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Subnanosecond magnetization dynamics driven by strain waves
Abstract
The magnetic properties of a magnetic material can be modified by elastic deformation - termed the magnetoelastic effect. This effect is considered an alternative approach to magnetic fields for the low-power control of magnetization states of nanostructures since it avoids charge currents that create heat dissipation. This article describes the effects of dynamic strain accompanying a surface acoustic wave on magnetic nano-elements. We use a technique based on stroboscopic x-ray microscopy to simultaneously image the evolution of both strain and magnetization at the nanometer length and picosecond time scales. The study shows that there is a delayed response of the magnetization to dynamic strain, adjustable by the magnetic properties of the material. The presented analysis provides insights into dynamic magnetoelastic coupling in nanostructures with implications for the design of strain-controlled nanodevices.
... In artificial multilayer systems, exchange bias or synthetic antiferromagnetic coupling, which is derived from the quantum interference generated via the heterojunction between ferromagnetic and antiferromagnetic layers, is generally used to control the magnetic properties [7,8,[10][11][12][13][14][15][16][17][18]. As for the uniaxial magnetic anisotropy that arises from the heterojunction, some investigations have been reported in the systems consisting of a thin ferromagnetic layer and a ferroelectric substrate, e.g., lithium niobate (LiNbO 3 ) [31][32][33][34][35][36][37][38][39]. Several of our previous studies have examined unusual magnetic characteristics found in a 30 nm thick Ni wire fabricated on a single crystal Y-cut 128 • LiNbO 3 substrate [33][34][35][36]. ...
... We found that uniaxial magnetic anisotropy was induced in the Ni layer on the LiNbO 3 substrate due to the formation of the heterojunction; the uniaxial magnetic anisotropy forced the Ni magnetizations to stochastically align parallel or antiparallel to the X-axis of the LiNbO 3 substrate (indicated by an arrow in region (i) of Figure 1b). As previously reported [33][34][35][36][37][38], a stripe domain structure is naturally formed when the wire is aligned perpendicular to the X-axis of the LiNbO 3 substrate, even in the absence of an external magnetic field, due to the competition between magnetic shape anisotropy and uniaxial magnetic anisotropy induced by the heterojunction (region (ii) of Figure 1b) [33][34][35][36][37][38]. The magnetic domain width is dependent on the width of wire, being almost equal to the width of the wire. ...
... We found that uniaxial magnetic anisotropy was induced in the Ni layer on the LiNbO 3 substrate due to the formation of the heterojunction; the uniaxial magnetic anisotropy forced the Ni magnetizations to stochastically align parallel or antiparallel to the X-axis of the LiNbO 3 substrate (indicated by an arrow in region (i) of Figure 1b). As previously reported [33][34][35][36][37][38], a stripe domain structure is naturally formed when the wire is aligned perpendicular to the X-axis of the LiNbO 3 substrate, even in the absence of an external magnetic field, due to the competition between magnetic shape anisotropy and uniaxial magnetic anisotropy induced by the heterojunction (region (ii) of Figure 1b) [33][34][35][36][37][38]. The magnetic domain width is dependent on the width of wire, being almost equal to the width of the wire. ...
The competition between magnetic shape anisotropy and the induced uniaxial magnetic anisotropy in the heterojunction between a ferromagnetic layer and a ferroelectric substrate serves to control magnetic domain structures as well as magnetization reversal characteristics. The uniaxial magnetic anisotropy, originating from the symmetry breaking effect in the heterojunction, plays a significant role in modifying the characteristics of magnetization dynamics. Magnetoelastic phenomena are known to generate uniaxial magnetic anisotropy; however, the interfacial electronic states that may contribute to the uniaxial magnetic anisotropy have not yet been adequately investigated. Here, we report experimental evidence concerning the binding energy change in the ferromagnetic layer/ferroelectric substrate heterojunction using X-ray photoemission spectroscopy. The binding energy shifts, corresponding to the chemical shifts, reveal the binding states near the interface. Our results shed light on the origin of the uniaxial magnetic anisotropy induced from the heterojunction. This knowledge can provide a means for the simultaneous control of magnetism, mechanics, and electronics in a nano/microsystem consisting of ferromagnetic/ferroelectric materials.
