Easy Direction Research Articles

Magnetic and magneto-optical properties of films of multiferroic GdMnO3 grown on LSAT [(LaAlO3)0.3 (Sr2AlTaO6)0.7] (100) and (111)

The magnetic properties of multiferroic GdMnO3 depend on strain which is produced in a thin film by growing an epitaxial film on a suitable substrate. We report an investigation of the magnetic and optical properties of GdMnO3, as a function of strain, produced by growing epitaxial films on the substrates on (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT) (100) and (111). Magnetic measurements have shown that at 5K the easy direction of the film is in-plane for LSAT (100) and the canted moment is significantly smaller than the value found in bulk material but larger than that found for GdMnO3 on SrTiO3 (100). The coercive field of the GdMnO3/LSAT (100) has also been found to be smaller than for bulk single crystal samples but comparable to a thinner film of GdMnO3 grown on SrTiO3. The magnetic properties of the film grown on LSAT (111) are very different. The transition to the canted phase is less pronounced and there is no magnetic hysteresis at low temperatures. The susceptibility data are fitted with the Curie's law and the measured magnetic moments for the film on LSAT (100) were similar to bulk values but significantly different for films on LSAT (111) and SrTiO3 (100). The magnetic circular dichroism spectroscopy showed two features: the charge transfer transition between Mn d states at ~2eV and the band edge transition from the oxygen p band to the d states at ~3eV. Magnetic circular dichroism also shows that the transition at around 2eV is stronger in LSAT (100) than in LSAT (111) implying that the structure of the film of GdMnO3 is closer to that of LaMnO3 when grown on LSAT (100).

Soft Magnetic Properties of Thin Nanocrystalline Particles Due to the Interplay of Random and Coherent Anisotropies

We investigated coercive fields of 200 nm $\times1200$ nm $\times 5$ nm rectangular nanocrystalline thin films as a function of grain size $D$ using finite-element simulations. To this end, we created granular finite-element models with grain sizes ranging from 5 to 60 nm, and performed micromagnetic hysteresis calculations along the $y$ -axis (easy direction) as well as along the $x$ -axis (hard direction). We then used an extended random anisotropy model to interpret the results and to illustrate the interplay of random anisotropy and shape-induced anisotropy, which is coherent on a much larger scale, in thin films.

Temperature Effects on the Magnetoimpedance in Glass-Coated Amorphous Wires

The temperature dependence of magnetoimpedance (MI) in glass-coated amorphous microwires of two compositions Co66.94Fe3.83Ni1.44B11.51Si14.59Mo1.69 and Co23.67Fe7.14Ni43.08B13.85Si12.26 having high (613 K) and low (335 K) Curie temperatures, respectively, was investigated. The wires of the first type have a small negative magnetostriction and are widely used as MI sensing elements. For sensor operation, the temperature stability is of primary importance, however, it was found that the MI change at moderate temperatures of 20 °C–90 °C and low magnetic fields can be nearly 200%. This is explained by high temperature sensitivity of the residual stress and stress-dependent magnetostriction, which resulted in abrupt change in the easy anisotropy direction. The wires of the second type with low Curie temperature ${T}_{c}$ and positive magnetostriction demonstrate large and sensitive change in impedance only in the vicinity of ${T}_{c}$ due to a corresponding drop in the magnetization saturation and magnetostriction, whilst the bistable type of the magnetization reversal remains. The frequency dispersion of the dynamical permeability shifts toward lower frequencies when approaching ${T}_{c}$ suppressing the high-frequency properties and MI. Such temperature dependence of MI is well controllable and is proposed for applications in temperature sensors.

Magnetic Properties of Nanocrystalline N-NFO Thin Films

The pure as well as nitrogen incorporated nickel ferrite (NFO and N-NFO) thin films were grown by radio frequency magnetron sputtering technique. The structural investigations of as-grown N-NFO films using GIXRD and Raman spectroscopy revealed nanocrystalline nature of the films. The microstructural and surface studies of the films using SEM and atomic force microscope micrographs showed increase in crystallite size and decrease in surface roughness with increase of sputtering power. The magnetization (M-H) measurements at 20 and 300 K revealed an increase in magnetization and decrease in coercivity values with increase of sputtering power. However, no saturation of magnetization ( $M_{s}$ ) was visible for the thin film samples, indicating superparamagnetic behavior due to nanocrystallinity. The comparison of in-plane and out-of-plane magnetizations of the N-NFO film (both at 20 and 300 K) showed that the easy magnetization direction lied in the plane of the film. The optical transmission spectra revealed decrease of direct band gap with increase of sputtering power.

