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Unidirectional and uniaxial anisotropies in the MnN/CoFeB exchange bias system
Abstract
We report on broadband and in-plane angle dependent ferromagnetic resonance measurements of MnN (30nm) / CoFeB (tCoFeB) exchange bias bilayer systems with CoFeB thicknesses ranging from 5 to 20 nm. We find that the strong unidirectional anisotropy in this system is accompanied by a strong uniaxial in-plane anisotropy and a small interfacial perpendicular anisotropy (K⊥,i= 0.043±0.001 erg/cm^2). The strength of the interfacial exchange bias coupling in this system ∆σeb= 0.193±0.005 erg/cm^2 is only 2.5 times larger than the interfacial uniaxial coupling strength ∆σu= 0.076±0.003 erg/cm^2. The relaxationin this system also shows a strong unidirectional and uniaxial in-plane anisotropy. While the observed thickness dependence of the uniaxial anisotropy of the relaxation is consistent with an interfacial two-magnon scattering contribution, we find that this is not the case for the unidirectional relaxation.
... The non-linearity observed along the hard IP axes could be due to a significant contribu- there is no contribution to the linewidth from field dragging. However, one cannot rule out the possible contribution originating from spin pumping [52,53], eddy currents [54], and for the IP measurements, two-magnon scattering which can scale approximately linearly with frequency [49,[55][56][57]. To sum up these contributions we use the term "effective Gilbert damping parameter", α ef f . ...
... Over the last several years, there have been a number of publications exploring the properties of the equiatomic AF alloy MnN. For instance, broadband ferromagnetic resonance and in-plane angle-dependent measurements have been used to determine the in-plane anisotropies and relaxation of MnN/CoFeB bilayers [11]. One of the attractive properties of this material is that scarcity is not an issue. ...
There is significant interest worldwide to identify new antiferromagnetic materials suitable for device applications. Key requirements for such materials are: relatively high magnetocrystalline anisotropy constant, low cost, high corrosion resistance and the ability to induce a large ex-change bias, i.e. loop shift, when grown adjacent to a ferromagnetic layer. In this article, a review on recent developments on the novel antiferromagnetic material MnN is presented. This material shows potential as a replacement for the commonly used antiferromagnet of choice, i.e. IrMn. Although the results so far look promising, further work is required for the optimisation of this material.
... The second term quantifies the Gilbert like contributions to the linewidth. We use the term "effective Gilbert damping parameter" or α eff to indicate that besides the intrinsic damping due to spin-orbit coupling other damping mechanisms like spin pumping [57], eddy currents [58] and two-magnon scattering [59,60] can lead to contributions that scale linearly or approximately linearly with frequency. The experimental data were fitted using equation [4] to extract H Δ 0 and for the 16 nm thick FeGa film whereas it was 71 ± 1 Oe for the 24 nm thick FeGa film along the hard axis which is significantly lower than reported by Kuanr et al. [61], Butera et al. [28] and Lou et al. [29] indicative of the high quality of the epitaxial films in our study. ...
Fabrication of voltage tunable high-frequency devices using magnetostrictive ferromagnet-piezoelectric hybrid structures requires materials with low damping and narrow linewidth to achieve low microwave loss. Galfenol exhibits superior magnetostriction, stability over a wide range of temperatures, and is free of rare earth elements, making it an economical and promising candidate, if the microwave losses can be minimized. We report the fabrication, structural and magnetic characterization, and study of dynamic properties of epitaxial FeGa thin films exhibiting an ultra-low residual linewidth, ΔH0=13±1 Oe – a requirement for low microwave loss and effective damping parameter, αeff, as low as 〖0.0065〗_(-0.0001)^(+0.0005 )for the 16 nm thick film. Similarly, we found αeff= 〖0.0039〗_(-0.0007)^(+0.0028 )and ΔH0=71±1 Oe for the 24 nm thick film, demonstrating its potential for applications in magnetic memory and high frequency devices.
