriers, and the correlation length Lₐᵥ associated to the characteristic size of depinning avalanches. Based on these ideas, we interpret the measured ς exponents as effective values and experimentally quantify for the first time the depinning correlation length Lₐᵥ for different fields and temperatures. Moreover, we have found that Lₐᵥ is finite even for H<Hd in accordance with previous theoretical ideas for DW dynamics at finite temperatures.For the (Ga,Mn)(As,P)/(Ga,Mn)As sample, we have studied the field- and current-driven DW motion when both stimuli are applied separately and simultaneously. In order to compare the strength of these two driving forces, we have analyzed the conditions of balance between them when they push oppositely. We show that there is a constant proportionality factor (1.3±0.2)mT/(GA/m²) over a large temperature range close to the Curie point. We find that this same factor successfully describes the DW dynamics in the creep regime close to the depinning transition both when field and current are applied separately and simultaneously. This suggests that the effective force exerted on DWs can be described by a sum of the forces due to field and current. However, this relation does not stand at relatively low velocities, which could be associated with the non-isotropic nature of current-driven DW motion. The results presented in this thesis shed light on the universal nature of driven DWs and broadens our knowledge on the effective features of thedriving forces.
... For further analysis of the TaO x thickness dependence of iDMI induced by the interface with TaO x , let us consider the Ta/CoFeB/TaO x system, where it is known that the Ta/CoFeB interface leads to a small positive iDMI around 0.03 erg/cm 2 [7,43]. Therefore, we conclude that the optimally oxidized/overoxidized CoFeB/TaO x interface (thin TaO x ) induces a positive iDMI around +0.14 erg/cm 2 . ...
Article
The perpendicular magnetic anisotropy (PMA) and the interfacial Dzyaloshinskii–Moriya interaction (iDMI) are investigated in as grown and 300 °C annealed Co-based ultrathin systems. For this, Co films of various thicknesses (0.8 nm ≤ tCo ≤ 5.7 nm) were deposited by magnetron sputtering on thermally oxidized Si substrates using Pt, W, Ir, Ti, Ru and MgO buffer or/and capping layers. X-ray diffraction was used to investigate their structural properties and vibrating sample magnetometry (VSM) was used to determine the magnetic dead layer thickness and the magnetization at saturation (Ms). VSM revealed that the Ms for the Pt and the Ir buffered and capped films is the largest. Microstrip line ferromagnetic resonance (MS-FMR), used to extract the gyromagnetic ratio of the thicker Co films, revealed the existence of a second order PMA term, which is thickness dependent. Brillouin light scattering (BLS) in the Damon–Eshbach configuration was used to investigate the thickness dependence of the iDMI effective constant from the spin wave vector dependence of the frequency difference between Stokes and anti-Stokes lines. BLS and MS-FMR techniques were combined to measure the spin wave frequency variation as a function of the in-plane applied magnetic field (where the second order PMA contribution vanishes). The thickness dependence of the effective magnetization was then deduced and used to investigate PMA. For all the systems, PMA results from interface and volume contributions that we determined. The largest interface PMA constants were obtained for Pt- and Ir-based systems due to the electron hybridization of Co with these heavy metals having high spin orbit coupling. Annealing at 300 °C increases both the interface PMA and iDMI for the Pt/Co/MgO most probably due to de-mixing of interpenetrating oxygen atoms from the Co layer and the formation of a sharp Co/O interface.
... Much smoother bubble domain walls occur for the He + irradiated samples with slight irregularities in the bubble domain circumference with increasing irradiation dose. An increase in interface width was reported in [49] with He + irradiation yielding an increase in D. One has to consider that the irradiation changes M s as well as K eff , two parameters that enter the fit for obtaining H DMI and the evaluation of D from H DMI . ...
Preprint
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Different models have been used to evaluate the interfacial Dzyaloshinskii-Moriya interaction (DMI) from the asymmetric bubble expansion method using magneto-optics. Here we investigate the most promising candidates over a range of different magnetic multilayers with perpendicular anisotropy. Models based on the standard creep hypothesis are not able to reproduce the domain wall (DW) velocity profile when the DW roughness is high. Our results demonstrate that the DW roughness and the interface roughness of the sample layers are correlated. Furthermore, we give guidance on how to obtain reliable results for the DMI value with this popular method. A comparison of the results with Brillouin light scattering (BLS) measurements on the same samples shows that the BLS approach often results in higher measured values of DMI.
... 83 Interestingly, ion (He þ ) irradiation was also shown to lead to an increase in interfacial DMI and domain wall velocity in Ta/CoFeB/MgO due to disorder introduced at the interface. 84 ...
Article
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Solitonic magnetic excitations such as domain walls and, specifically, skyrmionics enable the possibility of compact, high density, ultrafast, all-electronic, low-energy devices, which is the basis for the emerging area of skyrmionics. The topological winding of skyrmion spins affects their overall lifetime, energetics, and dynamical behavior. In this Perspective, we discuss skyrmionics in the context of the present-day solid-state memory landscape and show how their size, stability, and mobility can be controlled by material engineering, as well as how they can be nucleated and detected. Ferrimagnets near their compensation points are promising candidates for this application, leading to a detailed exploration of amorphous CoGd as well as the study of emergent materials such as Mn4N and inverse Heusler alloys. Along with material properties, geometrical parameters such as film thickness, defect density, and notches can be used to tune skyrmion properties, such as their size and stability. Topology, however, can be a double-edged sword, especially for isolated metastable skyrmions, as it brings stability at the cost of additional damping and deflective Magnus forces compared to domain walls. Skyrmion deformation in response to forces also makes them intrinsically slower than domain walls. We explore potential analog applications of skyrmions, including temporal memory at low density—one skyrmion per racetrack—that capitalizes on their near ballistic current–velocity relation to map temporal data to spatial data and decorrelators for stochastic computing at a higher density that capitalizes on their interactions. We summarize the main challenges of achieving a skyrmionics technology, including maintaining positional stability with very high accuracy and electrical readout, especially for small ferrimagnetic skyrmions, deterministic nucleation, and annihilation and overall integration with digital circuits with the associated circuit overhead.