Detailed modeling of local anisotropy and transverse K u interplay regarding hysteresis loop in FeCuNbSiB nanocrystalline ribbons

This article focuses on the modeling of the hysteresis loop featured by Fe-Cu-Nb-Si-B nanocrystalline alloys with transverse induced anisotropy. The magnetization reversal process of a magnetic correlated volume (CV), characterized by the induced anisotropy Ku, and a deviation of the local easy magnetization direction featuring the effect of a local incoherent anisotropy Ki, is analyzed, taking account of magnetostatic interactions. Solving the equations shows that considering a unique typical kind of CV does not enable accounting for both the domain pattern and the coercivity. Actually, the classical majority CVs obeying the random anisotropy model explains well the domain pattern but considering another kind of CVs, minority, mingled with classical ones, featuring a magnitude of Ki comparable to Ku, is necessary to account for coercivity. The model has been successfully compared with experimental data.

Phase diagram of the O(n) model with defects of “random local anisotropy” type

It is shown that the Imry-Ma theorem stating that in space dimensions d<4 the introduction of an arbitrarily small concentration of defects of the “random local anisotropy” type in a system with continuous symmetry of the n-component vector order parameter (O(n) model) leads to the long-range order collapse and to occurrence of a disordered state, is not true if an anisotropic distribution of the defect-induced random easy axes directions in the order parameter space creates a global anisotropy of the “easy axis” type. For a weakly anisotropic distribution of the easy axes, in space dimensions 2≤d<4 there exists some critical defect concentration, when exceeded, the inhomogeneous Imry-Ma state can exist as an equilibrium one. At lower defect concentration the long-range order takes place in the system. For a strongly anisotropic distribution of the easy axes, the Imry-Ma state is suppressed completely and the long-range order state takes place at any defect concentration.

Structure and magnetic properties of Fe-Co-B alloy thin films prepared on cubic (001) single-crystal substrates

Fe70Co30 and (Fe70Co30)0.95B5 (at. %) alloy films of 5 nm thickness are prepared by sputtering on cubic (001) oxide substrates at 200 °C. The lattice mismatch between film and substrate is varied from –4.2%, 0%, to +3.5% by employing MgO, MgAl2O4, and SrTiO3 substrates, respectively. Fe70Co30 and (Fe70Co30)0.95B5 single-crystal films with bcc structure grow epitaxially on all the substrates in the orientation relationship of (001)[110]film || (001)[100]substrate. The in-plane and out-of-plane lattice constants, a and c, are in agreement within small differences ranging between +1.1% and –0.9% with the value of bulk bcc-Fe70Co30 crystal, even though there exist the lattice mismatches of –4.2% and +3.5%. The result indicates that misfit dislocations are introduced around the film/substrate interface when films are deposited on MgO and SrTiO3 substrates. The single-crystal films show in-plane magnetic anisotropies with the easy magnetization direction of bcc[100], which are reflecting the magnetocrystalline anisotropy of bulk Fe70Co30 crystal.

Variation of magnetic easy direction of bcc structured ferromagnetic films up to thirty spin layers

Third order Heisenberg Hamiltonian was employed to determine the effect of number of spin layers on magnetic easy direction. 3-D graph of energy versus stress induced anisotropy and angle was plotted to find magnetic easy and hard directions. First values of stress induced anisotropy corresponding to minima and maxima of this 3-D plot were determined. Then the graph of energy versus angle was plotted at each of these stress induced anisotropy values in order to determine magnetic easy and hard directions. The same approach was applied to find the easy and hard directions for different values of spin exchange anisotropy. Magnetic easy direction gradually rotates from out of plane to in plane direction of the film, as the number of spin layers is increased from 10 to 30. The magnetic anisotropy energy (energy difference between magnetic easy and hard directions) gradually increases as the number of layers is increased. However, the angle between easy and hard directions doesn’t change considerably in this range of thickness. Our theoretical data agree with experimental data obtained by some other researchers. Easy axis oriented magnetic thin films are useful in magnetic memory devices due to the higher magnetic anisotropy and higher energy density.

