成分和厚度的依赖

通过调整Mn的成分，系统地研究了Ni81Fe19/Ni100-xMnx双层膜的磁学性质，特别是交换偏置场(Hex)的变化.当Ni100-xMnx中Mn的原子百分比在534%到600%之间时，对于150nm的Ni81Fe19，得到了最大的交换偏置场175kA/m，同时由于Mn对Ni81Fe19层的扩散所造成的磁矩的降低小于20%；高角x射线衍射证明Ni100-xMnx的晶格常数随着Mn成分的改变而变化，Mn含量越多，其晶格常数越大；制备态Ni100-xMnx膜晶格常数与θ相NiMn膜晶格常数的接近程度与NiMn膜θ相形成的容易程度相对应.也研究了交换偏置场随着Ni１００-xMnx厚度的变化，第一次得到了当Ni100-xMnx中Mn的原子百分比为706%时，Ni81Fe19(150nm)/Ni100-xMnx(90nm)双层膜在经过240℃，5h退火后，可以有80kA/m的交换偏置场，此时铁磁层磁矩的大小几乎不变. 关键词： Ni81Fe19/Ni100-xMnx 交换偏置场     

Training effect in an exchange bias system: the role of interfacial domain walls

Polarized neutron reflectivity (PNR) is used to obtain the magnetic depth profile of an antiferromagnetically coupled ferrimagnetic/ferrimagnetic bilayer, Gd40Fe60/Tb12Fe88. This system shows a transition from positive to negative exchange bias field H(E) as the cooling field H(cf) is increased from small to large positive value. It also exhibits training behavior upon field cycling which affects H(E) and the coercive field H(C). From the PNR measurements at room temperature and at 15 K, we confirm that the magnetic configuration inside the TbFe layer is frozen when the sample is cooled in various H(cf). The thickness and pitch of the magnetic twist inside the TbFe layer depend on H(cf) and give rise to the observed differences in the bias field. Irreversible reorganization of the TbFe magnetization at the interface occurs upon GdFe magnetization reversal and is found to explain the training effect as well as the overall reduction in coercivity.     

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.     

基于IrMn/Fe/Pt交换偏置结构的无场自旋太赫兹源

与目前商用的太赫兹源相比,自旋太赫兹源具有超宽频谱、固态稳定以及成本低廉等优点,这使其成为下一代太赫兹源的主要研究焦点.但使用自旋太赫兹源时,通常需要外加磁场使铁磁层的磁化强度饱和,才能产生太赫兹波,这制约了其应用前景.基于此,本文制备了一种基于Ir Mn/Fe/Pt交换偏置结构的自旋太赫兹波发生器,通过Ir Mn/Fe中的交换偏置场和Fe/Pt中的超快自旋流注入与逆自旋霍尔效应相结合,在无外加磁场下产生了强度可观的太赫兹波.在Ir Mn和Fe的界面中插入超薄的Cu,可以使Fe在厚度很薄时零场下实现饱和磁化,并且其正向饱和场最高可达–10 m T,从而进一步提升无场下的太赫兹发射效率.零场下出射的太赫兹波的动态范围超过60 d B,达到可实用化的水平.通过旋转样品,发现产生的太赫兹波的偏振方向也会随之旋转,并且始终沿着面内垂直于交换偏置场的方向.此外,在此交换偏置结构的基础上,引入了一层自由的铁磁金属层Fe,设计了一种以Ir Mn/Fe/Pt/Fe为核心结构的自旋阀太赫兹源,发现产生的太赫兹强度在两层铁磁层反平行排列时比平行排列以及不引入自由铁磁金属层时均大约提升了40%.结果表明,基...     

Exchange bias with Fe substitution in LaMnO<Subscript>3</Subscript>

We observe the negative shift of the magnetic hysteresis loop at 5 K, while the sample is cooled in external magnetic field in case of 30% of Fe substitution in LaMnO₃. The negative shift and training effect of the hysteresis loops indicate the phenomenon of exchange bias. The cooling field dependence of the negative shift increases with the cooling field below 7.0 kOe and then, decreases with further increase of cooling field. The temperature dependence of the negative shift of the hysteresis loops exhibits that the negative shift decreases sharply with increasing temperature and vanishes above 20 K. Temperature dependence of dc magnetization and ac susceptibility measurements show a sharp peak (T_p) at 51 K and a shoulder (T_f) around 20 K. The relaxation of magnetization shows the ferromagnetic and glassy magnetic components in the relaxation process, which is in consistent with the cluster-glass compound.     

