Antiferromagnetic Vector Research Articles

Stabilization of weak ferromagnetism by strong magnetic response to epitaxial strain in multiferroic BiFeO3.

Multiferroic BiFeO3 exhibits excellent magnetoelectric coupling critical for magnetic information processing with minimal power consumption. However, the degenerate nature of the easy spin axis in the (111) plane presents roadblocks for real world applications. Here, we explore the stabilization and switchability of the weak ferromagnetic moments under applied epitaxial strain using a combination of first-principles calculations and group-theoretic analyses. We demonstrate that the antiferromagnetic moment vector can be stabilized along unique crystallographic directions ([110] and [–110]) under compressive and tensile strains. A direct coupling between the anisotropic antiferrodistortive rotations and the Dzyaloshinskii-Moria interactions drives the stabilization of the weak ferromagnetism. Furthermore, energetically competing C- and G-type magnetic orderings are observed at high compressive strains, suggesting that it may be possible to switch the weak ferromagnetism “on” and “off” under the application of strain. These findings emphasize the importance of strain and antiferrodistortive rotations as routes to enhancing induced weak ferromagnetism in multiferroic oxides.

Electric-field-induced structural and magnetic transformations in BiFeO3 multiferroics

The influence of an electric field on the structural changes and related transformation of the magnetic subsystem in a BiFeO3 multiferroic material has been investigated in the framework of the phenomenological model based on the Ginzburg-Landau theory. The hysteresis dependences of the electric polarization, the counter-rotation of oxygen octahedra, the magnetization, and the antiferromagnetic vector have been obtained as a function of the electric field applied along the crystallographic directions [110] and [001] of the pseudocubic structure. The model parameters are consistent with the results of the ab initio calculations of the structural and magnetic transitions in this material. It has been shown that, in the region of the existence of a spatially modulated spin state, an abrupt change in the polarization vector leads to a stepwise transformation of the antiferromagnetic structure. Within the proposed approach, the electric-field-controlled change in the spin structure of the multiferroic material is adequately described in the range of weak and strong magnetic fields.

ChemInform Abstract: The Phenomenological Theory of Magnetization Relaxation (Review Article)

AbstractReview: 38 refs.

Magnetoresistive memory with recording by electric field: Is the weak ferromagnetism necessary?

Possible approaches to creating a magnetoresistive memory with recording by electric field (MERAM) are considered. The memory based on a particular geometry of multiferroic BiFeO3 is shown to be the most promising. The relatively small values of the weak ferromagnetic moment and linear magnetoelectric coupling constant in BiFeO3 are not significant obstructions for creation of such a MERAM. The key factors are the coupling between electrical polarization and the antiferromagnetism vector in the multiferroic (not between polarization and magnetization vectors), as well as the exchange coupling between the antiferromagnetism vector and the ferromagnet layer magnetization caused by the spin-flop orientation of the latter two vectors at the interface. This is inherently an antiferromagnetic-ferroelectric device mechanism.

Isostructural magnetic phase transitions and the magnetocaloric effect in Ising ferromagnets

It is shown that the first-order isostructural magnetic phase transition between antiferromagnetic phases with different magnitudes of the antiferromagnetism vector induced by an external magnetic field in Ising antiferromagnets can be related to entropy. It is found that, depending on the temperature, the entropy jump and the corresponding heat release can change their signs at the phase transition point. In the low-temperature region of the metamagnetic first-order phase transition the entropy jump is positive, while the entropy jump is negative near the triple point for isostructural magnetic phase transitions.

Magnetic Structure of Ground and Field Induced Ordered States of Low-Dimensional γ-CoV2O6

