Uniaxial Term Research Articles

Magnetic and Electronic Properties of Weyl Semimetal Co2MnGa Thin Films.

Magnetic Weyl semimetals are newly discovered quantum materials with the potential for use in spintronic applications. Of particular interest is the cubic Heusler compound Co2MnGa due to its inherent magnetic and topological properties. This work presents the structural, magnetic and electronic properties of magnetron co-sputtered Co2MnGa thin films, with thicknesses ranging from 10 to 80 nm. Polarized neutron reflectometry confirmed a uniform magnetization through the films. Hard x-ray photoelectron spectroscopy revealed a high degree of spin polarization and localized (itinerant) character of the Mn d (Co d) valence electrons and accompanying magnetic moments. Further, broadband and field orientation-dependent ferromagnetic resonance measurements indicated a relation between the thickness-dependent structural and magnetic properties. The increase of the tensile strain-induced tetragonal distortion in the thinner films was reflected in an increase of the cubic anisotropy term and a decrease of the perpendicular uniaxial term. The lattice distortion led to a reduction of the Gilbert damping parameter and the thickness-dependent film quality affected the inhomogeneous linewidth broadening. These experimental findings will enrich the understanding of the electronic and magnetic properties of magnetic Weyl semimetal thin films.

Formation of Defect-Dicubane-Type NiII 2LnIII 2 (Ln = Tb, Er) Clusters: Crystal Structures and Modeling of the Magnetic Properties.

In the field of molecular nanoclusters, cubane and defect-dicubane, or butterfly structures, are typical examples of tetranuclear metal core architectures. In this work, a halogenated and anionic Schiff-base ligand (L2–) is utilized as it is predisposed to chelate within a cluster core to both 3d and 4f metal ions, in different binding configurations (H2L = 4-chloro-2-(2-hydroxy-3-methoxybenzyliden amino)phenol). The phenolate oxygen atoms of the deprotonated ligand can act in μ-O and μ3-O bridging binding modes for the intramolecular assembly of metal ions. Based on that, two tetranuclear and isostructural compounds [Ni2Tb2(L)4(NO3)2(DMF)2]·2CH3CN (1) and [Ni2Er2(L)4(NO3)2(DMF)2]·0.5CH3CN (2) were synthesized and structurally characterized. Magnetic susceptibility and magnetization data indicate the occurrence of dominant intramolecular ferromagnetic interactions between the spin centers. Particular emphasis is given to the theoretical description of the magnetic behavior, taking into account the Ln–Ni and Ni–Ni coupling paths and the magnetic anisotropy of the LnIII and NiII ions. The study is distinguished for its discussion of two distinct models, whereby model A relies on the uniaxial B20 Stevens term describing the lanthanide anisotropy and model B is based on point-charge model calculations. Importantly, the physical meaning of the obtained parameters for both models was critically scrutinized.

Magnetic interaction and anisotropy axes arrangement in nanoparticle aggregates can enhance or reduce the effective magnetic anisotropy

The magnetic response of nanostructures plays an important role on biomedical applications being strongly influenced by the magnetic anisotropy. In this work, we investigate the role of the temperature, particle concentration and nanoparticles arrangement forming aggregates in the effective magnetic anisotropy of Mn-Zn ferrite-based nanoparticles. Electron magnetic resonance and coercivity temperature dependence analyses, were critically compared for the estimation of the anisotropy. We found that the temperature dependence of the anisotropy follows the Zener-Callen model, while the symmetry depends on the particle concentration. At low concentration one observes only an uniaxial term, while increasing a cubic contribution has to be added. The effective anisotropy was found to increase the higher the particle concentration on magnetic colloids, as long as the easy axis was at the same direction of the nanoparticle chain. Increasing even further the concentration up to a highly packed condition (powder sample) one observes a decrease of the anisotropy, that was attributed to the random anisotropy axes. Further, room temperature anisotropy decreased on the order of 50% in comparison to the low temperature values, thus impacting the theoretical estimations of the magnetic response of nanoparticles.

