Strain-induced Anisotropy Research Articles

Excitation of the Gyrotropic Mode in a Magnetic Vortex by Time-Varying Strain.

We demonstrate the excitation of the gyrotropic mode in a magnetostrictive vortex by time-varying strain. The vortex dynamics is driven by a time-varying voltage applied to the piezoelectric substrate and detected electrically by spin rectification at subthreshold values of rf current. When the frequency of the time-varying strain matches the gyrotropic frequency at a given in-plane magnetic field, the strain-induced in-plane magnetic anisotropy leads to a resonant excitation of the gyration dynamics in the magnetic vortex. We show that nonlinear gyrotropic dynamics can be excited already for moderate amplitudes of the time-varying strain.

Strain-induced giant enhancement and transformation of magnetocrystalline anisotropy in 1T-FeSe2 monolayer

The development of magnetic materials with large magnetic anisotropy energy (MAE) is crucial for magnetic storage and spintronics applications. The electronic structure and magnetic properties of 1T-FeSe2 monolayer have been systematically investigated by first-principles computational methods. The monolayer 1T-FeSe2 is confirmed to be a ferromagnetic semimetal with 100% spin polarization and exhibits pronounced magnetic anisotropy. It shows an easy magnetization plane in the two-dimensional plane and its MAE is as high as 8.134 meV. The MAE mainly originates from the contributions of transitions between Fe dyz and dz2, dxy and dx2−y2 orbitals, while the contributions of transitions between p x and p y, p x and p z orbitals of Se are also obvious. The biaxial strain significantly regulates the magnetic properties and electronic structures of the 1T-FeSe2 monolayer. Interestingly, the MAE increases significantly to 77.680 meV when the tensile strain increases to 3.5%, we can expect to achieve a reasonable value of MAE up to 98.951 meV by applying 5% tensile strain. In addition, the easy magnetization plane changes to perpendicular easy magnetization axis direction at −6% compressive strain or 7% tensile strain, which leads to a significant change in the band structure near the Fermi level. The 100% spin-polarized ferromagnetic semi-metallic behavior and the huge strain-induced magnetic anisotropy make the 1T-FeSe2 monolayer material promising for spintronics and magnetic data storage.

Switching the spin cycloid in BiFeO3 with an electric field

Bismuth ferrite (BiFeO3) is a multiferroic material that exhibits both ferroelectricity and canted antiferromagnetism at room temperature, making it a unique candidate in the development of electric-field controllable magnetic devices. The magnetic moments in BiFeO3 are arranged into a spin cycloid, resulting in unique magnetic properties which are tied to the ferroelectric order. Previous understanding of this coupling has relied on average, mesoscale measurements. Using nitrogen vacancy-based diamond magnetometry, we observe the magnetic spin cycloid structure of BiFeO3 in real space. This structure is magnetoelectrically coupled through symmetry to the ferroelectric polarization and this relationship is maintained through electric field switching. Through a combination of in-plane and out-of-plane electrical switching, coupled with ab initio studies, we have discovered that the epitaxy from the substrate imposes a magnetoelastic anisotropy on the spin cycloid, which establishes preferred cycloid propagation directions. The energy landscape of the cycloid is shaped by both the ferroelectric degree of freedom and strain-induced anisotropy, restricting the spin spiral propagation vector to changes to specific switching events.

Continuous strain-mediated perpendicular magnetic anisotropy in epitaxial (111) CoFe2O4 thin films grown on LiTaO3 substrates

Perpendicular magnetic anisotropy (PMA) in magnetic thin films has attracted much attention due to its potential applications in spintronics devices. Here, we report the continuous strain-mediated PMA in epitaxial (111) CoFe2O4 (CFO) thin films grown on (0001) LiTaO3 substrates. A large variation in lattice strain (∼0.9%) in a continuous way is realized in the CFO thin films by changing substrate temperature during deposition due to the difference in the thermal expansion coefficient between CFO and LiTaO3. As a result, the PMA of the (111) CFO thin films can be continuously mediated by the strain with uniaxial magnetic anisotropy energy in the range of 0.12-14.69×106 erg/cm3. Furthermore, the strain as well as the consequent PMA in the (111) CFO thin films can be maintained within the thickness of 25–205 nm, which is consistent with the scenario of the magnetoelastic effect. Our results reveal that the CFO/LiTaO3 system can be regarded as an ideal platform to realize robust PMA and its continuous strain tuning in the (111) CFO thin films by virtue of strain-induced magnetic anisotropy.

