Based on Brown's static equations of micromagnetics, we compute the uniaxial polarization of the scattered neutron beam of a bulk magnetic material. The theoretical expressions are compared to experimental data on a soft magnetic nanocrystalline alloy. The micromagnetic SANS theory provides a general framework for polarized real-space neutron methods, and it opens up a new avenue for magnetic neutron data analysis on magnetic microstructures.
Micromagnetic small-angle neutron scattering theory is well established for analyzing spin-misalignment scattering data of bulk ferromagnets. Here, this theory is extended to allow for a global uniaxial magnetic anisotropy (texture) of the material, in addition to the already included random zero-average local anisotropy. Macroscopic cross-sections and spin-misalignment response functions are computed analytically for several practically relevant mutual anisotropy and external magnetic field orientations in both parallel and perpendicular scattering geometries for field magnitudes both above a…
We use first-principles theory to predict that the application of uniaxial compressive strain leads to a transition from an antiferromagnetic insulator to a ferromagnetic half-metal phase in LaMnO$_3$. We identify the Q2 Jahn-Teller mode as the primary mechanism that drives the transition, indicating that this mode can be used to tune the lattice, charge, and spin coupling. Applying $\simeq$ 6 GPa of uniaxial pressure along the [010] direction activates the transition to a half-metallic $\textit{pseudo-cubic}$ state. The half-metallicity opens the possibility of producing colossal magnetoresis…
Describing the origin of uniaxial magnetic anisotropy (UMA) is generally problematic in systems other than single crystals. We demonstrate an in-plane UMA in amorphous CoFeB films on GaAs(001) which has the expected symmetry of the interface anisotropy in ferromagnetic films on GaAs(001), but strength which is independent of, rather than in inverse proportion to, the film thickness. We show that this volume UMA is consistent with a bond-orientational anisotropy, which propagates the interface-induced UMA through the thickness of the amorphous film. It is explained how, in general, this mechani…
We investigate the effect of an applied uniaxial strain on the ferromagnetic instability due to long- range Coulomb interaction between Dirac fermions in graphene. In case of undeformed graphene the ferromagnetic exchange instability occurs at sufficiently strong interaction within the Hartree- Fock approximation. In this work we show that using the same theoretical framework but with an additional applied uniaxial strain, the transition can occur for much weaker interaction, within the range in suspended graphene. We also study the consequence of strain on the formation of localized magnetic …
We develop a model for ferromagnetic resonance in systems with competing uniaxial and cubic anisotropies. This model applies to (i) magnetic materials with both uniaxial and cubic anisotropies, and (ii) magnetic nanoparticles with effective core and surface anisotropies; We numerically compute the resonance frequency as a function of the field and the resonance field as a function of the direction of the applied field for an arbitrary ratio of cubic-to-uniaxial anisotropy. We also provide some approximate analytical expressions in the case of weak cubic anisotropy. We propose a method that use…
We study the critical dynamics of three-dimensional ferromagnets with uniaxial anisotropy by taking into account exchange and dipole-dipole interaction. The dynamic spin correlation functions and the transport coefficients are calculated within a mode coupling theory. It is found that the crossover scenario is determined by the subtle interplay between three length scales: the correlation length, the dipolar and uniaxial wave vector. We compare our theoretical findings with hyperfine interaction experiments on Gd and find quantitative agreement. This analysis allows us to identify the universa…
The permeability is calculated for a thin ferromagnetic film with the stripe domain structure and out-of-plane uniaxial magnetic anisotropy. Analytical expressions for the frequency dependence of components of permeability tensor are derived with the use of the Smit-Beljers method, with the thickness of domain walls and the domain wall motion being neglected. The effect of the domain width and the angle between the anisotropy axis and the film plane on the frequency dependence of the permeability is analyzed. General equations relating the static permeability components and the ferromagnetic r…
In this work, we reported the observation of a novel planar topological Hall effect (PTHE) in single crystal of Fe3GeTe2, a paradigmatic two-dimensional ferromagnet with strong uniaxial anisotropy. The Hall effect and magnetoresistance varied periodically when the external magnetic field rotated in the ac (or bc) plane, while the PTHE emerged and maintained robust with field swept across the hard-magnetized ab plane. The PTHE covers the whole temperature region below Tc (~150 K) and a comparatively large value is observed at 100 K. Emergence of an internal gauge field was proposed to explain t…
Critical phenomena in uniaxial ferromagnetic thin films in the presence of random magnetic fields have been studied within the framework of effective field theory. When the type of the random field distribution is bimodal, the system exhibits tricritical behavior. Furthermore, the critical value of surface to bulk ratio of exchange interactions at which the transition temperature becomes independent of film thickness is insensitive to the presence of disorder whether the distribution is bimodal or trimodal. Regarding the universality properties, neither $p$, nor $h$ variations in the system ca…
It is demonstrated that the nearest neighbor Mn pair on the GaAs (001) surface has a lower energy for the [-110] direction comparing to the [110] case. According to the group theory and the Luttinger's method of invariants, this specific Mn distribution results in bulk uniaxial in-plane and out-of-plane anisotropies. The sign and magnitude of the corresponding anisotropy energies determined by a perturbation method and ab initio computations are consistent with experimental results.