... Examples of continuous excitation, using periodical HF signals, are XPEEM experiments with surface acoustic waves (SAW) [20][21][22][23][24]. Some experiments are still possible using the standard feedthroughs up to 500 MHz, matching the ALBA X-ray repetition rate. ...
... Lower sub harmonics (125 MHz, 250 MHz) can be accessed in combination with a gating system in the electron beam path [20]. Although at 500 MHz, only few percent of the original RF power arrives to the sample, effects driven by the dynamic strain of the SAW like magnetic domain wall motion and their delay [21,22], magneto-acoustic waves [23] and work function oscillations in Pt [24] have been observed. However, dynamical effects at higher frequencies, like the intrinsic ferromagnetic resonance (FMR), have remained elusive due to the continued fast decay of transmission in that range [7]. ...
We describe a setup that is used for high-frequency electrical sample excitation in a cathode lens electron microscope with the sample stage at high voltage as used in many synchrotron light sources. Electrical signals are transmitted by dedicated high-frequency components to the printed circuit board supporting the sample. Sub-miniature push-on connectors (SMP) are used to realize the connection in the ultra-high vacuum chamber, bypassing the standard feedthrough. A bandwidth up to 4 GHz with -6 dB attenuation was measured at the sample position, which allows to apply sub-nanosecond pulses. We describe different electronic sample excitation schemes and demonstrate a spatial resolution of 56 nm employing the new setup.
... Very recently, the generation of magnetoacoustic modulations by launching surface acoustic waves on piezoelectric single-crystal substrates coated by magnetic thin films of Fe, Co, and Ni was achieved, simultaneously being imaged us-* k.m.seemann@gmail.com ing monochromatized circularly polarized synchrotron light and x-ray magnetic circular dichroism photoemission electron microscopy (XMCD-PEEM) [3,[8][9][10]. While the behavior of SAWs in such simple magnetic materials is relatively well understood, the interplay of SAWs with more complex magnetic structures presents a real challenge. ...
In prototype ferromagnet-antiferromagnet interfaces we demonstrate that surface acoustic waves can be used to identify complex magnetic phases arising upon evolution of exchange springs in an applied field. Applying sub-GHz surface acoustic waves to study the domain structure of the ferromagnetic layer in exchange-biased bilayers of Ir20Mn80−Co60Fe20B20, we are able to associate the magnetoelastic resonance with the presence of the exchange spin-spirals in both the ferromagnetic and antiferromagnetic layer. Our findings offer a complementary, integrative insight into emergent magnetic materials for applications of noncollinear spin textures in view of low-energy-consumption spintronic devices.
... Recent interest has focused on the interaction between electrically generated acoustic modulations and magnetization dynamics. Clear observation of the interactions such as changes in SAW propagation caused by the back action of the magnetization dynamics [8][9][10] or the variation of magnetic states caused by SAWs [11][12][13][14][15][16][17][18], including observations of the oscillatory magnetic signal associated to a magnetoacoustic wave [16,19,20] have been reported. Although it is clear that there exists a SAW-induced resonance magnetization precession [20], there is no quantification yet of the magnetization oscillation and its associated wavelength. ...
Using hybrid piezoelectric-magnetic systems we have generated large amplitude magnetization waves mediated by magnetoelasticity with up to 25 degrees variation in the magnetization orientation. We present direct imaging and quantification of both standing and propagating acoustomagnetic waves with different wavelengths, over large distances up to several millimeters in a nickel thin film.
... Recent interest has focused on the interaction between electrically generated acoustic modulations and magnetization dynamics. Clear observation of the interactions such as changes in SAW propagation caused by the back action of the magnetization dynamics [8][9][10] or the variation of magnetic states caused by SAWs [11][12][13][14][15][16][17][18], including observations of the oscillatory magnetic signal associated to a magnetoacoustic wave [16,19,20] have been reported. Although it is clear that there exists a SAW-induced resonance magnetization precession [20], there is no quantification yet of the magnetization oscillation and its associated wavelength. ...