Strain-induced magnetic anisotropy of multi-domain epitaxial EuPd2 thin films

Europium intermetallic compounds show a variety of different ground states and anomalous physical properties due to the interactions between the localized 4f electrons and the delocalized electronic states. Europium is also the most reactive of the rare earth metals which might be the reason why very few works are concerned with the properties of Eu-based thin films. Here we address the low-temperature magnetic properties of ferromagnetic EuPd2 thin films prepared by molecular beam epitaxy. The epitaxial (111)-oriented thin films grow on MgO (100) with eight different domain orientations. We analyze the low-temperature magnetic hysteresis behavior by means of micromagnetic simulations taking the multi-domain morphology explicitly into account and quantify the magnetic crystal anisotropy contribution. By ab initio calculations we trace back the microscopic origin of the magnetic anisotropy to thin film-induced uniform biaxial strain.

Ultra-highly efficient SOT-writing in MTJs with strain-induced magnetic anisotropy

In order to break through limits of conventional MRAMs, MTJs with strain-induced magnetic anisotropy were intensively tested as SOT-MRAM cells. Small critical switching-current of 10–25 μA and switching-voltage of about 0.055 V, and almost no retention energy dependence of them were predicted and confirmed by experiments. Finally, high write efficiency of 1750 kBT/V (4.1 kBT/μA) and high write-power efficiency of 100 [kBT/(μA·V)] were obtained.

Excellent temperature dependence of retention energy and large tunnel magnetoresistance of MTJs with strain-induced magnetic anisotropy for SOT-MRAMs with high write efficiency

Excellent temperature dependence of retention energy and large tunnel magnetoresistance of MTJs with strain-induced magnetic anisotropy for SOT-MRAMs with high write efficiency

Nanosized antiferromagnetic domains in NiO/Ag(001) films probed by polarised X-ray and unpolarised low-energy electrons

Surface antiferromagnetism of NiO film, epitaxially deposited on Ag(001), is investigated using the absorption of linearly polarised X-ray and the diffraction of low-energy unpolarised electrons. Polarisation-dependent X-ray absorption studies reveal a preferential stabilisation of the magnetic domains with the main components of magnetic axes parallel to the interface. Furthermore, the role of interfacial strain-induced magnetocrystalline anisotropy below a critical film thickness (∼ 20 ML) could be demonstrated by the magnetic linear dichroism results. Utilising the exchange scattered electrons from various antiferromagnetic (111) planes, the magnetic twin domains could be probed by a low energy electron microscope operating in the dark-field mode. The linearly polarised X-ray, on the other hand, is sensitive to the magnetic axes orientations of Ni2+ ions and therefore, provides access to the spin domains as well as the twin domains. Comparison of results of these two independent imaging techniques demonstrates that the in-plane component of the magnetic easy axes contained by the complementary T-domains are parallel to each other.

Energy Efficient All-Electric-Field-Controlled Multiferroic Magnetic Domain-Wall Logic.

Magnetic domain wall (DW)-based logic devices offer numerous opportunities for emerging electronics applications allowing superior performance characteristics such as fast motion, high density, and nonvolatility to process information. However, these devices rely on an external magnetic field, which limits their implementation; this is particularly problematic in large-scale applications. Multiferroic systems consisting of a piezoelectric substrate coupled with ferromagnets provide a potential solution that provides the possibility of controlling magnetization through an electric field via magnetoelastic coupling. Strain-induced magnetization anisotropy tilting can influence the DW motion in a controllable way. We demonstrate a method to perform all-electrical logic operations using such a system. Ferromagnetic coupling between neighboring magnetic domains induced by the electric-field-controlled strain has been exploited to promote noncollinear spin alignment, which is used for realizing essential building blocks, including DW generation, propagation, and pinning, in all implementations of Boolean logic, which will pave the way for scalable memory-in-logic applications.