High coercive LTP-MnBi for high temperature applications: From isolated particles to film-like structures

Ferromagnetic low-temperature phase (LTP) of manganese bismuth Mn50Bi50 (at.%) alloy has been prepared by DC magnetron sputtering onto glass by alternative deposition of Bi and Mn with in-situ annealing. Modifying the temperature during deposition and/or during post-growth annealing leads to different microstructures ranging from quasi-isotropic isolated MnBi micro-particles to larger platelet-like individual ones up to highly coalesced textured film-like areas. These microstructural variations determine the magnetic behaviour, including change in the easy axis direction and possibility of varying coercivity from 4 to 16 kOe at room temperature. In all the cases, coercivity has a positive temperature coefficient, reaching 29 kOe at 500 K, being among the largest reported coercivity for MnBi. Isolated LTP-MnBi microstructures, being homogeneously distributed onto glass, have been grown while avoiding the presence of unreacted Bi regions between the MnBi structures. The possibility of tuning microstructural and magnetic properties, in combination with their time stability, makes them interesting candidates for high temperature permanent magnet applications.

Anisotropy and hysteretic behavior of single-crystal Fe triangular nanomagnets

We discuss, from the results of finite elements micromagnetic simulations, the remanence acquisition process taking place in Fe nano-prisms having triangular bases and incorporating non-zero magnetocrystalline anisotropy. Our conclusions include the identification of the occurrence of large remanences when the applied field is oriented along a direction for which a magnetocrystalline easy direction approximately balances a configurational hard one. Those remanences are adequate to implement magnetostatic interactions based applications.

Dzyaloshinskii-Moriya interaction at an antiferromagnetic interface: First-principles study of Fe/Ir bilayers on Rh(001)

We study the magnetic interactions in atomic layers of Fe and $5d$ transition metals such as Os, Ir, and Pt on the (001) surface of Rh using first-principles calculations based on density functional theory. For both stackings of the $5d$/Fe bilayer on Rh(001) we observe a transition from an antiferromagnetic to a ferromagnetic nearest-neighbor exchange interaction upon $5d$ band filling. In the sandwich structure $5d$/Fe/Rh(001) the nearest-neighbor exchange is significantly reduced. For Fe/Ir bilayers on Rh(001) we consider spin spiral states in order to determine exchange constants beyond nearest neighbors. By including spin-orbit coupling we obtain the Dzyaloshinskii-Moriya interaction (DMI). The magnetic interactions in Fe/Ir/Rh(001) are similar to those of Fe/Ir(001) for which an atomic scale spin lattice has been predicted. However, small deviations between both systems remain due to the different lattice constants and the Rh vs Ir surface layers. This leads to slightly different exchange constants and DMI and the easy magnetization direction switches from out-of-plane for Fe/Ir(001) to in-plane for Fe/Ir/Rh(001). Therefore a fine tuning of magnetic interactions is possible by using single $5d$ transition-metal layers which may allow to tailor antiferromagnetic skyrmions in this type of ultrathin films. In the sandwich structure Ir/Fe/Rh(001) we find a strong exchange frustration due to strong hybridization of the Fe layer with both Ir and Rh which drastically reduces the nearest-neighbor exchange. The energy contribution from the DMI becomes extremely large and DMI beyond nearest neighbors cannot be neglected. We attribute the large DMI to the low coordination of the Ir layer at the surface. We demonstrate that higher-order exchange interactions are significant in both systems which may be crucial for the magnetic ground state.

Magnetostriction and spin reorientation studies on [formula omitted] (x = 0, 0.12, 0.2, 0.24, 0.32, 0.36) compounds

Polycrystalline Sm0.9-xNdxPr0.1Fe1.93 (x=0, 0.12, 0.2, 0.24, 0.32 and 0.36) intermetallic compounds were found to stabilize in MgCu2-type cubic Laves phase structure. The easy direction of magnetization (EMD) was seen to be along 〈111〉 direction, accompanied by a rhombohedral distortion in all the compounds, at room temperature. The spin reorientation temperatures were observed at 59K (TSR1) and 190K (TSR2) from the temperature dependent magnetization studies. The spin reorientation temperature (TSR2) was due to the change of EMD from 〈011〉 to 〈111〉. The order–disorder transition (Curie temperature: TC) was found to decrease with increasing Nd content. The anisotropy compensated Sm0.9Pr0.1Fe1.93 compound was found to exhibit the largest induced magnetostriction (λ‖-λ⊥) value of -1822×10-6, from the present studies, at an applied field of 10kOe.

