Uniaxial Magnetic Anisotropy Research Articles

Detailed investigation of structural and magnetic properties of multiphase binary Pd-Co alloys prepared by modified polyol process

Pd-Co based magnetic alloy nanoparticles were prepared by the modified polyol process and stabilized by polyvinylpyrrolidone (PVP) and 3-aminopropyl-trimethoxysilane (APES) capping agents and further reduction of metal salts with Sodium borohydride (NaBH4) at high temperature to form desired NPs. We have a detailed investigation of the effect of Pd concentration in the Pd-Co alloy NPs on structural and magnetic properties. XRD and Rietveld refinement analyses were confirmed that the multiphase structures of fcc-PdCo, fcc-Co, and hcp-Co phases coexist at low Pd loading samples. Over %50 of Pd loading resulted in a single fcc-PdCo phase with reduced lattice parameter to 4.0079 Å and d(111)-space to 2.31 Å. TEM and SEM images reveal well dispersed and uniformly distributed NPs with an average particle sizes of below 7 nm. The elemental compositions and the characteristic OH, CH, CO stretching peaks of capping agents were confirmed by EDS and FT-IR spectrums, respectively. M(T) and M(H) curves revealed that there are multi magnetic phase transitions in the Pd-Co structure as a function of Pd loading from superparamagnetic to ferromagnetic phase or back to superparamagnetic phase by reducing the temperature from 300 K to 5 K. We observed that the blocking temperature (TB) could not be detected due to 5% Pd loading below 300 K, while it reduced up to 55 K at high Pd loading of 62%. The coercive field (Hc) was increased to ~1900 Oe for Pd0.62Co0.38 sample due to the smallest particle size as 5.26 nm. The highest amount of Co resulted in maximum saturation magnetization (Ms) up to 65.5 emu/g for 5% Pd concentration. We measured the Mr/Ms ratios were less than 0.5, which is due to the internal stress that results in the uniaxial magnetic anisotropy in the structure. The maximum Keff and μf.u. values were found to be over 12.9 × 106 erg/cm3 and 0.72 μB at 5 K for 5% Pd concentration, respectively.

Theoretical Study of the Input Impedance and Electromagnetic Field Distribution of a Dipole Antenna Printed on an Electrical/Magnetic Uniaxial Anisotropic Substrate

The present work considers the investigation of the effects of both electrical and magnetic uniaxial anisotropies on the input impedance, resonant length, and fields distribution of a dipole printed on an anisotropic grounded substrate. In this study, the associated integral equation, based on the derivation of the Green’s functions in the spectral domain, is numerically solved employing the method of moments. In order to validate the computing method and the evaluated calculation code, numerical results are compared with available data in the literature treating particular cases of electrical uniaxial anisotropy; reasonable agreements are reported. Novel results of the magnetic uniaxial anisotropy effects on the input impedance and the evaluated electromagnetic field are presented and discussed. This work will serve as a stepping stone for further works for a better understanding of the electromagnetic field behavior in complex anisotropic and bi-anisotropic media.

Soliton-Mediated Magnetic Reversal in an All-Oxide-Based Synthetic Antiferromagnetic Superlattice.

All-oxide-based synthetic antiferromagnets (SAFs) are attracting intense research interest due to their superior tunability and great potentials for antiferromagnetic spintronic devices. In this work, using the La2/3Ca1/3MnO3/CaRu1/2Ti1/2O3 (LCMO/CRTO) superlattice as a model SAF, we investigated the layer-resolved magnetic reversal mechanism by polarized neutron reflectivity. We found that the reversal of LCMO layer moments is mediated by nucleation, expansion, and shrinkage of a magnetic soliton. This unique magnetic reversal process creates a reversed magnetic configuration of the SAF after a simple field cycling. Therefore, it can enable vertical data transfer from the bottom to the top of the superlattice. The physical origin of this intriguing magnetic reversal process could be attributed to the cooperation of the surface spin-flop effect and enhanced uniaxial magnetic anisotropy of the bottom LCMO layer. This work may pave a way to utilize all-oxide-based SAFs for three-dimensional spintronic devices with vertical data transfer and high-density data storage.