A theoretical study is made of the magnetization versus applied field curves of ferromagnetic/nonmagnetic multilayers constructed according to a Fibonacci quasiperiodic sequence. The ferromagnetic films are assumed to have uniaxial anisotropy and are coupled by both bilinear and biquadratic effective exchange. The effects of quasiperiodicity in the magnetic phases are illustrated numerically for Fe/Cr systems.
We present anisotropic thermal expansion measurements on single crystalline ErNi2B2C. All three, superconducting, antiferromagnetic and weak ferromagnetic phase transitions are unambiguously distinguished in the data. Anisotropic uniaxial pressure dependencies of the transitions are estimated based on the Ehrenfest relation, leading to a conclusion, in particular, that weak ferromagnetic state will be suppressed by small, order of several kbar, hydrostatic pressure.
In the present study, we demonstrate the measurement of resistivity tensor ($ρ$) along the magnetic axes 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 fiel…
In search of novel, improved materials for magnetic data storage and spintronic devices, compounds that allow a tailoring of magnetic domain shapes and sizes are essential. Good candidates are materials with intrinsic anisotropies or competing interactions, as they are prone to host various domain phases that can be easily and precisely selected by external tuning parameters such as temperature and magnetic field. Here, we utilize vector magnetic fields to visualize directly the magnetic anisotropy in the uniaxial ferromagnet CeRu$_2$Ga$_2$B. We demonstrate a feasible control both globally and…
Probing and steering bulk and surface phonon polaritons in uniaxial materials using fast electrons: hexagonal boron nitride
Materials Physics Center, CSIC-UPV/EHU（材料物理中心，CSIC-UPV/EHU）；CIC NanoGUNE BRTA（纳米技术研究中心 NanoGUNE）；Department of Electricity and Electronics, EHU/UPV（电气与电子工程系，巴斯克国立大学）；IKERBASQUE, Basque Foundation for Science（伊卡巴斯基克，巴斯克科学基金会）；Donostia International Physics Center DIPC（圣塞巴斯蒂安国际物理中心）
We present a computational study of the magnetic anisotropy energy for a given concentration of the Mn ions in the GaAs host, in the framework of the density functional theory. We focus on the influence of a different kind of strains on the uniaxial magnetic anisotropy constants $K_1$ and $K_2$, which reflect the magnetic anisotropy energy out- and in- (001) plane, respectively. We have shown that the general trends for the applied biaxial strain on anisotropy constants are consistent with the experimental data. We have predicted the critical strains, for which the magnetization vector changes…
The perovskite rare-earth titanates are model Mott insulators with magnetic ground states that are very sensitive to structural distortions. These distortions couple strongly to the orbital degrees of freedom and, in principle, it should be possible to tune the superexchange and the magnetic transition with strain. We investigate the representative system (Y,La,Ca)TiO$_3$, which exhibits low crystallographic symmetry and no structural instabilities. From magnetic susceptibility measurements of the Curie temperature, we demonstrate direct, reversible and continuous control of ferromagnetism by …
We report our study of twin domains in $La_{2-x}Sr_x CuO_4$ under uniaxial pressure. Using bulk-sensitive x-ray microdiffraction in Laue geometry, we image the distribution of twin domains at room temperature. When compressive uniaxial pressure is applied along one of the in-plane crystallographic axes, the domain population changes dramatically. We observe that the twin domain with shorter lattice parameter along the direction of pressure is unstable under compression, and disappears completely with only moderate pressure. On the other hand, application of tensile pressure changes the domain …