Ferromagnetic resonance spectroscopy (FMR) is a versatile characterization technique that probes the magnetization dynamics in the frequency domain. Despite its long history, there continues to be significant improvement regarding the sensitivity and frequency range achievable in ferromagnetic resonance experiments. In this chapter we will briefly introduce the basic principles of ferromagnetic resonance spectroscopy and the long history of this technique. We will discuss the equation of motion for the magnetization that leads to the ferromagnetic resonance condition in the form of the Kittel equation and the Smit-Beljers equation, followed by a discussion of the resonance lineshape. We will cover four different experimental methods that are used to detect ferromagnetic resonance: cavity-based, shorted waveguide, transmission line, and electrically detected ferromagnetic resonance. We will give three examples that highlight some of the unique capabilities of broadband ferromagnetic resonance spectroscopy.
Bilayer and trilayer films with ferromagnetic CoPt and antiferromagnetic MnN constituent layers have been prepared by facing-target magnetron sputtering with a 6 Oe magnetic field applied to an in-plane direction of the growing film. Primitive exchange coupling, which signifies the exchange coupling occurred without conventional field cooling, has been investigated systematically by varying the component layer thickness. It has been shown that the interfacial spin configuration that initiates exchange coupling can be established at the interface when the MnN layer grows on the surface of the CoPt layer under a small field. The primitive exchange coupling shows an increasing tendency with increasing the MnN layer thickness, and a decreasing tendency with increasing the coupled CoPt layer thickness. And the coercivity of the coupled CoPt layer remain almost unchanged within the whole thickness range of both MnN and CoPt, which suggests the coercivity of the coupled CoPt layer is not only affected by exchange coupling.
Ta/MnN/CoFeB systems show high exchange bias of about 1800 Oe at room temperature; however, their thermal stability is limited by nitrogen diffusion that occurs during annealing processes [Quarterman et al., Phys. Rev. Mater. 3, 064413 (2019) and Dunz et al., AIP Adv. 8, 056304 (2018)]. In this study, we investigate the consequences of nitrogen diffusion in Ta/MnN/CoFeB exchange bias stacks in dependence on the Ta buffer layer thickness. Furthermore, we test the effects of introducing a TaN x layer between MnN and Ta as a diffusion barrier. Our findings show that the Ta buffer layer plays a decisive role in determining the exchange bias in the Ta/MnN/CoFeB system. It acts as a crystallographic seed layer for better growth of MnN and as a nitrogen sink during the annealing process. We show that both of these functions are crucial for the outcome of high exchange bias. Additionally, our results reveal that the measures decreasing nitrogen diffusion, even though being beneficial in terms of thermal stability, mostly lead to decreased crystallinity and thus weaker exchange bias.
We report on the experimental investigation of the perpendicular magnetic anisotropy in
Pt/[Co/Ni]_N/Ir multilayers. Broadband ferromagnetic resonance (FMR) and polar angle dependent FMR measurements reveal the presence of a strong fourth-order contribution to the perpendicular anisotropy. By numerically solving the Smit-Beljers relation, we are able to fit the polar angle dependence of the resonance field and extract both the second and fourth-order anisotropy constants. While this model provides a qualitative description for all samples, we note that, for samples near or in the easy cone state of the phase diagram, systematic deviations emerge in the polar angle dependent data. The exchange stiffness of the multilayers is found to be lower than the weighted average of the exchange stiffness reported for Co and Ni in the literature. We find that the effective Gilbert damping scales with the inverse multilayer thickness, indicating a significant interfacial contribution to the damping process.
Spinelectronics is becoming an increasingly important future technology for
the realization of nonvolatile, fast, low-power computer memory and is already
well-established in hard disk drive read heads and magnetic sensors. The often
required magnetic films that do not respond to external magnetic field, the
so-called fixed layers, are achieved by pinning them to an antiferromagnet via
the exchange bias effect, which causes a shift of the magnetic loop. In the
present letter we report on the new polycrystalline exchange bias system
MnN/CoFe, which shows exchange bias of up to 1480 Oe at room temperature. The
corresponding interfacial exchange energy is $J_S =
0.38\,\mathrm{erg}/\mathrm{cm}^2$ and the maximum blocking temperature at which
the exchange bias vanishes is about 300{\deg}C. Thus, the bilayers show
exchange bias properties that partly excel those of standard PtMn/CoFe,
IrMn/CoFe, or NiMn/NiFe bilayers, however without the need for noble metals or
other critical materials.
