铁磁/反铁磁双层膜中自旋波交换偏置的理论与实验分析

考虑铁磁/反铁磁界面粗糙度的随机性及局域外单向的界面能的随机性,考虑原子尺度在交换偏置强度上的至关作用,提出随机场效应概念.对(Fe/FeF2)研究表明,交换偏置和自旋波线宽频率在铁磁/反铁磁双层界面是依赖于外部磁场的.应用玻恩近似,计算了交换偏置对随机场的依赖,由于界面的粗糙度,证明了在反铁磁层出现时在界面自旋波线宽的增加,但自旋波随机场线宽随着外磁场增加而减小,在较大的外场下,从实验能观察到扩展的峰.     

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.     

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.     

铁磁球/反铁磁纳米体系磁滞回线的Monte Carlo模拟

利用经典Heisenberg模型和Monte Carlo方法研究外磁场和反铁磁磁晶各向异性、交换相互作用对铁磁球均匀嵌入到反铁磁基体中的铁磁/反铁磁纳米体系磁滞回线的影响.模拟结果显示,外加反向最大磁场不同时,磁滞回线形状不同.当磁场正向增加时,体系的磁化强度会产生一个跃变,但跃变高度与反向场最大值无关.反铁磁磁晶各向异性越大,体系的交换偏置现象越明显,且磁化强度回到饱和值所需的外磁场越大.随着反铁磁基体交换相互作用的增大,在正向和负向磁场区域还可能出现新的磁滞现象.     

外应力场下双层铁磁薄膜中的铁磁共振性质

用铁磁共振方法得到了双层铁磁薄膜的色散关系解析表达式.发现共振场依赖于层间耦合强度和应力场.假定层间为反铁磁性耦合，且铁磁层Ａ有较强的平面内各向异性.随着外磁场的增强，铁磁层Ｂ中的磁化强度突然由最初的反平行转为平行，从而导致色散曲线的阶跃，并且发现光学模阶跃幅度比声学模大.随着应力场的增强，Ｂ层中磁化强度反转所需的外磁场减弱.此外，在不同的交换耦合强度和应力场下，光学模共振场对外磁场方向的依赖性较强. 关键词： 双层铁磁薄膜 界面相互作用 应力各向异性场 铁磁共振     

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

采用一种新的种子层材料:(Ni081Fe019)1-xCrx,通过改变种子层中Cr原子的含量,使得在其上生长的NiFeFeMn双层膜的织构和晶粒尺寸产生极大的差异,系统研究了NiFeFeMn双层膜中FeMn晶粒尺寸和织构对交换偏置的影响.实验结果表明,在FeMn的γ相(111)织构较好的前提下,交换偏置场的大小与织构的差异没有关系;FeMn的晶粒尺寸对交换偏置场有很大影响,较小的反铁磁层晶粒对交换偏置场有利,过大的反铁磁层晶粒不利于交换偏置场.将(Ni081Fe019)05Cr05与传统的种子层材料Ta进 关键词： 交换偏置 晶粒尺寸 织构 种子层     

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.     

Ni81Fe19/Cr82Al18双层膜中的交换偏置

研究了直流磁控溅射法制备的Ni₈₁Fe₁₉/Cr₈₂Al₁₈双层膜中的交换耦合.样品的室温矫顽力与1/t^{3/2_FM(t_FM为铁磁层厚度）近似成正比例关系,从而表明在Ni₈₁Fe₁₉/Cr₈₂Al₁₈中交换耦合为铁磁/反铁磁界面的随机相互作用.另外还讨论了反铁磁层厚度对交换偏置的影响. 关键词：}     

退火对IrMn基磁隧道结多层膜热稳定性的影响

采用磁控溅射方法制备了结构为IrMn/CoFe/AlO_x/CoFe的磁性隧道结多层膜,样品置于真空磁场中进行退火处理. 将在不同温度退火的磁隧道结结构多层膜置于负饱和场中等待,研究退火温度对样品热稳定性的影响. 结果表明:退火提高了多层膜反铁磁层的单轴各向异性能,增加了样品的交换偏置;随着负饱和场等待时间的延长,被钉扎层的磁滞回线向正场偏移,交换偏置单调减小,但退火减弱了这种趋势. 关键词： 磁隧道结 交换偏置 磁化反转     