... Enabling the creation of regions with high and low energy that could then be used to pin, guide [25][26][27] or nucleate [17,28,29] such textures at desired locations in the device. It has already been well established that Ga + and other types of ion irradiation can be used to locally tune the magnetic parameters of single magnetic layers in a areas as small as 40 nm [30][31][32][33], but its effect on magnetic multilayers is not yet understood. Very recently, a study investigating the effect of low energy, broad beam He + ion irradiation on [Pt | Co | Ta] ×10 multilayers reported that the magnetic parameters can indeed be controlled, similar to a single magnetic layer [34]. ...
Preprint
Skyrmions are topologically protected chiral spin textures that have shown promise as data carriers in future spintronic applications. They can be stabilized by the interfacial Dzyaloshinskii-Moriya interaction (iDMI) in material systems with inversion asymmetry and spin-orbit coupling, such as Ir$|$Co$|$Pt multilayers. The ability to locally tune such interface interactions, and hence the skyrmion energy, could greatly enhance the nucleation and control of skyrmions in racetrack type devices. In this work, we investigate local tuning of the iDMI and perpendicular magnetic anisotropy (PMA) using focussed Ga$^{+}$ ion beam irradiation, in an Ir$|$Co$|$Pt multilayer system. We show that the magnitude of the interface contribution to both effects can be significantly reduced by the irradiation with Ga$^{+}$ ions. This leads to a reduction by a factor two of the domain wall energy density, while still preserving the N\'{e}el character of the domain walls. Hence, we postulate that Ga$^{+}$ ion irradiation is an effective way to locally reduce the energy barrier for skyrmion nucleation, providing a novel pathway for targeted skyrmion nucleation in racetrack type devices.
... The Landau-Lifshitz-Gilbert dynamic equation is augmented with Slonczewski-like spin-orbit torque that takes into account the contribution of the current flowing through the HM [41,42]. The following material parameter values, typical of oxide/Co-Fe-B/HM multilayers [43][44][45][46][47], are used in the micromagnetic simulations: A ex = 20 pJ/m (exchange stiffness), M s = 1 MA/m (saturation magnetization), K u = 0.8 MJ/m 3 (uniaxial anisotropy), D int = 1.8 mJ/m 2 (interfacial Dzyaloshinskii-Moriya), λ s = 3.7 × 10 −5 (magnetostriction), α = 0.3 (damping constant), and θ SH = −0.33 (spin Hall angle). Figure 3 shows the effect of the transversal strain gradient on skyrmion dynamics. ...
Article
Relying on both electromechanical and micromagnetic simulations, we propose a method to control the trajectory of current-driven skyrmions using an electric field in hybrid piezoelectric-magnetic systems. By applying a voltage between two lateral electrodes, a transverse strain gradient is created, as a result of the nonuniform electric field profile in the piezoelectric material. Due to magnetoelastic coupling, this transverse gradient leads to a lateral force on the skyrmions that can be used to suppress the skyrmion Hall angle for any given current density, if a proper voltage is applied. We show that this method works under realistic conditions, such as the presence of disorder in the ferromagnet, and that skyrmion trajectories can be controlled with moderate voltages. Moreover, our method allows the maximum current density that can be injected before the skyrmion is annihilated at the nanostrip edge to be increased, which leads to an increase in the maximum achievable velocities.
Article
We study the influence of He⁺ irradiation induced interface intermixing on magnetic domain wall (DW) dynamics in W-CoFeB (0.6 nm)-MgO ultrathin films, which exhibit high perpendicular magnetic anisotropy and large Dzyaloshinskii-Moriya interaction (DMI) values. Whereas the pristine films exhibit strong DW pinning, we observe a large increase in the DW velocity in the creep regime upon He⁺ irradiation, which is attributed to the reduction of pinning centers induced by interface intermixing. Asymmetric in-plane field-driven domain expansion experiments show that the DMI value is slightly reduced upon irradiation, and a direct relationship between DMI and interface anisotropy is demonstrated. Our findings provide insights into the material design and interface control for DW dynamics, as well as for DMI, enabling the development of high-performance spintronic devices based on ultrathin magnetic layers.
Article
Digital data, generated by corporate and individual users, is growing day by day due to a vast range of digital applications. Magnetic hard disk drives (HDDs) currently fulfill the demand for storage space, required by this data growth. Although flash memory devices are replacing HDDs in applications like mobile phones, laptops, and desktops, HDDs cover the majority of digital data stored in the cloud and servers. Since the capacity growth of HDDs is slowing down, it is essential to look for a potential alternative. One such alternative is domain wall (DW) memory, where magnetic domains in the form of two-dimensional or three-dimensional wires are used to store the information. DW memory (DWM) devices should satisfy the four basic operations, such as writing (nucleating domains or inserting DWs in memory element), storing (stabilizing DWs), shifting (moving DWs), and reading (reading magnetization direction). An external magnetic field or spin-transfer torque can be used to write the information. Spin–orbit torque or electric field may be used for shifting the DWs. The information can be read using tunneling magnetoresistance. The domains may be stored along the tracks using artificial pinning potentials. The absence of moving parts makes the DWM consume less power as compared to HDDs, and be more robust. The potential to stack many layers to store information in three dimensions makes them potentially a large storage capacity device. In addition to memory, DW devices also offer a route for making synaptic devices for neuromorphic computing. Despite these potential advantages of DWM, significant advances in research are needed before DWM could become commercially viable. One of the major challenges associated with DWM is DW dynamics. Many problems, such as controlled DW motion, the stability of domains, reducing the dimensions of the DW devices are still to be addressed. Artificial pinning sites fabricated through either geometrical or non-geometrical methods have been proposed for controlling DW motion. This review paper presents a survey of the investigations carried out so far and the future perspective of such devices.
Article
We propose to simultaneously determine bulk and interfacial Dzyaloshinskii-Moriya interactions (DMIs) by using the in-plane field dependence of the precessional flow of chiral domain walls (DWs). It is found that the effective fields of bulk and interfacial DMIs have respectively transverse and longitudinal components that affect differently the motion of chiral DWs in magnetic narrow heterostructure strips. The in-plane field dependence of the average precessional wall velocity can take either a dome shape or a canyon shape, depending on whether the driving force is an in-plane current or an out-of-plane magnetic field. The responses of their center shifts to the reversal of topological wall charge and current/field direction uniquely determine the nature and strength of DMI therein. Operable procedures are proposed and applied to explain existing experimental data.