We report the fabrication of polycrystalline Fe/MnPd bilayers, which show the exchange-bias effects. We show that the exchange-bias effect, described by unidirectional anisotropy, is also accompanied by induced uniaxial and fourfold in-plane anisotropy contributions in the system. Using the Stoner-Wohlfarth model, all in-plane anisotropy contributions to the total free energy are determined for the polycrystalline samples from the fit of the in-plane angular dependence of Heb and the coercivity Hc. A mechanism explaining the presence of the induced higher-order anisotropy contributions in an exchange-bias system is proposed.
The static and dynamic properties of magnetic Pd/Fe(001) ultrathin film crystalline structures prepared on GaAs(001) templates were investigated by ferromagnetic resonance (FMR) from 10 to 73 GHz. It will be shown that the formation of a self-assembled nanoscale network of misfit dislocations in crystalline structures can be detected during the growth by fan-out diffraction features in reflection high electron energy diffraction. This network of defects leads to a strong extrinsic magnetic damping. The out-of-plane measurements of the FMR linewidth have revealed that the extrinsic damping is caused by two-magnon scattering. The contribution to the FMR linewidth from two-magnon scattering is strongly anisotropic and follows the rectangular symmetry of the glide planes of the misfit dislocation network. It will be shown that the observed strong anisotropy in two-magnon scattering can be interpreted by Fourier components of magnetic defects. The angular dependence of the Fourier components results in an effective channelling of scattered spin waves.
The magnetic properties and the spin-wave damping in FM/AF bilayers of CoFe/IrMn have been investigated with magneto-optic Kerr effect magnetometry, ferromagnetic resonance (FMR), and Brillouin light scattering (BLS). As observed in other systems, the values of the interlayer exchange field obtained with different techniques are discrepant, but they all exhibit the expected t−1 dependence with the FM film thickness t. On the other hand the spin-wave relaxation rates measured by BLS and FMR are fit with a t−2 dependence plus a constant term. This is interpreted as the sum of two independent contributions, an intrinsic mechanism dominated by Gilbert damping and an extrinsic mechanism dominated by two-magnon scattering due to fluctuations of the interlayer exchange coupling caused by interface roughness. © 2003 American Institute of Physics.
The average value of the magnetocrystalline anisotropy field, Hk, is an important parameter for the characterization of magnetic recording media but is difficult to measure accurately due in part to the effect of interactions between the grains. In order to evaluate Hk we have studied two model CoCrPtTa magnetic films using a number of complementary techniques: high field ferromagnetic resonance (FMR) (35.0–45.0 kOe), low field (<20 kOe) vector vibrating-sample magnetometry and torque magnetometry. The FMR measurements were performed at a number of discrete frequencies in the range 75–93 GHz using a new quasi-optical spectrometer developed at the University of St. Andrews. The values of Hk derived by FMR (10.8 kOe) are approximately 10% greater than those obtained from conventional magnetometry (9.6 kOe). This difference is believed to be due to the presence of intergranular exchange coupling which reduces the measured value of anisotropy when the applied field is not sufficiently large to completely align the magnetic moments. © 2002 American Institute of Physics.
We present detailed studies of the enhanced coercivity of exchange-bias bilayer Fe/MnPd, both experimentally and theoretically. We have demonstrated that the existence of large higher-order anisotropies due to exchange coupling between different Fe and MnPd layers can account for the large increase of coercivity in the Fe/MnPd system. The linear dependence of coercivity on inverse Fe thickness is well explained by a phenomenological model by introducing higher-order anisotropy terms into the total free energy of the system. (C) 1999 American Institute of Physics. [S0003-6951(99)04031-0].
We report on the exchange bias effect as a function of the in-plane direction of the applied field in twofold symmetric, epitaxial Ni80Fe20/Fe50Mn50 bilayers grown on Cu(110) single-crystal substrates. An enhancement of the exchange bias field, H-eb, up to a factor of 2 is observed if the external field is nearly, but not fully aligned perpendicular to the symmetry direction of the exchange bias field. From the measurement of the exchange bias field as a function of the in-plane angle of the applied field, the unidirectional, uniaxial and fourfold anisotropy contributions are determined with high precision. The symmetry direction of the unidirectional anisotropy switches with increasing NiFe thickness from [1 (1) over bar 0] to [001]. (C) 1999 American Institute of Physics. [S0021-8979(99)01907-6].
