反铁磁耦合软磁/硬磁双层膜体系的交换弹性反磁化特性

基于微磁学理论研究了软磁/硬磁反铁磁交换耦合双层结构体系的反磁化特性,利用一维原子链模型模拟了其反磁化过程。研究表明:当考虑了软磁层的磁晶各向异性能后,随着软磁层厚度的增大,交换弹性反磁化过程从可逆过程转变为不可逆过程。存在一个临界的软磁层厚度tc,当软磁层厚度tstc时,交换弹性反磁化过程为不可逆过程。形核场Hb随软磁层厚度的变化仅当体系的反磁化过程为可逆的交换弹性反磁化过程时才满足经验公式Hb=Hb0/tsn。     

NiFe/IrMn多层膜制备及MgO掺杂研究

采用磁控溅射制备了Ta/NiFe/IrMn/Ta薄膜,研究了反铁磁IrMn的溅射功率和铁磁层NiFe厚度对多层膜交换偏置场的影响。在反铁磁IrMn中插入MgO,发现MgO含量对交换偏置场有一定影响。随着MgO含量的增加,多层膜的交换偏置场逐渐增大,当MgO的含量约为2.5%交换偏置场达到最大值。随着MgO含量进一步增加交换偏置场下降。在IrMn中插入适量的MgO可以有效地增加交换偏置场。     

反铁磁层厚度对垂直磁各向异性钴/铂/铁锰多层膜磁性质的影响

在室温下用超高真空磁控溅射系统制备了一系列的Pt(4.0nm)/[Co(0.5nm)/Pt(0.3nm)]3-FeMn(tAFnm)多层膜样品,研究了反铁磁层厚度对于易轴垂直于样品表面的Co/Pt/FeMn多层膜磁性质的影响。在室温下利用样品的剩磁进行了X射线磁圆二色测量(XMCD),结果表明在铁磁/反铁磁界面有反铁磁层铁锰的净磁矩,这些净磁矩仅来自于铁元素。铁的磁矩倾向于垂直于膜面排列。磁测量结果表明,随着铁锰层厚度的增加,交换偏置场HEB增加直到饱和,而HC先增加,然后轻微减少,在tAF7.5nm以后,HEB和HC都几乎不变了。没有观察到磁锻炼效应。     

铁磁/反铁磁双层膜中的磁化性质与界面微结构

采用Monte Carlo模拟方法,通过反铁磁层中非磁性掺杂方式调节铁磁层与反铁磁层界面微结构,讨论了其界面微结构对体系的磁滞回线的不对称性、交换偏置场及矫顽场的影响.模拟结果显示:在同一掺杂浓度下,体系的磁滞回线的不对称性及交换偏置场强烈地依赖于掺杂方式,但矫顽场几乎不受影响,其相应的温度特性亦依赖于掺杂方式.它表明铁磁/反铁磁双层膜体系中的磁滞回线的不对称性以及交换偏置场与其界面微结构密切相关,同时实验上人们可通过界面微结构的改变获得交换偏置场大、矫顽场小且热稳定性好的自旋阀结构.     

磁性薄膜材料中的交换耦合

介绍了在磁性薄膜材料中的交换耦合的研究进展。制备了铁磁/反铁磁/铁磁3层结构不同成分的薄膜。利用X射线粉末衍射(XRD)、扫描电镜(SEM)、透射电镜(TEM)等测试分析技术,系统研究了磁性多层薄膜的相组成、界面及微观结构等。利用超导量子干涉仪(SQUID)研究薄膜的磁、电性能和交换耦合。在Co/反铁磁/Fe结构中发现了非常明显的与温度相关的铁磁/反铁磁界面耦合与铁磁/铁磁层间耦合之间的竞争效应。含有不同铁磁层Fe、Co、Fe20Ni80的3层膜FM1/Cr2O3/FM2对交换耦合随温度的变化存在较强的影响,发现铁磁层的磁晶各向异性和跟Cr2O3接触的自旋非对称性反射系数体系的界面和层间耦合有很大的影响。铁磁层对FM/AFM的交换耦合强度的影响甚大,这种影响和铁磁层的各向异性的相关性要强于和铁磁层饱和磁化强度的相关性。通过面内预加场和场冷的方式,在易轴互相垂直的[Pt/Co]n/NiFe/NiO异质结中实现了交换偏置的4种状态,可等温调控偏置。研究了Co/NiO反点阵列和连续膜的交换偏置,与连续膜相比,纳米反点阵列既能增大偏置,也能减小偏置,具有更高的热稳定性。     

纳米复合磁体的界面结构、交换耦合和反磁化的研究进展

纳米复合磁体的磁能积能得到大幅度提高,前提是晶粒之间存在良好的交换耦合作用,而交换耦合作用与软、硬磁相之间的界面密切相关。对Nd_2Fe_(14)B、Sm-Co、FePt基纳米复合磁体界面交换耦合和反磁化的研究展开论述。在不同的条件下,界面结构的匹配性、界面原子扩散、晶间的非晶相、界面非磁性层、界面晶格弛豫等可能有利于改善界面的结构、增强交换耦合作用,进而对反磁化过程产生影响。反磁化的不可逆过程主要发生在硬磁相内,但与软、硬磁相界面特性密切相关。不可逆反磁化在一定程度上决定了磁体的矫顽力,它可通过改善界面结构进行调控。本文旨在对纳米复合磁体界面的作用深入理解并期望能对磁体磁性能的优化提供参考。     

