Variation Of Magnetic Anisotropy Research Articles

Dimensionality engineering dependence of vertical magnetization shift and magnetic anisotropy evolution in manganite superlattices

The manganite superlattices consisting of a ferromagnetic La0.7Sr0.3MnO3 (LSMO) metal and a G-type antiferromagnetic SrMnO3 (SMO) insulator have been investigated in this work. Two very intriguing effects have been observed in these LSMO/SMO superlattices. The first one is the coexistence of the robust exchange bias and the evident vertical magnetization shift in the thicker superlattices. The second is the magnetic anisotropy variation from planar to perpendicular with increasing SMO layer thickness. These two novel phenomena have been confirmed by the electric transport anisotropy measurements and the microscopic magnetic domain switching images. Moreover, the X-ray linear dichroism experiments and first-principle calculations have revealed the orbital reconstruction dependence on the SMO layer thickness. It is noteworthy that the vertical magnetization shift and perpendicular magnetic anisotropy are usually observed in intrinsic out-of-plane magnetic easy axis materials such as SrRuO3-based heterostructures. However, the spins within LSMO films preferably orient towards the in-plane direction. Therefore, a new microscopic method for vertical magnetization shift and magnetic anisotropy manipulation has been developed, enabling the discovery of emergent phenomena as well as the control of the magnetic properties.

Reversible Transformation between Isolated Skyrmions and Bimerons.

Skyrmions and bimerons are versatile topological spin textures that can be used as information bits for both classical and quantum computing. The transformation between isolated skyrmions and bimerons is an essential operation for computing architecture based on multiple different topological bits. Here we report the creation of isolated skyrmions and their subsequent transformation to bimerons by harnessing the electric current-induced Oersted field and temperature-induced perpendicular magnetic anisotropy variation. The transformation between skyrmions and bimerons is reversible, which is controlled by the current amplitude and scanning direction. Both skyrmions and bimerons can be created in the same system through the skyrmion-bimeron transformation and magnetization switching. Deformed skyrmion bubbles and chiral labyrinth domains are found as nontrivial intermediate transition states. Our results may provide a unique way for building advanced information-processing devices using different types of topological spin textures in the same system.

In situ manipulation of perpendicular magnetic anisotropy in half-metallic NiCo2O4 thin film by proton insertion

Herein we report the manipulation of perpendicular magnetic anisotropy (PMA), a very important technique for spintronics devices, which is achieved in an all-solid-state redox (reduction/oxidation) device. The device consisted of NiCo2O4 (NCO) thin film with a c-axis orientation and a proton-conducting polymer (Nafion). The PMA of NCO can be manipulated in situ with low voltage (V ≤ 0.7 V) due to the utilization of a proton-conducting solid electrolyte. The magnetic anisotropy variation was quantitatively evaluated by an anomalous Hall measurement. The magnetic anisotropy field of the NCO thin film was successfully manipulated in the range of 5%, although the rotation of the easy axis of magnetization was not observed. This modulation is attributed to the strain and electronic configuration variation by proton insertion. This manipulation method for PMA with the solid electrolyte enables a variety of reservoir states and contributes to lower power consumption and high-precision machine learning.

Neutron diffraction study and temperature variation of magnetic anisotropy in Bi substituted nickel ferrite

Bismuth doped NiFe2O4 (NFO) polycrystalline samples: Ni1-xBixFe2O4 with x = 0.00, 0.05 and 0.10, were synthesized using solid-state reaction method. The crystal structural, magnetic structure and magnetic properties of parent NFO and Bi-doped NFO samples were characterized using X-ray diffraction (XRD), neutron powder diffraction (NPD) and magnetization (M) versus magnetic field (H) isotherms, respectively. Rietveld refinement of XRD and NPD patterns confirms the formation of a single-phase cubic spinel crystal structure. No observable change in the lattice parameter was found (within the error limits) with the 5 at % Bi doping to NFO, whereas ≈0.15% change in the lattice parameter was observed in case of 10 at % Bi-doped NFO. Room temperature magnetic structure studies using NPD reveal a net magnetic moment of 1.55(3) μB/f. u., in the case of NiFe2O4. The net magnetic moment at room temperature changes to 1.61(4) μB/f.u. and 1.47(5) μB/f.u. for Ni0.95Bi0.05Fe2O4 and Ni0.90Bi0.10Fe2O4, respectively. The same observation was also inferred from the spontaneous magnetization (Msp) obtained from approach-to-saturation (ATS) analysis of M-H isotherms. Temperature (T) dependence of Msp follows a T2 decline. Approximately 14% increase in magnetic anisotropy was observed in Ni0.95Bi0.05Fe2O4 compared to NiFe2O4 in the temperature range 5 K -300 K.

