Exchange bias in patterned FeMn/NiFe bilayers

Electron beam lithography and ion beam etching have been used to pattern a wire-like array in FeMn/NiFe bilayers. The variation of hysteresis loops with the etching depth in FeMn layer has been presented, and it has been found that with increasing etching depth the coercivity increases and M–H loops show an asymmetric kink. Detailed studies of the magnetic behaviors of the asymmetric kink in the patterned sample with 3.5 nm thick FeMn layer have been performed, and a magnetization component perpendicular to the wire direction has been observed.     

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     

Structural analysis of ion irradiated polycrystalline NiFe/FeMn exchange bias systems

Structural investigations of the ion irradiated polycrystalline NiFe/FeMn exchange bias bilayer system were carried out by means of transmission electron microscopy. Key structural parameters like average grain size, lattice constant, and texture, as well as their dependence upon ion irradiation were determined. This information was extracted from a detailed analysis of a series of bright field images, dark field images, and diffraction patterns. Furthermore a previously established model was tested which ascribes changes in the magnetic properties upon irradiation with 5 keV He+ ions to the creation of point defects within the antiferromagnetic layer and to the intermixing between the ferromagnetic and antiferromagnetic layers. The obtained results indirectly support this model by excluding a change of the aforementioned structural parameters as a possible source of the observed modifications of the magnetic properties.     

Influence of the roughness on the exchange bias effect of NiFe/FeMn/NiFe trilayers

We have studied the effect of roughness on the exchanged biased NiFe/FeMn/NiFe trilayers system. The samples were prepared under three different argon working pressures (2, 5 and 10 mTorr) to obtain different roughness degrees. The root mean square roughness of the NiFe/FeMn interfaces enhances as the argon working pressure during the deposition increases from 2 to 10 mTorr. High-angle X-ray diffraction reveals that the samples have 1 1 1 texture and besides, possible changes in grain size could be an extra contribution to the interfacial roughness. Magnetometry measurements have shown that the coervive field enhances as the root mean square roughness of the NiFe/FeMn interfaces increases, while the dependence of the exchange bias field runs in the opposite way.     

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     

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.     

Hysteretic behavior of angular dependence of exchange bias in FeNi/FeMn bilayers

For FeNi/FeMn bilayers, the angular dependence of exchange bias shows hysteresis between clockwise and counterclockwise rotations, as a new signature. The hysteresis decreases for thick antiferromagnet layers. Calculations have clearly shown that the orientation of antiferromagnet spins also exhibits hysteresis between clockwise and counterclockwise rotations. This furnishes an interpretation of the macroscopic behavior of the ferromagnetic layer in terms of the thermally driven evolution of the magnetic state of the antiferromagnet layer.     

Magnetic exchange bias enhancement through seed layer variation in FeMn/NiFe layered structures

The exchange bias and crystalline texture of the multilayer structure (Ta/Al/seed/Fe₅₀Mn₅₀/Ni₈₁Fe₁₉/Al₂O₃/Ni₈₁Fe₁₉/Al/Ta with seed=Ni₈₁Fe₁₉ or Ni₈₁Fe₁₉/Cu) has been characterized. Measurements indicate an abrupt decrease in exchange bias of the Ni₈₁Fe₁₉ pinned layer for samples with very thin seed layers, and exchange bias as high as 325 Oe for thicker seed layers. Fluctuation of exchange bias with thickness was greatly reduced for the Ni₈₁Fe₁₉/Cu seed configuration. X-ray diffraction measurements demonstrate a correlation between exchange bias and strong (1 1 1) texture of FeMn. The results suggest a high sensitivity of Ni₈₁Fe₁₉ roughness and texture on deposition conditions, and corroborate previous observations of roughness in ultrathin NiFe films.     

(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     

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     

Exchange bias studies of NiFe/FeMn/NiFe trilayer by ion beam etching

Effect of low energy ion beam etching on exchange bias in NiFe/FeMn/NiFe trilayer is investigated in multilayers prepared by rf magnetron sputtering. Stepwise etching and magnetization measurement of FeMn layer in an NiFe/FeMn bilayer show increase of bias as etching proceeds and FeMn thickness decreases. The bias show a maximum around 7 nm FeMn thickness and then fall sharply below 5 nm, broadly in line with the exchange bias variation at increasing FeMn thickness but in reverse order, particularly at low FeMn thickness. Progressive etching of top NiFe layer in the NiFe/FeMn/NiFe trilayer shows an initial gradual increase in bias followed by a sharp increase below 7 nm thickness of top NiFe layer, with a maximum at 2 nm thickness for both NiFe layers and greater bias for seed NiFe layer.     

Co-Ni/FeMn双层膜中磁化强度对交换偏置的影响

固定CoNiFeMn双层膜中反铁磁层的厚度,改变CoNi铁磁层的成分来调节磁化强度,从而研究铁磁层的饱和磁化强度对CoNiFeMn双层膜中交换偏置的影响.研究表明,CoNiFeMn界面的交换耦合能U不是一个常量,而是随(MFM)12的增加而线性增加.其原因是铁磁层磁矩通过界面相互作用在反铁磁层中形成的局域交换磁场,在磁场冷却时影响反铁磁层的自旋结构或磁畴结构及双层膜中的交换偏置 关键词： 交换偏置 磁化强度     

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.     

Origin of the training effect and asymmetry of the magnetization in polycrystalline exchange bias systems

The training effect and asymmetry in exchange-coupled polycrystalline CoO/Co bilayers with in-plane magnetization has been investigated. This system is selected for its large training effect and initial asymmetry of the magnetic hysteresis after field cooling, which is removed after training. Applying an in-plane magnetic field perpendicular to the cooling field largely restores the untrained state with its pronounced asymmetry. The possibility to reinduce the asymmetry strongly depends on the magnitude of the perpendicular field, providing the key to identify the physical origin of training and removal of the asymmetry. These effects result from misalignment between the ferromagnetic magnetization and the uncompensated magnetization of the granular antiferromagnet.     

Reversing the training effect in exchange biased CoO/Co bilayers

We performed a detailed study of the training effect in exchange biased CoO/Co bilayers. High-resolution measurements of the anisotropic magnetoresistance (AMR) display an asymmetry in the first magnetization reversal process and training in the subsequent reversal processes. Surprisingly, the AMR measurements as well as magnetization measurements reveal that it is possible to partially reinduce the untrained state by performing a hysteresis measurement with an in-plane external field perpendicular to the cooling field. Indeed, the next hysteresis loop obtained in a field parallel to the cooling field resembles the initial asymmetric hysteresis loop, but with a reduced amount of spin rotation occurring at the first coercive field. This implies that the antiferromagnetic domains, which are created during the first reversal after cooling, can be partially erased.     

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.     

Suppression of interlayer segregation in spin-valve NiFe/Cu/NiFe/FeMn multilayers

Experimental results show that Cu atoms can float out to or segregate to the NiFe/FeMn interface for Ta/NiFe/Cu/NiFe/FeMn/Ta spin-valve multilayers, which results in a drop of the exchange-coupling field (H_ex) of NiFe/FeMn in the spin-valve multilayers. However, when a small amount of Bi atoms is deposited between the Cu and the pinned NiFe layers, Cu segregation to the NiFe/FeMn interface can be suppressed. At the same time, H_ex of NiFe/FeMn in the spin-valve multilayers with a Bi interfacial layer can be effectively increased. PACS 75.70.Cn; 82.80.Pv