Interfacial resonance state probed by spin-polarized tunneling in epitaxial Fe/MgO/Fe tunnel junctions

The direct impact of the electronic structure on spin-polarized transport has been experimentally proven in high-quality Fe/MgO/Fe epitaxial magnetic tunnel junctions, with an extremely flat bottom Fe/MgO interface. The voltage variation of the conductance points out the signature of an interfacial resonance state located in the minority band of Fe(001). When coupled to a metallic bulk state, this spin-polarized interfacial state enhances the band matching at the interface and therefore increases strongly the conductivity in the antiparallel magnetization configuration. Consequently, the tunnel magnetoresistance is found to be positive below 0.2 V and negative above. On the other hand, when the interfacial state is either destroyed by roughness-related disorder or not coupled to the bulk, the magnetoresistance is almost independent on the bias voltage.     

Interface induced exchange bias effect in La0.67Sr0.33MnO3/SrIrO3 multilayer

Epitaxial superlattices of ferromagnetic/paramagnetic La_0.67Sr_0.33MnO₃/SrIrO₃ materials have been prepared on SrTiO₃ (100) substrate using pulse laser deposition technique. An unexpected onset of interface magnetic interaction has been observed around 40 K. Interestingly, magnetic exchange bias effect has been observed in both field cooled and zero field cooled magnetization loops, however, the shifting of loop is opposite in both measurements. Exchange bias field vanishes as temperature increases to interface magnetic ordering temperature. Moreover, exchange bias field is found to decrease with increasing cooling field. We believe that tuning of magnetic exchange at interface during field cooling induces this evolution in nature of exchange bias field.     

Effect on alloying at the Fe/Ni(001) interfaces on the interlayer exchange coupling

We investigate the stability of various ordered FeNi alloys at the interfaces of Fe/Ni superlattices by using ab initio density functional calculation. We consider an Fe_0.5Ni_0.5 ordered alloy of one or two monolayers thick at different positions beyond the interface and the possibility of an interdiffusion of a complete monolayer of Ni(Fe) in Fe(Ni) slab. An interfacial atomic layer of Fe_0.5Ni_0.5 exchanged with its adjacent Ni monolayers, leading to a buffer zone of Ni₃Fe composition is found to be the most stable structural configuration. For this atomic arrangement we investigate the magnetic profile and the resulting interlayer exchange coupling between the Ni slabs for Fe spacer thickness of 0 to 4 monolayers.     

Uncompensated moments in the MnPd/Fe exchange bias system

The element-specific magnetic structure of an epitaxially grown Mn_52Pd_48/Fe bilayer showing exchange bias was investigated with atomic-layer depth sensitivity at the antiferromagnet/ferromagnet interface by soft-x-ray magnetic circular dichroism and magnetic reflectivity. A complex magnetic interfacial configuration, consisting of a 2-monolayer-thick induced ferromagnetic region, and pinned uncompensated Mn moments that reach far deeper (approximately 13 A), both in the antiferromagnet, were found. For the latter, a direct relationship with the magnitude of the exchange bias is verified by similar measurements perpendicular to the field cooling direction.     

Perpendicular interlayer coupling in Ni80Fe20/NiO/Co trilayers

An in-plane perpendicular magnetic coupling between Ni80Fe20 and Co has been found in NiFe/NiO/Co trilayers for a NiO thickness ranging from 4 to 25 nm by magneto-optical Kerr effect and x-ray magnetic circular dichroism measurements. In the easy magnetization direction of the Co layer, the Co coercive field H(C) increases when the thickness of the NiO layer t(NiO) increases. Because of the coupling, H(C) is always larger than for NiO/Co bilayers with the same thicknesses. The saturation field of the NiFe layer H(S) decreases when t(NiO) increases, indicating a weakening of the coupling. Numerical simulations show that the presence of interface roughness combined with a small value of the NiO anisotropy can explain the observed 90 degrees coupling.     