Article
Magnetic Tunnel Junctions (MTJ) are employed in a range of technologies such as data storage, sensing, and so on due to their compact nature and high field sensitivity. In magnetic films and structures, ion irradiation is capable of modifying the crystal structure, phases and magnetic properties including magnetic anisotropy, which enables tunability of the sensing axis of devices such as MTJ sensors. Irradiation can also adjust the magnetic properties of specific layers without affecting the others, making it one of the most effective methods to post-process multi-layered devices. In this article we review the literature of light ion irradiation effects on MTJs and their component materials, focusing on the effects most relevant to MTJ sensors, such as magnetic anisotropy and magnetization. We first briefly review the effects of the ion-solid interaction, then discuss the causes of magnetic anisotropy, and lastly, we summarize the state of the field of ion-induced modification of magnetic properties in ferromagnetic thin films, antiferromagnetic structures, and MTJs.
Article
We have investigated the spin–orbit torque-driven magnetization switching in W/CoFeB/MgO Hall bars with perpendicular magnetic anisotropy. He⁺ ion irradiation through a mask has been used to reduce locally the effective perpendicular anisotropy at a Hall cross. Anomalous Hall effect measurements combined with Kerr microscopy indicate that the switching process is dominated by domain wall (DW) nucleation in the irradiated region followed by rapid domain propagation at a current density as low as 0.8 MA/cm² with an assisting in-plane magnetic field. Thanks to the implemented strong pinning of the DW at the transition between the irradiated and the non-irradiated region, an intermediate Hall resistance state is induced, which is further verified by finite element simulations. Such a method to control electrically multi-level resistances using He⁺ ion irradiation shows great potential in realizing neuromorphic and memristor devices.
Article
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The possibility of tuning the Dzyaloshinskii-Moriya interaction (DMI) by electric (E)-field gating in ultrathin magnetic materials has opened up new perspectives in terms of controlling the stabilization of chiral spin structures. The most recent efforts have used voltage-induced charge redistribution at the interface between a metal and an oxide to modulate the DMI. This approach is attractive for active devices but tends to be volatile, making it energy-demanding, and it is limited by Coulomb screening in the metal. Here we demonstrate nonvolatile E-field manipulation of the DMI by ionic-liquid gating of Pt/Co/HfO2 ultrathin films. The E-field effect on the DMI scales with the E-field exposure time, and we propose that it is linked to the migration of oxygen species from the HfO2 layer into the Co and Pt layers and subsequent anchoring. This effect permanently changes the properties of the material, showing that E fields can be used not only for local gating in devices but also as a highly scalable materials design tool for postgrowth tuning of the DMI.
Article
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Magnetic skyrmions are topologically protected spin textures, stabilised in systems with strong Dzyaloshinskii-Moriya interaction (DMI). Several studies have shown that electrical currents can move skyrmions efficiently through spin-orbit torques. While promising for technological applications, current-driven skyrmion motion is intrinsically collective and accompanied by undesired heating effects. Here we demonstrate a new approach to control individual skyrmion positions precisely, which relies on the magnetic interaction between sample and a magnetic force microscopy (MFM) probe. We investigate perpendicularly magnetised X/CoFeB/MgO multilayers, where for X = W or Pt the DMI is sufficiently strong to allow for skyrmion nucleation in an applied field. We show that these skyrmions can be manipulated individually through the local field gradient generated by the scanning MFM probe with an unprecedented level of accuracy. Furthermore, we show that the probe stray field can assist skyrmion nucleation. Our proof-of-concepts results pave the way towards achieving current-free skyrmion control. Skyrmions are topologically nontrivial spin textures which could be used as energy efficient carriers of information in future memory devices, but first reliable and efficient control of their movement is required. Here, the authors demonstrate a method to control the movement of individual skyrmions by making use of the magnetic interaction between a sample and a magnetic force microscopy probe.
Chapter
This chapter begins with a review of the ordinary Hall effect and the anomalous Hall effect. The latter can be further viewed as the spin Hall effect in ferromagnetic materials. From such effects, the chapter introduces the concept of spin‐polarized current and pure spin current, namely, the flows of spin angular momentum carried by flows of spin‐polarized conduction electrons. The magnetization in a ferromagnetic film can change direction without an external field, but by a stream of high‐density electrons carrying a particular polarization of magnetic (or spin) moments. The chapter explains the spin‐torque‐transfer mechanism. The total spin angular momentum of the electrons and magnetization conserves. In other words, the magnetization in the wall absorbs the change of the electron spin, leading to a translational motion of the domain wall.
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Article
Domain wall dynamics in ferromagnets is complicated by internal degrees of freedom of the domain walls. We develop a model of domain walls in disordered thin films with perpendicular magnetic anisotropy capturing such features, and use it to study the depinning transition. For weak disorder, excitations of the internal magnetization are rare, and the depinning transition takes on exponent values of the quenched Edwards-Wilkinson equation. Stronger disorder results in disorder-dependent exponents concurrently with nucleation of an increasing density of Bloch lines within the domain wall.
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Chiral solitons are one dimensional localized magnetic structures that are metastable in some ferromagnetic systems with Dzyaloshinskii–Moriya interactions and/or uniaxial magnetic anisotropy. Though topological textures in general provide a very interesting playground for new spintronics phenomena, how to properly create and control single chiral solitons is still unclear. We show here that chiral solitons in monoaxial helimagnets, characterized by a uniaxial Dzyaloshinskii–Moriya interaction, can be stabilized with external magnetic fields. Once created, the soliton moves steadily in response to a polarized electric current, provided the induced spin-transfer torque has a dissipative (nonadiabatic) component. The structure of the soliton depends on the applied current density in such a way that steady motion exists only if the applied current density is lower than a critical value, beyond which the soliton is no longer stable.
Article
In monoaxial helimagnets, the Dzyaloshinskii–Moriya interaction favors inhomogeneous distributions of the magnetization with chiral modulations of solitonic character. In addition to the helical magnetic state at zero field, a chiral soliton lattice can be stabilized when a magnetic field perpendicular to the chiral axis is applied. When the magnetic field is increased, the system undergoes a phase transition to the uniform state at a critical field Bc. Above Bc, a single chiral soliton comprises the lowest level excitation over the stable uniform state, surviving as a metastable configuration. How to retain a single chiral soliton metastable state has not been addressed yet. Using micromagnetic simulations, we analyze this possibility by injecting spin polarized currents and put forward a feasible protocol to obtain a state with a single chiral soliton from the chiral soliton lattice. Our proposal could be relevant in the experimental study of metastable solitons for technological applications.
