Spin polarization and magnetoresistance through a ferromagnetic barrier in bilayer graphene

We study spin dependent transport through a magnetic bilayer graphene nanojunction configured as a two-dimensional normal/ferromagnetic/normal structure where the gate voltage is applied on the layers of ferromagnetic graphene. Based on the four-band Hamiltonian, conductance is calculated by using the Landauer-Buttiker formula at zero temperature. For a parallel configuration of the ferromagnetic layers of bilayer graphene, the energy band structure is metallic and spin polarization reaches its maximum value close to the resonant states, while for an antiparallel configuration the nanojunction behaves as a semiconductor and there is no spin filtering. As a result, a huge magnetoresistance is achievable by altering the configurations of ferromagnetic graphene around the band gap.     

Modeling of domain wall displacement in crystals with dislocations

The domain wall displacement in ferromagnetic crystals with dislocations that produce a continuous potential field with wavelength greater than the domain wall width is investigated by the machine modeling method. It is established that the domain wall shape and the start field are determined during magnetization by the potential field characteristics that depend on the specific location of the dislocations. It is shown that the start field is proportional to the ratio between the mean stress and the variance of the potential field stress at the site of domain wall fastening. The results of the modeling agree with values of the start field obtained experimentally and the behavior of the domain wall during magnetization of iron microcrystals.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 77–83, December, 1990.     

Nonlinear spin-polarized transport through a ferromagnetic domain wall

A domain wall separating two oppositely magnetized regions in a ferromagnetic semiconductor exhibits, under appropriate conditions, strongly nonlinear I-V characteristics similar to those of a p-n diode. We study these characteristics as functions of wall width and temperature. As the width increases or the temperature decreases, direct tunneling between the majority spin bands reduces the effectiveness of the diode. This has important implications for the zero-field quenched resistance of magnetic semiconductors and for the design of a recently proposed spin transistor.     

Current driven magnetization dynamics in ferromagnetic nanowires with a Dzyaloshinskii-Moriya interaction

We study current-induced magnetization dynamics in a long thin ferromagnetic wire with a Dzyaloshinskii-Moriya interaction (DMI). We find a spiral domain wall configuration of the magnetization and obtain an analytical expression for the width of the domain wall as a function of the interaction strengths. Our findings show that above a certain value of DMI a domain wall configuration cannot exist in the wire. Below this value we determine the domain wall dynamics for small currents, and calculate the drift velocity of the domain wall along the wire. We show that the DMI suppresses the minimum value of current required to move the domain wall. Depending on its sign, the DMI increases or decreases the domain wall drift velocity.     

Spin waves in static non-periodic magnetic structures

We have studied the propagation of spin waves in a number of static non-periodic magnetic structures. We have established that (1) a ferromagnetic spin wave can ride over a domain wall with little reflection if its wavelength is less than twice the thickness of the wall; (2) in a ferromagnet with a set of parallel but irregularly spaced domain walls the spin wave linewidth is determined by the product of the scattering strength of the walls and the degree of randomness of the wall spacings; and (3) spin waves of rather narrow linewidths can exist in continuously varying irregular spin structures.     

Magnon spectrum of a symmetric-spin nanocontact on a ferromagnetic ultrathin film

A theoretical model is presented for the study of the magnetic properties and the coherent magnon transport via monatomic chains in ultrathin magnetic films. In particular, we studied a finite number of monatomic chains joining two slabs of ferromagnetic material. Each slab consists of five atomic layers of a cubic lattice with magnetically ordered spins coupled by the Heisenberg exchange. The system is supported on a non-magnetic substrate and otherwise considered free from magnetic interactions. The spin dynamics of the ultrathin film is studied by the matching method. The individual and the total magnon transmissions of the ultrathin ferromagnetic film, scattering coherently at the nanojunction zone, and the localized spin states in the boundary domain are calculated and analyzed. The interatomic magnetic exchange is varied on the boundary domain specifically for three cases of magnetic exchange to investigate the consequences of magnetic softening and hardening for the calculated properties. Numerical results show characteristic interference effects between the incident spinwaves and the localized spin states of the nanocontact. The calculated properties are presented for arbitrary incidence of the magnons on the boundary, for all accessible frequencies in the propagating bands, and for the interatomic magnetic exchange of the magnetic film. The localized magnon branches created by the nanocontact domain are observed in the Brillouin zone.     

Magnetic field dependence of the domain wall resistance in a quantum wire

For an ideal one-dimensional ferromagnetic wire with a magnetic domain wall (DW), contribution of the DW to the resistivity of the system has been investigated. We have studied the resistance due to the magnetic impurities in the domain wall which was suspended in a weak magnetic field for two types of chiralities. The analysis has been based on Boltzmann transport equation, within the relaxation time approximation. Through this formalism, both increasing and decreasing of the resistance due to the DW have been predicted in presence of Zeeman interaction as an extrinsic mechanism.     

