铁磁/反铁磁双层膜系统中交换各向异性及其厚度依赖性

研究铁磁/反铁磁双层膜系统中交换偏置场和矫顽场的冷却磁场依赖性.结果表明,随着冷却磁场的增加,交换偏置场由负值向正值转变.在转变点附近,矫顽场有-个特别的增强,并达到最大值.结果同相关实验-致.研究铁磁层和反铁磁层厚度对交换偏置场和矫顽场的影响.发现,正负交换偏置场和矫顽场随着铁磁层厚度的增大而减小,但随反铁磁层厚度的变化关系复杂.在正交换偏置场的情形,随反铁磁层厚度的增大,交换偏置场增强,矫顽场减弱;在负交换偏置场的情形,随反铁磁层厚度的增大,交换偏置场减弱,矫顽场增强.     

非磁性掺杂下的铁磁/反铁磁系统中的交换偏置和矫顽场

采用Monte Carlo方法,分别讨论了在铁磁/反铁磁双层膜和铁磁/反铁磁单层混合膜中,掺入非磁性物质后,掺杂浓度对交换偏置以及矫顽场的影响.计算结果表明:随着掺杂浓度的增大,双层膜和单层膜交换偏置都有先增大后减小的现象,而其矫顽场则先减小后增大.在相同掺杂浓度下,对随机掺杂和规则掺杂两种不同掺杂方式的结果比较发现:铁磁/反铁磁双层膜中,规则掺杂下产生的交换偏置和矫顽场都得到了增强;对于单层混合膜,随机掺杂下的交换偏置更强,规则掺杂下的矫顽场更大.研究发现对于双层膜规则掺杂可明显地导致其磁滞回线的不对称性,说明铁磁/反铁磁系统中磁滞回线的不对称性与界面自旋微结构密切相关.     

铁磁/反铁磁双层膜中交换偏置及矫顽场的温度特性研究

讨论了铁磁-反铁磁双层膜中交换偏置和矫顽场随温度变化的关系。在本模型中,温度的依赖性来源于系统态的热激发以及相关磁学参量的温度依赖性。数值结果显示:低温下,交换偏置和矫顽场随温度的升高而减少,但是随着界面的交换耦合的增强或铁磁层各向异性的减少,其交换偏置变得平坦。随着温度的升高,交换偏置减少直至零;而矫顽场却达到峰值后再减为零。这些结果与实验结果定性一致。根据数值计算结果,可以预见软的铁磁层耦合上硬的反铁磁层,在恰当的交换耦合强度下,可构建具有大的交换偏置、小矫顽场;并在某温度区几乎不随温度变化的磁存贮器件.     

铁磁/反铁磁双层膜系统中的交换偏置及矫顽场的温度特性

提出了一个讨论铁磁／反铁磁双层膜中的交换偏置及矫顽场温度特性的物理模型，该模型，假设铁磁层为具有单畴各向异性的单畴膜而反铁磁层由许多相互独立具有多晶各向异性的颗粒组成，其温度依赖性主要来源于系统态的热不稳定，包括反铁磁颗粒易轴取向的热涨落和相关磁学量的温度依赖性等。计算结果表明其交换偏置随温度的增加非线性地减少而其矫顽场在体阻截温度处达极大值，且其体阻截温度随反铁磁颗粒粒径的增加而增加。我们的计算结果和相关实验结果一致，通过本的讨论，我们建议通过铁磁膜耦合上大粒径硬反铁磁颗粒膜可获得高交换偏置、低矫顽场且近独立于温度的相关磁学器件。     

掺杂下铁磁/反铁磁双层膜中交换偏置的增强

采用了Monte-Carlo方法，讨论了反铁磁层中不同非磁性掺杂浓度下，铁磁/反铁磁双层膜中交换偏置的温度特性. 模拟结果显示：反铁磁层中非磁性掺杂能导致铁磁/反铁磁双层膜中交换偏置的增强. 同时，交换偏置随非磁性掺杂浓度的变化存在极大值，即同一温度下交换偏置随掺杂浓度的变化是非单调的. 并且，随着温度的升高交换偏置的最大值所对应的掺杂浓度向浓度低的方向移动. 它和Hong Jung-Il等人的实验结果完全一致. 究其原因在于反铁磁层相应的自旋排布、磁畴结构等随掺杂浓度的改变发生大的变化，当其正向磁畴和负向磁畴都形成连通的网络结构时，系统的交换偏置达最大. 比较了随机掺杂与规则掺杂的模拟结果. 模拟结果表明规则掺杂能够获得比随机掺杂更大的交换偏置，进一步表明了铁磁/反铁磁双层膜中交换偏置的特性与铁磁/反铁磁界面磁畴结构密切相关.     