Magnetic anisotropies and its optical manipulation in epitaxial bismuth ferrite/ferromagnet heterostructure

Exchange-coupled BiFeO3/ferromagnet thin film heterostructures are promising for a fast and power efficient control of magnetization of the ferromagnetic layer. In the paper, the results of the study of magnetic properties of the epitaxial Fe2B/BiFeO3 heterostructure on (001)-SrTiO3 substrate with ferromagnetic resonance (FMR) spectroscopy are presented. The hierarchy of magnetic anisotropies that determine the angular variation of the resonance field in-plane and in-/out-of the plane of the system include the tetragonal four-fold and uniaxial terms for the Fe2B layer, uniaxial term for antiferromagnetic BiFeO3 layer and the exchange coupling at the interface. We find out that the exchange bias direction can be switched by a strong enough applied magnetic field. Both the exchange bias and an in-plane FMR resonance field of the heterostructure are strongly affected by the illumination with λ = 405 nm light. An optical tunability of the exchange bias and ferromagnetic resonance fields of the heterostructure has been demonstrated.

Exchange stiffness and damping constants in diluted CoxFeyB1−x−y thin films

The magnetic properties of 20 nm thick sputtered CoFeB thin films of different compositions have been investigated and compared to those of traditional 3D ferromagnetic materials (Co, Fe and Ni). A vibrating sample magnetometer has been used to measure their magnetization at saturation (Ms). Their dynamic magnetic properties were studied using microstrip line ferromagnetic resonances, as well as Brillouin light scattering (BLS) techniques. The effective magnetizations and gyromagnetic factors are firstly measured from resonance spectra obtained for in-plane and perpendicular applied fields. The angular dependence of the resonance field then allows us to derive parameters describing the in-plane magnetic anisotropy, which is found to result from a superposition of small uniaxial and fourfold terms. Frequency and angular dependencies of the ferromagnetic resonance linewidth have been used to determine the Gilbert damping coefficient for each sample. The perpendicular surface standing modes, observed in BLS spectra, allow the evaluation of the exchange stiffness constant, which is found to vary linearly with Ms (for CoFeB), in agreement with the simple model presented here. Finally, the thickness dependence of the Gilbert damping parameter of Co15Fe45B40 revealed a contribution due to spin pumping leading to a spin mixing conductance of 31 nm−2. The BLS measurements on the 5 nm thick Co15Fe45B40 film evidenced a large frequency asymmetry attributed to the interfacial Dzyaloshinskii–Moriya interaction (DMI) originating from the interface with Pt. The surface DMI constant has been estimated to be = −0.33 pJ m−1.

Thermodynamic conditions during growth determine the magnetic anisotropy in epitaxial thin-films of La0.7Sr0.3MnO3

The suitability of a particular material for use in magnetic devices is determined by the process of magnetization reversal/relaxation, which in turn depends on the magnetic anisotropy. Therefore, designing new ways to control magnetic anisotropy in technologically important materials is highly desirable. Here we show that magnetic anisotropy of epitaxial thin-films of half-metallic ferromagnet La0.7Sr0.3MnO3 (LSMO) is determined by the proximity to thermodynamic equilibrium conditions during growth. We performed a series of x-ray diffraction and ferromagnetic resonance (FMR) experiments in two different sets of samples: the first corresponds to LSMO thin-films deposited under tensile strain on (0 0 1) SrTiO3 by pulsed laser deposition (PLD; far from thermodynamic equilibrium); the second were deposited by a slow chemical solution deposition (CSD) method, under quasi-equilibrium conditions. Thin films prepared by PLD show fourfold in-plane magnetic anisotropy, with an overimposed uniaxial term. However, the uniaxial anisotropy is completely suppressed in the CSD films. This change is due to a different rotation pattern of MnO6 octahedra to accommodate epitaxial strain, which depends not only on the amplitude of tensile stress imposed by the STO substrate, but also on the growth conditions. Our results demonstrate that the nature and magnitude of the magnetic anisotropy in LSMO can be tuned by the thermodynamic parameters during thin-film deposition.

Temperature dependence of perpendicular magnetic anisotropy in CoFeB thin films

We study perpendicular magnetic anisotropy in thin films of Ta/Co20Fe60B20/MgO by ferromagnetic resonance and find a linear temperature dependence for the first and second order uniaxial terms from 5 to 300 K. Our data suggest the possible hybridization of Fe-O orbitals at the CoFeB/MgO interface for the origin of the first order anisotropy. However, we also find that non-interfacial contributions to the anisotropy are present. An easy-cone anisotropy is found for the entire temperature range in the narrow region of film thicknesses around the spin reorientation transition 1.2–1.35 nm.