X-ray Photoemission Spectroscopy Study of Uniaxial Magnetic Anisotropy Induced in a Ni Layer Deposited on a LiNbO3 Substrate.

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.

Simultaneous tuning of the magnetic anisotropy and thermal stability of alpha ^{''}-phase Fe_{16}N_{2}

Simultaneously enhancing the uniaxial magnetic anisotropy (K_u) and thermal stability of alpha ^{''}-phase Fe_{16}N_{2} without inclusion of heavy-metal or rare-earth (RE) elements has been a challenge over the years. Herein, through first-principles calculations and rigid-band analysis, significant enhancement of K_u is proposed to be achievable through excess valence electrons in the Fe_{16}N_{2} unit cell. We demonstrate a persistent increase in K_u up to 1.8 MJ m^{text {-}3}, a value three times that of 0.6 MJ m^{text {-}3} in alpha ^{''}-Fe_{16}N_{2}, by simply replacing Fe with metal elements with more valence electrons (Co to Ga in the periodic table). A similar rigid-band argument is further adopted to reveal an extremely large K_u up to 2.4 MJ m^{text {-}3} in (Fe_{0.5}Co_{0.5})_{16}N_{2} obtained by replacing Co with Ni to Ga. Such a strong K_u can also be achieved with the replacement by Al, which is isoelectronic to Ga, with simultaneous improvement of the phase stability. These results provide an instructive guideline for simultaneous manipulation of K_u and the thermal stability in 3d-only metals for RE-free permanent magnet applications.

Correlation of uniaxial magnetic anisotropy axes and principal resistivities in polycrystalline ferromagnetic films

In the present study, we demonstrate the measurement of resistivity tensor (ρ) in the plane of a polycrystalline film of ferromagnetic permalloy (Py). To this end, conventional Hall-bar and a more recent extended van der Pauw methods were utilized for determining 2D ρ in the film plane. The samples were prepared by normal incidence sputter deposition within an in situ magnetic field to induce in-plane uniaxial magnetic anisotropy in the film. Since ρ might be affected by the internal magnetization of the film, we performed measurements by rotation of a saturating magnetic field in the film plane. Both methods indicate that the average resistivity is lower along the magnetic easy axis of the film compared to the hard axis. Since X-ray diffraction results indicated no dominating texture in the film, we concluded that there is a correlation between uniaxial magnetic axes and principal resistivity axes. This is an important finding that allows determining the direction of magnetic anisotropy axes without magnetometry. The results also verify atomic or pair ordering to be the origin of uniaxial magnetic anisotropy in the Py since resistivity is sensitive to the level of order in solids. The extended van der Pauw utilized here can be easily performed on the as-deposited samples, which is of practical interest.

Spin reorientation transition in ultrathin Co films on the vicinal surface Au(788)

In this work, we investigate the structural, morphological and magnetic properties of epitaxial Cobalt ultrathin films grown on the vicinal surface Au(788). The aim was to study the magnetization reversal and determine the influence of the regular arrangement of atomic steps, typical of a vicinal surface. The influence of the Co thickness on the spin reorientation from out-of-plane to in-plane magnetization were investigated by means of the magneto-optical Kerr effect (MOKE). Preparation and surface quality of the Co films were characterized using the surface experimental techniques LEED and STM. A smooth spin reorientation transition, between 8 and 12 atomic monolayers (ML) of Co, was observed from out-of-plane magnetization, between 5 ML and 12 ML of Co, to in-plane magnetization, above 13 ML of Co. Furthermore, the angular dependence of the magnetic hysteresis loops indicates uniaxial magnetic anisotropy parallel to the atomic steps in the surface plane of the system Au/Co/Au(788) for thicknesses between 13 ML and 20 ML of Co.

