Exchange bias for ferromagnetic/antiferromagnetic bilayers with the uniaxial anisotropy being misaligned with the exchange anisotropy

Using the principle of minimal energy and S-W model, the exchange bias for ferromagnetic/antiferromagnetic bilayers has been investigated when the uniaxial anisotropy is misaligned with the exchange anisotropy. According to the relation between the energy of the bilayer and the orientation of ferromagnetic magnetization, it is found that the bilayer will be in the monostable state or bistable state when the external field is absent in the initial magnetization state. The monostable state or bistable state of the bilayer, which determines the angular dependence of exchange bias directly, is controlled by the competition between the exchange anisotropy and uniaxial anisotropy. When the applied field is parallel to the intrinsic easy axes and intrinsic hard axes, one of the switching fields of the hysteresis loop shows an abrupt change, while the other keep continuous by analyzing the magnetization reversal processes. Consequently, the exchange bias field and the coercivity will show a jump phenomenon. The numerical calculations indicate that both the magnitude and direction of the exchange anisotropy will significantly affect the angular dependence of exchange bias. The jump phenomenon of exchange bias is an intrinsic property of the bilayer, which is dependent on the interfacial exchange-coupling constant, the orientation of the exchange anisotropy, the thickness and uniaxial anisotropy constant of the ferromagnetic layer.     

Domain wall nucleation in epitaxial exchange-biased Fe/IrMn bilayers with highly misaligned anisotropies

The angular dependence of the magnetization reversal in epitaxial Fe/IrMn bilayers with collinear and non-collinear cubic and unidirectional anisotropies is investigated. Multistep loops with different magnetization reversal processes are observed for either positive or negative angles with respect to the Fe easy axis. The angular dependence of the switching fields displays the broken symmetry of the induced non-collinearity. The experimental results are reproduced with a generalized domain wall nucleation model that includes the induced anisotropy configuration and the peculiar asymmetric magnetic switching behavior. These results highlight the importance of the relative angle between anisotropies in epitaxial exchange bias systems with incoherent rotation reversal mechanism, opening a new pathway for tailoring the magnetic properties of such systems.     

外应力对铁磁/反铁磁体系交换偏置的影响及阶跃现象

采用能量极小原理及S-W模型研究了外应力对铁磁/反铁磁（FM/AFM）双层薄膜体系交换偏置的影响.不施加外磁场时,根据能量与铁磁层磁化强度方向之间的关系,指出体系存在单稳态和双稳态两种不同的状态,是由交换各向异性与单轴各向异性之间的竞争控制的.体系处于单稳态还是双稳态直接决定着交换偏置的角度依赖关系.分析磁化过程发现,外磁场沿内禀易轴及内禀难轴方向施加时,磁滞回线的一支转换场发生突变,而另一支转换场则保持不变,最终导致交换偏置场和矫顽场出现阶跃行为.数值计算表明,交换偏置场和矫顽场在阶跃点均具有较大的数值 关键词： 单稳态 双稳态 外应力     

Asymmetric reversal modes in ferromagnetic/antiferromagnetic multilayers

Experimentally an asymmetry of the reversal modes has been found in certain exchange bias systems. From a numerical investigation of the domain state model evidence is gained that this effect depends on the angle between the easy axis of the antiferromagnet and the applied magnetic field. Depending on this angle the ferromagnet reverses either symmetrically, e.g., by a coherent rotation on both sides of the loop, or the reversal is asymmetric with a nonuniform reversal mode for the ascending branch, which may even yield a zero perpendicular magnetization.     