Static and dynamic magnetic properties of Co2FeAl-based stripe arrays

25nm to 50nm Co2FeAl (CFA) thick wire arrays with varying widths and spacing have been patterned from continuous CFA films deposited on MgO(001) using e-beam lithography and Ar ion milling. Magneto-optical Kerr effect, transverse bias initial inverse susceptibility and torque measurements reveal that the in-plane magnetic anisotropy of the wires is dominantly monitored by a uniaxial term, in contrast with the continuous films where it is governed by the superposition of a fourfold term and of a smaller uniaxial term. The microstrip ferromagnetic resonance spectra performed using a magnetic field H, applied in the plane of the studied sample along various directions, or perpendicularly to this plane, gave us access to various quantized modes originating from the patterning. In addition, Brillouin light scattering also exhibits quantized modes. A large part of the experimental observations can be quantitatively interpreted as resulting from the demagnetizing terms induced by the geometrical patterning. However, the presented model, simply built on the effect of the demagnetizing field, is not able to give account of all the quantized modes present in the resonance spectra. When H is parallel to the wires, a more complete description is used: it considers the wave-vector quantization induced by the patterning. For the magnetic modes concerned by both approaches, the correspondence between the 2 models is easily established. When H is not parallel to the wires quantitative descriptions of the behavior of the field dependence of the observed modes still can often be performed. Finally, in all the studied patterned samples, the uniform magnetic mode, termed “film mode”, relative to the parent continuous film is observed by ferromagnetic resonance: such a behavior, which has been reported previously, remains to be completely interpreted.

Degenerate Effect on the Mobility of Holes in Graphane: A Study Based on Density Functional Theory Coupled with Deformation Potential Theory.

The traditional deformation potential method is not able to calculate the charge mobility of heavily doped degenerate semiconductors, in which inter-band scattering is not negligible. To theoretically predict the charge mobility of such semiconductors, an improved deformation potential method is required, in which the deformation potential constant is decomposed into two parts (hydrostatic and uniaxial terms) based on k⋅p theory to incorporate the inter-band scattering between degenerate valence bands. We propose a new method to calculate the heavy- and light-hole mobilities of graphane. The proposed method produces more appropriate values than the traditional methods. Hence, the new method can be applied to other 2D materials with degenerate bands.

ELECTRONIC, STRUCTURAL AND MAGNETIC PROPERTIES OF Co2FeAl THIN FILMS FOR POTENTIAL SPINTRONIC APPLICATIONS

This paper presents an overview concerning the electronic, structural and magnetic properties of Co 2 FeAl (CFA) thin films. We first used ab initio calculations of the electronic structure in order to discuss the half-metallicity of this compound. Involving a correlated structural-magnetic analysis, we then illustrate, experimentally, the effect of the thickness as well as the annealing temperature on the magnetic and structural properties of CFA films epitaxially grown on MgO (001) single crystal substrates. The X-ray diffraction shows that in our samples having the CFA(001)[110]// MgO (001)[100] epitaxial relation, the chemical order is enhanced as the thickness and the annealing temperature (T a ) are increased. Ferromagnetic resonance measurements reveal further dynamic magnetic properties. The gyromagnetic factor, estimated at 29.2 GHz/T, is both thickness and annealing temperature independent. The in-plane anisotropy results from the superposition between a dominant fourfold symmetry term, as expected for cubic crystal symmetry of the alloy, and a small uniaxial term. The fourfold anisotropy decreases with increasing thickness and annealing temperature. The exchange stiffness constant is thickness independent but increases with T a . In addition, the effective magnetization varies linearly with T a and with the inverse CFA thickness. This is due to the presence of perpendicular uniaxial anisotropy, estimated around -1.8 erg/cm2 at T a = 600°C and 1.05 erg/cm2 at T a = 265°C, respectively. Frequency and angular dependences of the FMR linewidth show two magnon scattering and mosaicity contributions which depend on the CFA thickness and T a . A Gilbert damping coefficient as low as 0.0011 is found for samples annealed at 600°C. Finally, we illustrate that these films can be used as ferromagnetic electrodes in sputtered epitaxial magnetic tunnel junctions (MTJ) based on MgO (001) tunnel barriers. These MTJs show an improvable TMR ratio around 95% at room temperature.

