Symmetry theory of the flexomagnetoelectric effect in the Bloch lines

It was shown that there are 48 magnetic point groups of the Bloch lines including 22 (11 time-invariant and 11 time-noninvariant) enantiomorphic and 26 non-enantiomorphic groups. The Bloch lines with the time-noninvariant enantiomorphism have identical types (parities) of the magnetization and polarization dependences. The list of soliton-like Bloch lines is derived from the symmetry classification. The tip electrode method of the creation of these Bloch lines is suggested for the potential applications in the magnetoelectric memory devices. The method of the experimental determination of the flexomagnetoelectric properties of the Bloch lines carried by the Bloch domain wall has been suggested. New type of the flexomagnetoelectric coupling, which is determined by the spatial derivatives of the electric polarization, can be found in the vicinity of the Curie temperature or compensation point of the ferrimagnets. The multi-state Bloch line magnetoelectric/multiferroic memory is proposed. It can be considered as a concept of the magnetoelectric enhancement of existing Bloch line memory invention.     

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.     

Discrete double core skyrmions in magnetic thin films

We study the Belavin-Polyakov double core skyrmions in two-dimensional isotropic Heisenberg ferromagnets on a discrete square lattice by means of spin dynamic simulations using periodic boundaries conditions. It is also investigated the presence of external in-plane magnetic fields and lattice defects. We have shown that these discrete skyrmions not cylindrically symmetric have a high degree of stability, similar to their continuum counterparts, even if small spatial inhomogeneities are present into the system.     

Enhanced pinning of superconducting vortices at circular magnetic dots in the magnetic-vortex state

Low-temperature magnetic force microscopy (LT-MFM) was used to study the distribution of superconducting vortices in Nb above a square array of 1 μm-sized circular ferromagnetic dots in a magnetic-vortex state. The force that the MFM tip exerts on the individual vortex in the depinning process was used to estimate the spatial modulation of the pinning potential. It was found, that the superconducting vortices which are preferably located on top of the Py dots experience a pinning force, about 15 times stronger as compared to the pinning force in the pure Nb film. This strong pinning exceeds the repulsive interaction between the superconducting vortices and allows vortex clusters to be located above the dots.     

On the free energy of the flexomagnetoelectric interactions

It was shown that free energy density of the local flexomagnetoelectric effect is determined by the four phenomenological constants in case of the cubic (hexoctahedral) crystal. The well-known single-constant Lifshitz invariant term is correct only when fixed electric polarization induces the inhomogeneity of the magnetization. Proposed phenomenological theory was applied to the magnetic domain walls. The domain wall structure has been investigated in details. The four-constant phenomenological theory conforms to the symmetry based predictions (Bar'yakhtar et al., 1984, [12]). The proposed experimental verification of the four-constant flexomagnetoelectric phenomenology is a detection of the shift of the Néel domain walls under the strong homogeneous electric field.     

Flexomagnetoelectric interaction in cubic,tetragonal and orthorhombic crystals

The phenomenological theory of the flexomagnetoelectric coupling in crystals of the cubic, tetragonal and orthorhombic crystal systems has been suggested. Secondary role of the crystal structure chirality was shown. Oppositely, significant role of the crystallographic point group type (symmetric, alternating, dihedral or cyclic) in the flexomagnetoelectric coupling has been derived. It was shown, that conceptually new features of the flexomagnetoelectric effects are expected in the crystals of the cyclic groups (crystal classes 4/m, 4? and 4). Proposed verification of the theory is investigation of the domain wall bend details (changes of the effect symmetry). Special case of such verification near the compensation point is suggested. First-principles mechanisms of the flexomagnetoelectric interaction were discussed.     