外应力场下铁磁/反铁磁双层膜系统中的交换偏置

采用自由能极小的方法研究了铁磁/反铁磁双层膜系统在外应力场下的交换各向异性.本模型中铁磁层具有单轴磁晶各向异性和立方磁晶各向异性，而反铁磁层仅具有单轴磁晶各向异性，但其厚度趋于半无穷.理论上解析地给出了系统的等效交换偏置和钉扎角(它显示了反铁磁层对铁磁层磁化的钉扎作用)与外应力场之间的关系.数值计算表明：系统的等效交换偏置与外磁场的方向有关，而与其大小无关；然而外应力场的大小和方向均对系统的等效交换偏置有影响，其根源在于外应力场的大小和方向都影响着钉扎角. 关键词： 铁磁/反铁磁双层膜 交换偏置 钉扎角 应力场     

铁磁/反铁磁/铁磁三层膜系统中反铁磁自旋结构及其交换各向异性

研究铁磁/反铁磁/铁磁三层膜中界面存在二次以及双二次交换耦合下反铁磁磁矩转动及其交换各向异性.结果表明,其反铁磁膜中的磁矩转动存在可逆"恢复行为"、不可逆"连续倒转行为"以及不可逆"中断倒转行为"三种情形,三种情形的出现强烈地依赖于两界面处的线性耦合和双二次耦合.钉扎界面的交换耦合与旋转界面的交换耦合相互竞争,当钉扎界面耦合占主导时,反铁磁磁矩发生可逆"恢复行为",系统出现交换偏置.在旋转界面耦合占主导情形下,其线性耦合与双二次耦合也相互竞争,分别导致反铁磁磁矩发生不可逆"连续倒转行为"和不可逆"中断倒转行为",系统都不出现交换偏置,但矫顽场都得以增强.相关结论为实验上观测的磁滞能耗以及界面垂直耦合提供了可能的解释.     

NiFe/FeMn双层膜的交换耦合

采用平面霍尔效应测量方法，对NiFe/FeMn双层膜的交换耦合进行了研究. 结果表明，在NiFe/FeMn体系中不存在spin-flop模型给出的单轴各向异性场. 而导致交换耦合场可逆与不可逆测量结果之间较大差异的原因是反铁磁颗粒的不稳定性或铁磁层的分畴现象. 关键词： 反铁磁/铁磁双层膜 交换偏置场 可逆与不可逆测量     

铁磁/反铁磁双层膜中冷却场对交换偏置场的影响

用铁磁畴壁模型研究了非补偿界面铁磁/反铁磁双层膜中冷却场（包括大小及其方向）对交换偏置场h_E的影响.结果表明：当冷却场的方向与反铁磁层磁易轴一致时,h_E大小与冷却场大小无关.当冷却场的方向偏离磁易轴时,h_E的大小随偏离角度的增大有缓慢的改变,但当冷却场的方向偏离到临界角度γ_c处,h_E的大小发生突变,其γ_c的大小随冷却场的增大而增大.特别是当冷却场的偏离角度大于γ_c后,h_E出现由负转正的现象,其转变点还与冷却场的大小有关.另外,h_E与铁磁层原子层数N_F的关系会发生由h_E∝N^-1_F向h_E∝N^-λ_F的转变,其中λ>1.其发生转变的条件与N_F、冷却场大小和方向密切相关. 关键词： 铁磁/反铁磁双层膜 交换偏置 冷却场     

外应力场下铁磁/反铁磁双层膜系统中的自旋波

采用自由能极小的方法研究了铁磁/反铁磁双层膜系统在外应力场下的一致进动自旋波性质，即铁磁共振现象. 本模型中铁磁层很薄可看成单畴结构，但具有单轴磁晶各向异性和立方磁晶各向异性；而反铁磁层仅具有单轴磁晶各向异性，但其厚度趋于半无穷. 推导出了该系统的铁磁共振频率和频谱宽度的解析式. 结果表明，外应力场和界面交换耦合或反铁磁磁强度仅在弱磁场下对系统的铁磁共振有影响，且系统的铁磁共振行为按磁场强度可分为两支，其区分弱磁场和强磁场的临界场依赖于外应力场的方向. 另一方面，应力场方向的改变可借助于反铁磁层磁畴变化对铁磁层磁晶各向异性轴有影响. 关键词： 铁磁/反铁磁双层膜 界面耦合强度 铁磁共振 应力场     

Theoretical investigation of exchange bias

The exchange bias of bilayer magnetic films consisting of ferromagnetic (FM) and antiferromagnetic (AFM) layers in an uncompensated case is studied by use of the many-body Green's function method of quantum statistical theory. The effects of the layer thickness and temperature and the interfacial coupling strength on the exchange bias H_E are investigated. The dependence of the exchange bias H_E on the FM layer thickness and temperature is qualitatively in agreement with experimental results. When temperature varies, both the coercivity H_C and H_E decrease with the temperature increasing. For each FM thickness, there exists a least AFM thickness in which the exchange bias occurs, which is called pinning thickness.     