Effects of the packing factor on magnetic anisotropy in a 3x3 array of square hollow Ni nanopillars

A study about the magnetic anisotropy in a 3x3 array, constituted by square nickel nanometric pillars (NPs), is present. The NPs geometry was defined by wide D = 30 nm and length L = 120 nm. The center to center NPs distance a = 35, 40, 50 and 80 nm, were chosen to create variations in magnetic anisotropy, originated by the hard dipolar interactions. A concentric central cavity was included, within the variables in the study, with lateral dimension of d = 0, 10 and 20 nm. The data was generated through micromagnetic simulation by using a ferromagnetic resonance (FMR) standard problem. The OOMMF package was used by applying an external magnetic field in parallel and perpendicular directions, relative to the pillars symmetry axis. The anisotropy field (HA) was obtained by the typical FMR uniaxial model. The HA values decrease with the center to center NPs distance for all cavity size. Based on previous reported studies in nanowires, the volumetric NPs packing (P) was related to HA. Thus, an empirical dependence of HA was proposed to describe the linear decrease with P. Even with the geometric characteristics of array it was evident, by the calculations with perpendicular field, that the simplified uniaxial model is no longer appropriate. Therefore, an analysis using the model composing two anisotropy constants (K1 and K2) was included. This proposal and the traditional uniaxial model are not suitable as the distance between the pillars decreases.

Understanding thickness dependent magnetic properties of Tb-Fe thin films

In this paper, we have attempted to understand the magnetic behaviour of Tb-Fe thin films grown with different thicknesses through detailed electron microscopy, magnetization measurements and magnetic domain imaging studies. Tb-Fe thin films over a range of thicknesses such as 50, 100, 200, 400, 600 and 800 nm were deposited on Si substrates using electron beam evaporation technique. Grazing incidence X-ray diffraction studies revealed that all the films were amorphous in nature. Microstructural studies using transmission electron microscopy showed columnar channel of voids extending from substrate to film surface. Magnetization measurements along the in-plane and out-of-plane direction indicated presence of out-of-plane magnetic anisotropy in all the films. Saturation magnetization and coercivity measured along the out-of-plane direction were found to vary with the thickness of the films. A very high coercivity of ~2000 mT was obtained for the film having thickness of 400 nm. Magnetic anisotropy calculations indicated a strong perpendicular magnetic anisotropy for the 400 nm thick film. The variation of magnetic anisotropy with film thickness was correlated with the magnetic domain patterns observed in the films using magnetic force microscopy.

Tunable microwave properties of a skyrmion in an isolated nanodisk

A magnetic skyrmion is a topological object having particle-spin like configuration, which can be stabilized in ferromagnetic systems with Dzyaloshinskii-Moriya interaction. In this report, we investigate tunable microwave properties of magnetic skyrmions in a circular nanodisk using micromagnetic simulations. An in-plane ac magnetic field excites gyrotropic dynamics with a clockwise and a counter-clockwise mode. We have shown the dependence of microwave responses on the size of the skyrmion by tuning the Dzyaloshinskii-Moriya interaction, magnetic anisotropy, and out-of-plane external bias magnetic field. A remarkably large frequency shift of 4.2 GHz has been observed for a change of 40 mT bias field. Furthermore, a broad microwave tunability of 10.8 GHz has been shown with a variation of magnetic anisotropy of 1 × 105 J/m3. The manifestation of inertial mass of the skyrmion, its dependence on the resonance responses and the sizes of the skyrmion are explained by using Thiele’s equation. The inertial mass of the skyrmion is estimated to be in the order of 10−23 kg. The results open up the potential use of the skyrmions in reconfigurable nano-magnonic devices with large tunability.