The structural and magnetic properties of γ-CoV2O6 low-dimensional magnetic oxide have been investigated with an emphasis on its magnetic structure. Our main results have been obtained by powder neutron diffraction measurements carried out under magnetic fields of 0, 0.52, and 1 T, corresponding to the antiferromagnetic, ferrimagnetic, and ferromagnetic configurations of the magnetic moments. The magnetic moments are not collinear, but lie mostly along the b direction (i.e., inside structural edge-sharing CoO6 chains), which corresponds to the experimental easy magnetization axis. In the ground state, the moments are ferromagnetically ordered along the b and c directions, and antiferromagnetically ordered along a. The ferromagnetic sheets (bc plane) containing the chains are separated by nonmagnetic sheets containing VO6 octahedra and VO4 tetrahedra. The ground state magnetic structure can be described in a (4a, 2b, c) supercell using two antiferromagnetic propagation vectors kAF1 = (1/2, 0, 0) and kAF2 = (1/4, 1/2, 0). The ferri- and ferro-magnetic structures are characterized by kFi = (1/3, 0, −1/3) and k = (0, 0, 0) propagation vectors, respectively. These results are in agreement with macroscopic magnetic measurements performed on single crystals.

Spin dynamics of antiferromagnets under action of femtosecond laser pulses (Review Article)

Investigations of excitation of spin dynamics in transparent antiferromagnets (AFM) under the action of femtosecond laser pulses are reviewed. A variety of observed effects is considered in the context of a unified approach. The analysis is based on the nonlinear sigma model for the antiferromagnetism vector L with taking into account effective fields induced in a magnetic material due to the interaction between light and the spin system of the magnetic material. The contributions of various magneto-optical effects (both standard Faraday or Cotton-Mouton (Voigt) effects and specific L-dependent effects for AFM) are considered as characteristic contributions to the effective field within the sigma model. The most typical experimental data for real AFM are discussed.

Spin waves in easy-axis antiferromagnets with precessing domain walls

Equations for the antiferromagnetism vector are used to study the spectrum and scattering of spin waves on a domain wall with precessing spins in an easy-axis antiferromagnet with a constant magnetic field directed along the easy axis. It is shown that this kind of magnetic field can be completely eliminated from the equations of motion, so that they can be reduced to a Lorentz invariant form. The spectral problem for weak excitation of a precessing domain wall is solved and exact solutions are found for the linearized equations describing the propagation of spin waves in antiferromagnets with this kind of domain wall. An explicit expression is found for the reflection coefficient of spin waves from a domain wall as a function of the wave vectors of the incident and transmitted waves, along with its dependence on the spin wave frequency. The range of frequencies within which the spin waves are fully reflected is found and it is shown that the reflection coefficient falls off sharply above the upper limit of this range. These results can be generalized to the case of a moving domain wall in a three-dimensional crystal.

Феноменологічна теорія релаксації у двопідґратковому фериті

The dissipative function of a two-sublattice ferrite was constructed. The relaxation times for the acoustic and optical branches of spin waves are calculated, as well as the relaxation times for the magnetization and antiferromagnetism vectors. The process of antiferromagnetism vector relaxation is shown to be the quickest one. The corresponding relaxation time is governed by the exchange relaxation constant and, due to the exchange interactions between atoms in the sublattices, becomes shorter owing to the dynamics of the antiferromagnetism vector. The process of ferrite magnetization relaxation is the slowest one. In the exchange approximation, the magnetization relaxation time tends to infinity, as the length of magnetization non-uniformities grows. The results obtained are compared with the experimental data on the relaxation phenomenon in GdFeCo alloy of rare-earth and transition metals.

The phenomenological theory of magnetization relaxation (Review Article)

The review is devoted to systematic description of results on relaxation of magnetization in magnetically ordered crystals previously obtained by the authors. The ideas of the phenomenological theory of magnetism formulated by Landau and Lifshitz are analyzed. A general method of constructing the dissipation function for both magnetically ordered systems and paramagnets is described. In the case of magnetically ordered systems the dissipation of exchange and relativistic nature is considered. It is found that for constructing the dissipation function it is necessary to take into account not only the symmetry of a crystal, but also the laws of conservation of magnetization. It is shown that in the case of a ferromagnet, the ground state is characterized by a continuous degeneracy parameter; the Landau-Lifshitz relaxation term gives qualitatively incorrect results (abnormally large attenuation of spin waves). According to the proposed method the spectra of spin waves and their attenuation were calculated and analyzed for ferromagnets with uniaxial, tetragonal and cubic symmetry as well as for two-sublattice uniaxial ferrites. It was found that the relaxation of the magnetization vector has a two-step character in ferromagnets and a multistep character in ferrites. In ferrites, the fastest process is the relaxation of length of the antiferromagnetic vector. It is shown that this relaxation is caused by the exchange interaction between sublattices of a ferrite and is enhanced by the exchange interactions within the sublattices. The relaxation of the total magnetization of a ferrite is much slower and, as in the case of a simple ferromagnet, is described by non-uniform exchange interactions and relativistic interactions. The results obtained are in a good agreement with recent experimental data.