Strain-induced magnetic anisotropy in Heusler alloys studied from first principles

We report the microscopic origin of strain-mediated changes in the magnetocrystalline anisotropy energy of the Co2FeSi, Co2MnSi, and Fe3Si Heusler alloys from the viewpoint of first-principles electron theory. Both Co2FeSi and Co2MnSi have similar anisotropy changes upon induced strain within the (001) plane, where the quadrupole moment due to Co minority-spin states dominates the anisotropy modulation, and, thus, giant magnetoelectric couplings in multiferroic heterointerfaces containing these compounds. In contrast, the strain-induced anisotropy modulation in Fe3Si has mixed contributing factors not limited to the anisotropy term of the orbital magnetic moment and the quadrupole term.

Multiscale modeling of the strain-induced α → β phase transition and piezoelectric activity in semi-crystalline poly(vinylidene fluoride) over a large-strain range

The present contribution investigates within a hierarchical multi-scale approach the relation between the macroscopic properties of semi-crystalline poly (vinylidene fluoride) (PVDF) and the transition from the α-phase to the β-phase crystalline structure in the course of a mechanical loading. A continuum-based constitutive model is proposed to capture the large-strain material behavior of PVDF, the amorphous molecular network orientation/relaxation process, the strain-induced morphological anisotropy induced by the crystallographic texturing and the strain-induced α → β phase transition; the mechanical coupling between the deformation modes in α-PVDF, β-PVDF and amorphous domains is obtained by considering the interfacial interactions in the micro-macro homogenization procedure. Fundamental parameters from molecular dynamics simulations, such as the amorphous, α crystalline and β crystalline properties along with the onset of α → β phase transition caused by the straining of α-PVDF, are transferred to the continuum scale. By completing the scale bridging method by the identification of the other model parameters, the model outputs are found in good agreement with the existing tensile experiments of a PVDF upon large-strain plastic deformation. The profound influence of the strain-induced α → β phase transition and the crystallographic texturing operated upon large-strain plastic deformation in the piezoelectric activity is finally investigated numerically.

The Effect of Size and Strain on Micro Stripe Magnetic Domain Structure of CoFeB Thin Films

The prerequisite for flexible magnetic electronic devices is the knowledge of the preparation technology of flexible magnetic films and the evolution of the film properties under strain. In this work, CoFeB amorphous ferromagnetic films with stripe domains were prepared on flexible polyimide (PI) substrates by oblique sputtering. The results show that oblique sputtering induces the formation of columnar crystal structure in CoFeB films, which increases the perpendicular magnetic anisotropy of the films, thus leading to the appearance of stripe magnetic domain structures. On this basis, the CoFeB films with stripe domains were processed on a microscopic scale to investigate the size effect and strain regulation on the microscopic domain structure of the magnetic films. The characterization of the magnetic domain structure shows that the stripe domain contrast is reduced by the striped structure prepared by lithography. The triangular, circular and ring patterns deflect the alignment of the stripe domain to different degrees. The experimental results show that the deflection of the stripe domains is caused by the anisotropy of the shapes produced by the different patterns and that the size of the microstructure needs to be close to the period of the stripe domains for the size effect to be significant. In addition, the strain-induced magnetoelastic anisotropy effectively rotates the orientation of the stripe domains, and the variation in domain contrast demonstrates that tensile/compressive strains vary the magnitude of the out-of-plane stray field of the film. Our results provide some insight into the modulation of the physical properties of flexible magnetic films.

A strain-controlled magnetostrictive pseudo spin valve

Electric-field control of magnetism via an inverse magnetostrictive effect is an alternative path toward improving energy-efficient storage and sensing devices based on a giant magnetoresistance effect. In this Letter, we report on lateral electric-field driven strain-mediated modulation of magnetotransport properties in a Co/Cu/Py pseudo spin valve grown on a ferroelectric 0.7Pb[Mg1/3Nb2/3)]O3–0.3PbTiO3 substrate. We show a decrease in the giant magnetoresistance ratio of the pseudo spin valve with the increase in the electric field, which is attributed to the deviation of the Co layer magnetization from the initial direction due to strain-induced magnetoelastic anisotropy contribution. Additionally, we demonstrate that strain-induced magnetic anisotropy effectively shifts the switching field of the magnetostrictive Co layer, while keeping the switching field of the nearly zero-magnetostrictive Py layer unaffected due to its negligible magnetostriction. We argue that magnetostrictively optimized magnetic films in properly engineered multilayered structures can offer a path to enhancing the selective magnetic switching in spintronic devices.