Thickness dependent magnetic properties of ferromagnetic films (Fe, Co) interfaced with Ta

In the present work, magnetic properties of ferromagnetic films viz. Co and Fe interfaced with Ta have been studied using magneto-optical Kerr effect (MOKE), X-ray reflectivity (XRR) and soft X-ray absorption spectroscopy (SXAS) on wedge shaped samples. Thickness of ferromagnetic film at different points of wedge sample as determined from XRR were employed to study the evolution of magnetic properties with increasing film thickness using MOKE measurements. Co film exhibits a uniaxial magnetic anisotropy on Ta which varies as a function of increasing film thickness, while no thickness dependent magnetic anisotropy has been observed for Fe film grown on Ta. The variation of coercivity with increasing film thickness is in agreement with the observed variation of magnetic anisotropy. SXAS measurements qualitatively support alloying / intermixing at interface. Thicknesses of interfacial magnetic dead layer as seen from variation of Kerr intensity with film thickness is relatively higher for Co as compared to Fe, while interfaced with Ta.

Electric Tuning of Magnetic Anisotropy and Exchange Bias of La0.8Sr0.2CoO3/La0.67Sr0.33MnO3 Bilayer Films

Heterostructures composed of dissimilar perovskite oxides with different properties provide an opportunity to observe emergent phenomena, and have promising applications. A key feature of oxide heterostructures is interfacial electronic and orbital reconstruction. In this paper, we demonstrate a dramatic variation in magnetic anisotropy caused by electric tuning of the charge-transfer process in a ${\mathrm{La}}_{0.8}{\mathrm{Sr}}_{0.2}{\mathrm{Co}\mathrm{O}}_{3}/{\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{Mn}\mathrm{O}}_{3}$ bilayer structure. By repeatedly changing the valence state of $\mathrm{Co}$ ions in the ${\mathrm{La}}_{0.8}{\mathrm{Sr}}_{0.2}{\mathrm{Co}\mathrm{O}}_{3}$ top layer using ionic-liquid gating, reversible switching of the magnetic easy axis of the bottom ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{Mn}\mathrm{O}}_{3}$ layer between the out-of-plane and the in-plane direction is achieved, accompanying a modulation of the interfacial exchange coupling. $\mathrm{Mn}$-to-$\mathrm{Co}$ charge transfer and its effect on the interfacial orbital occupancy are further confirmed by x-ray absorption spectroscopy and x-ray-linear-dichroism analysis. The considerable interfacial charge transfer causes an overlap of the $\mathrm{Mn}$ and $\mathrm{Co}$ 3d orbitals, resulting in orbital reconstruction in the ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{Mn}\mathrm{O}}_{3}$ layer and thus magnetic anisotropy. This work demonstrates a promising method for tuning the orbital occupancy and related properties of perovskite heterostructures.

Microscopic Origin of Magnetization Reversal in Nanoscale Exchange-Coupled Ferri/Ferromagnetic Bilayers: Implications for High Energy Density Permanent Magnets and Spintronic Devices.

Giant exchange bias shifts of several Tesla have been reported in ferrimagnetic/ferromagnetic bilayer systems, which could be highly beneficial for contemporary high energy density permanent magnets and spintronic devices. However, the lack of microscopic studies of the reversal owing to the difficulty of measuring few nanometer-wide magnetic structures in high fields precludes the assessment of the lateral size of the inhomogeneity in relation to the intended application. In this study, the magnetic reversal process of nanoscale exchange-coupled bilayer systems, consisting of a ferrimagnetic TbFeCo alloy layer and a ferromagnetic [Co/Ni/Pt]N multilayer, was investigated. In particular, minor loop measurements, probing solely on the reversal characteristics of the softer ferromagnetic layer, reveal two distinct reversal mechanisms, which depend critically on the thickness of the ferromagnetic layer. For thick layers, irreversible switching of the macroscopic minor loop is observed. The underlying microscopic origin of this reversal process was studied in detail by high-resolution magnetic force microscopy, showing that the reversal is triggered by in-plane domain walls propagating through the ferromagnetic layer. In contrast, thin ferromagnetic layers show a hysteresis-free reversal, which is nucleation-dominated due to grain-to-grain variations in magnetic anisotropy of the Co/Ni/Pt multilayer and an inhomogeneous exchange coupling with the magnetically hard TbFeCo layer, as confirmed by micromagnetic simulations.