磁性斯格明子的发现及研究现状

磁性斯格明子是一种具有拓扑保护的纳米尺度磁畴结构,其小尺寸、易被电流驱动、低能耗等优点使其成为磁信息存储和自旋电子学器件的核心候选材料,近年来备受人们关注。文章将着重介绍：(1)磁性斯格明子产生的机理;(2)探测磁性斯格明子的方法或者手段;(3)近期国际上研究的几种磁性斯格明子材料;(4)磁性斯格明子诱导产生新颖的霍尔效应——拓扑霍尔效应;(5)结论与展望。     

Consequences of the Dzyaloshinskii-Moriya interaction

Recently there has been an explosion of research related to the Dzyaloshinskii-Moriya interaction (DMI) in magnetic and multiferroic materials. This article reviews the key themes in this research and provides insight into the consequences of the DMI through simple theoretical models. The topics covered include new magnetic structures such as skyrmions and changes in domain wall structures along with their motion under a variety of driving fields. In addition, the influence of DMI on linear and nonlinear spin wave behavior is discussed. Multiferroic materials and new two-dimensional materials with DMI are briefly discussed. Finally, we also present an overview of different DMI materials and their characteristic parameters and potential applications.     

Topological textures and their bifurcation processes in 2D ferromagnetic thin films

In this paper, by the use of the topological current theory, the topological structures and the dynamic processes in thin-film ferromagnetic systems are investigated directly from the viewpoint of topology. It is found that the topological charge of a thin-film ferromagnetic system can be changed by annihilation or creation processes of opposite polarized vortex–antivortex pairs taking place at space–time singularities of the normalized magnetization vector field of the system, the variation of the topological charge is integer and can further be expressed in terms of the Hopf indices and Brouwer degrees of the magnetization vector field around the singularities. Moreover, the change of the topological charge of the system is crucial to vortex core reversal processes in ferromagnetic thin films. With the help of the topological current theory and implicit function theorem, the processes of vortex merging, splitting as well as vortex core reversal are discussed in detail.     

Symmetry and magnetic properties of transition metal clusters

The magnetic properties of 55-atom Fe, Co and Ni clusters are studied using the local spin-density formalism. The dependence of magnetic moment on cluster symmetry is found to be also cluster size dependent. The symmetry of cluster plays an important role in determining the charge distribution. The surface magnetism enhancement are found to be decreased from Fe to Ni.     

Low-energy electronic spin excitations between filling factors ν=1 and studied by optically detected nuclear magnetic resonance

We report measurements of the spin relaxation time (T_1n) for nuclei in the potential well confining a high-mobility two-dimensional electron system at a single GaAs–GaAlAs heterojunction. At low temperatures nuclear spin relaxation is dominated by electron–nuclear spin scattering: we find that T_1n displays sharp maxima at incompressible states throughout the hierarchy of the fractional quantum Hall effect. This behaviour is consistent with the existence of low-energy spin excitations only where the electron system is compressible. Our measurements also provide evidence for a gap in the spin excitation spectrum at .     

交换作用对CeF3晶体磁性和磁光效应的影响

研究了顺磁性晶体CeF₃内交换作用有效场随温度的变化关系，给出了其形式为H_in=(-0.68-0.002T)×10^-6M.结合该关系，用量子理论的计算得到了与实验值吻合较好的磁化率倒数、Verdet常数倒数随温度的变化关系，并且用光磁效应的概念和规律对结果作了进一步分析，指出光诱导促进了该晶体内电子间的交换作用. 关键词： 3晶体')" href="#">CeF₃晶体 交换作用 磁光效应 光磁效应     

Magnetocaloric effect, magnetic domain structure and spin-reorientation transitions in HoCo5 single crystals

A study on the magnetocaloric effect (MCE) has been carried out on the HoCo₅ single crystal using a direct method in the temperature range 78-300 K and in a magnetic field of 1.3 T at various orientations of the single crystal with respect to the direction of external magnetic field. Our research has shown that magnetic anisotropy strongly affects magnitude and sign of the MCE in the regions of spin-reorientation phase transitions. In addition, the magnetic domain structure of the HoCo₅ single crystal in (0 0 1) plane in the temperature range from room temperature down to 4.2 K was investigated.     