Preparation and characterization of iridescent Ni1-xCox-containing anodic aluminum oxide films

In this study, NiCo@PAA composite films with bright structural colors were successfully prepared via AC electrodeposition and their optical and magnetic properties were investigated. It was found that the results of the reflectance spectrum test are consistent with those observed in our experiment, the aspect ratio of NiCo alloy nanowires is very small shown by the scanning electron microscopy results, as the results of the magnet test indicate, the NiCo alloy nanowires have an easy magnetization direction which is parallel with the direction of the nanowires, the coercive force increases slowly with the increase of Co content and the coercive force also increases at low temperatures, and the magnetization reversal mechanism of the NiCo alloy nanowires conforms to the coherent rotation model.

Strain-induced band gap engineering in layered TiS3

By combining ab initio calculations and experiments, we demonstrate how the band gap of the transition metal trichalcogenide TiS3 can be modified by inducing tensile or compressive strain. In addition, using our calculations, we predicted that the material would exhibit a transition from a direct to an indirect band gap upon application of a compressive strain in the direction of easy electrical transport. The ability to control the band gap and its nature could have a significant impact on the use of TiS3 for optical applications. We go on to verify our prediction via optical absorption experiments that demonstrate a band gap increase of up to 9% (from 0.99 to 1.08 eV) upon application of tensile stress along the easy transport direction.

Peierls-Nabarro modeling of dislocations in UO2

Under conditions of high stress or low temperature, glide of dislocations plays an important role in the deformation of UO2. In this paper, the Peierls-Nabarro model is used to calculate the core widths and Peierls stresses of ½<110> edge and screw dislocations gliding on {100}, {110}, and {111}. The energy of the inelastic displacement field in the dislocation core is parameterized using generalized stacking fault energies, which are calculated atomistically using interatomic potentials. We use seven different interatomic potential models, representing the variety of different models available for UO2. The different models broadly agree on the relative order of the strengths of the different slip systems, with the 1/2<110>{100} edge dislocation predicted to be the weakest slip system and 1/2<110>{110} the strongest. However, the calculated Peierls stresses depend strongly on the interatomic potential used, with values ranging between 2.7 and 12.9 GPa for glide of 1/2<110>{100} edge dislocations, 16.4–32.3 GPa for 1/2<110>{110} edge dislocations, and 6.8–13.6 GPa for 1/2<110>{111} edge dislocations. The glide of 1/2<110> screw dislocations in UO2 is also found to depend on the interatomic potential used, with some models predicting similar Peierls stresses for glide on {100} and {111}, while others predict a unique easy glide direction. Comparison with previous fully atomistic calculations show that the Peierls-Nabarro model can accurately predict dislocation properties in UO2.

Electric-field tuning of ferromagnetic resonance in CoFeB/MgO magnetic tunnel junction on a piezoelectric PMN-PT substrate

We investigated the strain-induced modulation of in-plane magnetic anisotropy in single crystal Pb(Mg1/3Nb2/3)0.7Ti0.3O3 (PMN-PT) with a (011) orientation/seed layer/IrMn/CoFeB/MgO/CoFeB/capping layer structure using spin-torque ferromagnetic resonance (ST-FMR) at room temperature. An in-plane biaxial strain is produced by an electric field applied across the substrate, which results in the tuning of the magnetic anisotropy of the CoFeB layer. As the change in the electric field value progresses, the MR loop becomes slanted, indicating the change in the easy axis direction. A strain-induced FMR frequency shift dependence of 145 MHz⋅cm⋅kV−1 is obtained by analyzing the experimental FMR spectra. An analytical model of the potential dynamic mechanism based on the strain induced reorientation of magnetization was discussed. Our results suggest that PMN-PT based magnetic tunnel junctions can have applications in electric-field driven spintronic devices.

Magnetic anisotropy of (100)- and (110)-oriented epitaxial CrO2 films grown on SnO2- the effects of strain and interface

Abstract The crystal structures and magnetic properties of CrO 2 films of different orientations and thicknesses grown on SnO 2 were investigated and compared with those on TiO 2 . Results reveal that large anisotropic strains were found in (100) films, but only small strains existed in (110) films. The magnetic anisotropies of CrO 2 (110) films on SnO 2 and TiO 2 were found to be similar. As for CrO 2 (100) films, magnetic anisotropies were quite different-at the same thickness, the magnetic anisotropy of the film on SnO 2 substrate was higher than that on TiO 2 . However, the easy axes of the films were found to be always along c axis even at very small thickness, and no easy axis switching was observed. First-principle calculations reveal that magnetic anisotropy energy is not only concerned with anisotropic strain but also with the substrate. The competition between the anisotropic preference of interface and the effect of strains may be one important factor for determining the actual easy axis direction. The combined effects of strain and interface may also be one reason for the magnetic anisotropic difference between CrO 2 (100) films on SnO 2 and TiO 2 .