织构和界面粗糙度对NiFe/FeMn双层膜交换偏置场的影响

采用磁控溅射方法制备NiFe/FeMn双层膜（分别以Ta、Cu作为缓冲层，Ta作为保护层）。实验发现，以Ta为缓冲层的NiFe/FeMn双层膜的交换偏置场比以Cu为缓冲层的NiFe/FeMn双层膜的交换偏置场大，而矫顽力却很小。我们从织构、界面粗糙度两方面对其中的原因进行了分析。以Ta为缓冲层的NiFe/FeMn双层膜有好的织构且NiFe/FeMn界面较平滑，这引起了较强的交换偏置场和较低的矫顽力。     

Pt插层对NiFe/FeMn薄膜交换耦合的影响

采用磁控溅射方法制备了以Pt为缓冲层和保护层的NiFe/FeMn薄膜.在NiFe/FeMn界面插入Pt,发现交换偏置场(Hex)随着插层Pt厚度(tPt)的增加而减小.一个重要的现象是当Pt插层厚度为0.4nm时,在Het-tPt衰减曲线并非单纯指数下降,而是出现一个"凸起".通过对样品磁矩随Pt插层厚度的变化规律进行分析,发现随Pt插层厚度的增加,样品的磁矩先逐渐增大,然后又有所下降,并且稳定在某一值;表明在样品制备过程中,NiFe与FeMn之间的相互作用(如界面反应),使得在NiFe/FeMn界面存在磁死层,Pt的插入抑制了NiFe/FeMn界面磁死层的产生,有利于交换耦合;另一方面,Pt的插入隔离了NiFe和FeMn的直接接触,使得FeMn对NiFe的钉扎作用减弱,不利于交换耦合.两个方面的共同作用,使得当Pt插层为某一合适厚度时,Hex-tPt曲线出现"凸起".     

插层Bi对NiFe/FeMn双层膜交换偏置场的影响及其XPS分析

在Ta／Cu／NiFe／FeMn／Ta薄膜中,我们曾发现Cu在NiFe层的表面偏聚导致NiFe／FeMn薄膜的交换偏置场降低。为了抑制Cu的表面偏聚,我们在Ta／Cu／NiFe／FeMn／Ta薄膜中在Cu／NiFe界面沉积Bi插层。实验发现,沉积适当厚度的Bi插层可以将NiFe／FeMn双层膜的交换偏置场提高1倍。XPS分析表明,在Cu／NiFe界面沉积的插层Bi有效地抑制了Cu在NiFe表面的偏聚,提高了交换偏置场。     

基于NiMn的铁磁/反铁磁系统中交换偏置研究进展

NiMn合金薄膜作为一种高性能的反铁磁材料在巨磁电阻器件中具有重要的应用前景,引起了物理学及材料学等领域广大科研工作者的浓厚兴趣。综述了NiMn合金薄膜的基本性质,并对NiMn基铁磁/反铁磁双层膜的交换偏置的基本特征进行了分析。     

Broadband microwave absorption in [NiFe/FeMn]n exchange-coupled multilayer films

[NiFe/FeMn]_n exchange-coupled multilayer films have been fabricated on the silicon substrate by magnetron sputtering deposition. The static and dynamic magnetic properties of multilayer films have been investigated with varying numbers of layers. The results show that the linewidth and permeability of imaginary resonance peak are increased with increasing numbers of layers. For the NiFe/FeMn/NiFe sample, the resonance frequency shows a different shift with applying external magnetic field along the direction of easy and hard magnetization axis of the sample, respectively, indicating a different magnetic reversal process in two ferromagnetic layers. It proved that the increase of linewidth was originated from the different interface exchange coupling.     

Influence of metal spacer on the properties and microstructure of multilayer films and analyses

Ta/NiFe/nonmagnetic metal spacer/FeMn/Ta films were prepared by magnetron sputtering. The dependence of the exchange coupling field (H_ex) between the FeMn and NiFe layers on the thickness of a nonmagnetic metal spacer layers was investigated systematically. The results show that H_ex decreases rapidly with increasing thicknesses of the Bi and Ag spacer layers. It decreases gradually, however, with an increase in the thickness of the Cu spacer layer. We found empirically that H_ex corresponds to the lattice match between spacer layer atoms and NiFe layer atoms. However, the results of X-ray photoelectron spectroscopy show that when a small amount of Bi atoms are deposited on the NiFe/FeMn interface, they migrate to the FeMn layer surface and hardly influence H_ex.     