具有垂直各向异性(Pt/Co)n/FeMn多层膜的交换偏置

采用磁控溅射方法制备了以Pt为缓冲层和保护层的具有垂直各向异性(Pt/Co)n/FeMn多层膜.研究结果表明,多层膜的垂直交换偏置场Hex和反铁磁层厚度的关系与其具有平面各向异性的交换偏置场随反铁磁层厚度变化趋势相近.随着铁磁层调制周期数的增加,垂直交换偏置场Hex相应减小,并且与铁磁层的调制周期数近似成反比关系.(Pt/Co)3/FeMn的垂直交换偏置场Hex已经达到22.3kA/m.为了进一步提高Hex,在Co/FeMn的界面插入Pt层,当Pt层厚度为0.4nm时,Hex达到最大值39.8kA/m.     

Revealing the origin of the vertical hysteresis loop shifts in an exchange biased Co/YMnO(3) bilayer

We have investigated exchange bias effects in bilayers composed of the antiferromagnetic o-YMnO(3) and ferromagnetic Co thin film by means of SQUID magnetometry, magnetoresistance, anisotropic magnetoresistance and the planar Hall effect. The magnetization and magneto-transport properties show pronounced asymmetries in the field and magnetization axes of the field hysteresis loops. Both exchange bias parameters, the exchange bias field H(E)(T) as well as the magnetization shift M(E)(T), vanish around the Néel temperature T(N)???45?K. We show that the magnetization shift M(E)(T) is also measured by a shift in the anisotropic magnetoresistance and planar Hall resistance having a similar temperature dependence as the one obtained from magnetization measurements. Because the o-YMnO(3) film is highly insulating, our results demonstrate that the M(E)(T) shift originates at the interface within the ferromagnetic Co layer. To show that the main results obtained are general and not because of some special characteristics of the o-YMO(3) layer, similar measurements were done in Co/CoO micro-wires. The transport and magnetization characterization of the micro-wires supports the main conclusion that these effects are related to the response of the ferromagnetic Co layer at the interface.     

铁磁/反铁磁双层膜中的磁锻炼效应

采用Monte Carlo 方法,研究铁磁/反铁磁双层膜中的磁锻炼效应.结果表明,反铁磁层中冷场诱发的界面净磁化(钉扎效应)的磁弛豫可导致系统中的交换偏置场的磁锻炼效应.进一步研究表明,反铁磁层中掺杂可调控交换偏置场的磁锻炼效应,原因在于反铁磁层中掺杂能有效地改变冷场诱发的净磁化的磁弛豫过程.     

Exchange couplings in magnetic films

Recent advances in the study of exchange couplings in magnetic films are introduced.To provide a comprehensive understanding of exchange coupling,we have designed different bilayers,trilayers and multilayers,such as anisotropic hard/soft-magnetic multilayer films,ferromagnetic/antiferromagnetic/ferromagnetic trilayers,[Pt/Co]/NiFe/NiO heterostructures,Co/NiO and Co/NiO/Fe trilayers on an anodic aluminum oxide(AAO) template.The exchange-coupling interaction between soft-and hard-magnetic phases,interlayer and interfacial exchange couplings and magnetic and magnetotransport properties in these magnetic films have been investigated in detail by adjusting the magnetic anisotropy of ferromagnetic layers and by changing the thickness of the spacer layer,ferromagnetic layer,and antiferromagnetic layer.Some particular physical phenomena have been observed and explained.     

Magnetization reversal of elliptic Co/Si/Co nanodisks in the field of a magnetic-force microscope probe

The processes of local magnetization reversal of elliptic Co/Si/Co nanodisks under the action of a nonuniform magnetic field of a magnetic-force microscope (MFM) probe have been investigated. The specific features of the distribution of the phase MFM contrast from particles with ferromagnetic and antiferromagnetic configurations of the magnetic moments in neighboring Co layers have been discussed. It has been shown experimentally that, under the action of the probe field, there occur orientational transitions of two types: transitions from the ferromagnetic configuration to the antiferromagnetic configuration due to the reorientation of the magnetization of the upper layer and transitions in the antiferromagnetic configuration with a change in the orientation of the magnetic moment in both ferromagnetic layers. The presented results of micromagnetic simulation of the processes of transformation of the magnetization in such particles under the action of the MFM probe field explain the main regularities of the magnetization reversal processes.     