Spin waves in ferromagnets are coherent dispersive waves, with typical frequencies in the low GHz regime and wavelengths from hundreds of nanometers to a few micrometers. Recent interest for spin waves is motivated by the possibility of their integration in nano-scale devices for high-speed and low-power signal processing. However, generation of spin waves with high amplitudes---and their detection---is challenging due to the mismatch of wavelengths with electromagnetic waves in free space, which is of the order of several centimeters. Using hybrid piezoelectric/magnetic systems we have generated large amplitude spin waves mediated by magneto-elasticity with up to 25 degrees variation in the magnetization orientation. We present direct imaging and quantification of both standing and propagating spin waves with different wavelengths, over large distances up to several millimeters, orders of magnitude longer than previously achieved.
... Finally, Foerster et al. discuss our understanding of the dynamic response of mechanical waves interacting with magnetic spin waves. 52 This more fundamental research description provides a pathway to explore new dynamic coupling phenomenon while articulating the complexities associated with making nanometer-to micrometer-sized scale measurements. This research has important ramifications in the intersection between mechanical waves (velocity ∼ km/s), magnetic spin waves (velocity ∼km/s), and electromagnetic waves (velocity ∼10 5 km/s). ...
There is currently a need for an efficient approach to control magnetism at small scales (<1 mm). Work on these magnetoelectric concepts dates back to the 19th century, when researchers believed that a material could convert electrical to magnetic energy, similar to Oersted's discovery, made by passing a current through a wire. Today, there are significant magnetoelectric research opportunities in both materials discovery and theoretical modeling efforts to advance this important area of magnetic control. Applications for these strain-mediated magnetoelectric materials range from replacing existing inefficient magnetic memory approaches to spearheading new discoveries, such as micrometer-size electromagnetic motors enabling robotic manipulation. This article and the other articles in this issue provide the motivation, background information, research opportunities, and novel applications for studying strain-mediated magnetoelectric materials. The issue is designed to encourage additional research on magnetoelectrics due to its potential impact on society through the efficient control of magnetism at the micro- and nanoscale.
The phonon wavelength, being much shorter than that of photons at the same frequency, offers phononic devices a unique niche in radio frequency (RF) applications. However, the current limitations of these devices, particularly their restricted functionality, hinder their broader integration and application. Currently, many functions are achieved using alternative signal forms like electric and photonic signals, requiring bulky converters to transform between phonon signals and other forms. The development of functional phononic devices paves the way for integrated phononic circuits, which aim to minimize the need for signal conversion while accomplishing all necessary functions. In this perspective, a brief overview of several types of functional phononic devices is provided that hold promise for integration, such as phononic modulators, amplifiers, lasers, nonreciprocal devices, and those inspired by topological physics. It is envisioned that through continued developments in materials, fabrication techniques, and designs, it's possible to realize integrated phononic circuits which will be applied in miniaturized communication devices with reduced size, weight, power consumption, and cost (SWaP‐C), as well as in other fields including quantum information science, sensing, biomedical engineering, and beyond.
Magnetoelastic effects in antiferromagnetic CuMnAs are investigated by applying dynamic strain in the 0.01% range through surface acoustic waves in the GaAs substrate. The magnetic state of the CuMnAs/GaAs is characterized by a multitude of submicron-sized domains, which we image by x-ray magnetic linear dichroism combined with photoemission electron microscopy. Within the explored strain range, CuMnAs shows magnetoelastic effects in the form of Néel vector waves with micrometer wavelength, which corresponds to an averaged overall spin-axis rotation up to 2.4∘ driven by the time-dependent strain from the surface acoustic wave. Measurements at different temperatures indicate a reduction of the wave amplitude when lowering the temperature. However, no domain wall motion has been detected on the nanosecond timescale.
Surface acoustic waves (SAWs) provide an efficient dynamical coupling between strain and magnetization in micro- and nanometric systems. Using a hybrid device composed of a piezoelectric, GaAs, and a ferromagnetic Heusler alloy thin film, Fe3Si, we are able to quantify the amplitude of magnetoacoustic waves generated with SAWs via magnetic imaging in an x-ray photoelectron microscope. The cubic anisotropy of the sample, together with a low damping coefficient, allows for the observation of resonant and nonresonant magnetoelastic coupling. Additionally, via micromagnetic simulation, we verify the experimental behavior and quantify the magnetoelastic shear strain component in Fe3Si, which appears to be large (b2=10±4MJm−3).