Surface magnetic domains of a UCoGa single crystal during magnetization/demagnetization processes in increasing/decreasing magnetic fields were investigated by means of magnetic-force-microscopy (MFM) images at low temperatures. The observed domain structure is typical for a ferromagnet with strong uniaxial anisotropy. The evolution of magnetic domains during cooling of the crystal below TC has also been manifested by MFM images. Analysis of the available data reveals that the high uniaxial magnetocrystalline energy in combination with the relatively small ferromagnetic exchange interaction in…
Electric field control of magnetic anisotropy in ferromagnets has been intensively pursued in spintronics to achieve efficient memory and computing devices with low energy consumption. Compared with ferromagnets, antiferromagnets hold huge potential in high-density information storage for their ultrafast spin dynamics and vanishingly small stray field. However, the switching of magnetic anisotropy of antiferromagnets via electric field remains elusive. Here we use ferroelastic strain from piezoelectric materials to switch the uniaxial magnetic anisotropy and the Néel order reversibly in antife…
Conductive domain walls (CDWs) in insulating ferroelectrics have recently attracted considerable attention due to their unique topological, optical, and electronic properties, and offer potential applications such as in memory devices or re-writable circuitry. The electronic properties of domain walls (DWs) can be tuned by the application of strain, hence controlling the charge carrier density at DWs. In this work, we study the influence of uniaxial stress on the conductivity of DWs in the bulk single crystal lithium niobate (LiNbO$_3$). Using conductive atomic force microscopy (cAFM), we obse…
We propose that stable two-dimensional (2D) ferromagnets can be made of regular single-layered lattices of single-molecule nanomagnets with enough uniaxial magnetic anisotropy on appropriate substrates by controlling the inter-nanomagnet magnetic interaction. Our Monte Carlo simulated results show that such ideal 2D ferromagnets are thermodynamically stable when the anisotropy is strong enough. If the anisotropy energy equals 80 K, approximately that of the Mn12, the T_c varies from zero to 15 K depending on different inter-nanomagnet coupling constants. Such stable spin systems, experimentall…
We performed first - principles relativistic full-potential linearized augmented plane wave calculations for strained tetragonal ferromagnetic La(Ba)MnO$_3$ with an assumed experimental structure of thin strained tetragonal La$_{0.67}$Ca$_{0.33}$MnO$_3$ (LCMO) films grown on SrTiO$_3$[001] and LaAlO$_3$[001] substrates. The calculated uniaxial magnetic anisotropy energy (MAE) values, are in good quantitative agreement with experiment for LCMO films on SrTiO$_3$ substrate. We also analyze the applicability of linear magnetoelastic theory for describing the stain dependence of MAE, and estimate …
We demonstrate sensitivity of the Mn 3d valence states to strain in the ferromagnetic semiconductors (Ga,Mn)As and (Al,Ga,Mn)As, using x-ray magnetic circular dichroism (XMCD). The spectral shape of the Mn $L_{2,3}$ XMCD is dependent on the orientation of the magnetization, and features with cubic and uniaxial dependence are distinguished. Reversing the strain reverses the sign of the uniaxial anisotropy of the Mn $L_3$ pre-peak which is ascribed to transitions from the Mn 2p core level to p-d hybridized valence band hole states. With increasing carrier localization, the $L_3$ pre-peak intensi…
Insulating uniaxial room-temperature ferromagnets are a prerequisite for commonplace spin wave-based devices, the obstacle in contemporary ferromagnets being the coupling of ferromagnetism with large conductivity. We show that the uniaxial $A^{1+2x}$Ti$^{4+}$$_{1-x}$O$_3$ (ATO), $A=$Ni$^{2+}$,Co$^{2+}$ and $0.6<x \leq 1$, thin films are electrically insulating ferromagnets already at room-temperature. The octahedra network of the ATO and ilmenite structures are similar yet different octahedra-filling proved to be a route to switch from the antiferromagnetic to ferromagnetic regime. Octahedra c…