Magnetic and dielectric properties of the PbFeBO4 single crystal

PbFeBO4 single crystals up to 4×2.5×0.5mm3 in size were grown by spontaneous crystallization from a solution in a melt and their magnetic and dielectric properties were investigated. The high Neel temperature TN=114K and the absence of the broad maximum of susceptibility above the Neel temperature suggest that the PbFeBO4 crystal is a three-dimensional antiferromagnet. The magnetic susceptibility anisotropy in the ordered state indicates that the antiferromagnetic vector is directed along the orthorhombic c axis. The anomalies of dielectric permittivities ε' and ε″ were observed in both polycrystalline and single-crystal samples within the temperature ranges where the short- and long-range magnetic orders are established. This confirms the correlation between the magnetic and electric subsystems in the crystal.

Spin polarization of Ru in superconducting Ba(Fe0.795Ru0.205)2As2studied by x-ray resonant magnetic scattering

We have employed the x-ray resonant magnetic scattering (XRMS) technique at the Ru ${L}_{2}$ edge of the Ba(Fe${}_{1\ensuremath{-}x}$Ru${}_{x}$)${}_{2}$As${}_{2}$ ($x=0.205$) superconductor. We show that pronounced resonance enhancements at the Ru ${L}_{2}$ edge are observed at the wave vector which is consistent with the antiferromagnetic propagation vector of the Fe in the undoped BaFe${}_{2}$As${}_{2}$. We also demonstrate that the XRMS signals at the Ru ${L}_{2}$ edge follow the magnetic ordering of the Fe with a long correlation length, ${\ensuremath{\xi}}_{ab}>2850\ifmmode\pm\else\textpm\fi{}400$ \AA{}. Our experimental observation shows that the Ru is spin polarized in Ba(Fe${}_{1\ensuremath{-}x}$Ru${}_{x}$)${}_{2}$As${}_{2}$ compounds.

Domain non-reciprocity of electromagnetic waves in a centrosymmetric TbMnO3

The polariton spectrum in a centrosymmetric TbMnO3 is considered at low temperatures in a commensurate antiferromagnetic state after the spin-flop transition in a magnetic field. It is shown that a linear dependence of the electric polarization P on a magnetic field H, observed in this state, is caused by ordering spins of the terbium ions. The magnetoelectric energy PGH (G is the antiferromagnetic vector of terbium) induces in a centrosymmetric TbMnO3 without a linear magnetoelectric effect the non-reciprocal spectrum of TM and TE polaritons. In antiferromagnetic 180° terbium domains, the speed of electromagnetic waves turns out to be different, and directions of the electric field precession and the polarization plane rotation have opposite signs. The order of these effects were found to be 10−3.

Electronic band structure and momentum dependence of the superconducting gap in Ca1−xNaxFe2As2from angle-resolved photoemission spectroscopy

Electronic structure of newly synthesized single crystals of calcium iron arsenide doped with sodium with Tc ranging from 33 to 14 K has been determined by angle-resolved photoemission spectroscopy (ARPES). The measured band dispersion is in general agreement with theoretical calculations, nonetheless implies absence of Fermi surface nesting at antiferromagnetic vector. A clearly developing below Tc strongly band-dependant superconducting gap has been revealed for samples with various doping levels. BCS ratio for optimal doping, $2\Delta/k_{\rm B}T_{\rm c}=5.5$, is substantially smaller than the numbers reported for related compounds, implying a non-trivial relation between electronic dispersion and superconducting gap in iron arsenides.