We describe a setup that is used for high-frequency electrical sample excitation in a cathode lens electron microscope with the sample stage at high voltage as used in many synchrotron light sources. Electrical signals are transmitted by dedicated high-frequency components to the printed circuit board supporting the sample. Sub-miniature push-on connectors (SMP) are used to realize the connection in the ultra-high vacuum chamber, bypassing the standard feedthrough. A bandwidth up to 4 GHz with -6 dB attenuation was measured at the sample position, which allows to apply sub-nanosecond pulses. We describe different electronic sample excitation schemes and demonstrate a spatial resolution of 56 nm employing the new setup.
The control of magnetism by acoustically induced strain has driven significant research activities, with the ultimate goal of pursuing novel, ultrafast, compact, and energy-efficient electronic and spintronic applications. Here, we aim to present for the first time a comprehensive review of this field, which has seen a surge of interest in recent years. We review fundamental understanding of magnetoelastic coupling phenomena and mechanisms, diverse experimental configurations, recent advances in modeling and microscopic tools to intuitively describe them, and the experimental and theoretical exploration of devices and technological innovations. These include acoustic spintronics, surface acoustic wave (SAW)-assisted spin transfer torque (STT) switching, SAW-assisted all-optical switching (AOS), SAW-driven spin textures (e.g., Skyrmions and domain walls), acoustic Terahertz emitters, SAW magnetic field sensors, magnetoelastic antenna, on-demand magnonic crystals, and so on. Focusing on the translation of many fundamental research breakthroughs into potential technological applications, we identify the key challenges and opportunities in the field, which we hope may motivate further research efforts of moving scientific discoveries toward real applications.
The magnetoelastic effect—the change of magnetic properties caused by the elastic deformation of a magnetic material—has been proposed as an alternative approach to magnetic fields for the low-power control of magnetization states of nanoelements since it avoids charge currents, which entail ohmic losses. Here, we have studied the effect of dynamic strain accompanying a surface acoustic wave on magnetic nanostructures in thermal equilibrium. We have developed an experimental technique based on stroboscopic X-ray microscopy that provides a pathway to the quantitative study of strain waves and magnetization at the nanoscale. We have simultaneously imaged the evolution of both strain and magnetization dynamics of nanostructures at the picosecond time scale and found that magnetization modes have a delayed response to the strain modes, adjustable by the magnetic domain configuration. Our results provide fundamental insight into magnetoelastic coupling in nanostructures and have implications for the design of strain-controlled magnetostrictive nano-devices.
The spectroscopic LEEM-PEEM experimental station at the CIRCE helical undulator beamline, which started user operation at the ALBA Synchrotron Light Facility in 2012, is presented. This station, based on an Elmitec LEEM III microscope with electron imaging energy analyzer, permits surfaces to be imaged with chemical, structural and magnetic sensitivity down to a lateral spatial resolution better than 20 nm with X-ray excited photoelectrons and 10 nm in LEEM and UV-PEEM modes. Rotation around the surface normal and application of electric and (weak) magnetic fields are possible in the microscope chamber. In situ surface preparation capabilities include ion sputtering, high-temperature flashing, exposure to gases, and metal evaporation with quick evaporator exchange. Results from experiments in a variety of fields and imaging modes will be presented in order to illustrate the ALBA XPEEM capabilities.
The control of the magnetization of a material with an electric field would make the design and the integration of novel electronic devices possible. This explains the renewed interest in multiferroic materials. Progress in this field is currently hampered by the scarcity of the materials available and the smallness of the magnetoelectric effects. Here we present a proof-of-principle experiment showing that engineering large strains through nanoscale size reduction is an efficient route for increasing magnetoelectric coefficients by orders of magnitude. The archetype magnetoelectric material, Cr2O3, in the form of epitaxial clusters, exhibits an unprecedented 600% change in magnetization magnitude under 1 V. Furthermore, a multiferroic phase, with both magnetic and electric spontaneous polarizations, is found in the clusters, while absent in the bulk.
