Easy-axis Anisotropy Research Articles

Field-induced quantum magnetism in the verdazyl-based charge-transfer salt[o−MePy−V−(p−Br)2]FeCl4

We successfully synthesized a verdazyl-based charge-transfer salt $[$$o$-MePy-V-($p$-Br)$_2]$FeCl$_4$, which has an $S_{\rm{V}}$=1/2 on the radical $o$-MePy-V-($p$-Br)$_2$ and an $S_{\rm{Fe}}$=5/2 on the FeCl$_4$ anion. $Ab$ $initio$ molecular orbital calculations indicate the formation of an $S_{\rm{V}}$=1/2 honeycomb lattice composed of three types of exchange interaction with two types of inequivalent site. Further, the $S_{\rm{V}}$=1/2 at one site is sandwiched by $S_{\rm{Fe}}$=5/2 spins through antiferromagnetic (AF) interactions. The magnetic properties indicate that the dominant AF interactions between the $S_{\rm{V}}$ = 1/2 spins form a gapped singlet state, and the remaining $S_{\rm{Fe}}$ = 5/2 spins cause an AF order. The magnetization curve exhibits a linear increase up to approximately 7 T, and an unconventional 5/6 magnetization plateau appears between 7 T and 40 T. We discuss the differences between the effective interactions associated with the magnetic properties of the present compound and ($o$-MePy-V)FeCl$_4$. We explain the low-field linear magnetization curve through a mean-field approximation of an $S_{\rm{Fe}}$ = 5/2 spin model. At higher field regions, the 5/6 magnetization plateau and subsequent nonlinear increase are reproduced by the $S_{\rm{V}}$ = 1/2 AF dimer, in which a particular internal field is applied to one of the spin sites. The ESR resonance signals in the low-temperature and low-field regime are explained by conventional two-sublattice AF resonance modes with easy-axis anisotropy. These results demonstrate that exchange interactions between $S_{\rm{V}}$ = 1/2 and $S_{\rm{Fe}}$ = 5/2 spins in $[$$o$-MePy-V-($p$-Br)$_2]$FeCl$_4$ realize unconventional magnetic properties with low-field classical behavior and field-induced quantum behavior.

Features of Dispersion Characteristics of Surface Spin Waves in Coupled Antiferromagnetic Films with Easy-Axis Anisotropy

Propagation of magnetostatic spin waves in a layered structure consisting of two vertically coupled via a nonmagnetic dielectric layer antiferromagnetic (AFM) films has been studied. The results of calculation of the dispersion dependence for the AFM films with an easy-axis anisotropy are presented. It has been shown that splitting of coupled modes occurs in the region of AFM resonance frequencies. The behavior of coupled modes as a function of the parameter of coupling between films has been studied.

Surface spin waves in coupled easy-axis antiferromagnetic films

Abstract We present results of studying properties of surface spin waves in vertically coupled antiferromagnetic films with easy-axis anisotropy. The dispersion curves and frequency region of the existence of the surface magnetostatic waves are obtained using theoretical model for different antiferromagnetic materials. This study was carried out by solving the dispersion relation for the surface magnetostatic waves. Behavior of two types of dispersion curves on the value of coupling parameter is investigated. We conclude, that results have potential applications for antiferromagnetic THz spintronics.

Quantized excitation spectra by magnon confinement in quasi-one-dimensional S=1 spin systems

We apply the infinite time-evolving-block-decimation algorithm to calculate the dynamical spin-structure factors of the quasi-one-dimensional (q1D) S=1 antiferromagnetic spin system with the single-ion anisotropy and the bond alternation. We find that excitation continuum originating from magnons is quantized, when the staggered field induced by the weak inter-chain interaction is taken into account. The excitation energies of the quantized excitation spectra are well explained by negative zeros of the Airy functions, when the easy-axis anisotropy is strong and the ground state is located deep in the N\'eel phase. This quantization of the magnon continuum is a counterpart of the spinon confinement, which has been recently discussed in q1D S=1/2 antiferromagnets. We further show that, when the staggered field exists, the quantized excitation spectra appear the phase boundary between the Haldane phase and the N\'eel phase of the phase diagram without the staggered field. However, the quantized excitation spectra disappear in the singlet dimer phase.