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.     

Field cooling induced changes in the antiferromagnetic structure of NiO films

The magnetic anisotropy in antiferromagnetic 500 A thick NiO films, before and after the establishment of an exchange bias field with Co84Fe16 ferromagnetic layers, was measured using magnetic linear dichroism in soft x-ray absorption. Both <111> textured NiO and untextured NiO films show exchange-bias induced in-plane magnetic anisotropy of nearly equal magnitude and with the Ni moment axis being nearly parallel to the exchange bias field direction. These results represent the first observation of the key step in the exchange biasing process, namely, repopulation of the antiferromagnetic domains whose magnetization axis is closest to the exchange bias field direction.     

Perpendicular exchange bias of Co/Pt multilayers

Exchange bias measurements of ferromagnetic/antiferromagnetic (F/AF) bilayers are typically performed with the magnetization of the F layer parallel to the AF interface. We describe measurements of Co/Pt multilayers with out-of-plane magnetic easy axis that are exchange biased with CoO. Field-cooling experiments with the applied field perpendicular and parallel to the sample plane exhibit loop shifts and enhanced coercivities. Modeling and comparison to biasing of samples with planar easy axis suggests such measurements provide a way to probe the spin projections at F/AF interfaces.     

Composition-controlled exchange bias training effect in FeCr/IrMn bilayers

For FeCr/IrMn bilayers, the exchange bias training effect and the magnetization reversal mechanism are correlated to each other and depend on the composition of the ferromagnetic layer. For high Fe contents, the asymmetric magnetization reversal is observed. During the training effect, both exchange field and coercivity decrease monotonically, suggesting a type I training effect. For low Fe contents, the domain wall depinning takes place for the two hysteresis loop branches. Only exchange field diminution happens in the training effect. The coercivity almost does not change in the process, corresponding to a type II training effect. It is suggested that the motion of antiferromagnetic spins is modified by the magnetization reversal mechanism in the ferromagnetic layer.     

Structural and magnetic properties of transition metal oxide/metal bilayers prepared by in situ oxidation

Metal oxide/metal bilayers have been prepared by in situ oxidation. Ultrathin layers with antiferromagnetic properties are formed at room temperature by controlled in situ exposure of clean Fe, Co and Ni layers to pure oxygen gas. X-ray diffraction, Auger electron spectroscopy and transmisson electron microscopy have been applied to provide a detailed analysis of the layered structure. The magnetic properties of these antiferromagnetic/ferromagnetic bilayers are characterized by an interfacial coupling (exchange bias) which drastically varies with the type of antiferromagnetic material.     

Magnetic Properties of Fe/Ni and Fe/Co Multilayer Thin Films

In this work, the magnetic and transport properties of Fe/SiO₂/Ni and Fe/SiO₂/Co multilayers grown on Si/SiO₂ substrates have been studied. The samples have been prepared by two-stage deposition process. In the first stage, Fe layer and SiO₂ interlayer of both samples are grown by ion beam deposition technique at room temperature. Then the samples are taken out to ambient atmosphere and loaded into a pulse laser deposition (PLD) chamber. Prior to the deposition of top layer, the samples are cleaned by annealing at 150 °C. In the second stage, Ni (or Co) layer is prepared by PLD technique at room temperature. The thickness of deposited layers has been measured by Rutherford back scattering (RBS). Magnetic properties of ferromagnetic bilayers have been investigated by room-temperature ferromagnetic resonance (FMR) and vibrating sample magnetometer (VSM) techniques. Standard four-point magneto-transport measurements at various temperatures have been performed. Two-step switching in the in-plane hysteresis loops of Fe/SiO₂/Ni and Fe/SiO₂/Co samples is observed. A crossing in the middle of hysteresis loops of both samples points to a weak antiferromagnetic interaction between the magnetic layers of the stacks. Saturation magnetization values have been obtained from the VSM measurements of samples with DC magnetic field perpendicular to the films surface. Magneto-transport measurements have shown the predominant contribution of anisotropic magnetic resistance both at room and low temperatures. FMR studies of Fe/SiO₂/Ni and Fe/SiO₂/Co samples have revealed additional non-uniform (surface and bulk SWR) modes, which behavior has been explained in the framework of the surface inhomogeneity model. An origin of the antiferromagnetic interaction has been discussed.