The discovery of the spin-torque effect has made magnetic nanodevices realistic candidates for active elements of memory devices and applications. Magnetoresistive effects allow the read-out of increasingly small magnetic bits, and the spin torque provides an efficient tool to manipulate - precisely, rapidly and at low energy cost - the magnetic state, which is in turn the central information medium of spintronic devices. By keeping the same magnetic stack, but by tuning a device's shape and bias conditions, the spin torque can be engineered to build a variety of advanced magnetic nanodevices. Here we show that by assembling these nanodevices as building blocks with different functionalities, novel types of computing architecture can be envisaged. We focus in particular on recent concepts such as magnonics and spintronic neural networks.
We demonstrate in situ 90° electric field-induced uniform magnetization rotation in single domain submicron ferromagnetic islands grown on a ferroelectric single crystal using x-ray photoemission electron microscopy. The experimental findings are well correlated with micromagnetic simulations, showing that the reorientation occurs by the strain-induced magnetoelectric interaction between the ferromagnetic nanostructures and the ferroelectric crystal. Specifically, the ferroelectric domain structure plays a key role in determining the response of the structure to the applied electric field, resulting in three strain-induced regimes of magnetization behavior for the single domain islands.
The control of magnetic order in nanoscale devices underpins many proposals for integrating spintronics concepts into conventional electronics. A key challenge lies in finding an energy-efficient means of control, as power dissipation remains an important factor limiting future miniaturization of integrated circuits. One promising approach involves magnetoelectric coupling in magnetostrictive/piezoelectric systems, where induced strains can bear directly on the magnetic anisotropy. While such processes have been demonstrated in several multiferroic heterostructures, the incorporation of such complex materials into practical geometries has been lacking. Here we demonstrate the possibility of generating sizeable anisotropy changes, through induced strains driven by applied electric fields, in hybrid piezoelectric/spin-valve nanowires. By combining magneto-optical Kerr effect and magnetoresistance measurements, we show that domain wall propagation fields can be doubled under locally applied strains. These results highlight the prospect of constructing low-power domain wall gates for magnetic logic devices.
Investigations into fast magnetization switching are of both fundamental and technological interest. Here we present a low-power, remote method for strain driven magnetization switching. A surface acoustic wave propagates across an array of ferromagnetic elements, and the resultant strain switches the magnetization from the easy axis into the hard axis direction. Investigations as a function of applied magnetic field as well as unidirectional anisotropy (the exchange bias) reveal excellent agreement with prediction, confirming the viability of this method.
We show that the resonant coupling of phonons and magnons can be exploited to generate spin currents at room temperature. Surface acoustic wave pulses with a frequency of 1.55 GHz and duration of 300 ns provide coherent elastic waves in a ferromagnetic thin-film-normal-metal (Co/Pt) bilayer. We use the inverse spin Hall voltage in the Pt as a measure for the spin current and record its evolution as a function of time and external magnetic field magnitude and orientation. Our experiments show that a spin current is generated in the exclusive presence of a resonant elastic excitation. This establishes acoustic spin pumping as a resonant analogue to the spin Seebeck effect.
Surface acoustic waves (SAW) in the GHz frequency range are exploited for the
all-elastic excitation and detection of ferromagnetic resonance (FMR) in a
ferromagnetic/ferroelectric (nickel/lithium niobate) hybrid device. We measure
the SAW magneto-transmission at room temperature as a function of frequency,
external magnetic field magnitude, and orientation. Our data are well described
by a modified Landau-Lifshitz-Gilbert approach, in which a virtual,
strain-induced tickle field drives the magnetization precession. This causes a
distinct magnetic field orientation dependence of elastically driven FMR that
we observe in both model and experiment.
Piezoelectric materials, which convert mechanical to electrical energy and vice versa, are typically characterized by the
intimate coexistence of two phases across a morphotropic phase boundary. Electrically switching one to the other yields large
electromechanical coupling coefficients. Driven by global environmental concerns, there is currently a strong push to discover
practical lead-free piezoelectrics for device engineering. Using a combination of epitaxial growth techniques in conjunction
with theoretical approaches, we show the formation of a morphotropic phase boundary through epitaxial constraint in lead-free
piezoelectric bismuth ferrite (BiFeO3) films. Electric field–dependent studies show that a tetragonal-like phase can be reversibly converted into a rhombohedral-like
phase, accompanied by measurable displacements of the surface, making this new lead-free system of interest for probe-based
data storage and actuator applications.