Symmetry interaction and many-body correlations

Many-body correlations generated by the symmetry potential naturally arise in the molecular-dynamics CoMD-II model. The effect of these correlations on the collision dynamics at Fermi energies is discussed. In particular we show that two-body correlations generated by the symmetry potential are able to produce large effects in simple observables such as the charge distributions. A comparison with the predictions, based on EOS static calculations, is also discussed.     

Direct visualization of the vortex distributions in a superconducting film with a Penrose array of magnetic pinning centers: Symmetry induced giant vortex state

Using scanning Hall probe microscopy a direct visualization of the flux distribution in a Pb film covering a fivefold Penrose array of Co dots is obtained. We demonstrate that stable vortex configurations can be found for fields H ∼ 0.8H₁, H₁ and 1.6H₁, where H₁ corresponds to one flux quantum per pinning site. The vortex pattern at 0.8H₁ corresponds to one vacancy in one of the vertices of the thin tiles whereas at 1.6H₁ the vortex structure can be associated with one interstitial vortex inside each thick tile. Strikingly, for H = 1.6H₁ interstitial and pinned vortices arrange themselves in ring-like structures (“vortex corrals”) which favor the formation of a giant vortex state at their center.     

高温度稳定性磁斯格明子的材料发现及性能调控

文章报道了阻挫型磁体Fe₃Sn₂中高温度稳定性磁斯格明子材料的发现以及利用电流实现斯格明子自旋手性翻转的一系列工作。作者首先基于合金化设计的思想,解决了晶体取向生长困难和易发生包晶反应这两个关键技术难题,生长出了高质量的Fe₃Sn₂单晶样品。通过原位洛伦兹电镜观测发现,该材料体系具有室温磁性斯格明子,并具有多种拓扑形态,而且在外部磁场作用下可以相互转化。作者进一步利用聚焦离子束(FIB)技术,采用空间几何受限方法,制备出了磁斯格明子单链排列的“赛道”纳米条带样品。实验发现,该样品中斯格明子可以在室温到630 K极宽温区内保持其尺寸及间距不变,这表明该材料中斯格明子具有极高的温度稳定性。在这些研究工作基础上,作者在“赛道”纳米条带样品中进一步实现了电流驱动的斯格明子自旋手性翻转。作者这一系列关于高温度稳定性磁斯格明子材料以及相关器件的探索工作,从材料和器件两个方面推进了磁斯格明子材料的实用化。     

磁斯格明子的微磁学研究进展和应用

磁斯格明子作为一种具有拓扑保护性质的准粒子受到了磁学与磁性材料领域科学家的广泛关注.本文对磁斯格明子的拓扑性质进行了概述,回顾了磁斯格明子的存在条件以及运输特性,综述了近年来利用微磁学模拟研究的磁斯格明子激发、操控、微波磁场响应以及基于磁斯格明子的器件设计,主要包括赛道存储器、自旋纳米振荡器、晶体管和逻辑门.通过本文的综述,希望为磁斯格明子在未来信息领域的应用提供参考.     

The relation between the radii and the densities of magnetic skyrmions

Compared with the traditional magnetic bubble, a skyrmion has a smaller size, and better stability and therefore is considered as a very promising candidate for future memory devices. When skyrmions are manipulated, erased and created, the density of skyrmions can be varied, however the relationship between the radii and the densities of skyrmions needs more exploration. In this paper, we study this problem both theoretically and by using the lattice simulation. The average radius of skyrmions as a function of material parameters, the strength of the external magnetic field and the density of skyrmions is obtained and verified. With this explicit function, the skyrmion radius can be easily predicted, which is helpful for the future study of skyrmion memory devices.     

Effect of tensile stress on the variation of magnetic field of low-alloy steel

The surface magnetic field intensity H_p(y) of low-carbon steel plate specimen was measured after tensile test and unloading; variation of H_p(y) was studied. It was found that H_p(y) signals first decreased with increase in stress, and then reversed to the initial field when the stress was greater than 160 MPa. Under yield stress, H_p(y) reached its maximum, and then decreased slightly with further increase in stress. The initial magnetic signals have great impact on the variation of magnetic field.