We theoretically investigate the interplay between local lattice distortions around $\rm{Mn^{2+}}$ ion impurity and the ion's magnetic polarization, mediated through spin-orbit coupling of hole. We show that the tetrahedral symmetry around $\rm{Mn^{2+}}$ ion impurity is spontaneously broken even in the paramagnetic regime. Modest local lattice distortions around the impurity $\rm{Mn^{2+}}$ ion, along with the growth strain, stabilize magnetization along $< 110 >$ directions, in the ferromagnetic regime. We explain the experimentally observed in-plane uniaxial magnetic anisotropy seen in this s…
Cr$_2$Ge$_2$Te$_6$ is a quasi-2D semiconducting van der Waals ferromagnet down to the bilayer with great potential for technological applications. Engineering the critical temperature to achieve room temperature applications is one of the critical next steps on this path. Here, we report high-resolution capacitance dilatometry studies on Cr$_2$Ge$_2$Te$_6$ single crystals which directly prove significant magnetoelastic coupling and provide quantitative values of the large uniaxial pressure effects on long-range magnetic order (${\partial}T_{\mathrm{C}}/{\partial}p_{\mathrm{c}}$ = 24.7 K/GPa an…
Decoupling the effects of geometry and nature of strain in LaMnO$_3$: Interplay of dynamic correlations and uniaxial strain driving magnetic phase transitions
We report a numerical implementation of the Landau-Lifshitz-Baryakhtar theory, which dictates that the micromagnetic relaxation term obeys the symmetry of the magnetic crystal, i. e. replacing the single intrinsic damping constant with a tensor of corresponding symmetry. The effect of anisotropic relaxation is studied in thin saturated ferromagnetic disk and ellipse with and without uniaxial magneto-crystalline anisotropy. We investigate the angular dependency of the linewidth of magnonic resonances with respect to the given structure of the relaxation tensor. The simulations suggest that the …
Ferromagnetic resonance (FMR) is used to study magnetic anisotropy of GaMnAs in a series of Ga$_{1 - x}$Mn$_{x}$As/Ga$_{1 - y}$Al$_{y}$As heterostructures modulation-doped by Be. The FMR experiments provide a direct measure of cubic and uniaxial magnetic anisotropy fields, and their dependence on the doping level. It is found that the increase in doping -- in addition to rising the Curie temperature of the Ga$_{1 - x}$Mn$_{x}$As layers -- also leads to a very significant increase of their uniaxial anisotropy field. The FMR measurements further show that the effective $g$-factor of Ga$_{1 - x}$…
The magnetic phase diagram at zero external field of an ensemble of dipoles with uniaxial anisotropy on a FCC lattice is investigated from tempered Monte Carlo simulations. The uniaxial anisotropy is characterized by a random distribution of easy axes and its magnitude $λ_u$ is the driving force of disorder and consequently frustration. The phase diagram, separating the paramagnetic, ferromagnetic, quasi long range ordered ferromagnetic and spin-glass regions is thus considered in the temperature, $λ_u$ plane. This system is aimed at modeling the magnetic phase diagram of supracrystals of magn…
We revisit the description of the magnetic field around antiferromagnetic magnetoelectrics in the context of recent developments regarding magnetoelectric monopoles. Using Maxwell's equations, we calculate the magnetic and electric fields associated with a free charge in a bulk uniaxial magnetoelectric, as well as in a finite sphere of magnetoelectric material. We show that a charge in the prototypical magnetoelectric Cr$_2$O$_3$, which is uniaxial with a diagonal magnetoelectric response, induces an internal magnetic field with both monopolar and quadrupolar components, but that only the quad…