Sensing domain wall pinning in the longitudinal magnetoresistance of a two-dimensional electron gas

We investigate the sensing of domain wall pinning in thin Co wires positioned on top of a two-dimensional electron gas (2DEG) heterostructure by measuring the longitudinal resistance of the 2DEG as the magnetic field is swept, in an analogy to the Barkhausen effect. For comparison, we also measure the magnetoresistance of the ferromagnetic film in the same device in a subsequent sweep. Compared to the Hall measurements, the longitudinal measurement has the advantage of sensing magnetic activity over longer lengths, while compared to the measurement of the magnetoresistance in the ferromagnetic wire, it offers complementary information related to the pinning and unpinning of the domain wall, due to its sensitivity only to the out-of-plane magnetic field component.     

Interaction of a shear wave with a moving domain wall in a ferromagnetic crystal with allowance for the external magnetic field

The boundary-value problem of the magnetoelastic wave interaction with a moving domain wall in a ferromagnetic crystal is solved in the nonexchange magnetostatic approximation with allowance for the external magnetic field. It is shown that the difference introduced by magnetic field between the ferromagnetic resonance frequencies of the domains does not cause any noticeably departure of the refraction characteristics of reflected and transmitted waves from those observed at zero frequency mismatch. By contrast, the magnitudes of the transmission and reflection coefficients strongly depend on the external magnetic field and on the mobility of the domain wall. The dependence of the magnitude of the reflection coefficient on the external magnetic field at a fixed angle of shear wave incidence is found to possess two ferromagnetic resonance peaks. The positions and heights of the peaks may vary depending on the mobility of the domain wall.     

Domain wall pinning in narrow ferromagnetic ring structures probed by magnetoresistance measurements

We present a magnetoresistance study of magnetization reversal and domain wall pinning effects in a mesoscopic narrow ferromagnetic Permalloy ring structure containing notches. The size and strength of the attractive pinning potential created by a notch is measured and the resistance minimum at remanence is found to occur when a single transverse domain wall is pinned at the notch, in agreement with the results of numerical simulations of the anisotropic magnetoresistance. When a field is applied in the direction corresponding to a potential well edge, a novel magnetic state with a very wide domain wall is stabilized, giving rise to a characteristic signature in the magnetoresistance at such angles.     

Model of hybrid interfacial domain wall in ferromagnetic/antiferromagnetic bilayers

A general model of a hybrid interfacial domain wall(HIDW) in ferromagnetic/antiferromagnetic exchange biased bilayers is proposed, where an interfacial domain wall is allowed to extend into either the ferromagnetic or antiferromagnetic layer or across both. The proposition is based on our theoretical investigation on thickness and field dependences of ferromagnetic domain wall(FMDW) and antiferromagnetic domain wall(AFDW), respectively. Good match of the simulation to the hysteresis loops of a series of Ni Fe/Fe Mn exchange-biased bilayers confirms the existence of the HIDW, where the AFDW part is found to preferentially occupy the entire antiferromagnetic layer while the FMDW shrinks with the increased magnetic field as expected. The observed asymmetry between the ascending and descending branches of the hysteresis loop is explained naturally as a consequence of different partition ratios between AFDW and FMDW.     

消磁场对纳米铁磁线磁畴壁动力学行为的影响

为了实现基于磁畴壁运动的自旋电子学装置, 掌握磁畴壁动力学行为是重要争论之一.研究了在外磁场驱动下L-型纳米铁磁线磁畴壁的动力学行为. 通过微磁学模拟,在各种外磁场的驱动下考察了纳米铁磁线磁畴壁的动力学特性; 在较强外磁场的驱动下, 在不同厚度纳米线上考察了纳米线表面消磁场对磁畴壁动力学行为的影响. 为了进一步证实消磁场对磁畴壁动力学的影响, 在垂直于纳米线表面的外磁场辅助下分析了磁畴壁的动力学行为变化. 结果表明, 随着纳米线厚度和外驱动磁场强度的增加, 增强了纳米线表面的消磁场的形成, 使得磁畴壁内部自旋结构发生周期性变化, 导致磁畴壁在纳米线上传播时出现Walker崩溃现象. 在垂直于纳米线表面的外磁场辅助下, 发现辅助磁场可以调节消磁场的强度和方向. 这意味着利用辅助磁场可以有效地控制纳米铁磁线磁畴壁的动力学行为.     

