The structure dependence of exchange bias in ferromagnetic/antiferromagnetic bilayers

The structure dependence of exchange bias in ferromagnetic/antiferromagnetic （FM/AF） bilayers has been investigated in detail by extending Slonczewski＇s ＇proximity magnetism＇ idea. Here three important parameters are discussed for FM/AF bilayers, i.e. interracial bilinear exchange coupling J1, interracial biquadratic （spin-flop） exchange coupling J2 and antiferromagnetic layer thickness tAF. The results show that both the occurrence and the variety of the exchange bias strongly depend on the above parameters. More importantly, the small spin-flop exchange coupling may result in an exchange bias without the interracial bilinear exchange coupling. However, in general, the spin-flop exchange coupling cannot result in the exchange bias. The corresponding critical parameters in which the exchange bias will occur or approach saturation are also presented.     

PtMn层厚度对NiFe/PtMn双层膜交换偏置形成及热稳定性的影响

采用磁控溅射的方法制备了一组以(Ni_0.81Fe_0.19)_1-xCr_x作为缓冲层的NiFe/PtMn双层膜样品，研究了NiFe/PtMn双层膜的形成过程和热稳定性.实验表明，Cr成分的不同会引起NiFe/PtMn双层膜中PtMn层晶粒尺寸的不同，使NiFe/PtMn双层膜的交换偏置场与PtMn层厚度之间呈现不同的变化关系.热稳定性实验表明，PtMn晶粒尺寸较大的样品，出现交换偏置现象所需要的临界厚度较小，热稳定性好，这与Mauri的理论模型一致. 关键词： NiFe/PtMn双层膜 交换偏置场 热稳定性     

(Ni0.81Fe0.19)1－xCrx作为种子层对NiFe/PtMn双层膜交换偏置的影响

将NiFe/PtMn双层膜生长在(Ni_0.81Fe_0.19)_1-xCr_x种子层材料上，通过改变种子层中Cr的原子含量，系统的研究了NiFe/PtMn双层膜中PtMn晶粒尺寸和织构对交换偏置的影响.对退火270℃，5h后的NiFe/PtMn双层膜磁性的研究表明,PtMn织构强弱对交换偏置场的影响不明显，而PtMn的晶粒尺寸是影响交换偏置场的主要因素，PtMn颗粒的相干长度在11.3nm左右时得到了较大的交换偏置场. 关键词： NiFe/PtMn双层膜 交换偏置场     

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     

Asymmetric exchange bias training effect in spin glass(FeAu)/FeNi bilayers

A significant exchange bias(EB) training effect has been observed in sputter deposited FeAu/FeNi bilayers, wherein the exchange field(HE) exhibits a special sign-changeable temperature dependence. Very interestingly, despite the absence of multiple easy axes in the FeAu spin glass(SG) layer, HEdrops abruptly between the first and second magnetic cycles,which is followed by a more gradual continuous change in the subsequent cycles. This training behavior cannot be described by the empirical n-1/2law because of the asymmetric magnetization reversal processes. We propose modifying Binek’s model to include the asymmetric changes of the pinning SG spins at the descending and ascending branches. This new model successfully describes the EB training effect in FeAu/FeNi bilayers.     

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     

The influence of annealing on the bimodal distribution of blocking temperatures of exchange biased bilayers

The exchange bias effect at the interface between antiferromagnetic (AFM) and ferromagnetic (FM) layers is of paramount importance in state‐of‐the‐art spintronic devices. However, a complete account of the physics behind exchange bias remains elusive and new effects are constantly unraveled. In particular, a bimodal distribution of blocking temperatures $ (T_{\rm B})$ was recently discovered, associated with the bulk of the AFM layer and interfacial AFM/FM regions with spin‐glass‐like properties. Here we study exchange bias in MnIr (25, 60 Å)/CoFe (50 Å) bilayers annealed at high temperatures (623 K and 673 K). We observe, for all samples and annealing temperatures, the existence of a large exchange bias variation at low temperatures associated with interfacial disorder. Such variation is more significative in the thinnest samples, where it is found to be independent on annealing temperature. On the other hand, in the thickest samples the contribution of the low temperature distribution largely increases with annealing temperature, due to enhanced disorder arising from Mn diffusion. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

NiFe/FeMn双层膜交换偏置的形成及热稳定性研究

研究了在铁磁(NiFe)/反铁磁(FeMn)双层膜之间，交换偏置的形成过程和热稳定性，特别是NiFe/FeMn的交换偏置作用与FeMn层晶粒尺寸的关系.和以前作者不同的是，本文方法采用非磁性Ni-Fe-Cr合金作缓冲层材料，改变Cr的含量就可以获得不同晶粒尺寸的反铁磁FeMn层.实验表明，晶粒尺寸较小的FeMn产生较强的铁磁/反铁磁交换偏置场；但是，对于较大晶粒的FeMn层，出现交换偏置作用所要的临界厚度较小.这符合Mauri提出的理论模型.交换偏置场的热稳定性实验表明，具有较大晶粒尺寸的FeMn层给出较 关键词： 交换偏置 热稳定性 反铁磁 晶粒尺寸     

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.     