Effects of Anisotropy Field and Easy Axis Dispersions on Squareness Ratio for HDDR-Processed Nd-Fe-B Powders

NdFeB magnetic powders that are produced by a hydrogenation decomposition desorption recombination (HDDR) process consist of small grains with highly anisotropic energy [1]–[3]. Therefore, HDDR-processed NdFeB magnet powders are expected to have a high squareness ratio and high coercivity $(H_{c})$ to obtain the high maximum energy product $(BH _{max})$ that is required in highly efficient motors of hybrid or electric vehicles. The squareness ratio is defined as the value of the magnetic field at 90% of the remanent magnetization divided by $H_{c}$. However, the squareness ratio is much lower than the expected value of 1.0, and the $H_{c}$ is lower than a third of the average anisotropy fields. A previous study using a micromagnetic simulator has shown that when an anisotropy field $(H_{k})$ dispersion of grains was assumed to be a Gaussian distribution with a coefficient variation $(\sigma H_{k} {/\lt \mathrm {H}} _{k} \gt )$ of 30%, the squareness ratio corresponded with an experimental value [4]. However, Nishio et al. showed that the $H_{k}$ of a single crystal was 7600 kA/m [5]. Therefore, when the $\sigma H_{k} {/\lt \mathrm {H}} _{k} \gt$ is 30%, the $\lt H_{k} \gt + 3 \sigma H_{k}$ is unrealistically higher than the $H_{k}$ of a single crystal. In this study, the $H_{k}$ dispersion of the grains was assumed to be a horizontally flipped lognormal distribution, and the effects of the $H_{k}$ dispersion of the grains on the squareness ratio were investigated by using a micromagnetic simulator. Moreover, Nishio et al. showed that an easy axis (c-axis) inclination angle was distributed within ± 20° [6], so we also investigated the effects of the c-axis dispersion on the squareness ratio.

Domain-wall dynamics for an in-plane magnetized thin film with a large perpendicular hard-axis anisotropy including the Dzyaloshinskii-Moriya interaction

We consider a thin ferromagnetic layer to which an external field or a current are applied along an in plane easy axis. The perpendicular hard axis anisotropy constant is large so that the out of plane magnetization component is smaller than the in plane components. A perturbation approach is used to obtain the profile and velocity of the moving domain wall. The dynamics of the in plane components of the magnetization is governed by a reaction diffusion equation which determines the speed of the profile. We find a simple analytic expression for the out of plane magnetization showing a symmetric distortion due to the motion in addition to the asymmetric component due to the Dzyaloshinskii--Moriya interaction. The results obtained complement previous studies in which either the Dzyalozhinskii vector or the out of plane hard axis anisotropy were assumed small. In the regime studied the Walker breakdown is not observed but the reaction diffusion dynamics predicts a slowing down of the domain wall for sufficiently large magnetic field. The transition point depends on the applied field, saturation magnetization and easy axis anisotropy.

Trapping of a Pseudotetrahedral CoIIO4 Core in Mixed-Valence Mixed-Geometry [Co5] Coordination Aggregates: Synthetic Marvel, Structures, and Magnetism.

A systematic one-step one-pot multicomponent reaction of Co(ClO4)2·6H2O, H3L (2,6-bis((2-(2-hydroxyethylamino)ethylimino)methyl)-4-methylphenol), and readily available carboxylate salts (RCO2Na; R = CH3, C2H5) resulted in the two structurally novel coordination aggregates [CoIICoIII4L2(μ1,3-O2CCH3)2(μ-OH)2](ClO4)4·4H2O (1) and [CoIICoIII4L2(μ1,3-O2CC2H5)2(μ-OH)(μ-OMe)](ClO4)4·5H2O (2). At room temperature, reactions of H3L in MeOH with cobalt(II) perchlorate salts led to coassembly of initially formed ligand-bound {CoII2} fragments following aerial oxidation of metal centers and bridging by in situ generated hydroxido/alkoxido groups and added carboxylate anions. Available alkoxido arms of the initially formed {L(μ1,3-O2CCH3)(μ-OH/OMe)Co2}+ fragments were utilized to trap a central CoII ion during the formation of [Co5] aggregates. In the solid state, both complexes have been characterized by X-ray crystallography, variable-temperature magnetic measurements, and theoretical studies. Both 1 and 2 show field-induced slow magnetic relaxation that arises from the single pseudo- T d CoII ion present. The structural distortion leads to an easy-axis magnetic anisotropy ( D = -31.31 cm-1 for 1 and -21.88 cm-1 for 2) and a small but non-negligible transverse component ( E/ D = 0.11 for 1 and 0.08 for 2). The theoretical studies also reveal how the O-Co-O bond angles and the interplanar angles control D and E values in 1 and 2. The presence of two diamagnetic {Co2(μ-L)} hosts controls the distortion of the central {CoO4} unit, highlighting a strategy to control single-ion magnetic anisotropy by trapping single ions within a diamagnetic coordination environment.