МАГНИТНЫЕ ЭЛЕМЕНТЫ НЕЙРОННЫХ СЕТЕЙ. СВОЙСТВА МАГНИТНЫХ НЕОДНОРОДНОСТЕЙ В СИСТЕМАХ ОГРАНИЧЕННОЙ ГЕОМЕТРИИ

This paper discusses the prospects for using magnetic nanostructures as elements of neural networks. At present neural network learning programs are actively used in analyzing and processing large data arrays; however, the development of computer technologies based on the neural network principle still remains open. Possibilities for using magnetic elements as physical carriers of information bits in these systems attract much attention from researchers and technologists due to the presence of several easily controlled parameters (order parameter) in the magnetic system, possibilities for the dimensionality reduction in magnetic elements by using magnetic nanostructures (domain boundaries, vortices, ckyrmions), superquick switching between magnetic states and some other factors. One of the key aspects of research in this regard is to determine basic controlled magnetic parameters in restricted geometries and to identify ways of controlling these parameters through internal and external factors. The paper presents a research on the magnetic ground state in restricted geometries. It deals with the magnetic state rebuilding in the system under changes in both external factors (applied magnetic field, sample dimensions) and internal ones (magnetic anisotropy constant, Dzyaloshinskii-Moriya interaction constant). Calculations were performed within the framework of micromagnetic modelling using the Object Oriented MicroMagnetic Framework ( OOMMF) sogtware. It is shown that the anisotropic exchange interaction (Dzyaloshinskii-Moriya interaction) has a significant effect on the magnetization distribution in restricted geometries. Namely, when changing the value of the Dzyaloshinskii-Moriya constant in the system with uniaxial magnetic anisotropy there is a series of phase transitions observed between magnetic states of different types: transitions from the homogenous magnetic state into the skyrmion-type vortex state (domain structure with the skyrmion-type unidomain state) with subsequent domain structure reversal when changing the value of the Dzyaloshinskii-Moriya constant. In the case of magnetic anisotropy of easy -axis type, chirality and properties of the structures in question do not depend on the constant symbol of the Dzyaloshinskii-Moriya interaction.

Fe−Ni−N based alloys as rare-earth free high-performance permanent magnet across [formula omitted] to [formula omitted] phase transition: A theoretical insight

Over the years, simultaneous enhancement of the energy density product and thermal stability of 3d-only metals, without including heavy metals or rare-earth elements, has been a tremendous challenge in the field of permanent magnetism. In this study, we investigate the structural stability and intrinsic magnetic properties of (Fe1-xNix)16N2 (x=0−1) across the α″ to L10 phase transition using the systematic density-functional theory and Monte Carlo simulations. We theoretically demonstrated an extremely large enhancement in the uniaxial magnetic anisotropy (Ku), up to 1.8 MJ·m-3 in the L10-type (Fe0.5Ni0.5)16N2, a value roughly three times those of α″-Fe16N2 (0.6 MJ·m-3) and L10-FeNi (0.68 MJ·m-3). Simultaneously, it is predicted that the resulting L10-type (Fe0.5Ni0.5)16N2 phase is energetically more stable than the α″-Fe16N2 phase. Further calculations reveal that in L10-type (Fe0.5Ni0.5)16N2,Ku can increase up to nearly 3.9 MJ·m-3 with additional interstitial N atoms. In conclusion, we predict that the Fe16N2-based compounds can be effectively used as efficient rare-earth free permanent magnets by simultaneously enhancing their structural stability and energy product (BH)max.

Effect of atom substitutions on the magnetic properties in Ce2Fe17: Toward permanent magnet applications

Due to the rapidly developing technologies and huge market demand, there has been increasing interest internationally in exploring permanent magnet formulations in addition to the well-known Nd2Fe14B and SmCo5/Sm2Co17. Given Fe's low materials cost and generally high magnetization, Fe-rich rare earth binaries such as Ce2Fe17 comprise a rich “hunting ground” for such new materials. While this compound suffers from a low ordering point and is a helimagnet, these difficulties are easily remedied by the substitution of appropriate amounts of cobalt for Fe, with room-temperature saturation magnetization as high as 1.5 T. Here, we try to switch the all-important magnetic anisotropy from planar to uniaxial behavior in Ce2Fe17 via 18h- and 6c-type atom substitutions with Si, Ir, and numerous other atoms. The uniaxial magnetocrystalline anisotropy is successfully achieved in the 6c-site-substituted Ce2Fe15Ir2 systems, along with large magnetization. We find that iridium substitution, in particular, induces a substantial uniaxial anisotropy of 11.25 MJ/m3, which is comparable to most of the current rare earth permanent magnets. Although the iridium substitution is costly, the finding of Ir-triggered uniaxial magnetic anisotropy indicates the potential of Ce–Fe-based alloys for permanent magnets.