Article
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Microstrip line ferromagnetic resonance (MS-FMR) and Brillouin light scattering (BLS) in the Damon-Eshbach geometry were used to investigate the perpendicular magnetic anisotropy (PMA), the damping and the interfacial Dzyaloshinskii-Moriya (iDMI) interaction in ferromagnetic (FM)/TaOx−based systems as a function of the ferromagnetic (FM=Co or Co8Fe72B20) and the TaOx thicknesses (oxidation level). The analysis of the experimental FMR and BLS data has shown that the effective magnetization, the Gilbert damping parameter α and the iDMI are inversely proportional to the CoFeB and the Co films thickness. The BLS investigation of the iDMI variation versus the TaOx thickness and oxidation level reveals a contribution of FM/TaOx mediated by the presence of a Rashba field at this interface. Finally, we evidenced a correlation between iDMI and PMA by varying the Cu spacer thickness in the Pt/Cu/Co/TaOx system and we showed that both PMA and iDMI are localized at the first atomic monolayers of the Pt/Co interface. The observed non-linear dependence of PMA versus iDMI constant is attributed to similar interface orbital hybridizations involved in both quantities.
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Magnetic skyrmions are localized chiral spin textures, which offer great promise to store and process information at the nanoscale. In the presence of asymmetric exchange interactions, their chirality, which governs their dynamics, is generally considered as an intrinsic parameter set during the sample deposition. In this work, we experimentally demonstrate that this key parameter can be controlled by a gate voltage. We observed that the current-induced skyrmion motion can be reversed by the application of a gate voltage. This local and dynamical reversal of the skyrmion chirality is due to a sign inversion of the interfacial Dzyaloshinskii-Moriya interaction that we attribute to ionic migration of oxygen under gate voltage. Micromagnetic simulations show that the chirality reversal is a continuous transformation, in which the skyrmion is conserved. This gate-controlled chirality provides a local and dynamical degree of freedom, yielding new functionalities to skyrmion-based logic devices.
Article
The influence of 30 keV Ga⁺ ion irradiation in the fluence range 0÷1·10¹⁶ ions/cm² on magnetic properties of Pt(bottom)/Co(3nm)/Au(top) trilayers grown by the molecular beam epitaxy was studied. Polar magnetooptical Kerr effect magnetometry and Brillouin light scattering spectrometry were used to measure the magnetic anisotropy and the strength of Dzyaloshinskii-Moriya interaction (DMI). The as-deposited sample exhibits in-plane magnetization with an effective anisotropy field of -0.35±0.02 T. Magnetic anisotropy gradually grows as the ion fluence is increased and out-of-plane magnetic anisotropy appears for the fluence above ∼3·10¹⁵ ions/cm². The frequency asymmetry in Stokes and anti-Stokes lines Δf, being a measure of DMI strength, was determined from Brillouin light scattering investigations as a function of applied fluence. The effective DMI constant is equal to -0.48±0.05 mJ/m² in the as-deposited sample. While increasing fluence the parameter Δf: (i) varies non-monotonically (with two maxima) for F smaller than F≈10¹⁵ ions/cm² and (ii) gradually decreases to zero for higher fluences. Our results can be used for the adjustment of DMI interaction in the manufacturing of magnonic nano-devices.
Article
In Ta/(Co,Fe)B/HfO2 stacks, a gate voltage drives, in a nonvolatile way, the system from an underoxidized state exhibiting in-plane anisotropy (IPA) to an optimum oxidation level resulting in perpendicular anisotropy (PMA) and further into an overoxidized state with IPA. The IPA→PMA regime is found to be significantly faster than the PMA→IPA regime, whereas only the latter shows full reversibility under the same gate voltages. The effective damping parameter also shows a marked dependence with gate voltage in the IPA→PMA regime, going from 0.029 to 0.012, and only a modest increase to 0.014 in the PMA→IPA regime. The existence of two magnetoionic regimes has been linked to a difference in the chemical environment of the anchoring points of oxygen species added to underoxidized or overoxidized layers. Our results show that multiple magnetoionic regimes can exist in a single device and that their characterization is of great importance for the design of high-performance spintronics devices.
Article
Synthetic antiferromagnets (SAFs) with perpendicular magnetic anisotropy (PMA) have recently attracted intensive attention for their potential applications in domain-wall-based racetrack memory. In this paper, swift heavy Fe ion irradiation is introduced to tune the magnetic properties of Pt/Co/Pt/Ru/Pt/Co/Ta SAFs from spin-flip to spin-flop transition. The evolutions of magnetic domain and domain wall motion are investigated to obtain the microscopic insight into the change of magnetic states. Both the experimental and the simulated results demonstrate that different responses of the effective PMA constant and the interlayer-exchange-coupling strength to ion irradiation determine the magnetic-phase transitions. The structure evolution from experiments and theoretical simulations suggests that the ion-irradiation-induced damage mainly happens at the Pt/Co interface. The damage leads to the different decrease rates of the effective PMA constant and the interlayer-exchange-coupling strength due to their intrinsic direct and Ruderman-Kittel-Kasuya-Yosida interaction nature, respectively. A quite good thermal stability of samples is obtained with ion fluence <1.5×1014ions/cm2. In this paper, we demonstrate that ion irradiation is a promising method to tailor magnetic properties of SAFs. Additionally, the ion-irradiation-induced well-controlled antiferromagnetically coupled domains could show potential applications in spintronic devices.
Article
We studied the impact of He ⁺ irradiation on the Dzyaloshinskii–Moriya interaction (DMI) in Ta/Co 20 Fe 60 B 20 /Pt/MgO samples. We found that irradiation of 40 keV He ⁺ ions increases the DMI by approximately 20% for fluences up to 2 × 10 ¹⁶ ions/cm ² before it decreases for higher fluence values. In contrast, the interfacial anisotropy shows a distinctly different fluence dependence. To better understand the impact of the ion irradiation on the Ta and Pt interfaces with the Co 20 Fe 60 B 20 layer, we carried out Monte-Carlo simulations, which showed an expected increase in disorder at the interfaces. A moderate increase in disorder increases the total number of triplets for the three-site exchange mechanism and consequently increases the DMI. Our results demonstrate the significance of disorder for the total DMI.