Perpendicular magnetic anisotropy in bulk and thin-film CuMnAs for antiferromagnetic memory applications

CuMnAs with perpendicular magnetic anisotropy is proposed as an active material for antiferromagnetic memory. Information can be stored in the antiferromagnetic domain state, while writing and readout can rely on the existence of surface magnetization. It is predicted, based on first-principles calculations, that easy-axis anisotropy can be achieved in bulk CuMnAs by substituting a few percent of As atoms by Ge, Si, Al, or B. This effect is attributed to the changing occupation of certain electronic bands near the Fermi level induced by hole doping. The calculated temperature dependence of the magnetic anisotropy does not exhibit any anomalies. Thin CuMnAs(001) films are also predicted to have perpendicular magnetic anisotropy.

Magnetic exchange parameters and anisotropy of the quasi-two-dimensional antiferromagnetNiPS3

Neutron inelastic scattering has been used to measure the magnetic excitations in powdered NiPS$_3$, a quasi-two dimensional antiferromagnet with spin $S = 1$ on a honeycomb lattice. The spectra show clear, dispersive magnons with a $\sim 7$ meV gap at the Brillouin zone center. The data were fitted using a Heisenberg Hamiltonian with a single-ion anisotropy assuming no magnetic exchange between the honeycomb planes. Magnetic exchange interactions up to the third intraplanar nearest-neighbour were required. The fits show robustly that NiPS$_3$ has an easy axis anisotropy with $\Delta = 0.3$ meV and that the third nearest-neighbour has a strong antiferromagnetic exchange of $J_3 = -6.90$ meV. The data can be fitted reasonably well with either $J_1 < 0$ or $J_1 > 0$, however the best quantitative agreement with high-resolution data indicate that the nearest-neighbour interaction is ferromagnetic with $J_1 = 1.9$ meV and that the second nearest-neighbour exchange is small and antiferromagnetic with $J_2 = -0.1$ meV. The dispersion has a minimum in the Brillouin zone corner that is slightly larger than that at the Brillouin zone center, indicating that the magnetic structure of NiPS$_3$ is close to being unstable.

Effect of biquadratic exchange on the phase diagram of a spin-1 transverse XY model with single-ion anisotropy

The two-time Green function method is employed to explore the effect of the biquadratic exchange interaction on the phase diagram of a $d$-dimensional spin-1 transverse XY model with single-ion anisotropy close to the magnetic-field-induced quantum critical point. We work at level of the Anderson-Callen-decoupling-like framework for both easy-plane and easy-axis single-ion anisotropy. The structure of the phase diagram is analyzed with analytical estimates and numerical calculations by adopting Tyablikov-like decouplings for the exchange higher order Green functions in the equation of motion. For dominant bilinear short-range exchange interaction the structure of the phase diagram close to the quantum critical point remains qualitatively the same of that in absence of biquadratic interaction including reentrant critical lines. When the biquadratic exchange becomes increasingly dominant its role appears more effective and tends to reduce or destroy the reentrant character of the critical lines.

Magneto-electronic properties and structural features of unusual bis(μ-aqua) bis(μ-sulfato) bridges in binuclear cobalt-based 4-aminopyridine

In order to improve the relationship between the structure of bridging dinuclear complexes and their magnetic behavior, we did synthesize metal-organic [Co2(μ2-H2O)2(μ2-SO4)2(C5H6N2)4]·2.86(C2H6OS)·2.88(C4H8O) 1. This dinuclear transition metal complex was characterized by infrared and UV–vis spectroscopy as well as single X-ray diffraction. The thermal and magnetic behavior of this complex was studied. Single crystal X-ray diffraction studies confirmed that 1 consists of discrete dinuclear cobalt entities and two free solvent molecules (DMSO, THF). The two cobalt ions are bridged by two water oxygen atoms and two bidentate sulfate groups to form a four bridged dinuclear core. The optical band gap was determined by diffuse reflection spectroscopy to 3.06 eV for an indirect forbidden transition. The magnetic measurements and DFT calculations suggest the presence of ferromagnetic exchange between Co(II) centers with a strong easy-axis anisotropy.