Study of the Fe−Ni interface by conversion electron Mössbauer spectroscopy

Nickel-gamma iron interfaces obtained by growing films of Fe on Ni substrates are studied using CEMS. We have found that a) intermixing is confined to two atomic layers, b) Fe atoms deposited on structural defects of the Ni (111) surfaces are magnetically polarized, c) Fe epitaxially grown on Ni is highly susceptible at room temperature. Moreover the Ni−Fe interface shows Neel-type magnetic anisotropy.     

Structure and magnetic properties of Fe/Fe oxide clusters

Fe clusters have been synthesised in ultra-high-vacuum chamber using a gas-stabilized cluster aggregation method that ensures good control of the cluster size and naturally oxidized in order to obtain Fe/Fe oxide core-shell nanoparticles. The morphology of an individual nanoparticle, as revealed by transmission electron microscopy, consists of a Fe core of an average diameter of 4.4 nm surrounded by an oxide shell of uniform thickness of about 1.2 nm in average. The nanoparticles may be assimilated with a ferro-/antiferromagnetic (FM/AF) system. The morpho-structural features have been correlated with magnetic measurements on the core-shell nanoparticles. A significant exchange bias effect has been measured, when the sample was field-cooled under an applied field of 3 T. As the morphology of core-shell nanoclusters is much more complicated than in FM/AF bilayers of regular thickness due to the particular geometry of the coronal AF layer, the shape and surface anisotropy have to be taken into account for a correct interpretation of the magnetic data.     

Structural and Magnetic Properties of [Fe/Ni]N Multilayers

[Fe/Ni]N multilayered structure grows epitaxially on the single crystalline MgO substrate. Due to the different directions of magnetic easy axes of Fe and Ni and the strong strain, large anisotropy dispersion is assumed. According to the layer model, the magnetization of Fe and Ni layers cannot follow each easy axis because of exchange coupling, and then the anisotropies are averaged out. The reduction of the effective anisotropy enhances with the decrease of periodic thickness. Thus, the coercivity of [Fe/Ni]N multilayers reduces with decreasing periodic thickness.     

Mössbauer study of the invar Fe–Ni and Fe–Ni–C alloys in magnetic field

F.C.C. Fe–30.3%Ni and Fe–30.5%Ni–1.5%C (wt.%) alloys were studied by means of Mössbauer spectroscopy in external magnetic field B _ext?=?2.5, 5, 7 T parallel to the gamma-beam. It is shown that distribution of effective magnetic field in the alloys is broad and that carbon expands the range of B _eff. The external magnetic field increases B _eff in the Fe–Ni alloy and decreases it more evidently in the Fe–Ni–C alloy. Antiferromagnetic spin coupling along the ferromagnetic component is proposed to explain data.     

Exchange bias and training effect in Ni/Ag-doped NiO bilayers

A series of polycrystalline Ag-doped Ni_1−xAg_xO/Ni bilayers with x up to 0.2 were prepared by magnetron sputtering. X-ray diffraction, atomic force microscopy and transmission electron microscopy analyses reveal that Ag doping significantly reduces the mean NiO grain size and leads to the appearance of Ag nanoparticles on the surface of the Ag-doped NiO films. As x increases, the exchange bias field and coercivity at room temperature decrease as a consequence of the reduced thermal stability of smaller NiO grains and the screening effect resulting from the interfacial Ag nanoparticles. At lower temperatures, a slight enhancement of the exchange bias field is observed in the Ag-doped sample, indicating that the Ag doping increases the uncompensated NiO spin density. In addition, our studies find that the training effect of the Ag-doped sample can be well described by a spin configurational relaxation model, regardless of the presence of Ag nanopartiles at the interface.     