Nematic liquid crystals possess three different phases: isotropic, uniaxial, and biaxial. The ground state of most nematics is either isotropic or uniaxial, depending on the external temperature. Nevertheless, biaxial domains have been frequently identified, especially close to defects or external surfaces. In this paper we show that any spatially-varying director pattern may be a source of biaxiality. We prove that biaxiality arises naturally whenever the symmetric tensor $\Sb=(\grad \nn)(\grad \nn)^T$ possesses two distinct nonzero eigenvalues. The eigenvalue difference may be used as a meas…
For the first time in a bulk proper uniaxial ferroelectrics, double antiferroelectric-like hysteresis loops have been observed in the case of Sn$_2$P$_2$S$_6$ crystal. The quantum anharmonic oscillator model was proposed for description of such polarization switching process. This phenomenon is related to three-well local potential of spontaneous polarization fluctuations at peculiar negative ratio of coupling constants which correspond to inter-site interaction in given sublattice and interaction between two sublattices of Sn$_2$P$_2$S$_6$ modeled crystal structure. Obtained data can be used …
The low temperature behavior of densely packed interacting spherical single domain nanoparticles (MNP) is investigated by Monte Carlo simulations in the framework of an effective one spin model. The particles are distributed through a hard sphere like distribution with periodic boundary conditions and interact through the dipole dipole interaction (DDI) with an anisotropy energy including both cubic and uniaxial symmetry components. The cubic component is shown to play a sizable role on the value of the blocking temperature $T_b$ only when the MNP easy axes are parallel to the cubic easy direc…
In an antiferromagnet (AF) with uniaxial anisotropy, spin-up and spin-down magnons coexist and form an intrinsic degree of freedom resembling electrons. When polarized by an adjacent ferromagnet (F), a magnonic pure spin current can be thermally generated in an AF. We explore thermal magnon transport in an insulating F/AF/F trilayer where propagating magnons inside the AF spacer can transfer angular momenta between the two Fs. We find that a sufficiently large temperature gradient can switch the downstream F via magnonic spin-transfer torque if it is initially antiparallel with the upstream F.…
In this work we studied a two dimensional ferromagnetic system using Monte Carlo simulations. Our model includes exchange and dipolar interactions, a cubic anisotropy term, and uniaxial out-of-plane and in-plane ones. According to the set of parameters chosen, the model including uniaxial out-of-plane anisotropy has a ground-state which consists of a canted state with stripes of opposite out-of-plane magnetization. When the cubic anisotropy is introduced zigzag patterns appear in the stripes at fields close to the remanence. An analysis of the anisotropy terms of the model shows that this conf…
Structural and magnetic properties have been studied for polycrystalline Zn_1-xMn_xO (x=0.02, 0.03, 0.05). Low-temperature (~500 oC) synthesis leaves unreacted starting ZnO and manganese oxides. Contrary to a recent report, no bulk ferromagnetism was observed for single-phase materials synthesized in air at temperatures above 900 oC. Single-phase samples show paramagnetic Curie-Weiss behavior.
In this letter we report observation of room temperature ferromagnetism in bulk Ga$_2$O$_3$ . We also observe large (10-60%) increase/decrease in photoluminescence. In the red(700 nm wavelength)/blue(500 nm), with the application of small magnetic field(0.4 Tesla). We argue, that ferromagnetism occurs entirely due to chains of oxygen(O(3) sites, Fig. 5) vacancies. We propose a simple model to explain, origin and location of moment, formation of ferromagnetic dislocation needles, and strong increase/decrease ofred/blue photoluminescence intensity with magnetic field.