磁性物质交换Hamiltonian中两项的竞争

对于一个N电子体系, 正确的交换Hamilton应该由两项组成，为H_ex=-2A1ii·s_j-2A₂ii·sj，而不是以往的铁磁学理论使用的H_ex=-2Aii ·s_j （其中A为A₁与A₂的代数和， A₁>0, A₂<0）, 以往的理论使用了一个不合理的交换Hamiltonian量.-2A₁ii·s_j与-2A₂ii< /sub>·s_j在数学上是同类项，但是在物理上不是 同类项，它们有不同的本征态和本征值.根据量子力学中的态叠加原理，这个电子系统的本 征态矢为X〉=1A²₁+A²₂(A₁ 1〉+A₂‖2〉),其中Dirac符号1〉表示系统所有电子 的自旋平行排列时的态（简称平行自旋态）矢量，2〉表示系统所有电子或最近邻电子的自 旋反平行排列时的态（简称反平行自旋态）矢量，H_ex的本征值（即系统的 交换能） 为E=-Nz（A₁-A₂）-2NzA₂²A₁ +A₂=-Nz（A₂-A₁）-2NzA²₁A< sub>1+A₂,其中z为最近邻电子数.当A₂=0时，X〉=1〉，E =-A₁, 系统具有Wei ss 铁磁性；当A₁ =0 时，X〉=2〉，E =-A₂，系统具有Neel 反铁磁性；当A₁ =A₂（即A=0）时，X〉=12 (1〉+2〉)，E=-A₁，系统处于自旋玻璃（spin glass）态；当A₁>A ₂时，X〉=1A²₁+A²₂［(A₁-A₂)1〉+A₂(1〉+2〉)］,平行自旋态与自旋 玻璃态共存；当A₁2时，X〉=1A²₁+A2₂［(A₂-A₁)2〉+A₁( 1〉+2〉)］，反平行自旋态与自旋玻 璃态共存.与原来理论中的Weiss铁磁态或Neel反铁磁态相比，平行自旋态与自旋玻璃态共存 或反平行自旋态与自旋玻璃态共存使系统的交换能降低.自旋玻璃态中电子自旋之间取向的 随机性或无序性是由交换Hamiltonian中-2A₁iis_j与-2A₂ii·s_j之间的竞争引起的，不是热运 动引起的. 关键词： 交换哈密顿量 铁磁态 反铁磁态 自旋玻璃态     

应力各向异性对铁磁/反铁磁双层薄膜磁性质的影响

 采用铁磁共振方法,研究了交换各向异性和应力各向异性对铁磁/反铁磁双层薄膜性质的影响。结果表明：界面交换作用导致单向各向异性,应力各向异性对材料的磁化难易程度有较大影响。在外磁场方向接近应力场方向时,共振频率向高值方向移动,其它区域共振频率则向低值方向移动。对频率线宽而言,接近应力场方向,频率线宽加宽,其它区域频率线宽则变窄。此外,当磁场变化时,应力的存在使得共振频率向低值方向移动,尤其在β＝π方向情况较为复杂,在弱场范围出现了两个区域：即在某磁场范围内,共振频率向高值方向移动,且频率线宽加宽;而其它范围的共振频率（线宽）是向高值方向移动（加宽）还是向低值方向移动（变窄）,取决于外磁场的相对强弱。     

金属离子掺杂对CuO基纳米复合材料的交换偏置调控

为了研究反铁磁基体中掺杂的金属离子对交换偏置效应的影响, 本文采用非均相沉淀法制备了纳米复合材料. X射线衍射图(XRD)和场发射扫描电子显微镜(SEM) 照片清晰表明CuO纳米复合样品具有统一的颗粒尺寸, 约为80 nm. 通过体系中掺杂磁性金属离子Ni和Fe, 实现了亚铁磁MFe₂O₄ (M=Cu, Ni)晶粒镶嵌在反铁磁(AFM) CuO 基体中. 在CuO基体中加入少量的Ni能改变两相交界面的磁无序从而生成类自旋玻璃相, 相应提高对铁磁相磁矩的钉扎作用. 同时, 场冷过程中反铁磁相内形成磁畴, 冻结在原始状态或磁场方向上, 畴壁也起到钉扎铁磁自旋的作用, 进而提高交换偏置效应. 随后加入的Ni 会生成各向异性能较大的NiO, 也能够提高交换偏置场. 在带场冷却下, 所有样品均发生垂直交换偏置, 也证明了样品在场冷过程中形成了自旋玻璃相, 正是由于亚铁磁与自旋玻璃相界面上的磁交换耦合, 才导致回线在整个测量范围内发生了向上偏移. 零场冷却和场冷却(ZFC/FC)情况下磁化强度与温度变化曲线(M-T)说明在这些复合材料中的交换偏置效应是由于存在亚铁磁颗粒和类自旋玻璃相界面处的交换耦合作用. 研究发现随着持续掺杂Ni离子, 交换偏置场先缓慢增加后又急剧增加, 生成各向异性能高的反铁磁相NiO 和反铁磁相内的畴态组织是这一结果的原因.     