All contributing magnetic anisotropies in (110)-oriented exchange biased Ni80Fe20/Fe50Mn50 double layers prepared by molecular beam epitaxy on Cu(110) single crystals have been determined by means of Brillouin light scattering. Upon covering the Ni80Fe20 films by Fe50Mn50, a unidirectional anisotropy contribution appears, which is consistent with the measured exchange bias field. The uniaxial and fourfold in-plane anisotropy contributions are largely modified by an amount, which scales with the Ni80Fe20 thickness, indicating an interface effect. The strong uniaxial anisotropy contribution shows an in-plane switching of the easy axis from [1(1) over bar0$] to [001] with increasing Ni80Fe20-layer thickness. The large mode width of the spin wave excitations, which exceeds the linewidth of uncovered Ni80Fe20 films by a factor of more than six, indicates large spatial variations of the exchange coupling constant. (C) 1998 American Institute of Physics.
The contribution that the technique of ferromagnetic resonance (FMR) has made to the understanding of the magnetic behaviour of ultrathin single films is reviewed. Experimental methods to measure FMR in situ in ultrahigh vacuum are presented. The temperature dependence of the magnetization, of the magnetic relaxation rate in the vicinity of the Curie temperature, and of the second- and fourth-order magnetic anisotropy energy (MAE) constants can be measured by FMR in situ for magnetic monolayers. Using the cases of Ni/Cu(001) and Gd/W(110) as examples, the role of the MAE for the quantitative description of temperature- and thickness-dependent reorientation transitions of the magnetization is discussed. Initial results for the anisotropy of the g-factor which is related to the anisotropy of the orbital moment (and the MAE) are presented.
The exchange bias effect, discovered more than fifty years ago, is a fundamental interfacial property, which occurs between ferromagnetic and antifer-romagnetic materials. After intensive experimental and theoretical research over the last ten years, a much clearer picture has emerged about this effect, which is of immense technical importance for magneto-electronic device applications. In this re-view we start with the discussion of numerical and analytical results of those models which are based on the assumption of coherent rotation of the magnetization. The behavior of the ferromagnetic and antiferromagnetic spins during the magnetization reversal, as well as the dependence of the critical fields on characteristic parameters such as exchange stiffness, magnetic anisotropy, interface disorder etc. are analyzed in detail and the most important models for exchange bias are reviewed. Finally recent experiments in the light of the presented models are discussed. 3.1 Fundamental Aspects of Exchange Bias: Introduction The exchange bias (EB) effect was discovered 50 years ago by Meiklejohn and Bean [1]. Meanwhile the EB effect has become an integral part of modern magnetism with implications for basic research and for numerous device ap-plications. The EB effect was the first of its kind which relates to an interface effect between two different classes of materials, here between a ferromagnet and an antiferromagnet. Later on the interlayer exchange coupling between ferromagnets interleaved by paramagnet layers was discovered [2], and the proximity effect between ferromagnetic and superconducting layers was de-scribed [3, 4]. Recent reviews on these topics can be found in [5, 6, 7] and also in this book. The EB effect manifests itself in a shift of the hysteresis loop to negative or positive direction with respect to the applied field. Its origin is related to the magnetic coupling across the common interface shared by a ferromagnetic (F) and an antiferromagnetic (AF) layer. Extensive research is being carried out to unveil the details of this effect, which has resulted in more
The out-of-plane angular dependence of the ferromagnetic resonance linewidth, ΔH, has been measured for thin magnetic films coupled to NiO and for uncoupled control films. In the control films, ΔH is described by nearly angle-independent damping parameters. In the NiO-coupled films, however, the damping was found to depend strongly on magnetization orientation, with linewidth values comparable to the control samples at normal orientation, but several times larger when the magnetization lies in plane. The additional linewidth in the NiO-coupled films follows the angular dependence of the number of nearly degenerate spin wave modes, in agreement with the predictions of a two-magnon scattering model of damping which incorporates a spin wave dispersion relation suitable for ultrathin films.© 1998 American Institute of Physics.
The magnetization relaxation of thin ferromagnetic Ni<sub>81</sub>Fe<sub>19</sub> films in close proximity and in direct contact with antiferromagnetic FeMn films was investigated using broadband ferromagnetic resonance. For unbiased films, a contribution to the effective-damping parameter α <sub>eff</sub> inversely proportional to the ferromagnetic film thickness is found, consistent with the presence of spin pumping. For biased films, we find a unidirectional contribution to the effective-damping parameter, which does not scale with the inverse film thickness. The unidirectional character of the magnetization relaxation is described in terms of two-magnon scattering.