Article
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The Dzyaloshinskii-Moriya interaction (DMI), which is essential for the stabilization of topologically non-trivial chiral magnetic textures such as skyrmions, is particularly strong in heterostructures of ultra-thin magnetic materials and heavy elements. We explore by density-functional theory calculations the possibility to modify the magnetic properties at Co/Pt interfaces with chemical disorder. In these systems, we find a particular robustness of the DMI against intermixing. Upon dusting the interface with a third element (all 4d transition metals and B, Cu, Au, and Bi), a strong reduction of the DMI is predicted. This opens up possibilities to tune the DMI through the degrees of intermixing and dusting.
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We investigate the spin Hall effect in perpendicularly magnetized Ta/Co40Fe40B20/MgO trilayers with Ta underlayers thicker than the spin diffusion length. The crystallographic structures of the Ta layer and Ta/CoFeB interface are examined in detail using X-ray diffraction and transmission electron microscopy. The thinnest Ta underlayer is amorphous, whereas for thicker Ta layers a disoriented tetragonal beta-phase appears. Effective spin-orbit torques are calculated based on harmonic Hall voltage measurements performed in a temperature range between 15 and 300 K. To account for the temperature dependence of damping-like and field-like torques, we extend the spin diffusion model by including an additional contribution from the Ta/CoFeB interface. Based on this approach, the temperature dependence of the spin Hall angle in the Ta underlayer and at Ta/CoFeB interface are determined separately. The results indicate an almost temperature-independent spin Hall angle of theta_SH-N = -0.2 in Ta and a strongly temperature-dependent theta_SH-I for the intermixed Ta/CoFeB interface.
Article
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We present a quantitative and comparative study of magnetic field driven glassy domain wall dynamics in different ferromagnetic ultrathin films over a wide range of temperature. We reveal a universal scaling function accounting for both drive and thermal effects on the depinning transition, including critical exponents. The consistent description we obtain for both the depinning and the creep motion well below the depinning threshold should shed light on the universal glassy dynamics of thermally fluctuating elastic objects pinned by disordered energy landscapes.
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We report a comparative study of magnetic field driven domain wall motion in thin films made of different magnetic materials for a wide range of field and temperature. The full thermally activated creep motion, observed below the depinning threshold, is shown to be described by a unique universal energy barrier function. Our findings should be relevant for other systems whose dynamics can be modeled by elastic interfaces moving on disordered energy landscapes.
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Magnetic skyrmions are topologically protected spin textures that exhibit fascinating physical behaviours and large potential in highly energy-efficient spintronic device applications. The main obstacles so far are that skyrmions have been observed in only a few exotic materials and at low temperatures, and fast current-driven motion of individual skyrmions has not yet been achieved. Here, we report the observation of stable magnetic skyrmions at room temperature in ultrathin transition metal ferromagnets with magnetic transmission soft X-ray microscopy. We demonstrate the ability to generate stable skyrmion lattices and drive trains of individual skyrmions by short current pulses along a magnetic racetrack at speeds exceeding 100 m s(-1) as required for applications. Our findings provide experimental evidence of recent predictions and open the door to room-temperature skyrmion spintronics in robust thin-film heterostructures.
Article
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We investigate the Dzyaloshinskii-Moriya interactions (DMIs) in perpendicularly magnetized thin films of Pt/Co/Pt and Pt/Co/Ir/Pt. To study the effective DMI, arising at either side of the ferromagnet, we use a field-driven domain wall creep-based method. The use of only magnetic field removes the possibility of mixing with current-related effects such as spin Hall effect or Rashba field, as well as the complexity arising from lithographic patterning. Inserting an ultrathin layer of Ir at the top Co/Pt interface allows us to access the DMI contribution from the top Co/Pt interface. We show that the insertion of a thin Ir layer leads to reversal of the sign of the effective DMI acting on the sandwiched Co layer, and therefore continuously changes the domain wall structure from the right- to the left-handed Neel wall. The use of two DMI-active layers offers an efficient way of DMI tuning and enhancement in thin magnetic films. The comparison with an epitaxial Pt/Co/Pt multilayer sheds more light on the origin of DMI in polycrystalline Pt/Co/Pt films and demonstrates an exquisite sensitivity to the exact details of the atomic structure at the film interfaces.
Article
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Spin waves in perpendicularly-magnetized Pt/Co/AlO$_x$/Pt ultrathin films with varying Co thicknesses (0.6-1.2 nm) have been studied with Brillouin light spectroscopy in the Damon-Eshbach geometry. The measurements reveal a pronounced nonreciprocal propagation, which increases with decreasing Co thicknesses. This nonreciprocity is attributed to an interfacial Dzyaloshinskii-Moriya interaction (DMI), which is significantly stronger than asymmetries resulting from surface anisotropies for such modes. Results are consistent with an interfacial DMI constant $D_s = -1.7 \pm 0.11$ pJ/m, which favors left-handed chiral spin structures. This suggests that such films below 1 nm in thickness should support novel chiral states like skyrmions.
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We report on the design, verification and performance of mumax3, an open-source GPU-accelerated micromagnetic simulation program. This software solves the time- and space dependent magnetization evolution in nano- to micro scale magnets using a finite-difference discretization. Its high performance and low memory requirements allow for large-scale simulations to be performed in limited time and on inexpensive hardware. We verified each part of the software by comparing results to analytical values where available and to micromagnetic standard problems. mumax3 also offers specific extensions like MFM image generation, moving simulation window, edge charge removal and material grains.
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Recent advances in the understanding of spin orbital effects in ultrathin magnetic heterostructures have opened new paradigms to control magnetic moments electrically. The Dzyaloshinskii-Moriya interaction (DMI) is said to play a key role in forming a Neel-type domain wall that can be driven by the spin Hall torque, a torque resulting from the spin current generated in a neighboring non-magnetic layer via the spin Hall effect. Here we show that the strength and sign of the DMI can be changed by modifying the adjacent heavy metal underlayer (X) in perpendicularly magnetized X|CoFeB|MgO heterstructures. Albeit the same spin Hall angle, a domain wall moves along or against the electron flow depending on the underlayer. We find that the sense of rotation of a domain wall spiral11 is reversed when the underlayer is changed from Hf to W and the strength of DMI varies as the number of 5d electrons of the heavy metal layer changes. The DMI can even be tuned by adding nitrogen to the underlayer, thus allowing interface engineering of the magnetic texture in ultrathin magnetic heterostructures.