In-Plane Uniaxial Anisotropy of Cobalt Ferrite Films Epitaxially Grown on MgAl<sub>2</sub>O<sub>4</sub> (110)

We have investigated the magnetic properties of cobalt ferrite films epitaxially grown on MgAl 2 O 4 (110) substrates. The unit cell of the cobalt ferrite film on the substrate with twofold surface symmetry is tetragonally distorted with c/a <; 1, and the film exhibits uniaxial magnetic anisotropy parallel to the in-plane [001] orientation, which is similar to what is observed for films grown on MgO (001). The magnetic easy axis is parallel to the in-plane [001] direction and the magnetic hard axis is parallel to the out-of-plane [110] direction. Careful measurement of the magnetic torque indicates that the anisotropy energy can be described by a combination of uniaxial, fourfold, and demagnetization terms. The uniaxial term is comparable with that obtained for cobalt-ferrite films epitaxially grown on MgO (001).

Epitaxial films of Heusler compound Co2FeAl0.5Si0.5 with high crystalline quality grown by off-axis sputtering

Co2FeAl0.5Si0.5 films with a surface roughness of 0.12 nm have been grown epitaxially on lattice-matched MgAl2O4 (001) substrates by off-axis sputtering. X-ray diffraction shows pronounced Laue oscillations, rocking curves as narrow as 0.0043°, and clear Co2FeAl0.5Si0.5 (111) peaks indicating L21 ordering. Magnetic characterizations show a clear magnetocrystalline anisotropy comprising cubic and epitaxy-induced uniaxial terms. Nuclear magnetic resonance measurements reveal L21 order of 81% in the Co2FeAl0.5Si0.5 films. Magnetotransport measurements show a distinct separation of anisotropic magnetoresistance and ordinary magnetoresistance. These results demonstrate the state-of-the-art crystalline quality and magnetic uniformity of the Co2FeAl0.5Si0.5 films.

Three-dimensional mapping of the anisotropic magnetoresistance in Fe3O4 single crystal thin films

The anisotropic magnetoresistance (AMR) effect with a magnetic field along arbitrary directions in single crystalline (001)-oriented Fe3O4 films was studied. A cubic symmetry term, an in-plane uniaxial term, and an out-of-plane uniaxial term could be quantitatively separated. The cubic term is independent of the current direction, and decreases with increasing temperature, but both in-plane and out-of-plane uniaxial terms are found to be strongly dependent on the current orientation. This three-dimensional magnetoresistance measurement provides a quantitative method for identifying the different contributions to the AMR effect.

Tailoring magnetic anisotropy in epitaxial half metallic La0.7Sr0.3MnO3 thin films

We present a detailed study on the magnetic properties, including anisotropy, reversal fields, and magnetization reversal processes, of well characterized half-metallic epitaxial La0.7Sr0.3MnO3 (LSMO) thin films grown onto SrTiO3 (STO) substrates with three different surface orientations, i.e., (001), (110), and (11−8). The latter shows step edges oriented parallel to the [110] (in-plane) crystallographic direction. Room temperature high resolution vectorial Kerr magnetometry measurements have been performed at different applied magnetic field directions in the whole angular range. In general, the magnetic properties of the LSMO films can be interpreted with just the uniaxial term, with the anisotropy axis given by the film morphology, whereas the strength of this anisotropy depends on both structure and film thickness. In particular, LSMO films grown on nominally flat (110)-oriented STO substrates presents a well defined uniaxial anisotropy originated from the existence of elongated in-plane [001]-oriented structures, whereas LSMO films grown on nominally flat (001)-oriented STO substrates show a weak uniaxial magnetic anisotropy, with the easy axis direction aligned parallel to residual substrate step edges. Elongated structures are also found for LSMO films grown on vicinal STO(001) substrates. These films present a well-defined uniaxial magnetic anisotropy, with the easy axis lying along the step edges, and its strength increases with the LSMO thickness. It is remarkable that this step-induced uniaxial anisotropy has been found for LSMO films up to 120 nm thickness. Our results are promising for engineering novel half-metallic magnetic devices that exploit tailored magnetic anisotropy.

Systematic study of anisotropic magnetoresistance effect in (311)A GaMnAs films

Anisotropic magnetoresistance effect has been studied systematically in (311)A-oriented as-grown and annealed GaMnAs films at different temperatures. Besides a uniaxial term, the measured angular dependence of longitudinal and transverse magnetoresistivities contains higher order terms arising from the crystal symmetry of GaMnAs. A third order crystalline term accounts for the observed antisymmetric planar Hall effect which is intrinsic to (311)A-oriented GaMnAs films. The uniaxial term persists above the Curie temperature, while the higher order terms can only be observed below the Curie temperature, with their magnitudes increasing with decreasing temperatures. It is revealed that the higher order crystalline terms originate from the long range ferromagnetic (FM) phase and the uniaxial one from isolated magnetic polarons (MPs) as well as the FM phase. With decreasing temperatures, the dominant contribution to the uniaxial term changes from isolated MPs to the FM phase. In addition, low temperature annealing of GaMnAs films is found to increase the magnitudes of all these terms.