Staggered field driven domain walls motion in antiferromagnetic heterojunctions

In this work, we study the antiferromagnetic (AFM) spin dynamics in heterostructures which consist of two coupled AFM layers, i.e., AFM1 layers (describing CuMnAs or Mn2Au) with field-like Néel spin-orbit torque (NSOT) and AFM2 layers with easy-axis anisotropy orthogonal to that in AFM1 layers. Our micromagnetic simulations demonstrate that through the interface coupling, the AFM2 domain wall (DW) can be effectively driven by the AFM1 DW which is driven by the electrical current induced NSOT [Gomonay et al., Phys. Rev. Lett. 117, 017202 (2016)]. Furthermore, the two DWs detach from each other when the torque increases above a critical value. The critical field and the highest possible velocity of the AFM2 DW depend on several factors, which are investigated and discussed in detail. Based on the calculated results, we propose a method of efficiently modulating the multi DWs in antiferromagnets, which definitely provides useful information for future AFM spintronics device design.

Stripe order and magnetic anisotropy in the S=1 antiferromagnet BaMoP2O8

Magnetic behavior of BaMoP$_2$O$_8$ with the spatially anisotropic triangular arrangement of the $S=1$ Mo$^{4+}$ ions is explored using thermodynamic measurements, neutron diffraction, and density-functional band-structure calculations. A broad maximum in the magnetic susceptibility around 46\,K is followed by the stripe antiferromagnetic order with the propagation vector ${\mathbf k}=(\frac{1}{2},\frac{1}{2},\frac{1}{2})$ formed below $T_N\simeq 21$ K. This stripe phase is triggered by a pronounced one-dimensionality of the spin lattice, where one of the in-plane couplings, $J_2\simeq 4.6$ meV, is much stronger than its $J_1\simeq 0.4$ meV counterpart, and stabilized by the weak easy-axis anisotropy. The ordered moment of 1.42(9) $\mu_B$ at 1.5 K is significantly lower than the spin-only moment of 2 $\mu_B$ due to a combined effect of quantum fluctuations and spin-orbit coupling.

Tuning transport and magnetic properties of CoxFe3-xO4 thin films by Co content

We systematically investigated the influence of the Co content on the magnetic and transport properties of spinel CoxFe3-xO4 (0 ≤ x ≤ 1) thin films on MgO (001) substrates prepared by molecular beam epitaxy. Our films exhibited the spinel structure as expected. The Verwey transition was clearly observed in Fe3O4 and Co0.6Fe2.4O4, while it disappeared with higher Co contents. Electrical resistivity increased with the Co content from 0.003 Ω-cm for x = 0 to 1.5 Ω-cm for x = 1 at 300 K. As expected, magnetization curves show an increase of the coercive fields of the samples with the Co content. In addition, the samples reveal complex magnetic properties that are explained by a reorientation of the anisotropy easy axis from the crystal 〈111〉 directions to the 〈100〉 directions with the Co content.

Control of magnetic anisotropy by orbital hybridization with charge transfer in (La0.67Sr0.33MnO3)n/(SrTiO3)n superlattice

The chemical asymmetry at the hetero-structure interface offers an effective opportunity to design desirable electronic structures by controlling charge transfer and orbital hybridization across the interface. However, controlling the hetero-interface remains a daunting task. Here, we report the modulation of interfacial coupling of (La0.67Sr0.33MnO3)n/(SrTiO3)n superlattices by manipulating the periodic thickness with n unit cells of SrTiO3 and n unit cells of La0.67Sr0.33MnO3 with a fixed thickness of ~120 unit cells. The easy axis of magnetic anisotropy rotates ~45° towards the out-of-plane direction from n = 10 to n = 2 at reduced temperature TRe = T/TS = 0.87, where TS is the temperature at the onset of magnetization. Transmission electron microscopy reveals an enlarged tetragonal ratio >1 with breaking of volume conservation around the (La0.67Sr0.33MnO3)n/(SrTiO3)n interface and electronic charge transfer from Mn to Ti 3d orbitals across the interface. Orbital hybridization accompanying the charge transfer results in preferred occupancy of {3d}_{3z^2-r^2} orbitals at the interface and induces a stronger electronic hopping integral and interfacial magnetic anisotropy along the out-of-plane direction, which contributes to the rotation towards the out-of-plane direction of an effective magnetic easy axis for n = 2. We demonstrate that interfacial orbital hybridization with charge transfer in the superlattice of strongly correlated oxides may be a promising approach to tailor electronic and magnetic properties in device applications.

The Imry–Ma Phase in a Nanocrystalline Ferromagnet

A possibility of appearing a disordered Imry–Ma state induced by fluctuations of the anisotropy easy axis direction in a nanocrystalline sample in the case of a weak exchange interaction between crystallites has been studied. A phase diagram of the system has been built in variables “the characteristic crystallite size–the exchange integral of the intercrystallite interaction.” The characteristic value of statistic fluctuations of the crystallographic anisotropy has been estimated, and the dependences of the coercive field on the crystallite size have been found for systems of various space dimensions.