铁磁/反铁磁双层膜系统中的磁畴动力学行为

比较了铁磁单层膜与铁磁/反铁磁双层膜结构中的磁畴演化行为, 发现由于反铁磁层膜对铁磁层膜的耦合作用使得系统的磁畴壁厚度、 磁畴壁等效质量、磁畴壁移动速度等发生了改变, 系统的矫顽场增强, 并出现了交换偏置场. 文章具体研究了反铁磁层耦合作用下其磁畴壁厚度、 等效质量以及磁畴壁移动速度等与反铁磁层的净磁化、 磁各向异性、界面耦合强度以及温度等的关系; 并研究了其对铁磁/反铁磁双层膜中的交换偏置场、矫顽场的影响. 进而 从磁畴结构的形成及其演化上揭示了铁磁/反铁磁双 层膜中出现交换偏置以及矫顽场增加的物理机制.     

Single domain wall dynamics in ferromagnetic lamination with variable conductivity

The influence of variable conductivity and thickness of two outer non-ferromagnetic layers on magnetization reversal of one central ferromagnetic layer is theoretically investigated. The model of a thin rigid 180°$180°$ domain wall moving transversely through the axially magnetized ferromagnetic layer is used to calculate induced eddy currents in lamination from which the domain wall mobility is determined. The effect of asymmetric distribution of eddy currents around moving domain wall results in acceleration of the wall near the edge of the lamination. The known domain wall mobility in ferromagnetic lamination can then be used to determine either the conductivity or the thickness of deposited outer non-ferromagnetic layers as proposed in discussion.     

Structure and dynamic properties of the twisted magnetic domain wall in an electric field

The structure of the domain wall in a magnetically uniaxial ferromagnetic film placed in an external electric field has been studied. It has been shown that the domain wall has a complex twisted structure whose characteristics (thickness, profile, and limit velocity of steady motion) depend on the film thickness, quality factor, and external electric field. The effect of the electric field on the domain wall is caused by inhomogeneous magnetoelectric coupling taking place in domain walls with a twisted structure.     

Electromagnetic excitation and dynamical response of ferromagnetic superconductors

Electromagnetic excitation modes in ferromagnetic superconductors are studied theoretically as the coupled modes of photon, electric current and magnetic moments, and their dispersion relations are obtained. With the use of these modes, the reflection coefficient is calculated for the electromagnetic wave incident on the surface of a semi-infinite crystal of ferromagnetic superconductor. It is shown that the reflection coefficient decreases rapidly in the neighborhood of the critical temperature where a spontaneous surface magnetization occurs. An anomaly in the surface impedance is also predicted.     

Mechanically driven domain wall movement in magnetoelastic nanomagnets

Magnetic domain walls are fundamental objects arising in ferromagnetic materials, largely investigated both through micromagnetic simulations and experiments. While current- and field-based techniques for inducing domain wall propagation have been widely studied for fundamental understanding and application-oriented purposes, the possibility to manipulate domain walls using mechanical stress in magnetoelastic materials has only recently drawn interest. Here, a complete analytical model describing stress-induced transverse domain wall movement in ferromagnetic nanostripe with variable cross-section is presented. This approach yields a nonlinear integro-differential equation describing the magnetization field. Its numerical implementation, based on the nonlinear relaxation method, demonstrates the possibility to precisely control the position of a domain wall through mechanical action.     

Indirect control of antiferromagnetic domain walls with spin current

The indirect controlled displacement of an antiferromagnetic domain wall by a spin current is studied by Landau-Lifshitz-Gilbert spin dynamics. The antiferromagnetic domain wall can be shifted both by a spin-polarized tunnel current of a scanning tunneling microscope or by a current driven ferromagnetic domain wall in an exchange coupled antiferromagnetic-ferromagnetic layer system. The indirect control of antiferromagnetic domain walls opens up a new and promising direction for future spin device applications based on antiferromagnetic materials.     

Ballistic transport through a sharp domain wall in a semiconducting ferromagnetic nanoconstriction and in the presence of the Dresselhaus spin-orbit coupling

We study the effect of the Dresselhaus spin-orbit interaction on the magnetoresistance (MR) of a quasi-one-dimensional ferromagnetic semiconductor containing a sharp domain wall. The MR is calculated in the ballistic regime, within the Landauer-Büttiker formalism. The results show that the Dresselhaus spin-orbit coupling which induces an effective magnetic field along the wire, reduces the domain wall MR.     

磁性金属纳米结构的畴壁特性与磁逻辑电路构筑

自旋电子学由于其丰富的物理内涵和广泛的应用前景受到学术界和工业界的高度重视,成为近年来凝聚态物理和信息技术领域关注的焦点。本文介绍了利用磁性金属纳米结构实现作为自旋电子器件基础的自旋注入的方法,特别涉及利用铁磁金属纳米点接触结构钉扎磁畴的特点,研究自旋极化电流与磁畴壁的相互作用规律, 理解纳米结构中畴壁的动力学行为,并以此为基础构筑结构简单、性能优异的全金属磁逻辑电路,从而实现了由电信号驱动,通过电信号检测,并与CMOS技术兼容的目的。