磁性纳米线反磁化过程的截椭球链模型

依据实际的纳米线形貌，提出了描述磁性纳米线反磁化过程的截椭球链模型.基于此模型，推导了纳米线反磁化过程中不可逆磁化的临界场和矫顽力在一致转动和对称扇形模式下的表达式.同时，还研究了椭球间接触角、单轴磁晶各向异性、外场与链轴方向夹角、截椭球个数和椭球形状因子对临界场和矫顽力的影响.利用该模型解释了镍纳米线的反磁化过程可能是对称扇形的模式. 关键词： 截椭球链模型 反磁化 一致转动 对称扇形     

GdFeCo磁光薄膜中RE-TM反铁磁耦合与激光感应超快磁化翻转动力学研究

使用飞秒时间分辨抽运-探测磁光克尔光谱技术,研究了激光加热GdFeCo磁光薄膜跨越铁磁补偿温度时稀土-过渡金属(RE-TM)反铁磁交换耦合行为和超快磁化翻转动力学. 实验观察到由于跨越铁磁补偿温度、净磁矩携带者交换而引起的磁化翻转反常克尔磁滞回线以及在同向外磁场下,反常回线上大于和小于矫顽力部分的饱和磁化强度不同,显示出GdFeCo中RE与TM之间的非完全刚性反铁磁耦合. 在含有Al导热底层的GdFeCo薄膜上观测到饱和磁场下激光感应磁化态翻转及再恢复的完整超快动力学过程. 与剩磁态的激光感应超快退磁化过 关键词： 补偿温度 磁化翻转 反铁磁耦合 GdFeCo     

Magnetization reversal in submicron disks: exchange biased vortices

Submicron, circular, ferromagnetic-antiferromagnetic dots exhibit different magnetization reversal mechanisms depending on the direction of the magnetic applied field. Shifted, constricted hysteresis loops, typical for vortex formation, are observed for fields along the exchange bias direction. However, for fields applied close to perpendicular to the exchange bias direction, magnetization reversal occurs via coherent rotation. Magnetic force microscopy imaging together with micromagnetic simulations are used to further clarify the different magnetic switching behaviors.     

Magnetization reversal of single domain permalloy nanowires

Magnetization reversal process and magnetoresistance (MR) hysteresis of single domain permalloy nanowires are numerically investigated by using OOMMF. It is shown that the abrupt jumps in the magnetoresistance are due to the domain formation and domain wall propagation so that a magnetic domain suddenly switches from one state into another. A nonmonotonic angular dependence of the jump (switching) field is found. Coherent rotation mode is responsible for the smooth variation of MR curves. The nucleation pattern of newly born domains depends on the tilted angle of external field.     

Out-of-plane coercive field of Ni80Fe20 antidot arrays

The out-of-plane magnetic anisotropy and out-of-plane magnetization reversal process of nanoscale Ni₈₀Fe₂₀ antidot arrays deposited by magnetron sputtering technique on an anodic aluminum oxide (AAO) membrane are investigated. The angular dependence of out-of-plane remanent magnetization of Ni₈₀Fe₂₀ antidot arrays shows that the maximum remanence is in-plane and the squareness of the out-of-plane hysteresis loop follow a |cos θ| dependence. The angular dependence of out-of-plane coercivity of Ni₈₀Fe₂₀ antidot arrays shows that the maximum coercivity lies on the surface of a cone with its symmetric axis normal to the sample plane, which indicates a transition of magnetic reversal from curling to coherent rotation when changing the angle between the applied magnetic field and the sample plane.     

Shape-induced oscillation of exchange bias in the Co nanopillar with lateral oxidation

A modified Monte Carlo Metropolis method is performed to simulate the hysteresis loops of a laterally oxidized Co nanopillar with an excised arc or an inclined circular plane. At low temperature after field cooling, an oscillatory exchange bias is observed with excised length or angle of inclination. However, the behaviors of coercivity exhibit a monotonous decrease with excision but an oscillation with tilt. The phenomena are intriguing and corroborated by means of the microscopic spin configurations and the magnetization reversal mechanisms. The occurrence of oscillatory exchange bias in such a Co/CoO composite nanopillar with direct exchange couplings is mainly because of the shape inducement in the nanopillar, which results in the energy fluctuations of interfacial coupling and the nonuniformity of domain distribution.     