A field-asymmetric offset of the hysteresis loop in ferromagnetic-antiferromagnetic sandwiches, one of the manifestations of so-called exchange anisotropy, can be predicted from the presence of random interface roughness giving rise to a random field acting on the interface spins. The antiferromagnet breaks up into domains of size determined by the competition of exchange and an additional uniaxial in-plane anisotropy, and this size sets the scale for averaging of the random field.
We report detailed investigations of a strong perpendicular magnetic anisotropy in Ni0.8Fe0.2 thin films originating from interfaces with adjacent Ru and SiO2 layers. The anisotropy is quantified using broadband ferromagnetic resonance measurements for three types of layered heterostructures, namely an asymmetric structure of SiO2/Ni0.8Fe0.2/Ru and symmetric structures of Ru/Ni0.8Fe0.2/Ru and SiO2/Ni0.8Fe0.2/SiO2. The results show a stronger perpendicular anisotropy at the Ni0.8Fe0.2/SiO2 interface than at the Ni0.8Fe0.2/Ru interface and a lower interfacial damping contribution for the Ni0.8Fe0.2/SiO2 interface compared to the Ni0.8Fe0.2/Ru interface.
We present angle-dependent measurements of the damping properties of epitaxial Fe layers with MgO, Al, and Pt capping layers. Based on the preferential distribution of lattice defects following the crystal symmetry, we make use of a model of the defect density to separate the contribution of two-magnon scattering to the damping from the isotropic contribution originating in the spin pumping effect, viscous Gilbert damping, and the magnetic proximity effect. The separation of the two-magnon contribution, which depends strongly on the defect density, allows for the measurement of a value of the effective spin mixing conductance that is closer to the value exclusively due to spin pumping. The influence of the defect density for bilayer systems due to the different capping layers and to the unavoidable spread in defect density from sample to sample is thus removed. This shows the potential of studying spin pumping phenomena in fully ordered systems in which this separation is possible, contrary to polycrystalline or amorphous metallic thin films.
We investigated an out-of-plane exchange bias system that is based on the antiferromagnet MnN. Polycrystalline, highly textured film stacks of Ta / MnN / CoFeB / Ta were grown on SiO$_x$ by (reactive) magnetron sputtering and studied by x-ray diffraction and Kerr magnetometry. Nontrivial modifications of the exchange bias and the perpendicular magnetic anisotropy were observed both as functions of film thicknesses as well as field cooling temperatures. In optimized film stacks, a giant perpendicular exchange bias of 3600 Oe and a coercive field of 350 Oe were observed at room temperature. The effective interfacial exchange energy is estimated to be $J_\mathrm{eff} = 0.24$ mJ/m$^2$ and the effective uniaxial anisotropy constant of the antiferromagnet is $K_\mathrm{eff} = 24$ kJ/m$^3$. The maximum effective perpendicular anisotropy field of the CoFeB layer is $H_\mathrm{ani} = 3400$ Oe. These values are larger than any previously reported values. These results possibly open a route to magnetically stable, exchange biased perpendicularly magnetized spin valves.
We develop a theory of the extrinsic contributions to the ferromagnetic resonance linewidth and frequency shift of ultrathin films. The basic mechanism is two magnon scattering by defects at surfaces and interfaces. In the presence of dipolar couplings between spins in the film, one realizes short wavelength spin waves degenerate with the ferromagnetic resonance (FMR) mode, provided the magnetization is parallel to the film surfaces. Defects on the surface or interface thus scatter the FMR mode into such short wavelength spin waves, producing a dephasing contribution to the linewidth, and a frequency shift of the resonance field. The mechanism described here is inoperative when the magnetization is perpendicular to the film.
Exchange-biased NiFe/FeMn/Co trilayers were grown by dc magnetron sputtering and analyzed by in-plane ferromagnetic resonance using Q-band microwaves. The experiments revealed that distinct Co and NiFe resonance modes were excited by the microwave field. A misalignment between the anisotropy axes of the magnetic layers was deduced from the angular variations of the resonance fields, which also showed the effects of uniaxial and unidirectional anisotropies. A phenomenological model was used to fit the experimental results taking also into account a rotatable anisotropy field associated to the domain structure of the FeMn layer and the magnetic history of the films.