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We have studied the motion of a magnetic domain wall (MDW) driven by a magnetic field H in a 2D ultrathin Pt/Co/Pt film showing perpendicular anisotropy and quenched disorder. MDW velocity measurements down to the so called creep regime show that the average energy barrier scales as $$1/H$$mu with mu = 0.24+/-0.04 and that the correlation function along a MDW is governed by a wandering exponent zeta = 0.69+/-0.07, in very good agreement with theories giving mu = 0.25 and zeta = 2/3. This is the first direct measurement of the creep regime for a moving interface in a disordered medium.
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We explore a new type of domain wall structure in ultrathin films with perpendicular anisotropy, that is influenced by the Dzyaloshinskii-Moriya interaction due to the adjacent layers. This study is performed by numerical and analytical micromagnetics. We show that these walls can behave like Neel walls with very high stability, moving in stationary conditions at large velocities under large fields. We discuss the relevance of such walls, that we propose to call Dzyaloshinskii domain walls, for current-driven domain wall motion under the spin Hall effect.
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The phenomenon of stripe domain nucleation is deeply investigated both theoretically and experimentally in FePd films by the rigorous micromagnetic theory of domain nucleation and x-ray resonant magnetic scattering. The critical domain width and the nucleation field are determined by measuring the magnetic satellite peak position and integrated intensities in a wide temperature interval up to 400 ° C 0.9T c at varying in-plane magnetic fields for each temperature value. We develop and demonstrate a procedure that allows us to deter-mine directly from the micromagnetic treatment the exchange stiffness constant A and the first order anisotropy constant K u as a function of temperature. The proposed procedure, based on linearized micromagnetic equa-tions at the critical field, is valid for magnetic films with perpendicular magnetic anisotropy, and is therefore effective to measure A and K u in a technologically relevant class of materials.
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The field-driven velocity of magnetic domain walls in nanostrips patterned in (Co/Ni) multilayer films with perpendicular anisotropy is studied by magnetooptical microscopy. By applying fields up to 250 mT, two peculiar features are revealed, which are beyond the simple one-dimensional model. First, above wall depinning, a velocity plateau is observed over a 150 mT field range. Then, at large fields, an important increase of velocity occurs. Micromagnetic simulations reproduce this behaviour and underline the nontrivial role of wall structure deformations in the appearance of this complexity. (C) 2011 The Japan Society of Applied Physics
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We show how, combining He ion irradiation and thermal mobility below 600 K, the transformation from chemical disorder to order in thin films of an intermetallic ferromagnet (FePd) may be triggered and controlled. Kinetic Monte Carlo simulations show that the initial directional short range order determines the transformation. Magnetic ordering perpendicular to the film plane was achieved, promoting the initially weak magnetic anisotropy to the highest values known for FePd films. Applications to ultrahigh density magnetic recording are suggested.
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We report on magnetic domain-wall velocity measurements in ultrathin Pt/Co(0.5-0.8 nm)/Pt films with perpendicular anisotropy over a large range of applied magnetic fields. The complete velocity-field characteristics are obtained, enabling an examination of the transition between thermally activated creep and viscous flow: motion regimes predicted from general theories for driven elastic interfaces in weakly disordered media. The dissipation limited flow regime is found to be consistent with precessional domain-wall motion, analysis of which yields values for the damping parameter, alpha.
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While edge pinning is known to play an important role in sub-μm wires, we demonstrate that strong deviations from the universal creep law can occur in 1 to 20μm wide wires. Magnetic imaging shows that edge pinning translates into a marked bending of domain walls at low drive and is found to depend on the wire fabrication process and aging. Edge pinning introduces a reduction of domain wall velocity with respect to full films which increasingly dominates the creep dynamics as the wire width decreases. We show that the deviations from the creep law can be described by a simple model including a counter magnetic field which links the width of the wire to the edge dependent pinning strength. This counter field defines a key nonuniversal contribution to creep motion in patterned structures.
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The magnetic properties of 20 nm thick sputtered CoFeB thin films of different compositions have been investigated and compared to those of traditional 3D ferromagnetic materials (Co, Fe and Ni). A vibrating sample magnetometer has been used to measure their magnetization at saturation (M s). Their dynamic magnetic properties were studied using microstrip line ferromagnetic resonances, as well as Brillouin light scattering (BLS) techniques. The effective magnetizations and gyromagnetic factors are firstly measured from resonance spectra obtained for in-plane and perpendicular applied fields. The angular dependence of the resonance field then allows us to derive parameters describing the in-plane magnetic anisotropy, which is found to result from a superposition of small uniaxial and fourfold terms. Frequency and angular dependencies of the ferromagnetic resonance linewidth have been used to determine the Gilbert damping coefficient for each sample. The perpendicular surface standing modes, observed in BLS spectra, allow the evaluation of the exchange stiffness constant, which is found to vary linearly with M s (for CoFeB), in agreement with the simple model presented here. Finally, the thickness dependence of the Gilbert damping parameter of Co15Fe45B40 revealed a contribution due to spin pumping leading to a spin mixing conductance of 31 nm⁻². The BLS measurements on the 5 nm thick Co15Fe45B40 film evidenced a large frequency asymmetry attributed to the interfacial Dzyaloshinskii–Moriya interaction (DMI) originating from the interface with Pt. The surface DMI constant has been estimated to be = −0.33 pJ m⁻¹.
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We demonstrate Ar+ irradiation at the 100 eV scale as a technique for spatially tuning the Dzyaloshinskii-Moriya interaction (DMI) in Pt/Co/Pt trilayers. The irradiation energy determines the sign and magnitude of the DMI independently from magnetic anisotropy, allowing us to vary the DMI while holding the coercive field constant. This independence, which we measure by super-resolution magneto-optical Kerr effect microscopy, emphasizes the different physical origins of these effects. We also observe and propose a physical model for a poorly understood peak in domain wall velocity at zero in-plane field in regions of our trilayers with low DMI. Our technique of structuring the DMI enables new fundamental investigations and technological applications in chiral nanomagnetics.
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We study the effect of sputter-deposition conditions, namely substrate temperature and chamber base pressure, upon the interface quality of epitaxial Pt/Co/Pt thin films with perpendicular magnetic anisotropy. Here we define interface quality to be the inverse of the sum in quadrature of roughness and intermixing. We find that samples with the top Co/Pt layers grown at 250 degrees C exhibit a local maximum in roughness-intermixing and that the interface quality is better for lower or higher deposition temperatures, up to 400 degrees C, above which the interface quality degrades. Imaging the expansion of magnetic domains in an in-plane field using wide-field Kerr microscopy, we determine the interfacial Dzyaloshinskii-Moriya interaction (DMI) in films in the deposition temperature range 100 degrees C to 300 degrees C. The net DMI is linked to the difference in top and bottom Co interface qualities; the net DMI increases as the difference between top and bottom Co interface quality increases. Furthermore, for sufficiently low base pressures, the net DMI increases linearly with the deposition temperature, indicating that fine-tuning of the DMI may be achieved via the deposition conditions.