Influence of magnetocrystalline anisotropy on the magnetization reversal mechanism in exchange bias Co/CoO bilayers

We investigated the reversal mechanism in a Co/CoO exchange bias bilayer with a pronounced magnetocrystalline anisotropy in the ferromagnet. The anisotropy, which is induced by the growth of a highly textured Co layer, imposes a distinct reversal mechanism along the magnetically easy and hard direction. It is shown that exchange bias can be induced along both directions, despite the magnetocrystalline anisotropy. The interplay between the magnetocrystalline anisotropy and exchange bias induces a different reversal mechanism for the subsequent reversals in the two crystallographic directions. Along the hard axis, the magnetization reverses according to the reversal mechanism observed before in polycrystalline exchange bias bilayers, i.e. domain wall nucleation and motion for the first reversal and coherent rotation for the subsequent ones. Along the easy axis, domain wall motion remains the dominant reversal mechanism and magnetization rotation has only a minor contribution.     

交换偏置系统中的反铁磁磁化与自旋波

采用自由能变分法研究了铁磁/反铁磁双层膜系统在反铁磁层存在净磁化下的自旋波谱.本模型中铁磁薄层具有单轴磁晶各向异性和立方磁晶各向异性,反铁磁层仅具有单轴磁晶各向异性,但厚度有限,推导出了系统铁磁共振频率的表达式.结果表明：系统的自旋波谱分光学模和声学模两种,其中光学模仅在反铁磁层存在净磁化时得到激发.自旋波谱可按外磁场强度的变化情况分为强弱两支;区分强磁场和弱磁场的临界场依赖于铁磁/反铁磁间的交换作用,反铁磁层的磁化强度以及反铁磁层的厚度等.交换偏置场对光学模的影响明显于声学模,而反铁磁的净磁化和其厚度对系统的影响紧密联系,难以区分.但当反铁磁层净磁化很小可忽略时,系统只存在声学模激发. 关键词： 铁磁/反铁磁双层膜 反铁磁层净磁化 光学模 声学模     

Rotation of the pinning direction in the exchange bias training effect in polycrystalline NiFe/FeMn bilayers

For polycrystalline NiFe/FeMn bilayers, we have observed and quantified the rotation of the pinning direction in the exchange bias training and recovery effects. During consecutive hysteresis loops, the rotation of the pinning direction strongly depends on the magnetization reversal mechanism of the ferromagnet layer. The interfacial uncompensated magnetic moment of antiferromagnetic grains may be irreversibly switched and rotated when the magnetization reversal process of the ferromagnet layer is accompanied by domain wall motion and domain rotation, respectively.     

Experimental evidences and driving mechanisms for anisotropic misalignments in exchange coupled systems

Experimental evidence for misalignments between F anisotropy axes, AF anisotropy axes and the exchange bias field direction is shown in a CoFe/Ni_0.38O_0.62 system. The angular dependence of the remanent magnetization, the exchange bias field and the coercive field is studied as a function of the diluted NiO thickness. The exchange coupling leads to misalignments between the applied field during growth, the exchange bias field and the coercive field directions. It shows that two different interfacial spin frustrations are present, corresponding to pinned and unpinned spins contributions of the diluted NiO.     

Exchange bias in ferromagnet/antiferromagnet bilayers

Since the exchange bias （EB） effect was discovered in the Co/CoO core-shell nanoparticles, it has been extensively studied in various ferromagnet （FM）/antiferromagnet （AFM） bilayers due to its crucial role in spintronics devices. In this article, we review the investigation of the EB in our research group. First, we outline basic features of the EB, including the effects of the constituent layer thickness, the microstructure and magnetization of the FM layers, and we also discuss asymmetric magnetization reversal process in wedged-FM/AFM bilayers. Secondly, we discuss the mechanisms of the positive EB and the perpendicular EB. Thirdly, we demonstrate the hysteretic behavior of the angular dependence of the EB and analyze the EB training effect. Finally, we discuss the roles of the rotatable anisotropy in the two phenomena.     