We review the phenomenology of exchange bias and related effects, with emphasis on layered antiferromagnetic (AFM)–ferromagnetic (FM) structures. A compilation of materials exhibiting exchange bias and some of the techniques used to study them is given. Some of the applications of exchange bias are discussed. The leading theoretical models are summarized. Finally some of the factors controlling exchange bias as well as some of the unsolved issues associated with exchange bias are discussed.
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DOI:https://doi.org/10.1103/PhysRev.97.555.2
The ability to understand and control the unique properties of interfaces has created an entirely new field of magnetism which already has a profound impact in technology and is providing the basis for a revolution in electronics. The last decade has seen dramatic progress in the development of magnetic devices for information technology but also in the basic understanding of the physics of magnetic nanostructures. This volume describes thin film magnetic properties and methods for characterising thin film structure topics that underpin the present 's pintronics' revolution in which devices are based on combined magnetic materials and semiconductors.
Ferromagnetic resonance and Brillouin light-scattering techniques have been used to investigate the spin-wave damping in ferromagnetic (FM)/antiferromagnetic bilayers exhibiting exchange bias. The measurements were done in the prototype system NiFe/NiO sputtered on Si(100) as a function of the NiFe film thickness. The linewidths measured with both techniques are more than one order of magnitude larger than in similar NiFe films without exchange bias and increase dramatically with decreasing FM film thickness. The data are consistently explained by a relaxation mechanism based on two-magnon scattering processes due to the local fluctuation of the exchange coupling caused by interface roughness. The local interface energy necessary to account for the measured linewidths is on the same order of the atomic exchange coupling.
We discuss the theory of extrinsic contributions to the linewidth of ultrathin ferromagnetic films, magnetized parallel to their surfaces. We invoke defect-induced two magnon scatterings, rendered active by unique aspects of dipolar contributions to spin wave dispersion in two dimensions. A two magnon shift in resonance frequency is present as well. We summarize our principal earlier conclusions, and present new results on the in-plane anisotropy of linewidth, for samples with stepped surfaces. © 2000 American Institute of Physics.
We report our progress on material improvement, device design, wafer processing, integration with CMOS, and testing of STT-RAM memory chips at 54 nm node with cell sizes of 14 and 28 F2 (F=54 nm). A dual tunnel barrier MTJ structure was found to have lower and more symmetric median spin transfer torque writing switching currents, and much tighter parallel to antiparallel switching current distribution. In-plane MTJ devices write endurance data, read and write soft error rates data and simulation fits, and solutions to the long write error rate tail at fast write speeds are discussed.
The exchange bias effect of NiFe–MnPd bilayers and a new system Fe–MnPd bilayers was analyzed. Through fitting the in-plane angular dependence of the exchange bias field Hex and the coercivity Hc obtained by magneto-optical Kerr effect (MOKE), the in-plane anisotropies of the two systems were obtained. We found large additional higher order anisotropies (uniaxial and four-fold) in the system due to ferromagnet (FM)–antiferromagnet (AF) exchange bias coupling. These anisotropies show a linear dependence on the inverse FM thickness indicating their interfacial origin caused by exchange bias effect. The thickness dependence of the exchange bias fields Hex and the coercivity Hc also exhibited a linear dependence on the inverse FM layer thickness.
Modern applications for thin film magnets involve unique requirements for the control and design of specific magnetic properties. The exchange bias effect in ferromagnet/antiferromagnet bilayers appears to be a useful feature for controlling one of the most important characteristics of a ferromagnet: coercivity. Prospects for control and enhancement of desirable effects depend upon a clear understanding of mechanisms governing exchange bias. The processes underlying the existence and properties of exchange bias are reviewed, with particular emphasis on the roles of interface structure and temperature. Results from numerical simulations are used to illustrate how exchange bias is modified by geometric structures at the interface and randomly placed defects. A general theoretical formulation of the bias problem is proposed, and an expression for the interface energy is derived. A key result is the existence of higher-order coupling terms when more than one sublattice of the antiferromagnet is present at the interface. Results from calculations of finite temperature effects on bias and coercivity are described, and the concept of viscosity in the antiferromagnet is discussed. A brief discussion is also included of how a dynamic linear response, such as ferromagnetic resonance or light scattering, can be used to determine relevant anisotropy and exchange parameters.