Epitaxial growth and magnetic exchange anisotropy in Fe3O4<Emphasis Type="Bold">/NiO bilayers grown on MgO(001) and Al</Emphasis>2O3(0001)

Epitaxial Fe3O4/NiO bilayers were epitaxially grown on MgO(001) and Al2O3(0001) substrates to investigate the influence of the fully spin compensated (001) and the non-compensated (111) NiO interface planes between the ferromagnetic (F) and antiferromagnetic (AF) layers on the AF/F exchange coupling. Bilayers of different magnetite thicknesses and constant NiO thickness were investigated. The structural characterizations indicate a perfect epitaxy of the two layers for the both growth directions in the two Fe3O4/NiO/MgO(001) and NiO/Fe3O4/Al2O3(0001) systems. An epitaxial ferrimagnetic (Ni,Fe)Fe2O4 phase is observed at the AF/F interface when the NiO oxide is grown on the top of the Fe3O4 layer while a perfectly flat AF/F interface is observed in the Fe3O4/NiO/MgO(001) system exhibiting only a very slight interdiffusion. Magnetic measurements indicate a relative strong bias at 300 K for the bilayers grown on Al2O3(0001), which decreases with the inverse of the ferrimagnetic layer thickness as theoretically expected. On the contrary, a zero exchange biasing is observed at 300 K for the bilayers grown on MgO(001).     

A study on electrodeposited Ni x Fe1−x alloy films

Ni _x Fe_1−x (0.22 ≤x ≤ 0.62) alloy films were grown by electrodeposition technique. A shift in diffraction peaks of NiFe and Ni₃Fe was detected with increasing Ni content. The highest positive magnetoresistance ratio was detected as 5% in Ni0.₅₁Fe0.₄₉. Positive and negative anisotropic magnetoresistance were observed in longitudinal and transverse geometries respectively. The highest anisotropic magnetoresistance ratio of 9.8% was also detected in Ni0.₅₁Fe0.₄₉. The angular variation of magnetoresistance was studied. Magnetisation loop curves show that NiFe alloy films have a linear decreasing anisotropy constant with increasing Ni deposit content and show a decreasing behavior of coercivity which indicates soft magnetic property with increasing Ni deposit content     

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.     

Co/Co3O4/PZT多铁复合薄膜的交换偏置效应及其磁电耦合特性

本文采用溶胶-凝胶工艺并结合脉冲激光沉积技术, 在Pt/Ti/SiO₂/Si衬底上制备了Co/Co₃O₄/PZT多铁复合薄膜. 对复合薄膜的微结构和组分进行了表征, 并系统研究了复合薄膜中的交换偏置效应及其对磁电耦合作用的影响. 研究结果表明, 复合薄膜在77 K具有明显的交换偏置效应, 交换偏置场达到80 Oe, 且交换偏置场及矫顽场均随温度降低而增大. 当温度降低到10 K时, 交换偏置场增至160 Oe. X射线光电子能谱(XPS)测试结果证实在Co和Co₃O₄界面处存在约5 nm厚的CoO层, 表明77 K下的交换偏置效应源自反铁磁的CoO层对Co的钉扎作用. 观察到复合薄膜的电容-温度曲线随着外加磁场大小和方向的改变而呈现出规律性的变化, 表明复合薄膜存在磁电耦合效应. 进一步研究发现, 在低温下复合薄膜呈现出各向异性的磁电容效应, 与磁场大小和方向密切相关. 复合薄膜的这种磁电耦合特性主要与复合体系的交换偏置效应及基于界面应力传递的磁电耦合作用有关, 本文对其中的物理机理进行了详细讨论与分析.     

Study of the magnetism and the magnetic anisotropy of Fe layers in Ni/Fe/Ni(1 1 1) by Mössbauer spectroscopy

The hyperfine field and the magnetic anisotropy of a Fe layer as a function of thickness have been investigated in several Ni/⁵⁷Fe_x/Ni(1 1 1) trilayers with relatively thick Ni layers by Mössbauer spectroscopy. For Fe layers with thickness below 16 Å, the Mössbauer spectra show always the presence of two ferromagnetic phases with high-spin state. In the range between 6 and 8 Å, also a ferromagnetic phase with low-spin state and a paramagnetic phase have been found. The evolution of the mean hyperfine field of the ⁵⁷Fe nuclei is used to study the Fe growth. A structural FCCBCC phase transition is found to begin with an iron thickness of 8 Å. The easy direction of the magnetization is found out-of-plane for Fe interlayer with FCC structure, and perfectly in plane for Fe interlayer with BCC structure.