A small-strain soil constitutive model for initial stiffness evaluation of laterally loaded piles in drained marine sand

Three-Dimensional Finite-Element Method (3-D FEM) analyses were performed to investigate the initial stiffness of the laterally loaded piles in drained marine sand. A small-strain soil constitutive model developed based on Menq model was implemented in the 3-D FEM model to simulate the response of laterally-loaded piles in the sand over a range of small displacements. The model incorporated how the shear modulus for sand increased with the mean effective stress and decreased non-linearly with the increasing shear strain. The procedure of calibrating the model was proposed based on fitting the measurements from both laboratory tests and in-situ site investigation This soil constitutive model was implemented within a 3-D FEM analysis using an isotropic material with strain-induced anisotropy behavior in order to simulate the response of laterally-loaded piles under monotonic loading. A comparison of model predictions with laboratory and field test results indicated that this model had the promise to simulate the non-linear relationship between lateral load and pile-head deflection for mudline deflections ranging from less than 0.25-percent of the pile diameter.

Reduced effective magnetization and damping by slowly relaxing impurities in strained γ-Fe2O3 thin films

Magnetically ordered insulators are of key interest for spintronics applications, but most of them have not yet been explored in depth regarding their magnetic properties, in particular with respect to their dynamic response. We study the static and dynamic magnetic properties of epitaxially strained γ-Fe2O3 (maghemite) thin films grown via pulsed-laser deposition on MgO substrates by SQUID magnetometry and cryogenic broadband ferromagnetic resonance experiments. SQUID magnetometry measurements reveal hysteretic magnetization curves for magnetic fields applied both in- and out of the sample plane. From the magnetization dynamics of our thin films, we find a small negative effective magnetization in agreement with a strain induced perpendicular magnetic anisotropy. Moreover, we observe a non-linear evolution of the ferromagnetic resonance-linewidth as a function of the microwave frequency and explain this finding with the so-called slow relaxor model. We investigate the magnetization dynamics and non-linear damping mechanisms present in our samples as a function of frequency and temperature and in particular, observe a sign change in the effective magnetization from the transition of the magnetic anisotropy from a perpendicular easy axis to an easy in-plane anisotropy for reduced temperatures. Its nonlinear damping properties and strain-induced perpendicular anisotropy render γ-Fe2O3 an interesting material platform for spintronics devices.

Strain-induced dielectric anisotropy of polymers for rapid and sensitive monitoring of the small elastic strain

Stretching is the most important way of forming polymer films, and the mechanism of stretch-induced structural evolution is a long-standing fundamental issue in the stretching process of polymer films. Current methods are limited in sampling frequency or transparent materials and difficult to in situ measure the molecular chain structure evolution. In this study, we proposed a new principle for measuring the stretching process of polymer materials based on dielectric anisotropy. Firstly, the strain-dielectric anisotropy linear relation during small elastic deformation was found, and an extremely small strain (∼0.1%) can be measured with a high sampling frequency (∼8.3 Hz). Then, the molecular chain orientation-induced dielectric anisotropy for the underlying mechanism was confirmed by molecular dynamics simulations and in situ polarized Raman spectroscopy experiments. The proposed method was successfully applied in measuring tube compliance and dynamic health monitoring by measuring small elastic strain accurately, exhibiting the potential for wide applications.