The ferromagnetic resonance (FMR) linewidth ΔH is a parameter used to characterize ultrathin film quality. The frequency‐independent part, ΔH(0), reflects the contribution of magnetic inhomogeneities. The linear part is caused by the intrinsic Gilbert damping mechanism. Recently we have found that the lattice reconstructions of substrate templates strongly affects the FMR linewidth. bcc Fe(001) films (∼10 ML) were grown on unreconstructed fcc Ag(001), bcc Cu(001), and on surface reconstructed bcc Cu(001) templates. Fe layers grown on unreconstructed Cu templates exhibited an isotropic FMR linewidth. Fe layers grown on reconstructed Cu templates, on the contrary, showed an anisotropic behavior in both the zero‐frequency FMR linewidth and the intrinsic damping parameter. Other magnetic parameters such as uniaxial and 4‐fold anisotropies are also discussed.
The theory of two‐magnon scattering is extended to disordered ferromagnetic metals. In order to treat properly the role of two‐magnon scattering in metals we have derived the formula for the rf susceptibility in the parallel (in plane) configuration by using the retarded Green’s function formalism. The roles of the elliptical polarization, the finite values of resonance k vectors, nonresonant spin‐wave interactions, eddy current effects as well as the contribution of spin‐orbit interaction to the intrinsic damping are included. We have evaluated the susceptibility in the presence of magnetostatic inhomogeneities, statistical variations of local properties and fluctuating exchange interactions.
Ferromagnetic resonance of Permalloy in the bilayer system Ni 0.80 Fe 0.20 /Mn 0.50 Fe 0.50 has been used as a probe of the exchange anisotropy at the interface between the ferromagnetic Permalloy layer and the antiferromagnetic manganese iron. For rotation of the applied magnetic field in the plane of the film, the resonance condition exhibits a unidirectional anisotropy which relates directly to the exchange bias field, as measured, for example, by the shift of the hysteresis loop. In the perpendicular orientation, and for Permalloy layer thicknesses greater than about 400 Å, one or more standing spin wave modes are seen in addition to the uniform FMR. The indexing of these modes permit an evaluation of the boundary condition at the ferromagnet/antiferromagnet interface, in terms of partial spin pinning.
The exchange field H ex transferred from a thick antiferromagnetic substrate to a thin exchange coupled ferromagnetic film is shown to reach a limiting value no matter how large the exchange coupling is. The limit is due to domain‐wall formation in the antiferromagnet. Numerical results based on a simple model for the interface are presented and compared to experimental results.
The Landau-Lifschitz-Gilbert (LLG) equation is used widely in device design to describe spin motions in magnetic nanoscale structures. The damping term in this equation plays an essential role in the description of the magnetization dynamics. The form of this term is simple and appealing, but it is derived through use of elementary phenomenological considerations. An important question is whether or not it provides a proper description of the damping of the magnetization in real materials. Recently, it was predicted that a mechanism called two magnon damping should contribute importantly to linewidths and consequently spin damping in ultrathin ferromagnetic films. This process yields ferromagnetic resonance (FMR) linewidths whose frequency dependence is incompatible with the linear variation expected from the Landau-Lifsehitz equation. This prediction has now been confirmed experimentally. Furthermore, subsequent experimental and theoretical studies have demonstrated that the damping rate depends strongly on wave vector as well. It is thus clear that for many samples, the LLG equation fails to account for the systematics of the damping of the magnetization in ultrathin ferromagnets,. at the linear response level. The paper will review the recent literature on this topic relevant to this issue. One must then inquire into the nature of a proper phenomenology to describe these materials. At the linear response level, the theory of the two magnon mechanism is sufficiently complete that one can describe the response of these systems without resort to LLG phenomenology. However, currently there is very great interest in the large amplitude response of the magnetization in magnetic nanostructures. In the view of the authors, it is difficult to envision a generally applicable extension of linear response theory into the large amplitude regime.
Using a scattering matrix approach, the precession of the magnetization of a ferromagnet is shown to transfer spins into adjacent normal metal layers. This ``pumping'' of spins slows down the precession corresponding to an enhanced Gilbert damping factor in the Landau-Lifshitz equation. The damping is expressed in terms of the scattering matrix of the ferromagnet-normal metal interface, which is accessible to model and first-principles calculations. Our estimates for permalloy thin films explain the trends observed in recent experiments.
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