Symmetry theory of the flexomagnetoelectric effect in the magnetic domain walls

A local flexomagnetoelectric (A.P. Pyatakov, A.K. Zvezdin, 2009) effect in the magnetic domain walls (DWs) of the cubic hexoctahedral crystal has been investigated on the basis of a symmetry analysis. The strong connection between magnetic symmetry of the DW and the type of the distribution of the electric polarization was shown. Results were systemized in the scope of the DW chirality. It was shown, that new type of the local flexomagnetoelectric coupling corresponds to the presence of the coupled electric charge in the DW. It was found that all time-noninvariant chiral DWs have identical type of spatial distribution of the magnetization and polarization. There are coincidence between the symmetry predictions and results obtaining from the known term of the flexomagnetoelectric coupling for transverse polarization components.     

Nonlinear waves in a chain of plane-parallel domain walls in ferromagnets

Taking into account the nonlinear interaction between plain domain walls (DWs) in a chain of DWs, one-and two-parameter solitons are obtained. These solitons are solitary shear waves propagating along the DW chain.     

Directed motion of domain walls in biaxial ferromagnets under the influence of periodic external magnetic fields

Directed motion of domain walls (DWs) in a classical biaxial ferromagnet placed under the influence of periodic unbiased external magnetic fields is investigated. Using the symmetry approach developed in this article the necessary conditions for the directed DW motion are found. This motion turns out to be possible if the magnetic field is applied along the easiest axis. The symmetry approach prohibits the directed DW motion if the magnetic field is applied along any of the hard axes. With the help of the soliton perturbation theory and numerical simulations, the average DW velocity as a function of different system parameters such as damping constant, amplitude, and frequency of the external field, is computed.     

Low-frequency resonance of domain walls in the iron garnet Tb3Fe5O12 near the magnetic compensation point

The vibrational motion dynamics of domain walls (DWs) in the iron garnet Tb₃Fe₅O₁₂, a low-frequency magnetic field, and the temperature range 200–295 K (which includes the magnetic compensation point of this ferrimagnet, T _c ≈ 249 K) is studied by a magnetooptical method. The temperature dependence of the DW vibration amplitude in this garnet crystal near T _c has a resonance character. A theoretical model of the magnetic resonance of DWs is proposed to interpret the obtained experimental results; according to this model, the DW mass tends to infinity and the resonance frequency tends to zero when temperature approaches the magnetic compensation point.     

Low frequency magnetic noise in epitaxial antiferromagnetically coupled Fe/Cr multilayers

For antiferromagnetically coupled epitaxial [Fe/Cr(001)]₁₀ multilayers we detected a strong enhancement of the magnetism-related electrical noise in the vicinity of the orientation transition between the easy and hard axes. Our measurements are performed at different temperatures and we also identified the noise caused by depinning of domain walls (DWs). We are able to detect and follow in real time the motion of rather extended (of the order of 100 μm) DWs by comparing the magnetic noise in the presence and absence of a DC transport current, respectively. The presence of large and small (<1 μm) DWs is confirmed by magnetic force microscopy images obtained at room temperature.     

复合外场中磁光材料(BiTm)3(FeGa)5O12薄膜畴壁运动特性的研究

通过对BiTm)₃(FeGa)₅O₁₂膜施工加低频交变磁场,匀速率增加的直流磁场和同时对该膜施加这两种磁场(复合外场),用照相划线读数方法和通过电荷耦合器件(CCD)-计算机作数字化处理获得磁畴壁(DW)的相对百分数。结果表明:(1)复合外场下DW运动规律中存在交互作用项;(2)低频交变磁场幅值(140A/m)远低于等效阻力场(10³A/m)时,DW可以运动,但不同步,频率大于1Hz时明显滞后,(3)利用图像转换有利于提高实验结果的分辨率。     

Tunable resistivity of individual magnetic domain walls

Despite the relevance of current-induced magnetic domain wall (DW) motion for new spintronics applications, the exact details of the current-domain wall interaction are not yet understood. A property intimately related to this interaction is the intrinsic DW resistivity. Here, we investigate experimentally how the resistivity inside a DW depends on the wall width Δ, which is tuned using focused ion beam irradiation of Pt/Co/Pt strips. We observe the nucleation of individual DWs with Kerr microscopy, and measure resistance changes in real time. A 1/Δ(2) dependence of DW resistivity is found, compatible with Levy-Zhang theory. Also quantitative agreement with theory is found by taking full account of the current flowing through each individual layer inside the multilayer stack.     