Thickness dependence of positive exchange bias in ferromagnetic/antiferromagnetic bilayers

For the ferromagnetic (FM)/antiferromagnetic (AFM) bilayers, both negative and positive exchange bias H_E have been observed for low and high cooling field H_CF, respectively. The thickness dependence of H_E and coercivity H_C have been investigated for the cases of negative and positive H_E. It is found that the negative H_E and the positive one have similar FM thickness dependence that is attributed to the interfacial nature of exchange bias. However, the AFM thickness dependence of positive H_E is completely contrary to that of the negative one, which clearly demonstrates that the AFM spins play different roles for the cases of positive and negative H_E. In particular, the AFM thickness of positive H_E was first highlighted by an AFM spin canting model. These results should be attributed to the interfacial spin configuration after field cooling procedure.     

Tuning exchange bias through zero field cooling from different remanent states above blocking temperature in Ni50Mn36Sb14 alloy

Changing remanent states above blocking temperature (T_B) in Ni₅₀Mn₃₆Sb₁₄ alloy has been proven to be an effective way of tuning the value and sign of exchange bias (EB) field. The hysteresis loops at 5 K exhibit double shifted shape, indicating that there are two opposite EB signs resulting from an imprint of domain pattern of ferromagnetic (FM) regions into anti‐ferromagetic (AFM) ones during cooling. All the results demonstrate that the interfacial spin configuration plays a crucial role on the origin of EB, while the high cooling field not only induces a single FM domain state above T_B but also tunes the fractions of FM and AFM interactions through martensitic transition. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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.     

Thermal relaxation of exchange bias field in an exchange coupled CoFe/IrMn bilayer

This paper reports that a CoFe/IrMn bilayer was deposited by high vacuum magnetron sputtering on silicon wafer substrate; the thermal relaxation of the CoFe/IrMn bilayer is investigated by means of holding the film in a negative saturation field at various temperatures. The exchange bias decreases with increasing period of time while holding the film in a negative saturation field at a given temperature. Increasing the temperature accelerates the decrease of exchange field. The results can be explained by the quantitative model of the nucleation and growth of antiferromagnetic domains suggested by Xi H W et al. [2007 Phys. Rev. B 75 014434], and it is believed that two energy barriers exist in the investigated temperature range.     

铁磁共振法测磁各向异性

采用铁磁共振方法,研究了铁磁/反铁磁双层膜系统中,因交换耦合以及磁晶各向异性而产生的有效各向异性场.结果表明:被测系统有无交换偏置场以及其正负号性质等均能在共振谱中得到辨析.结果还显示:沿着不同结晶方向施加外磁场,共振场的行为与磁晶各向异性以及铁磁/反铁磁交换耦合作用而诱发的单向各向异性等密切相关.将共振频率的变化看成外磁场(包括其方向和大小)的函数,研究得到了单向各向异性,立方各向异性等对共振频率的影响,并同实验结果做了很好的比较. 关键词： 铁磁/反铁磁双层膜 交换耦合 铁磁共振 单向各向异性     

Co/CoO双层膜的各向异性磁电阻与交换偏置行为研究

本文系统研究了Co/CoO双层膜的各向异性磁电阻(AMR)与交换偏置行为,并给出了外加微扰场对交换偏置磁锻炼效应恢复程度影响的实验结果.结果表明磁锻炼效应发生后,施加0.15T的倾斜微扰场可致使铁磁畴分裂进而诱导磁锻炼效应的恢复,揭示磁锻炼效应恢复程度与微扰场的角度有紧密关联.在微扰场作用下FM自旋在两个方向分裂,一部分自旋沿微扰场方向,另一部则被冷却场的AFM自旋钉扎住而沿原来方向不变.当微扰场和冷却场夹角大于30°时,FM畴被分裂,磁锻炼效应开始恢复,表明一个磁畴内的铁磁自旋偏离夹角最大为30°,而磁锻炼效应发生后,部分AFM自旋偏离冷却场方向的角度则小于30°,实验结果与相应的理论计算结果一致.此外,恢复程度随微扰场角度的增加而增加,最大恢复程度时角度为90°.同时,磁锻炼效应的恢复增加了交换偏置值,为器件设计提供理论支持,在自旋电子学基础和应用研究方面具有重要的指导意义.     

The synthesis and exchange bias effect of monodisperse NiO nanocrystals

Monodisperse NiO nanocrystals with an average particle size of 3 ± 0.4 nm are successfully synthesized by the thermal decomposition of Ni-oleylamine complex in an organic solvent under a continuous O2 flux. The crystalline structure and the morphology of the product are investigated by X-ray diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy. Magnetization and alternating-current (ac) susceptibility measurements indicate that the structure of the particles can be considered as consisting of an antiferromagnetically ordered core and a spin-glass-like surface shell. In addition, both the exchange bias field and the vertical magnetization shift can be observed in this system at 10 K after field cooling. This observed exchange bias effect is explained in terms of the exchange interaction between the antiferromagnetic core and the spin-glass-like shell.