We report on the coupling between ferroelectric and magnetic order parameters in a nanostructured BaTiO3-CoFe2O4 ferroelectromagnet. This facilitates the interconversion of energies stored in electric and magnetic fields and plays an important role in many devices, including transducers, field sensors, etc. Such nanostructures were deposited on single-crystal SrTiO3 (001) substrates by pulsed laser deposition from a single Ba-Ti-Co-Fe-oxide target. The films are epitaxial in-plane as well as out-of-plane with self-assembled hexagonal arrays of CoFe2O4 nanopillars embedded in a BaTiO3 matrix. The CoFe2O4 nanopillars have uniform size and average spacing of 20 to 30 nanometers. Temperature-dependent magnetic measurements illustrate the coupling between the two order parameters, which is manifested as a change in magnetization at the ferroelectric Curie temperature. Thermodynamic analyses show that the magnetoelectric coupling in such a nanostructure can be understood on the basis of the strong elastic interactions between the two phases.
Today's electronic gadgets, such as digital cameras and mp3 players, often contain three different types of memory, because
none of these memory types-static random access memory, dynamic random access memory, and Flash-can fulfill all memory requirements
of these devices. In his Perspective,
Ã
kerman
discusses magnetoresistive random access memory (MRAM), a promising candidate for a "universal memory" that combines all the
strengths and none of the weaknesses of the existing memory types. Several companies have succeeded in creating multi-megabyte
MRAM prototypes, suggesting that large-scale introduction of MRAM devices to the market is not far off.
Surface acoustic waves (SAWs) traveling on the surface of a piezoelectric substrate are capable of exciting magnetoelastic ferromagnets into resonance. In this work, we explore the effects of magnetoelastic film thickness on the coupling of SAWs into such magnetic thin films. We find that power absorption as a function of film thickness begins to saturate above thicknesses of ≈20 nm. This is contrary to current models that predict an exponential increase of the absorption as a function of increasing film thickness. We show that the saturation happens due to an increase in the damping of the film beyond this thickness range.
A variety of custom-built sample holders offer users a wide range of non-standard measurements at the ALBA synchrotron PhotoEmission Electron Microscope (PEEM) experimental station. Some of the salient features are: an ultrahigh vacuum (UHV) suitcase compatible with many offline deposition and characterization systems, built-in electromagnets for uni- or biaxial in-plane (IP) and out-of-plane (OOP) fields, as well as the combination of magnetic fields with electric fields or current injection. Electronics providing a synchronized sinusoidal signal for sample excitation enable time-resolved measurements at the 500 MHz storage ring RF frequency.
Precessional switching allows subnanosecond and deterministic reversal of magnetic data bits. It relies on triggering a large-angle, highly nonlinear precession of magnetic moments around a bias field. Here we demonstrate that a surface acoustic wave (SAW) propagating on a magnetostrictive semiconducting material produces an efficient torque that induces precessional switching. This is evidenced by Kerr microscopy and acoustic behavior analysis in a (Ga,Mn)(As,P) thin film. Using SAWs should therefore allow remote and wave control of individual magnetic bits at potentially GHz frequencies.
The propagation of mechanical disturbances in gases, fluids and solids has been studied for many centuries, and has attracted the attention of such celebrated physicists as Newton, Poisson, Hooke, Stokes and Maxwell. Although never enjoying the significance or popularity of electromagnetic wave propagation, it received increased interest in the late eighteenth and early nineteenth centuries with the investigations of RAYLEIGH [1], LOVE [2], STONELEY [3] and others into seismic wave propagation. It was during the course of such studies that exactly one hundred years ago Lord Rayleigh showed theoretically that a mechanical disturbance can propagate unperturbed on the surface of a semi-infinite isotropic solid, in many respects like waves on the sea. Rayleigh conjectured that such surface waves would be especially significant in seismic wave propagation because, unlike bulk acoustic waves, they would only diffract on the surface, not into the volume of the earth. In this conjecture he was partly correct, but it was shown later by LOVE [2] that surface waves arising from the layered structure of the earth’s surface (and having a different particle motion, as discussed later) are probably more important in this respect. A number of other studies have been undertaken over the years in an attempt to define the modes of propagation of specific mechanical structures, such as the work of Pochhamner and Chree on cylindrical bars, and of STONELEY [3] on the waves that can propagate at a plane boundary between two solids.