Mechanism of Néel Order Switching in Antiferromagnetic Thin Films Revealed by Magnetotransport and Direct Imaging.

We probe the current-induced magnetic switching of insulating antiferromagnet-heavy-metal systems, by electrical spin Hall magnetoresistance measurements and direct imaging, identifying a reversal occurring by domain wall (DW) motion. We observe switching of more than one-third of the antiferromagnetic domains by the application of current pulses. Our data reveal two different magnetic switching mechanisms leading together to an efficient switching, namely, the spin-current induced effective magnetic anisotropy variation and the action of the spin torque on the DWs.

Multiple magnetic droplet soliton modes

Droplet solitons are large amplitude localized spin-wave excitations that can be created in magnetic thin films with uniaxial anisotropy by a spin-polarized current flowing through an electrical nanocontact. Here, we report a low-temperature (4 K) experimental study that shows there are multiple and, under certain conditions, combinations of droplet modes, each mode with a distinct high-frequency spin precession (tens of gigahertz). Low-frequency ($\ensuremath{\lesssim}1\phantom{\rule{0.28em}{0ex}}\mathrm{GHz}$) voltage noise is used to assess the stability of droplet modes. It is found that droplets are stable only in a limited range of applied field and current, typically near the current and field at which they nucleate, in agreement with recent predictions. Applied fields in the film plane favor multiple droplet modes, whereas fields perpendicular to the film plane tend to stabilize a single droplet mode. Micromagnetic simulations are used to show that spatial variation in the energy landscape in the nanocontact region (e.g., spatial variation of magnetic anisotropy or magnetic field) can lead to quantized droplet modes and low-frequency mode modulation, characteristics observed in our experiments.

The Origin and Evolution of Magnetic Fabrics in Mafic Sills

Studying extinct volcanoes where erosion has exposed dykes and sills provides direct access to the fossil remnants of magma movement, however linking crystallised magma to emplacement dynamics is challenging. This study investigates how magma flow varies across the thickness of a thin (6 metres thick) mafic sill. We use a high-resolution sampling regime to measure micro-scale variations in magnetic anisotropy, which is associated with the orientation of the magnetic particles present within the crystalline rock. Fieldwork was conducted on exposed sills of the British and Irish Palaeogene Igneous Province, Isle of Skye, Scotland. Here Jurassic sedimentary rocks have been intruded by a series of sills, of picrite to crinanite composition, from the Little Minch Sill Complex (c.60 Ma). Anisotropy of magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetisation (AARM) signals have been used to separate a crinanite sill into distinct magnetic groupings. We identified two AMS groups (the upper and lower sill margins, and the central region) and four AARM groups (the lower margin, the middle region, a region just below the upper margin, and the upper margin). Both AMS and AARM signals originate from titanomagnetite of multi-domain or vortex-state to single-domain sized grains, respectively. The AMS and AARM fabrics are aligned with each other in the margin regions preserving a history of magma flow towards the North during initial emplacement. However, in the sill interior the magnetic fabrics are oblique to each other, thus reflecting multiple origins. We suggest the AMS fabrics have recorded magma flow during sill growth, and AARM fabrics have recorded melt percolation flow as the interstitial melt migrated upwards through a solidifying crystal mush. We demonstrate that when AMS and AARM are used in combination they enable a detailed understanding of magma flow and solidification dynamics to be obtained, from initial emplacement to solidification. Overall, our detailed sampling and analysis indicates that magnetic fabrics can be highly variable over small distances, supporting the suggestion of horizontal flow restriction and propagation path migration within growing sills, and that previous reports of magma flow and solidification dynamics based on under-sampled bodies may require reconsideration.