Generation and gyroscopic quasi-relativistic dynamics of antiferromagnetic vortices in domain walls of yttrium orthoferrite

An unusual nonlinear relation between the velocity of an antiferromagnetic (AFM) vortex along a domain wall (DW) on the DW velocity is detected. This relation has a maximum whose position depends on the topological charge of the vortex. As the DW velocity increases from the value corresponding to the maximum to its limiting value, the AFM-vortex velocity decreases and tends to zero. The total AFM-vortex velocity increases nonlinearly with the DW velocity and levels off at 20 km/s, which is equal to the velocity of spin waves in the linear section of their dispersion law. The experimental data are approximated satisfactorily. The dynamics of AFM vortices in DWs of yttrium orthoferrite, just as the dynamics of the DWs, is quasi-relativistic and gyroscopic.     

二维正方晶格Compass-Ising模型的方向序和磁有序

采用Monte Carlo方法研究二维正方晶格Compass-Ising模型的低温性质.通过调节过渡参量 ,计算了阻挫对方向序和磁有序的影响. 结果表明,阻挫的增加,易于形成方向序,同时会抑制磁有序. 当阻挫足够强时,磁有序将被破坏. 方向序和磁有序间的转变是一个逐渐过渡的过程,并不存在明显相变.     

Motion of bound domain walls in a spin ladder

The elementary excitation spectrum of the spin- \frac12\frac{1}{2} antiferromagnetic (AFM) Heisenberg chain is described in terms of a pair of freely propagating spinons. In the case of the Ising-like Heisenberg Hamiltonian spinons can be interpreted as domain walls (DWs) separating degenerate ground states. In dimension d > 1, the issue of spinons as elementary excitations is still unsettled. In this paper, we study two spin- \frac12\frac{1}{2} AFM ladder models in which the individual chains are described by the Ising-like Heisenberg Hamiltonian. The rung exchange interactions are assumed to be pure Ising-type in one case and Ising-like Heisenberg in the other. Using the low-energy effective Hamiltonian approach in a perturbative formulation, we show that the spinons are coupled in bound pairs. In the first model, the bound pairs are delocalized due to a four-spin ring exchange term in the effective Hamiltonian. The appropriate dynamic structure factor is calculated and the associated lineshape is found to be almost symmetric in contrast to the 1d case. In the case of the second model, the bound pair of spinons lowers its kinetic energy by propagating between chains. The results obtained are consistent with recent theoretical studies and experimental observations on ladder-like materials.     

Translational motion of domain walls in a strong magnetic field circularly polarized in the basal plane of a uniaxial ferromagnet

A substantially nonlinear theory is developed for the translational motion of domain walls (DWs) in ferromagnets with large easy-axis anisotropy under the influence of a strong magnetic field circularly polarized in the basal plane of the ferromagnet. This theory is a generalization of the well-known theories of DW drift that are limited to an approximation quadratic in the field magnitude. The analytical results are confirmed by computer simulation performed on the basis of the Landau-Lifshitz equations.     

Domain walls in magnetic multilayers with a biquadratic exchange

The structure of domain walls in magnetic multilayers is investigated taking into account the uniaxial anisotropy and biquadratic exchange between the layers. Analytical solutions are derived for different types of domain wall structures. The majority of the solutions obtained have no analogs in conventional magnetic materials. The thickness and the energy density per unit area are calculated for the domain walls under investigation. The range of parameters that correspond to more energetically favorable structures of domain walls is established.     

Interfacial roughness and temperature effects on exchange bias properties in coupled ferromagnetic/antiferromagnetic bilayers

Monte Carlo simulations have been used to study the relationship between the exchange bias properties and the interface roughness in coupled ferromagnetic/antiferromagnetic (FM/AFM) films of classical Heisenberg spins. It is shown that the variation of the exchange bias field versus the AFM anisotropy strongly depends on the FM/AFM interface. Unlike the flat interface, a non-monotonic dependence is observed for the roughest FM/AFM interface. This is explained by canted magnetic configurations at the FM/AFM interface, which appear after the first reversal due to the magnetic frustration. The temperature dependence of the exchange field is also dependent on the roughness. While the exchange field is roughly constant for the flat interface, a decrease is observed for the roughest interface as the temperature increases. This has been interpreted as a significant decrease of the effective coupling between the FM and the AFM due to the disordering of the moments at the FM/AFM interface because of the combination of magnetic frustration and temperature activation.     