Graphene has intrigued the science community by many unique properties not found in conventional materials. In particular, it is the strongest two-dimensional material ever measured, being able to sustain reversible tensile elastic strain larger than 20%, which yields an interesting possibility to tune the properties of graphene by strain and thus opens a new field called “straintronics”. In this article, the current progress on strain engineering of graphene is reviewed. We first summarize the strain effects on the electronic structure and Raman spectra of graphene. We then highlight the electron-phonon coupling greatly enhanced by the biaxial strain and the strong pseudomagnetic field induced by the non-uniform strain with specific distribution. Finally, the potential application of strain-engineering in the self-assembly of foreign atoms on graphene surface is also discussed. Given the short history of graphene straintronics research, the current progress has been notable, and many further advances in this field are expected.
A phenomenon that can be exploited for the manipulation of magnetization without the conventional current-generated magnetic fields is magnetoelastic coupling, which might, thus, pave the way for low-power data-storage devices. Here, we report a quantitative analysis of the magnetic uniaxial anisotropy induced by piezoelectric strain in Ni nanostructured squares. By applying strain, the magnetic domains in Ni nanostructured squares can be manipulated by the magnetoelastic effect in the Ni. The strain-induced anisotropy displaces the domain walls in the square leading to changes in the domain sizes. By comparing the experiments with micromagnetic simulations, the resulting uniaxial anisotropy is quantified. We find a good agreement for a magnetostrictive constant of λs=-26 ppm, confirming a full strain transfer from the piezoelectric to the Ni nanostructures and the retainment of a bulklike λs.
Electric-field-controlled tunneling magnetoresistance (TMR) of magnetic tunnel junctions is considered as the milestone of ultralow power spintronic devices. Here, reversible, continuous magnetization rotation and manipulation is reported for TMR at room temperature in CoFeB/AlOx /CoFeB/piezoelectric structure by electric fields without the assistance of a magnetic field through strain-mediated interaction. These results provide a new way of exploring electric-field-controlled spintronics.
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We report here on the influence of surface acoustic waves (SAWs) on the magnetization of a Mn12-acetate single crystal. The crystal was mounted on the surface of a piezoelectric LiNbO3 substrate containing an interdigital transducer for the excitation of SAWs. The magnetization of the crystal was measured using a rf superconducting quantum interference device with a time resolution of 1 μs. The piezoelectric material was excited by SAW pulses of different frequencies produced by applying microwave pulses to the transducer. Our data show that molecular magnets onto the LiNbO3 surface can be used as very sensitive detectors of the SAW frequency and intensity.
The vortex state, characterized by a curling magnetization, is one of the equilibrium configurations of soft magnetic materials and occurs in thin ferromagnetic square and disk-shaped elements of micrometre size and below. The interplay between the magnetostatic and the exchange energy favours an in-plane, closed flux domain structure. This curling magnetization turns out of the plane at the centre of the vortex structure, in an area with a radius of about 10 nanometres--the vortex core. The vortex state has a specific excitation mode: the in-plane gyration of the vortex structure about its equilibrium position. The sense of gyration is determined by the vortex core polarization. Here we report on the controlled manipulation of the vortex core polarization by excitation with small bursts of an alternating magnetic field. The vortex motion was imaged by time-resolved scanning transmission X-ray microscopy. We demonstrate that the sense of gyration of the vortex structure can be reversed by applying short bursts of the sinusoidal excitation field with amplitude of about 1.5 mT. This reversal unambiguously indicates a switching of the out-of-plane core polarization. The observed switching mechanism, which can be understood in the framework of micromagnetic theory, gives insights into basic magnetization dynamics and their possible application in data storage.