Anomalous Hall and Nernst effects in ferrimagnetic Mn4N films: Possible interpretations and prospects for enhancement

Ferrimagnetic Mn4N is a promising material for heat flux sensors, based on the anomalous Nernst effect (ANE), because of its sizeable uniaxial magnetic anisotropy (Ku) and low saturation magnetization (Ms). We experimentally and theoretically investigated the ANE and anomalous Hall effect in sputter-deposited Mn4N films. It was revealed that the observed negative anomalous Hall conductivity (σxy) could be explained by two different coexisting magnetic structures, that is, a dominant magnetic structure with high Ku, contaminated by another structure with negligible Ku, owing to an imperfect degree of order of N. The observed transverse thermoelectric power (SANE) of +0.5 μV/K at 300 K yielded a transverse thermoelectric coefficient (αxy) of +0.34 A/(m · K), which was smaller than the value predicted from the first-principles calculation. The interpretation for αxy based on the first-principles calculations led us to conclude that the realization of single magnetic structure with high Ku and optimal adjustment of the Fermi level are promising approaches to enhance SANE in Mn4N through the sign reversal of σxy and the enlargement of αxy up to a theoretical value of 1.77 A/(m · K).

Study of magnetic nanowires of amorphous Co20Fe60B20 prepared by oblique angle deposition on nanorippled substrate

An array of magnetic nanowires of amorphous Co20Fe60B20 alloy has been prepared by oblique angle deposition of material on nanorippled substrate. The angle of incidence is chosen such that the deposition takes place on only one facet of the nanoripples. Grazing incidence small angle x-ray scattering measurement has been done in order to characterize the morphology of the nanowires. It is found that the oblique angle deposition results in columnar growth, and each individual nanowire consists of nanocolumns having average diameter of 7 nm and a tilt angle of 37° with respect to the surface normal. Azimuthal angle dependence of longitudinal magneto-optical Kerr effect measurement shows that the nanowires possess a well-defined uniaxial magnetic anisotropy with the easy axis of magnetization tilted by an angle of 25° with respect to the direction of the ripples. The magnetization follows the Stoner-Wohlfarth model with a very narrow distribution in the direction of easy axis. It is suggested that, tilt of the easy axis can be understood in terms of the interplay of two shape anisotropy terms corresponding to the ensemble of strongly interacting nanocolumns through magnetostatic dipolar interaction, and the overall shape of a nanowire.

Role of the interfaces in the crystallization and hysteresis mechanisms of amorphous Fe-B thin films

In this work we study the crystallization processes of two sets of amorphous Fe80B20 films fabricated by Pulsed Laser Ablation Deposition on substrates with different nature, Corning glass® and MgO(001). We analyze their magnetic hysteresis mechanisms by means of magneto-optic techniques and Transmission Electron Microscopy. The as-deposited amorphous films present a highly homogeneous uniaxial magnetic anisotropy with the easy axis orientation dependent on the type of substrate and much weaker than that of bulk alloys with similar composition. The onset of crystallization for the films deposited on glass and MgO appears, respectively, at temperatures 150 °C and 250 °C below that of their bulk counterparts. We study the role of the substrate in the crystallization mechanism and the resulting nanostructure of the magnetic films. While the crystallization of the MgO-deposited films proceeds in a broad front growing from the substrate to the surface, that of the glass-deposited films takes place through the nucleation and growth of isolated crystallites. We analyze the evolution of the coercivity during the crystallization of the films. It follows a similar trend for both types, remaining in values close to those of the amorphous precursors until it rises steeply at a given annealing temperature. The threshold of the steep coercivity increase of the glass-deposited films is shifted to lower temperatures with respect to the MgO ones, in spite of the higher crystallization onset temperature of the former. The coercivity mechanisms have been analyzed, correlated to the films interfacial characteristics and compared to those of bulk alloys.