Effect of Cu surface segregation on properties of NiFe/FeMn bilayers

The films of NiFe/FeMn with Ta and Ta/Cu buffer layers were prepared by magnetron sputtering. Results show that the exchange bias field of NiFe/FeMn films with Ta/Cu buffer is lower than that of the films with Ta buffer. The crystalline texture, surface roughness and element distribution of these two sets of samples were examined, and there is no apparent difference for the texture and roughness. However, the segregation of Cu atoms on the surface of NiFe in the trilayer of Ta/Cu/NiFe has been observed by using the angle-resolved X-ray photoelectron spectroscopy. The decrease of the exchange bias field for NiFe/FeMn films with Ta/Cu buffer layers is mainly caused by the diffusion of Cu atoms through NiFe layer, which stayed at the interface of NiFe/FeMn film or even intruded into FeMn layer. The present results indicate that Cu segregation through NiFe layer should be suppressed in order to improve the exchange bias field in giant magnetoresistance spin valves with Cu spacer.     

不同缓冲层对NiFe/FeMn双层膜交换耦合场的影响

采用磁控溅射方法制备了NiFe/FeMn双层膜 ,分别以Ta ,Cu作为缓冲层 ,Ta作为保护层。实验发现 ,以Ta为缓冲层的NiFe/FeMn双层膜的交换耦合场比以Cu为缓冲层的NiFe/FeMn双层膜的交换耦合场大 ,而矫顽力却很小。本文分别从织构、界面粗糙度、界面偏聚等几方面对其中的原因进行了分析。除不同缓冲层引起的织构、界面粗糙度不同对交换耦合场有影响外 ,不同缓冲层引起的界面偏聚对交换耦合场也有影响     

Ion beam deposition and structural characterization of GMR spinvalves

NiO exchange-biased “bottom” spin valves of the type NiO/NiFe/Co/Cu/Co/NiFe and FeMn exchange-biased “top” spin valves of the type NiFe/Co/Cu/Co/NiFe/FeMn were deposited by ion-beam deposition (except the NiO layer). Their magnetic properties, magneto-transport, and microstructures are characterized and compared with corresponding GMR spin valves deposited by dc magnetron sputtering. High-resolution cross-sectional transmission electron microscopy and X-ray diffraction reveal microstructural differences between ion-beam-deposited and dc magnetron sputtered spin valves. In particular, film texture, surface morphology, GMR ratio, exchange bias, interlayer coupling strength, and coercivity vary widely, but property-structure-processing correlation can be identified. A GMR ratio of ~9.7% was obtained on random textured NiO exchange-biased bottom spin valves by ion-beam deposition     

纳米级FePt/Fe多层膜中微结构的优化与耦合效应

本文用磁控溅射法制备了一系列不同厚度Fe层的FePt/Fe多层膜,经过热处理后成功地制备出具有fct结构的FePt有序相和exchange-spring型永磁体.研究了不同热处理条件下多种膜层厚度与FePt/Fe多层膜结构、磁性及其内在关系,并探讨了FePt/Fe多层膜铁中的磁耦合失效问题.观察了不同热处理条件和层厚的FePt/Fe多层膜的结构变化过程,包括结构相变、有序度和晶粒度.研究了其磁性能随结构变化的规律,发现了耦合失效的模式.     

Coercivity variation in exchange-coupled Fe/FePt bilayer with perpendicular magnetization

The soft/hard Fe/FePt film with perpendicular magnetization has been deposited on a glass substrate. The (001) oriented L1₀ FePt film was obtained when annealed by rapid thermal process at 800 °C and a Fe layer was deposited at room temperature with thicknesses of 2 nm to 20 nm. Controlling the Fe layer thickness allowed modification of the hysteresis loops from out-of-plane rigid magnet to in-plane exchange-spring like magnet due to the nanometer scale interface coupling. When the Fe layer thickness increased to 2 nm, the out-of-plane coercivity is reduced to 5.9 kOe but the remanence ratio (0.98) is still high. The Fe (2 nm)/FePt film shows perpendicular magnetization with linear in-plane hysteresis loop. The remanence ratio is reduced to 0.85 when the Fe layer thickness increased to 5 nm. When the Fe layer thickness was varied up to 10-20 nm, the in-plane hysteresis loop shows exchange-spring like behavior with two-step magnetization reversal processes. The films with perpendicular coercivity were moderated by the thickness of soft magnetic layer.     

Exchange coupling and its applications in magnetic data storage

The continuing scaling of magnetic recording is facing more and more scientific and technological challenges because both the read sensor and recording bit are approaching sub-50 nm regime with the ever increasing areal density in hard disk drives. One of the key and indispensable elements for both high-sensitivity sensors and high-density media is the exchange bias between a ferromagnetic and an antiferromagnetic layer or the exchange coupling between two ferromagnets via a non-magnetic spacer. In the nanometer regime, the exchange coupling between ferromagnet and antiferromagnet or two ferromagnets through a conductive spacer is governed by the intergrain exchange interaction which has its origin in electron spins. Interlayer exchange coupling in multilayer or trilayer essentially originates from the quantum confinement effect. In this paper, we first review the physical origin and various theoretical models of the two types of exchange couplings, followed by a review of the applications of the exchange bias and interlayer exchange coupling in data storage with emphasis on the advanced read sensor and advanced media including perpendicular media and patterned media.