Strain-induced shape anisotropy in antiferromagnetic structures

We demonstrate how shape-induced strain can be used to control antiferromagnetic order in NiO/Pt thin films. For rectangular elements patterned along the easy and hard magnetocrystalline anisotropy axes of our film, we observe different domain structures and we identify magnetoelastic interactions that are distinct for different domain configurations. We reproduce the experimental observations by modeling the magnetoelastic interactions, considering spontaneous strain induced by the domain configuration, as well as elastic strain due to the substrate and the shape of the patterns. This allows us to demonstrate and explain how the variation of the aspect ratio of rectangular elements can be used to control the antiferromagnetic ground state domain configuration. Shape-dependent strain does not only need to be considered in the design of antiferromagnetic devices, but can potentially be used to tailor their properties, providing an additional handle to control antiferromagnets.

A multi-scale plastic-damage model for strain-induced morphological anisotropy in semi-crystalline polyethylene

In this article, we present a multi-scale plastic-damage model for strain-induced morphological anisotropy in semi-crystalline polyethylene formulated within a continuum-based micromechanical framework. The crystallographic shear in the crystalline lamellae and the molecular alignment/relaxation of the amorphous phase are two underlying inelastic processes integrated in the constitutive representation. The cavitation damage accumulation related to the progressive nucleation and anisotropic growth of nano-sized cavities in the amorphous phase is also integrated and treated separately for the elastic–viscoplastic intermolecular interactions and for the viscohyperelastic network interactions. The mechanical coupling between the deformation modes in the amorphous and crystalline domains is obtained by considering the crystalline–amorphous interfacial interaction in the micro-macro homogenization procedure. We compare the model output to tensile experimental observations from the literature of high-density polyethylene, in terms of stress–strain response and inelastic volumetric strain, during stretching and stretching-retraction-recovery sequences upon large-strain plastic deformation at different strain rates and temperatures. We analyze the effect of strain-induced morphological anisotropy on the internal cavitation damage distributions by means of pole figures. The key role of the cavitation damage on the macroscopic and internal responses is studied.

Polarized Moiré Phonon and Strain-Coupled Phonon Renormalization in Twisted Bilayer MoS2

The ability to control the atomic-scale behavior in quasi-two-dimensional crystals via twisting the vicinal layers and generating patterned moiré potentials provides an additional degree of freedom to study correlated physics. Under the existence of macroscopic external parameters such as strain, we study the high-frequency Raman active vibrational modes of chemical vapor deposition (CVD)-grown monolayer, stacked (AA′ and AB), and twisted bilayer MoS2. The polarized Raman scattering, being an efficient characterization tool for these moiré structures, is employed to investigate the effects of strain on the different phonon modes. The recently discovered moiré phonons (FA1g), i.e., zone center phonons in a twisted bilayer MoS2, stem from the folding of the off-center phonons of its monolayer constituents and are found to be Raman polarized. Moreover, the Raman shift impressions of these moiré phonons in twist angle in the range between 0 and 30° are reproduced, showing the strain-coupled modifications of the previously reported sinusoidal behavior. Our findings establish an effective method to probe the strain-induced anisotropy in twisted van der Waals systems using polarized Raman spectroscopy showing future directions to moiré-related physics.

Strain Control of Magnetic Anisotropy in Yttrium Iron Garnet Films in a Composite Structure with Yttrium Aluminum Garnet Substrate

This report is on the nature of strain in thin films of yttrium iron garnet (YIG) on yttrium aluminum garnet (YAG) substrates due to film-substrate lattice mismatch and the resulting induced magnetic anisotropy. Films with thickness 55 nm to 380 nm were deposited on (100), (110), and (111) YAG substrates using pulsed laser deposition (PLD) techniques and characterized by structural and magnetic characterization techniques. The in-plane strain determined to be compressive using X-ray diffraction (XRD). It varied from −0.12% to −0.98% and increased in magnitude with increasing film thickness and was relatively large in films on (100) YAG. The out-of-plane strain was tensile and also increased with increasing film thickness. The estimated strain-induced magnetic anisotropy field, found from XRD data, was out of plane; its value increased with film thickness and ranged from 0.47 kOe to 3.96 kOe. Ferromagnetic resonance (FMR) measurements at 5 to 21 GHz also revealed the presence of a perpendicular magnetic anisotropy that decreased with increasing film thickness and its values were smaller than values obtained from XRD data. The PLD YIG films on YAG substrates exhibiting a perpendicular anisotropy field have the potential for use in self-biased sensors and high-frequency devices.