Magnetization reversal studies of continuous and patterned exchange biased NiFe/FeMn thin films

In this article we present a detailed investigation of the structural and magnetic properties of exchange biased NiFe (ferromagnet)/FeMn (antiferromagnet) thin films. The influence of the shape anisotropy on exchange bias and the magnetization reversal mechanism in a sample with patterned lines is compared with a continuous two-dimensional reference sample. Polarized neutron reflectivity (PNR) is employed to study the magnetization reversal by analyzing the spin-flip and non-spin-flip reflectivities. PNR measurements show that the magnetization reversal in the reference two-dimensional film and patterned lines is by domain wall motion rather than coherent rotation of magnetization.     

Angular dependence of the coercivity and remanence of ordered arrays of Co nanowires

The angular dependence of the coercivity and remanence of ordered hexagonal arrays of Co nanowires prepared using anodic aluminum oxide templates was investigated. The experimental evolution of coercivity as a function of the angle, in which the external field is applied, is interpreted considering micromagnetic simulations. Depending on the angle between the axis of the wire and the applied magnetic field direction our results show that the magnetization reversal mode changes from vortex to a transverse domain wall. Besides, we observed that the dipolar interactions cause a reduction in coercive fields, mainly in the direction of easy magnetization of the nanowires. Good agreement between numerical and experimental data is obtained.     

Unidirectional anisotropy of the resistivity in 112-type EuBaCo<Subscript>2</Subscript>O<Subscript>5.5</Subscript> cobaltite

It is found that the resistivity of EuBaCo₂O_5.5 depends not only on the magnitude of an applied magnetic field but also on its sign. This phenomenon is new for cobalt and manganese oxides. The symmetric or shifted hysteresis loops (depending on the cooling method) for the resistivity correspond to similar loops for the magnetization. The angular dependence of the resistivity is described by the relation Δρ ~ sinθ, where the angle θ = 0 corresponds to the direction of magnetization, at which the sample has been cooled. The results are interpreted in terms of the exchange interaction between the ferromagnetic (FM) and antiferromagnetic (AFM) phases. At the cyclic variation of temperature or magnetic field, the resistivity of EuBaCo₂O_5.5 increases and does not return to the initial state. The metastable state of EuBaCo₂O_5.5 is related to the kinetic phenomena accompanying the first order FM–AFM phase transition.     

Influence of magnetic field sweep rate on the hysteresis loops of Ni0.8Fe0.2/Fe0.5Mn0.5 exchange bias bilayer

The influence of the magnetic field sweep rate on the hysteresis loops of exchange bias Ni0.8Fe0.2/Fe0.5Mn0.5 bilayers has been investigated with a vibrating sample magnetometer. It was found that the sweep rate of 13.6 kA/4πms is high enough to bring about obvious changes in the hysteresis loops of the exchange bias bilayer. High sweep rate in the magnetization reversal stage enlarges the coercivity of the sample, while high sweep rate in the saturation state reduces the coercivity. The above phenomena were attributed to magnetic viscosity in the ferromagnetic layer enhanced by the interface exchange interaction and domain magnetization reversals assisted by thermal fluctuation in the antiferromagnetic layer respectively.     

Exchange bias in nearly perpendicularly coupled ferromagnetic/ferromagnetic system

Exchange bias phenomena appear not only in ferromagnetic/antiferromagnetic systems but also in ferromagnetic/ferromagnetic systems in which two layers are nearly perpendicularly coupled. We investigated the origin of the symmetry-breaking mechanism and the relationship between the exchange bias and the system's energy parameters. We compared the results of computational Monte Carlo simulations with those of theoretical model calculation. We found that the exchange bias exhibited nonlinear behaviors, including sign reversal and singularities. These complicated behaviors were caused by two distinct magnetization processes depending on the interlayer coupling strength. The exchange bias reached a maximum at the transition between the two magnetization processes.