Phase diagram of the frustrated asymmetric ferromagnetic spin ladder

We present a systematic investigation on the ground state of an asymmetric two-leg spin ladder (where exchange couplings of the legs are unequal) with ferromagnetic (FM) nearest-neighbor interaction and diagonal anti-ferromagnetic frustration using the density matrix renormalization group method. When the ladder is strongly asymmetric with moderate frustration, a magnetic canted state is observed between an FM state and a singlet dimerized state. The phase boundaries are dependent on the asymmetric strength. On the other hand, when the asymmetric strength is intermediate, a so-called spin-stripe state (spins align parallel on the same legs, but antiparallel on rungs) is discovered, and the system experiences a first-order phase transition from the FM state to the spin-stripe state upon increasing frustration. Numerical evidence is presented for interpretation of the phase diagram in terms of frustration and the asymmetric strength.     

Generation of zonal magnetic fields by drift waves in a current carrying nonuniform magnetoplasma

It is shown that zonal magnetic fields (ZMFs) can be nonlinearly excited by incoherent drift waves (DWs) in a current carrying nonuniform magnetoplasma. The dynamics of incoherent DWs in the presence of ZMFs is governed by a wave-kinetic equation. The governing equation for ZMFs in the presence of nonlinear advection force of the DWs is obtained from the parallel component of the electron momentum equation and the Faraday law. Standard techniques are used to derive a nonlinear dispersion relation, which depicts instability via which ZMFs are excited in plasmas. ZMFs may inhibit the turbulent cross-field particle and energy transport in a nonuniform magnetoplasma.     

Origin of the 150-K anomaly in LaFeAsO: competing antiferromagnetic interactions, frustration, and a structural phase transition

From all-electron fixed-spin-moment calculations we show that ferromagnetic and checkerboard antiferromagnetic ordering in LaFeAsO are not stable and the stripe antiferromagnetic configuration with M(Fe)=0.48 microB is the only stable ground state. The main exchange interactions between Fe ions are large, antiferromagnetic, and frustrated. The magnetic stripe phase breaks the tetragonal symmetry, removes the frustration, and causes a structural distortion. These results successfully explain the magnetic and structural phase transitions in LaFeAsO recently observed by neutron scattering. The presence of competing strong antiferromagnetic exchange interactions suggests that magnetism and superconductivity in doped LaFeAsO may be strongly coupled, much like in the high-T(c) cuprates.     

Nonequilibrium states and anomalies of Young’s modulus in rare-earth cobaltites RBaCo4O7 (R = Dy-Er,Y, Lu) caused by short-range magnetic order

The elastic properties of rare-earth cobaltites RBaCo₄O₇ (R = Dy, Ho, Er, Y, Lu) have been investigated experimentally. It has been found that the temperature dependences of the Young’s modulus exhibit significant hysteresis and irreversible effects over a wide range (80–280 K) between the structural and magnetic phase transition temperatures. These effects indicate that the short-range magnetic order in the Co sub-system of the studied rare-earth cobaltites gradually develops below the structural phase transition, when the distortion of the structure relieves the frustration of exchange interactions both in the Kagome lattice and in the triangular lattice of the cobalt subsystem. At the magnetic phase transition temperature, there are weak and smoothed anomalies of the Young’s modulus, which correlate with the low dimensionality and frustration of the exchange interactions in the Co subsystem of the studied rare-earth cobaltites.     

Exchange coupling and helical spin order in the triangular lattice antiferromagnet CuCrO₂ using first principles

The magnetic and electronic properties of the geometrically frustrated triangular antiferromagnet CuCrO2 are investigated by first principles through density functional theory calculations within the generalized gradient approximations (GGA)+U scheme. The spin exchange interactions up to the third nearest neighbours in the ab plane as well as the coupling between adjacent layers are calculated to examine the magnetism and spin frustration. It is found that CuCrO2 has a natural two-dimensional characteristic of the magnetic interaction. Using Monte-Carlo simulation, we obtain the Neel temperature to be 29.9 K, which accords well with the experimental value of 24 K. Based on non-collinear magnetic structure calculations, we verify that the incommensurate spiral-spin structure with (110) spiral plane is believable for the magnetic ground state, which is consistent with the experimental observations. Due to intra-layer geometric spin frustration, parallel helical-spin chains arise along the a, b, or a + b directions, each with a screw-rotation angle of about 120°. Our calculations of the density of states show that the spin frustration plays an important role in the change of d-p hybridization, while the spin-orbit coupling has a very limited influence on the electronic structure.