Tailor-made nano-structured materials for perpendicular recording media and head—precise control of direct/indirect exchange coupling

Tailor-made nano-structured spin materials obtained by precisely controlled nano-scale fabrication technologies for use in ultra-high density hard disk drives (HDDs), as well as an understanding of their nanomagnetics, are essential from the view point of materials, processes, and physics. Artificial control of the exchange coupling among ferromagnetic layers through the RKKY interaction (indirect) and direct exchange coupling represented as the exchange bias at the ferromagnetic (FM)/antiferromagnetic (AFM) interface are of great interest and have received significant attention to induce new modulated spin structures in conventional simple FM materials. In particular, soft magnetic under layer (SUL) with strong synthetic antiferromagnetic (SAF) coupling between two adjacent soft magnetic layers, exchange coupled stacked media introducing exchange coupling between FM layers and giant exchange anisotropy at the FM/AFM interface have attracted significant attention from the view point of applications. Within the framework of the present paper, we discuss future technical trends for SUL, granular media and the spin-valve head from the viewpoint of direct and/or indirect exchange coupling based on our recent results.     

Depth magnetization profile of a perpendicular exchange coupled system by soft-x-ray resonant magnetic reflectivity

The magnetic profile across the interface of a perpendicular exchange coupled [NiO/CoO]3/Pt-Co/Pt(111) system is investigated. The magneto-optic Kerr effect reveals a strong coupling between the antiferromagnetic (AFM) oxide and the ferromagnetic (FM) Pt-Co layer, by an increasing coercivity and a rotation of the easy magnetization axis of the FM layer along the AFM spins. Soft x-ray resonant magnetic reflectivity is used to probe the spatial distribution of the out-of-plane magnetization inside the oxide above its ordering temperature. It extends over 1 nm and exhibits a change of sign.     

Temperature dependence of exchange bias and coercivity in ferromagnetic/antiferromagnetic bilayers

A model for the temperature dependence of exchange bias and coercivity in epitaxial ferromagnetic (FM)/ antiferromagnetic (AFM) bilayers is developed. In this model, the interface coupling includes two contributions, the direct coupling and the spin-flop coupling. The temperature dependence arises from the thermal disturbance to the system, involved in the thermal fluctuations of magnetization of AFM grains and the temperature modulation of the relevant magnetic parameters. In addition, the randomness of original orientations of easy axes of AFM grains after field cooling is taken into account. A self-consistent calculation scheme is proposed and numerical treatment is carried out. The results show that the temperature dependence of exchange bias and coercivity is closely related to the sizes of AFM grains and the interface exchange coupling constants. Especially, the exchange bias will have a peak and the blocking temperature will increase if the spin-flop coupling plays a role. On the other hand, the original orientation distribution of easy axes of AFM grains will affect exchange bias and coercivity prominently. The prediction has been well supported by experiments.Received: 12 May 2004, Published online: 31 August 2004PACS: 75.30.Et Exchange and superexchange interactions - 75.50.Ee Antiferromagnetics - 75.30.Gw Magnetic anisotropy     

Recent advances in exchange bias of layered magnetic FM/AFM systems

The exchange bias(EB) has been investigated in magnetic materials with the ferromagnetic(FM)/antiferromagnetic(AFM) contacting interfaces for more than half a century.To date,the significant progress has been made in the layered magnetic FM/AFM thin film systems.EB mechanisms have shown substantive research advances.Here some of the new advances are introduced and discussed with the emphasis on the influence of AFM layer,the interlayer EB coupling across nonmagnetic spacer,and the interlayer coupling across AFM layer,as well as EB related to multiferrioc materials and electrical control.     

交换偏置双层膜中的反铁磁自旋结构及其交换各向异性

研究了交换偏置双层膜中界面存在二次以及双二次交换耦合下反铁磁磁矩转动及其交换各向异性.结果表明，其反铁磁膜中的磁矩转动存在可逆“恢复行为”、不可逆“半转动行为”、不可逆“倒转行为”以及不可逆“半倒转行为”四种情形，四种情形的出现强烈地依赖于界面二次、双二次耦合以及反铁磁膜厚度.其中可逆恢复行为情况下，系统出现交换偏置，而不可逆的半转、半倒转以及倒转情形，系统不出现交换偏置.特别地,在界面处仅存在双二次耦合的情形下，其界面双二次耦合常数J₂≤0.1 σ关键词： 反铁磁自旋结构 交换各向异性 界面双二次耦合 交换偏置     

Exchange bias and domain evolution at 10 nm scales

For a fixed 2 μm×2 μm area of a Co/Pt-CoO perpendicular exchange bias system we image the ferromagnetic (FM) domains for various applied fields with 10-nm resolution by magnetic force microscopy (MFM). Using quantitative MFM we measure the local areal density of pinned uncompensated spins (pinUCS) in the antiferromagnetic (AFM) CoO layer and correlate the FM domain structure with the UCS density. Larger applied fields drive the receding domains to areas of proportionally higher pinUCS aligned antiparallel to FM moments. The data confirm that the evolution of the FM domains is determined by the pinUCS in the AFM layer, and also present examples of frustration in the system.