Intrinsic DMI-free skyrmion formation and robust dynamic behaviors in magnetic hemispherical shells

We performed finite-element micromagnetic simulations to examine the formation of skyrmions without intrinsic Dzyaloshinskii–Moriya interaction (DMI) in magnetic hemispherical shells. We found that curvature-induced DM-like interaction allows for further stabilization of skyrmions without the DMI in curved-geometry hemispherical shells for a specific range of uniaxial perpendicular magnetic anisotropy (PMA) constant Ku. The larger the curvature of the shell, the higher the Ku value required for the formation of the skyrmions. With well-stabilized skyrmions, we also found in-plane gyration modes and azimuthal spin-wave modes as well as an out-of-plane breathing mode, similarly to previously found modes for planar geometries. Furthermore, additional higher-frequency hybrid modes were observed due to coupling between the gyration and azimuthal modes. This work provides further physical insight into the static and dynamic properties of intrinsic DMI-free skyrmions formed in curved-geometry systems.

Electric control of magnetic properties in epitaxially grown FeRh/MgO/PMN-PT heterostructures

We epitaxially grew the metamagnetic FeRh films on ferroelectric PMN-PT(001) substrates and investigated the electric control of magnetic properties. An MgO buffer layer is inserted to reduce the considerably large lattice mismatch of 5.44% between FeRh and PMN-PT, leading to a nice epitaxial growth with a relationship of FeRh[110](001)||MgO[100](001)||PMN-PT[100](001). The as-grown FeRh film shows a typical antiferromagnetic-to-ferromagnetic phase transition by means of varying the temperature. Because a growth-induced uniaxial magnetic anisotropy is superimposed on the in-plane four-fold magnetocrystalline anisotropy, the epitaxial FeRh film displays different kinds of multi-step hysteresis loops at various field orientations. An electric field applied on the PMN-PT substrate can remarkably increase the coercivity of FeRh film, which presents a butterfly type shape, indicating a strain-mediated magnetoelectric coupling. Because of the inhibitory effect of the remnant compressive strain on the phase transition, the critical transition temperature and the magnetization are obviously enhanced and reduced, respectively, after removing the electric effect.

Direct observation of a magnetic domain change in Ni wire and film on a LiNbO3 substrate using X-ray magnetic circular dichroic photoemission electron microscopy

The magnetic domain structure modulation in Ni wires deposited on a LiNbO3 substrate is observed during the application of a DC current or heating. A striped domain structure is formed in the Ni wires that is aligned perpendicular to the X-axis of the LiNbO3 substrate owing to the competition between magnetic shape anisotropy and uniaxial magnetic anisotropy from the heterojunction. We observe that the striped domain structure disappears as the DC current is increased. In the same manner, it is also confirmed that the striped magnetic domain structure disappears as the substrate is heated. These results are considered to arise from the decrease in uniaxial magnetic anisotropy and magnetization owing to an oxidation reaction at the interface. Heterojunctions are identified to play an important role in controlling and modulating the magnetic properties of this system.

Robust perpendicular magnetic anisotropy of Co3Sn2S2 phase in sulfur deficient sputtered thin films