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We have characterized the strength of the interfacial Dyzaloshinskii-Moriya interaction (DMI) in ultrathin perpendicularly magnetized CoFeB/MgO films, grown on different underlayers of W, TaN, and Hf, using two experimental methods. First, we determined the effective DMI field from measurements of field-driven domain wall motion in the creep regime, where applied in-plane magnetic fields induce an anisotropy in the wall propagation that is correlated with the DMI strength. Second, Brillouin light spectroscopy was employed to quantify the frequency non-reciprocity of spin waves in the CoFeB layers, which yielded an independent measurement of the DMI. By combining these results, we show that DMI estimates from the different techniques only yield qualitative agreement, which suggests that open questions remain on the underlying models used to interpret these results.
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Micromagnetic simulations are used to investigate the effect of disorder on field-driven domain wall motion in perpendicularly magnetized CoFeB thin films. It is found that some degree of inhomogeneity in the form of an irregular grain structure needs to be introduced in the model in order to account for the domain wall velocities measured experimentally, even for applied fields much larger than the finite propagation field induced by weak disorder in the film. Moreover, the details of this grain structure have a large impact on domain wall motion in this flow regime. In particular, it is found that, for a fixed applied field, domain wall velocity rapidly increases with grain size up to a diameter of 40 nm, above which it slowly decreases. This is explained showing that the grain structure of the material introduces a new form of dissipation of energy via spin wave emission during domain wall propagation. We focus on the relation between grain size and domain wall velocity, finding that the frequency of emission of spin waves packets during domain wall motion depends on the grain size and affects directly the domain wall velocity of propagation.
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We carried out measurements of domain wall (DW) velocities driven by magnetic-field pulses in symmetric Pt/Co/Pt and asymmetric Pt/Co/AlOx, Pt/Co/GdOx and Pt/Co/Gd trilayers with ultrathin Co layers and perpendicular magnetic anisotropy. The maximum observed velocity is much larger in the asymmetric samples, where the interfacial Dzyaloshinskii-Moriya interaction (DMI) stabilises chiral Néel walls. In quantitative agreement with analytical models, in all samples the maximum observed DW speed scales as , where D is the strength of the DMI and the spontaneous magnetisation. In Pt/Co/Gd, where the anti-parallel coupling between the magnetic moments of Gd and Co leads to a decrease of the total magnetisation, very large DW speeds (up to 700 m/s) are obtained.
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Magnetic skyrmions, which are topologically-protected spin textures, are promising candidates for ultralow-energy and ultrahigh-density magnetic data storage and computing applications. To date, most experiments on skyrmions have been carried out at low temperatures. The choice of available materials is limited and there is a lack of electrical means to control skyrmions in devices. In this work, we demonstrate a new method for creating a stable skyrmion bubble phase in the CoFeB-MgO material system at room temperature, by engineering the interfacial perpendicular magnetic anisotropy of the ferromagnetic layer. Importantly, we also demonstrate that artificially-engineered symmetry-breaking gives rise to a force acting on the skyrmions, in addition to the current-induced spin-orbit-torque, which can be used to drive their motion. This room-temperature creation and manipulation of skyrmions offers new possibilities to engineer skyrmionic devices. The results bring skyrmionic memory and logic concepts closer to realization in industrially relevant and manufacturable thin film material systems.
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Magnetization dynamics are strongly influenced by damping. An effective damping constant {\alpha}eff is often determined experimentally from the spectral linewidth of the free induction decay of the magnetization after the system is excited to its non-equilibrium state. Such an {\alpha}eff, however, reflects both intrinsic damping as well as inhomogeneous broadening. In this paper we compare measurements of the magnetization dynamics in ultrathin non-epitaxial films having perpendicular magnetic anisotropy using two different techniques, time-resolved magneto optical Kerr effect (TRMOKE) and hybrid optical-electrical ferromagnetic resonance (OFMR). By using an external magnetic field that is applied at very small angles to the film plane in the TRMOKE studies, we develop an explicit closed-form analytical expression for the TRMOKE spectral linewidth and show how this can be used to reliably extract the intrinsic Gilbert damping constant. The damping constant determined in this way is in excellent agreement with that determined from the OFMR method on the same samples. Our studies indicate that the asymptotic high-field approach that is often used in the TRMOKE method to distinguish the intrinsic damping from the effective damping may result in significant error, because such high external magnetic fields are required to make this approach valid that they are out of reach. The error becomes larger the lower is the intrinsic damping constant, and thus may account for the anomalously high damping constants that are often reported in TRMOKE studies. In conventional ferromagnetic resonance (FMR) studies, inhomogeneous contributions can be readily distinguished from intrinsic damping contributions from the magnetic field dependence of the FMR linewidth. Using the analogous approach, we show how reliable values of the intrinsic damping can be extracted from TRMOKE.
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This study presents the effective tuning of perpendicular magnetic anisotropy in CoFeB/MgO thin films by He+ ion irradiation and its effect on domain wall motion in a low field regime. Magnetic anisotropy and saturation magnetisation are found to decrease as a function of the irradiation dose which can be related to the observed irradiation-induced changes in stoichiometry at the CoFeB/MgO interface. These changes in the magnetic intrinsic properties of the film are reflected in the domain wall dynamics at low magnetic fields (H) where irradiation is found to induce a significant decrease in domain wall velocity (v). For all irradiation doses, domain wall velocities at low fields are well described by a creep law, where Ln(v) vs. H −1∕4 behaves linearly, up to a maximum field H*, which has been considered as an approximation to the value of the depinning field H dep . In turn, H* ≈ H dep is seen to increase as a function of the irradiation dose, indicating an irradiation-induced extension of the creep regime of domain wall motion.