Controllable magnetic anisotropy of ferromagnet/antiferromagnet bilayers coupled with piezoelectric strain

Coupling of magnetoelastic and piezoelectric effects is achieved with the ferromagnetic (FM) Co60Fe20B20/antiferromagnetic (AFM) Co0.7Ni0.3O bilayers deposited on the surface of piezoelectric single crystalline substrate. Piezoelectric strain of the substrate is successfully transferred to the magnetic layers, and controllable variation of magnetic anisotropy in response to the elastic deformation is demonstrated in the M-H loops measured at various azimuthal angles with respect to the direction of AFM spin alignment. Magnetic behaviors of the exchange coupled bilayers can be understood by considering the competition of many magnetic anisotropy energies within the magnetic film based on the modified Meiklejohn and Bean (MB) model. From the comparison of experimentally measured results with the modified MB model, it is found that the strain in the film from the substrate effectively introduces additional uniaxial anisotropy in the FM layer with its anisotropy direction and magnitude determined by the applied strain.

Variation of magnetic anisotropy and temperature-dependent FORC probing of compositionally tuned Co-Ni alloy nanowires

The magnetic microstructure of Co-Ni binary alloy nanowires electrodeposited with controlled composition into nanoporous aluminum oxide templates can be represented as an ensemble of stochastic magnetic domains, whose size is determined by the magnetic correlation length. Using a method based on the approaching of magnetization to saturation, we defined the dimension of regions with magnetic orientation coherency as the stochastic domain size. Based on the experimental measurements of magnetization curves near saturation and first order reversal curves (FORC), we described a relationship between the macroscopic and microscopic parameters of the nanowires depending on the crystal structure observed by high-resolution transmission electron microscopy in terms of the random anisotropy model. The alloy composition strongly determines the crystal structure, in particular, the grain size and hcp/fcc phase distribution, and influences the effective magnetic anisotropy energy providing the direction of easy magnetization in the nanowire arrays. The characterization of Co-Ni arrays by FORC method at room and low temperatures revealed the transformation of magnetic behavior and certain contributions to the energy of the effective magnetic anisotropy.

Magneto-ionic effect in CoFeB thin films with in-plane and perpendicular-to-plane magnetic anisotropy

The magneto-ionic effect is a promising method to control the magnetic properties electrically. Charged mobile oxygen ions can easily be driven by an electric field to modify the magnetic anisotropy of a ferromagnetic layer in contact with an ionic conductor in a solid-state device. In this paper, we report on the room temperature magneto-ionic modulation of the magnetic anisotropy of ultrathin CoFeB films in contact with a GdOx layer, as probed by polar micro-Magneto Optical Kerr Effect during the application of a voltage across patterned capacitors. Both Pt/CoFeB/GdOx films with perpendicular magnetic anisotropy and Ta/CoFeB/GdOx films with uniaxial in-plane magnetic anisotropy in the as-grown state exhibit a sizable dependence of the magnetic anisotropy on the voltage (amplitude, polarity, and time) applied across the oxide. In Pt/CoFeB/GdOx multilayers, it is possible to reorient the magnetic anisotropy from perpendicular-to-plane to in-plane, with a variation of the magnetic anisotropy energy greater than 0.2 mJ m−2. As for Ta/CoFeB/GdOx multilayers, magneto-ionic effects still lead to a sizable variation of the in-plane magnetic anisotropy, but the anisotropy axis remains in-plane.

Structural, thermal and magnetic studies of MgxZn1−xFe2O4 nanoferrites: Study of exchange interactions on magnetic anisotropy

Polycrystalline nanoferrites with chemical formula MgxZn1−xFe2O4 (x=0.5, 0.6, 0.7) have been synthesized by co-precipitation technique and then subsequently heated to 800°C in order to investigate structural, thermal and magnetic properties. The samples are characterized by using XRD, FTIR, TGA-DSC, SQUID and Mössbauer spectroscopy techniques. The synergic effect of heat treatment with substitution of Mg2+, results in random variation of lattice parameter (a) and crystallite size (D). FTIR studies revealed the formation of cubic spinel structure. The broadening at octahedral bands for compositions x=0.6 and 0.7 attributes to distribution of ferrite particles of different sizes in these samples. The characteristic feature of hysteresis loops reflects the nature of ferrite particles in the state of superparamagnetism. The saturation magnetization at room temperature has been reported for composition x=0.7 is 44.03emu/g. The variation of coercivity is due to variation in magnetic anisotropy which is predominately affected by the exchange interactions arising from the nature of nanoparticles. The blocking temperatures are in the range of 10–30K and their variation is in the line of change in magnetocrystalline anisotropy but not due to variation in crystallite sizes. The Zeeman splitting at tetrahedral (A) and octahedral (B) sites for composition x=0.6 is expected due to increase in size of core of the nanoparticles/or increasing of magnetocrystalline anisotropy. The range of isomer shift values and quadruple splitting values are evident for the presence of Fe3+ ions and the absence of Fe2+ ions in the present systems. The present ferrite nanoparticles in the superparamagnetic state are the potential candidates for biomedical applications like cancer treatment through hyperthermia. The results are interpreted in terms of cation redistribution presuming exchange coupling energy variation on magnetocrystalline anisotropy.