In-plane magnetic anisotropies inFe3O4films on vicinal MgO(100)

Ferromagnetic resonance was used to study the influence of vicinal (miscut) angle and film thickness on in-plane fourfold and uniaxial magnetic anisotropies in epitaxial ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ films grown on vicinal MgO(100) surfaces. The in-plane fourfold anisotropy constant ${K}_{4\ensuremath{\parallel}}$ is approximately the same for all films but the dominant in-plane uniaxial constant ${K}_{2\ensuremath{\parallel}}$ varies linearly with the inverse ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ layer thickness and approximately quadratically with the vicinal angle. A second, weaker, in-plane uniaxial term is evident for the film on a larger miscut (10\ifmmode^\circ\else\textdegree\fi{}) substrate. The easy axis of the dominant in-plane uniaxial term is perpendicular to the step edges. The dominant in-plane uniaxial anisotropy has one term inversely proportional to the film thickness that is associated with anisotropy localized at the interface and a second term that is independent of film thickness; the latter may arise from the preferential alignment of antiphase boundaries with the step edges.

An extensive comparison of anisotropies in MBE grown (Ga,Mn)As material

This paper reports on a detailed magnetotransport investigation of the magnetic anisotropies of (Ga,Mn)As layers produced by various sources worldwide. Using anisotropy fingerprints to identify the contributions of the various higher-order anisotropy terms, we show that the presence of both a [100] and a [110] uniaxial anisotropy in addition to the primary ([100] + [010]) anisotropy is common to all medium doped (Ga,Mn)As layers typically used in transport measurement, with the amplitude of these uniaxial terms being characteristic of the individual layers.

Effects of spin non-collinearities in magnetic nanoparticles

In a many-spin approach that takes account of the internal structure, microscopic interactions and single-site anisotropies, we investigate the effect of spin non-collinearities induced by the boundary and surface anisotropy on the behaviour of individual magnetic nanoparticles. Through analytical and numerical calculations, we show that there are mainly two regimes separated by some critical value of the surface-anisotropy constant K s which controls the intensity of spin non-collinearities: (i) the so-called Stoner–Wohlfarth or Néel–Brown regime of a macrospin undergoing a coherent switching, and (ii) the many-spin regime where the strong spin non-collinearities invalidate the coherent mechanism, and where the particle's magnetic state and switching mechanisms can no longer be modelled by a macrospin. For small-to-intermediate values of K s , and within two models of surface anisotropy (transverse and Néel), the behaviour of the nanoparticle can be modelled by that of a modified macrospin with an effective potential energy containing a uniaxial and cubic-anisotropy terms. This effective spin model provides a crossover between the two regimes above.

Electrical and magnetic properties of [formula omitted]-Fe 2O 3 epitaxial films

We have investigated electrical and magnetic properties of γ -Fe 2O 3 (0 0 1) epitaxial films. Resistivity of the film was ∼ 10 2 Ω cm at room temperature. The temperature dependent resistivity seemed to follow the three-dimensional variable-range-hopping model. Moreover, ferromagnetic resonance experiment revealed that magnetic anisotropy of cubic and uniaxial terms were - 1.3 × 10 4 and 2.7 × 10 5 erg / cm 3 , respectively. This result is qualitatively consistent with our Mössbauer experiment and magnetization measurements.

Fine-structure anomalies in EPR spectra ofGd3+ centresformed in TlCdF3 single crystals

Electron paramagnetic resonance (EPR) measurements have been made on as-grown single crystals ofperovskite-type TlCdF3 at room temperature in the cubic phase. Signals from two kinds of tetragonalGd3+ centre are observed. One centre observed in the Gd-only-doped crystal is ascribed to aGd3+ ionat a Cd2+ site being associated with a vacancy at the nearestCd2+ site. The other centre observed in the Gd, Li co-doped crystal is ascribed to aGd3+ ion at aCd2+ site beingassociated with a Li+ ion at the nearest Cd2+ site. The fine-structure terms are discussed by separating them into the uniaxial andcubic terms. Anomalous values of the axial and cubic parameters are obtained forTlCdF3 in contrast with the parameters for other host crystalsACdF3 (A = K,Rb, Cs).