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The recent observation of current-induced domain wall (DW) motion with large velocity in ultrathin magnetic wires has opened new opportunities for spintronic devices. However, there is still no consensus on the underlying mechanisms of DW motion. Key to this debate is the DW structure, which can be of Bloch or N\'eel type, and dramatically affects the efficiency of the different proposed mechanisms. To date, most experiments aiming to address this question have relied on deducing the DW structure and chirality from its motion under additional in-plane applied fields, which is indirect and involves strong assumptions on its dynamics. Here we introduce a general method enabling direct, in situ, determination of the DW structure in ultrathin ferromagnets. It relies on local measurements of the stray field distribution above the DW using a scanning nanomagnetometer based on the Nitrogen-Vacancy defect in diamond. We first apply the method to a Ta/Co40Fe40B20(1 nm)/MgO magnetic wire and find clear signature of pure Bloch DWs. In contrast, we observe left-handed N\'eel DWs in a Pt/Co(0.6 nm)/AlOx wire, providing direct evidence for the presence of a sizable Dzyaloshinskii-Moriya interaction (DMI) at the Pt/Co interface. This method offers a new path for exploring interfacial DMI in ultrathin ferromagnets and elucidating the physics of DW motion under current.
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A complete understanding of domain wall motion in magnetic nanowires is required to enable future nanowire based spintronics devices to work reliably. The production process dictates that the samples are polycrystalline. In this contribution, we present a method to investigate the effects of material grains on domain wall motion using the GPU-based micromagnetic software package MuMax3. We use this method to study current-driven vortex domain wall motion in polycrystalline Permalloy nanowires and find that the influence of material grains is fourfold: an extrinsic pinning at low current densities, an increasing effective damping with disorder strength, shifts in the Walker breakdown current density, and the possibility of the vortex core to switch polarity at grain boundaries.
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We have studied the domain-wall dynamics in Ta-CoFeB-MgO ultra-thin films with perpendicular magnetic anisotropy for various Co and Fe concentrations in both the amorphous and crystalline states. We observe three motion regimes with increasing magnetic field, which are consistent with a low fields creep, transitory depinning, and high fields Walker wall motion. The depinning fields are found to be as low as 2 mT, which is significantly lower than the values typically observed in 3d ferromagnetic metal films with perpendicular magnetic anisotropy. This work highlights a path toward advanced spintronics devices based on weak random pinning in perpendicular CoFeB films.
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We study perpendicularly magnetized Ta/CoFeB/MgO films and investigate whether their irradiation with light ions can improve their properties by inducing a different crystallization dynamics. We report the magnetization, anisotropy, g-factor, and damping dependence upon irradiation fluence and discuss their evolutions with collisional mixing simulations and its expected consequence on magnetic properties. We show that after a short irradiation at 100 °C, the anisotropy increases close to the value obtained by conventional high temperature annealing. Higher irradiation-induced increase of anisotropy can be obtained but with a detrimental effect on the damping that can be understood from spin-orbit contributions.
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We demonstrate here that ultrathin ferromagnetic Pt/Co/Pt films with perpendicular magnetic anisotropy exhibit a sizeable Dzyaloshinskii-Moriya interaction (DMI) effect. Such a DMI effect modifies the domain-wall (DW) energy density and consequently, results in an asymmetric DW expansion driven by an out-of-plane magnetic field under an in-plane magnetic field bias. From an analysis of the asymmetry, the DMI effect is estimated to be strong enough for the DW to remain in the N\'eel-type configuration in contrast to the general expectations of these materials. Our findings emphasize the critical role of the DMI effect on the DW dynamics as the underlying physics of the asymmetries that are often observed in spin-transfer-related phenomena.
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Domain structures in CoFeB-MgO thin films with a perpendicular easy magnetization axis were observed by magneto-optic Kerr-effect microscopy at various temperatures. The domain-wall surface energy was obtained by analyzing the spatial period of the stripe domains and fitting established domain models to the period. In combination with superconducting quantum interference device measurements of magnetization and anisotropy energy, this leads to an estimate of the exchange stiffness and domain-wall width in these films. These parameters are essential for determining whether domain walls will form in patterned structures and devices made of such materials.
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Advanced thin films for today's industrial and research needs require highly specialized methodologies for a successful quantitative characterization. In particular, in the case of multilayer and/or unknown phases a global approach is necessary to obtain some or all the required information. A full approach has been developed integrating novel texture and residual stress methodologies with the Rietveld method (Acta Cryst. 22 (1967) 151) (for crystal structure analysis) and it has been coupled with the reflectivity analysis. The complete analysis can be done at once and offers several benefits: the thicknesses obtained from reflectivity can be used to correct the diffraction spectra, the phase analysis help to identify the layers and to determine the electron density profile for reflectivity; quantitative texture is needed for quantitative phase and residual stress analyses; crystal structure determination benefits of the previous. To achieve this result, it was necessary to develop some new methods, especially for texture and residual stresses. So it was possible to integrate them in the Rietveld, full profile fitting of the patterns. The measurement of these spectra required a special reflectometer/diffractometer that combines a thin parallel beam (for reflectivity) and a texture/stress goniometer with a curved large position sensitive detector. This new diffraction/reflectivity X-ray machine has been used to test the combined approach. Several spectra and the reflectivity patterns have been collected at different tilting angles and processed at once by the special software incorporating the aforementioned methodologies. Some analysis examples will be given to show the possibilities offered by the method.
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A thermodynamic theory of “weak” ferromagnetism of α-Fe2O3, MnCO3 and CoCO3 is developed on the basis of landau's theory of phase transitions of the second kind. It is shown that the “weak” ferromagnetism is due to the relativistic spin-lattice and the magnetic dipole interactions. A strong dependence of the properties of “weak” ferromagnetics on the magnetic crystalline symmetry is noted and the behaviour of these ferromagnetics in a magnetic field is studied.
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A theory of anisotropic superexchange interaction is developed by extending the Anderson theory of superexchange to include spin-orbit coupling. The antisymmetric spin coupling suggested by Dzialoshinski from purely symmetry grounds and the symmetric pseudodipolar interaction are derived. Their orders of magnitudes are estimated to be (Deltagg) and (Deltagg)2 times the isotropic superexchange energy, respectively. Higher order spin couplings are also discussed. As an example of antisymmetric spin coupling the case of CuCl2.2H2O is illustrated. In CuCl2.2H2O, a spin arrangement which is different from one accepted so far is proposed. This antisymmetric interaction is shown to be responsible for weak ferromagnetism in alpha-Fe2O3, MnCO3, and CrF3. The paramagnetic susceptibility perpendicular to the trigonal axis is expected to increase very sharply near the Néel temperature as the temperature is lowered, as was actually observed in CrF3.
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