We demonstrate that the layered room temperature ferromagnet Fe7Se8 and the topological insulator Bi2Se3 form crystallographically oriented bulk composite intergrowth crystals. The morphology of the intergrowth in real space and reciprocal space is described. Critically, the basal planes of Bi2Se3 and Fe7Se8 are parallel and hence the good cleavage inherent in the bulk phases is retained. The intergrowth is on the micron scale. Both phases in the intergrowth crystals display their intrinsic bulk properties: the ferromagnetism of the Fe7Se8 is anisotropic, with magnetization easy axis in the pl…
Thin ferromagnetic films with in-plane magnetic anisotropy are promising materials for obtaining high microwave permeability. The paper reports on the Mössbauer study of the field induced in-plane uniaxial anisotropy in electro-deposited $FeCo$ alloy films. The $FeCo$ alloy films have been prepared by electro-deposition method with and without external magnetic field applied parallel to the film plane during deposition. The vibrating sample magnetometry and Mössbauer spectroscopy measurements at room temperature indicate that the film deposited in external field shows an in-plane uniaxial anis…
A phenomenological theory of magnetic states in noncentrosymmetric tetragonal antiferromagnets is developed, which has to include homogeneous and inhomogeneous terms (Lifshitz-invariants) derived from Dzyaloshinskii-Moriya couplings. Magnetic properties of this class of antiferromagnets with low crystal symmetry are discussed in relation to its first known members, the recently detected compounds Ba2CuGe2O7 and K2V3O8. Crystallographic symmetry and magnetic ordering in these systems allow the simultaneous occurrence of chiral inhomogeneous magnetic structures and weak ferromagnetism. New types…
The reflection coefficient of bulk spin waves from multilayer ferromagnetic structure with periodically modulated parameters of exchange interaction, uniaxial and rhombic magnetic anisotropy and saturation magnetization is calculated with a non-ideal coupling between layers. The strong dependence of spin-wave reflection coefficient on frequency, magnetic field and parameter of interfacial coupling is revealed. It allows changing the reflection intensity from 0 to 1 by changing either only external magnetic field value or frequency. The proposed model of boundary conditions gives the opportunit…
The identification of general principles for stabilizing magnetic skyrmion phases in bulk materials over wide ranges of temperatures is a prerequisite to the development of skyrmion-based spintronic devices. Lacunar spinels with the formula GaM4X8 with M=V, Mo; X=S, Se are a convenient case study towards this goal as they are some of the first bulk systems suggested to host equilibrium chiral skyrmions far from the paramagnetic transition. We derive the magnetic phase diagrams likely to be observed in these materials, accounting for all possible magnetic interactions, and prove that skyrmion s…
The effects of uniaxial strain on the structural, orbital, optical, and magnetic properties of LaMnO_3 are calculated using a general elastic energy expression, along with a tight-binding parameterization of the band theory. Tensile uniaxial strain of the order of 2 % (i.e., of the order of magnitude of those induced in thin films by lattice mismatch with substrates) is found to lead to changes in the magnetic ground state, leading to dramatic changes in the band structure and optical conductivity spectrum. The magnetostriction effect associated with the Neel transition of bulk(unstrained) LaM…
Magnetism in lanthanum cobaltite (LCO, LaCoO$_3$) appears to be strongly dependent on strain, defects, and nanostructuring. LCO on strontium titanate (STO, SrTiO$_3$) is a ferromagnet with an interesting strain relaxation mechanism that yields a lattice modulation. However, the driving force of the ferromagnetism is still controversial. Experiments debate between a vacancy-driven or strain-driven mechanism for the ferromagnetism of epitaxial LCO. We found that a weak lateral modulation of the superstructure is sufficient to promote ferromagnetism. We find that ferromagnetism appears under unia…
The attainability of modification of the apparent magnetic anisotropy in (III,Mn)V ferromagnetic semiconductors is probed by means of the finite-elements-based modelling. The most representative case of (Ga,Mn)As and its in-plane uniaxial anisotropy is investigated. The hysteresis loops of the continuous films of a ferromagnetic semiconductor as well as films structured with the elliptic antidots are modelled for various eccentricity, orientation, and separation of the anti dots. The effect of anti-dots on the magnetic anisotropy is confirmed but overall is found to be very weak. The subsequen…