Exchange-induced spin reorientation in La1-xGdxMn2Si2

Magnetic properties of the La1-xGdxMn2Si2 (0 ≤ x ≤ 1) compounds with layered ThCr2Si2-type structure have been studied using quasi-single-crystalline and polycrystalline samples. Concentration dependences of critical temperatures of magnetic phase transitions, effective magnetic moments and paramagnetic Curie temperatures have been determined for different compositions. Using the obtained results, we plotted the concentration magnetic phase diagram. The type of the interlayer Mn-Mn magnetic ordering is found to depend on the intralayer Mn-Mn distance. With increasing the Gd concentration, the ordering of the Mn sublattice changes from ferromagnetic (F) to antiferromagnetic (AF) at x ≈ 0.2–0.27. We found that the concentration F-AF magnetic phase transition is accompanied by a change of magnetic anisotropy from the easy-axis to the easy-plane type at low temperatures when the Gd moments are magnetically ordered. The transition can hardly be expected since the Mn sublattice has a strong easy-axis type anisotropy in LaMn2Si2, while the Gd has zero orbital momentum and typically weakly contribute to the magnetic anisotropy. The spin reorientation in the La1-xGdxMn2Si2 has been attributed to a competition between the Gd-Mn and Mn-Mn exchange interactions.

Antiferromagnetic Resonance in GdB6

The electron spin resonance has been measured for the first time both in the paramagnetic phase of the metallic GdB6 antiferromagnet (TN = 15.5K) and in the antiferromagnetic state (T < TN). In the paramagnetic phase below T* ~ 70 K, the material is found to exhibit a pronounced increase in the resonance linewidth and a shift in the g-factor, which is proportional to the linewidth Δg(T) ~ ΔH(T). Such behavior is not characteristic of antiferromagnetic metals and seems to be due to the effects related to displacements of Gd3+ ions from the centrosymmetric positions in the boron cage. The transition to the antiferromagnetic phase is accompanied by an abrupt change in the position of resonance (from μ0H0 ≈ 1.9 T to μ0H0 ≈ 3.9 T at ν = 60 GHz), after which a smooth evolution of the spectrum occurs, resulting eventually in the formation of the spectrum consisting of four resonance lines. The magnetic field dependence of the frequency of the resonant modes ω0(H0) obtained in the range of 28–69 GHz is well interpreted within the model of ESR in an antiferromagnet with the easy anisotropy axis ω/γ = (H 0 2 +2HAHE)1/2, where HE is the exchange field and HA is the anisotropy field. This provides an estimate for the anisotropy field, HA ≈ 800 Oe. This value can result from the dipole−dipole interaction related to the mutual displacement of Gd3+ ions, which occurs at the antiferromagnetic transition.

Precessing solitons in the stripe domain structure

New solutions have been found for the Landau-Lifshitz equation for a ferromagnet with easy-axis anisotropy that describe magnetic solitons strongly associated with stripe domain structure. They serve as elementary carriers of macroscopic shifts of the structure and are, under certain conditions, nuclei of the magnetic reversal of a material. It is shown that the inhomogeneous elliptic precession of magnetization in a soliton nucleus leads to perturbations of the adjacent domain walls of a structure. Modulated instability of solitons near the boundaries of their existence is investigated.

Galvanomagnetic methods of Curie temperature determination in (Ga,Mn)As

We critically discuss various experimental methods to determine Curie temperature TC of (Ga,Mn)As thin layers or other conducting magnetic materials by means of electric charge transport measurements. They all base on the influence of sample magnetization on the magnetoresistivity tensor ρ̂ and are an alternative to the method based upon an analysis of the temperature derivative of the sample resistance (Novák et al., 2008). These methods can be applied even when standard SQUID magnetometers are difficult or impossible to use – for example for extremely small samples or in the case of experiments performed at very specific physical conditions, e.g. at high hydrostatic pressure inside the clamp cell. We show that the use of the so called Arrott plot prepared with the use of high magnetic field isotherms ρxx(H0),ρxy(H0) (H0 – external magnetic field) may lead to substantial (of the order of 10 K) divergence of the obtained TC values depending on the assumptions which are necessary to make in this case and depending on the direction of a magnetic anisotropy easy axis. We also propose a number of ways how to obtain, basing on low magnetic field isotherms ρxx(H0),ρxy(H0), clear and characteristic features which are closely related to the ferromagnetic–paramagnetic phase transition.