Peculiarities of stochastic motion in antiferromagnetic nanoparticles

Antiferromagnetic (AFM) materials are widely used in spintronic devices as passive elements (for stabilization of ferromagnetic layers) and as active elements (for information coding). In both cases the switching between different AFM states, to a great extent depends on the environmental noise. In the present paper we derive stochastic Langevian equations for an AFM vector and a corresponding Fokker-Plank equation for a distribution function in the phase space of generalised coordinate and momentum. Thermal noise is modelled by a random delta-correlated magnetic field that interacts with the dynamic magnetisation of AFM particle. We scrupulously analyse a particular case of a collinear compensated AFM in the presence of spin-polarised current. The energy distribution function is found for normal modes in the vicinity of two equilibrium states (static and stationary) in sub- and super-critical regimes. It is shown that the noise-induced dynamics of AFM vector has some pecuilarities compared to the dynamics of magnetisation vector in ferromagnets.

A simple law governing coupled magnetic orders in perovskites

An energetic expression containing four different macroscopic terms is proposed to explain and understand coupled magnetic orders (and the directions of the simultaneously occurring ferromagnetic and/or antiferromagnetic vectors) in terms of anti-phase and/or in-phase tilting of oxygen octahedra in magnetic and multiferroic perovskites. This expression is derived from a suggested simple microscopic formula, and has its roots in the Dzyaloshinsky–Moriya interaction. Comparison with data available in the literature and with first-principles calculations we conduct here confirms the validity of such a simple and general law for any tested structural paraelectric and even ferroelectric phase, and for any chosen direction of any selected primary magnetic vector.

The Influence of Magnetoelectric Interactions on the Domain Walls in Multiferroics

The influence of magnetoelectric interactions on the magnetic structure, flexomagnetoelectric polarization and magnetization in thin multiferroics film has been investigated. The correlation between antiferromagnetic domain structure and ferroelectric domain pattern has been revealed. It has been shown the asymmetry of the antiferromagnetic vector distribution over multiferroics film in the case of 1090 and 710 ferroelectric domain walls. The direction of spins rotation in magnetic domain walls is determined by the type of ferroelectric domains and the antiferromagnetic vector in the centre of ferroelectric domain. The peculiarities of the micromagnetic distribution are reflected in the behavior of polarization and magnetization, which appears to be different for 1800, 1090 and 710 ferroelectric domains.

Domain walls, magnetization, and magnetoelectric effect in bismuth ferrite films

The specific features of the antiferromagnetic domain structure, magnetization, and polarization induced by an inhomogeneous micromagnetic distribution in films of bismuth ferrite multiferroics have been investigated. It has been shown that the magnetic domain structure correlates with the ferroelectric domain structure, and the character of the rotation of the antiferromagnetic vector depends on the type of ferroelectric domain walls. An asymmetry in the distribution of the antiferromagnetic vector has been observed for the cases of 109° and 71° ferroelectric domain walls. It has been demonstrated that there are differences in the distributions of the polarization and magnetization in bismuth ferrite films with ferroelectric domains separated by 109° and 71° walls. The basic mechanisms responsible for the magnetization in domain walls in multiferroics have been considered.

Theoretical Analysis of Inertia-like Switching in Magnets: Applications to a Synthetic Antiferromagnet

The magnetization dynamics of a synthetic antiferromagnet subjected to a short-magnetic-field pulse has been studied by using a combination of first principles calculations and atomistic spin-dynamics simulations. We observe switching phenomena on the time scale of tens of picoseconds, and inertia-like behavior in the magnetization dynamics. We explain the latter in terms of a dynamic redistribution of magnetic energy from the applied-field pulse to other possible energy terms, such as the exchange interaction and the magnetic anisotropy, without invoking concepts such as the inertia of an antiferro-magnetic vector. We also demonstrate that such dynamics can also be observed in a ferromagnetic material where the incident-field pulse pumps energy to the magnetic anisotropy.

The reversal of magnetization driven by the Dzyaloshinskii–Moriya interaction in Bi2FeMnO6

Ab initio calculations show that the coupling between antiferrodistortive (AFD) distortions and magnetization in perovskite Bi2FeMnO6 is prohibited to make magnetization rotate as effective Hubbard parameter is larger than 2.7eV, where anomalies in antiferromagnetic (AFM) vectors and band gap varying with on-site Coulomb interaction can be observed. This coupling is attributed to the antisymmetric Dzyaloshinskii–Moriya interaction driven by the eg–eg states AFM interaction and charge redistribution with respect to different AFD distortions.