Electric field modulation of reversible and non-volatile magnetic anisotropy transition in (0 1 1) 0.7[Pb(Mg1/3Nb2/3)O3]–0.3[PbTi0.3O3]/FeSi heterostructures

Electric-field-modulated magnetic properties are investigated on FeSi films with a (0 1 1) preferred orientation on (0 1 1) 0.7[Pb(Mg1/3Nb2/3)O3]–0.3[PbTi0.3O3] (PMN–0.3PT) single crystal substrates. When saturated and reversed coercive electric fields are alternately applied to the Pt/PMN–0.3PT/FeSi/Ta heterostructures, a reversible and non-volatile magnetic transition between four-fold and two-fold anisotropy is demonstrated by means of the angular dependence of remanent magnetization measurements. Based on the electric field control of the 90° magnetic easy axis direction transition of the FeSi film, we also achieve electric-field-mediated giant 180° magnetization switching with the assistance of the magnetic field pulse.

Role of magnetic exchange interaction due to magnetic anisotropy on inverse spin Hall voltage at FeSi3%/Pt thin film bilayer interface

In our recent studies inverse spin Hall voltage (ISHE) was investigated by ferromagnetic resonance (FMR) using bilayer FeSi3%/Pt thin film prepared by pulsed laser deposition (PLD) technique. In ISHE measurement microwave signal was applied on FeSi3% film along with DC magnetic field. Higher magnetization value along the film-plane was measured by magnetic hysteresis (M-H) loop. Presence of magnetic anisotropy has been obtained by M-H loop which showed easy direction of magnetization when applied magnetic field is parallel to the film plane. The main result of this study is that FMR induced inverse spin Hall voltage 12.6µV at 1.0GHz was obtained across Pt layer. Magnetic exchange field at bilayer interface responsible for field torque was measured 6×1014Ω−1m−2 by spin Hall magnetoresistance. The damping torque and spin Hall angle have been evaluated as 0.084 and 0.071 respectively. Presence of Si atom in FeSi3% inhomogenize the magnetic exchange field among accumulated spins at bilayer interface and feebly influenced by spin torque of FeSi3% layer. Weak field torque suppresses the spin pumping to Pt layer thus low value of inverse spin Hall voltage is obtained. This study provides an excellent opportunity to investigate spin transfer torque effect, thus motivating a more intensive experimental effort for its utilization at maximum potential. The improvement in spin transfer torque may be useful in spin valve, spin battery and spin transistor application.

Magnetocaloric effect across the metamagnetic transition in Dy5Si2Ge2 single crystal

A single crystal of rare earth intermetallic compound Dy5Si2Ge2 (Orthorhombic, Space group Pnma) has been prepared by Czochralski method. Magnetization of Dy5Si2Ge2 single crystal has been measured with magnetic field applied (i) along b axis and (ii) parallel to ac plane. Although a same Neél temperature (TN) of ∼55K is obtained when 50 Oe field is applied along b axis and in ac plane, the magnitude of magnetization is substantially different. This TN value is close to the antiferromagnetic ordering temperature of polycrystalline Dy5Si2Ge2 as well. The magnetization vs field data at 1.8K shows an ultra-sharp metamagnetic transition when field is applied along both the directions, in a critical field of about 28 kOe. It is observed that b axis is easy direction of magnetization. Magnetocaloric effect of this Dy5Si2Ge2 crystal has been estimated using magnetization vs field data obtained at various temperatures. A maximum magnetic entropy change (ΔSmmax) of ∼−15.6J/kg/K has been observed at 22K with field along b axis and ΔSmmax is ∼−11.2J/kg/K at 54K when field is parallel to ac plane, for a field change of 50 kOe. Occurrence of metamagnetic transition at temperatures below TN gives rise to a second peak at ∼22K in the temperature dependent isothermal magnetic entropy change that leads to the observed large magnetocaloric effect.