Perpendicular magnetic anisotropy (PMA) in magnetic thin films is a fundamental key feature in the design of spintronic devices. As one of magnetic Weyl semimetals, ${\mathrm{Co}}_{3}{\mathrm{Sn}}_{2}{\mathrm{S}}_{2}$ has been studied for its large anomalous Hall effect (AHE), uniaxial crystalline magnetic anisotropy, and half metallicity. In this study, we investigated the effect of off-stoichiometric composition on the PMA and AHE of ${\mathrm{Co}}_{3}{\mathrm{Sn}}_{2}{\mathrm{S}}_{x}$ thin films fabricated by the sputtering technique. The prepared thin films have off-stoichiometric S compositions $x$ of 1.54 (S poor) and 3.27 (S rich) as well as the nearly stoichiometric one of 2.02. In addition to the ${\mathrm{Co}}_{3}{\mathrm{Sn}}_{2}{\mathrm{S}}_{2}$ phase, the segregated Co metal is found to contribute to the measured magnetization in the S-poor and S-rich films. The coercive field of perpendicular magnetization in all the films is much larger than that in the ${\mathrm{Co}}_{3}{\mathrm{Sn}}_{2}{\mathrm{S}}_{2}$ bulk crystals despite the fact that effective perpendicular magnetic anisotropy constants (${K}_{\mathrm{u}}^{\mathrm{eff}}$) between the prepared films are significantly different. In addition, the ${K}_{\mathrm{u}}^{\mathrm{eff}}$ values of two samples with $x=2.02$ and 2.22 are comparable to those of the bulk crystals. In contrast to the isotropic magnetization behavior in the S-rich film, the S-poor film holds the PMA feature. This result means that the PMA is more robust in the S-poor film than in the S-rich film. For the electrical transport properties, a large tangent of Hall angle of about 0.2 is observed for both the nearly stoichiometric and the S-poor films. This large tangent of Hall angle demonstrates that the Weyl feature of ${\mathrm{Co}}_{3}{\mathrm{Sn}}_{2}{\mathrm{S}}_{2}$ phase is well maintained even in the S-poor thin films as well as the nearly stoichiometric films although the amount of Co segregation in both S-poor and S-rich films is similar. Our findings on the influence of off-stoichiometry on the PMA and AHE are beneficial to design magnetic devices incorporated with the Weyl features of ${\mathrm{Co}}_{3}{\mathrm{Sn}}_{2}{\mathrm{S}}_{2}$.

Effect of Ga composition on soft and high-frequency magnetic properties of Fe85.1−xGaxB14.9 thin films

The effects of Ga composition (x) on soft and high-frequency magnetic properties of 10-nm-thick B-doped Fe–Ga (Fe85.1−xGaxB14.9) thin films were investigated. A uniaxial magnetic anisotropy appeared regardless of the Ga composition. The damping constant values and magnetic inhomogeneous broadening at a zero frequency were much lower than those of the Fe–Ga polycrystalline film for all Ga compositions. These results indicate that adding B atoms to Fe–Ga thin films improves the soft and high-frequency magnetic properties of these films and suggest that Fe–Ga–B thin films are candidate magnetostrictive materials for high-frequency devices.

Effects of swift heavy ion-irradiation on magnetic properties of Co-doped TiO2

Ferromagnetic Co-doped TiO2 thin films at room temperature have been synthesized by RF sputtering and ion-irradiated with 100 MeV Au ions. The surface roughness of the film increases with increasing Au-ion fluence. Kerr microscopy measurements at room temperature give isotropic hysteresis loop with negligible coercivity, which gets modified into in-plane uniaxial magnetic anisotropy after anion fluence of 5×1011 ions/cm2. Further ion-irradiation reduced the coercivity of the film. X-ray magnetic circular dichroism (XMCD) measurements have been performed in total electron yield mode, to investigate the effects of ion-irradiation on magnetic properties of the Co-doped TiO2 thin films. From the XMCD analysis, we observed that the total magnetic moment of Co atoms in case of as-deposited film is 1.771±0.016μB. After the ion-irradiation it became 0.3818±0.014μB at room temperature.

High thermal stability and low driven current achieved by vertical domain wall motion memory with artificial ferromagnet

To enhance thermal stability while keeping low driven current is difficult in traditional domain wall (DW) motion devices. The increasing of energy barrier for thermal stability inevitably results in the enhancement of driven current. We numerically investigate depinning field (H depin) and critical current density (J c) for DW motion as a function of uniaxial magnetic anisotropy (K u) in vertical DW motion memory with artificial ferromagnet. It is found that H depin and J c show different K u dependence. The results indicate that it is promising to simultaneously achieve high thermal stability and low driven current in artificial ferromagnet based DW motion devices.