The role of electron confinement in Pd films for the oscillatory magnetic anisotropy in an adjacent Co layer

We demonstrate the interplay between quantum well states in Pd and the magnetic anisotropy in Pd/Co/Cu (0 0 1) by combined scanning tunneling spectroscopy (STS) and magneto optical Kerr effect (MOKE) measurements. Low temperature scanning tunneling spectroscopy reveals occupied and unoccupied quantum well states (QWS) in atomically flat Pd films on Co/Cu (0 0 1). These states give rise to sharp peaks in the differential conductance spectra. A quantitative analysis of the spectra reveals the electronic dispersion of the Pd (0 0 1) d-band (-type) along the -X direction. In situ MOKE experiments on Pd/Co/Cu (1, 1, 13) uncover a periodic variation of the in-plane uniaxial magnetic anisotropy as a function of Pd thickness with a period of 6 atomic layers Pd. STS shows that QWS in Pd cross the Fermi level with the same periodicity of 6 atomic layers. Backed by previous theoretical work we ascribe the variation of the magnetic anisotropy in Co to QWS in the Pd overlayer. Our results suggest a novel venue towards tailoring uniaxial magnetic anisotropy of ferromagnetic films by exploiting QWS in an adjacent material with large spin–orbit coupling.

Tunable spin-wave frequency gap in anisotropy-graded FePt films obtained by ion irradiation

The effect of graded anisotropy on static and dynamic magnetic properties of ${\mathrm{Ar}}^{+}$-irradiated FePt films has been investigated by static magnetometry, magnetic force microscopy, and Brillouin light scattering from thermally excited spin waves. A gradual variation of magnetic anisotropy with film thickness was obtained by ${\mathrm{Ar}}^{+}$ irradiation. The irradiation incidence angle influences the anisotropy profile: on decreasing $\ensuremath{\alpha}$, a decreasing thickness of the hard $\mathrm{L}{1}_{0}$ phase and an increasing thickness of the soft A1 phase were obtained. Accordingly, the zero-field spin-wave frequency gap was found to decrease. In the sample with the highest soft-phase thickness the spin-wave frequency gap takes a substantial value (${\ensuremath{\nu}}_{0}\ensuremath{\approx}6$ GHz), which could be reproduced assuming the presence of a nonzero ``rotatable'' anisotropy (i.e., any direction in the film plane can be established as the easy axis by the application of a saturating magnetic field along this direction). The hypothesis is supported by both magnetometry and magnetic force microscopy data.

Effects of piezoelectric-poling-induced strain on the magnetic properties of FeBSiC/PMN-PT (001) multiferroic heterostructures

Amorphous FeBSiC thin films were deposited on PMN-PT (Pb(Mg1/3Nb2/3)O3-PbTiO3)(001) single crystal substrate by DC magnetron sputtering. The effects of piezoelectric-poling-induced strain on the magnetic properties of the FeBSiC/PMN-PT heterostructures were investigated. Studies indicated that the non-linear strains in PMN-PT were induced by poling at different electric fields, with tensile strain for out-of-plane direction and compressive strain for in-plane direction, respectively. Moreover, the in-plane compressive strain increased the saturation magnetization of the FeBSiC thin film, while the out-of-plane tensile strain decreased it. These results could be attributed to the 109° ferroelastic domain switching at applied electric field, which induced large variation in magnetic anisotropy due to magnetoelectric coupling in the FeBSiC/PMN-PT heterostructures. This study demonstrates the non-volatile tuning of magnetism by non-linear strains of lattice induced by the piezoelectric-poling, which shows promising applications for energy efficient magnetoelectric devices.