A Study of Exchange Anisotropy

白交换各向异性是指反铁磁体和铁磁体之间的矢量交换相互作用。如果反铁磁体有适当的偏置，这种相互作用会使磁滞回线偏离通常的H=O对称位置。这种位移的大小被称为交换场。这种相互作用也使磁滞回线变宽，使矫顽力场增大。交换各向异性是在大约40年前被发现的，但直到20世纪80年代才意识到这种效应可以用于固定磁性薄膜的方向，这对磁阻传感器和自旋阀很重要，而磁阻传感器和自旋阀在磁数据存储系统中很重要。从那时起，从事交换各向异性研究的科学家和工程师数量激增，但大多是从非常实用的角度出发的。结果是关于交换各向异性的经验信息很多，但对模具现象的基本理解仍然很少。为了使交换各向异性存在，必须在反铁磁体/铁磁体界面处存在反铁磁体的磁极化，并且在反铁磁体中存在磁各向异性。关于反铁磁体界面极化的起源，目前有两种假说。一种是在界面处调用不平衡自旋，另一种是在界面处调用反铁磁自旋结构。不平衡自旋图像表明极化与反铁磁体的畴结构有关。实验证据支持这两种假设。事实上，这两种情况都可能发生，只是在不同的情况下。为了理解交换各向异性现象，必须能够观察到(1)反铁磁体的界面极化，(2)反铁磁体的畴结构，(3)反铁磁体的磁各向异性。不幸的是，直到最近，只有第三个参数是可以直接测量的。他们已经意识到，x射线磁二色性可以用来直接测量其他“隐藏”参数。x射线磁圆二色性具有元素特异性和单层灵敏度，可用于直接测量界面极化。x射线磁线性二色性决定了有序系统的轴，但不决定净磁化强度。因此，它可以测量反铁磁体和铁磁性中的自旋系统取向。他们建议使用线性二色性来观察铁磁和反铁磁薄膜的反铁磁畴结构和相对取向。他们最初在研究中提出使用的反铁磁体是立方NiO，因为它具有非常简单的晶体结构和已知的简单自旋结构。他们开发了一种MBE沉积NiO和样品所需磁性金属的技术。MBE技术可以很好地控制薄膜的厚度，并且可以生产出高质量的外延薄膜。

9810185 White Exchange anisotropy is the name given to the vector exchange interaction between an antiferromagnet and a ferromagnet. If the antiferromagnet is appropriately biased, this interaction produces a shift of the hysteresis loop away from the usual position of symmetry about H=O. The magnitude of the shift is known as the exchange field, He. The interaction also produces a broadening of the hysteresis loop, an increase in the coercive field, Hc. Exchange anisotropy was discovered some 40 years ago, but remained a scientific curiosity until it was realized in the 1980's that this effect could be useful for pinning the direction of magnetic thin films, important for the magnetoresistive sensors and spin valves important today in magnetic data storage systems. Since then there has been an explosion in the number of scientists and engineers working on exchange anisotropy, but mostly from a very applied point of view. The result is that there is a lot of empirical information on exchange anisotropy, but still very little fundamental understanding of die phenomenon. For exchange anisotropy to exist there must be a magnetic polarization of the antiferromagnet at the antiferromagnet/ferromagnet interface, and magnetic anisotropy in the antiferromagnet. There are two present hypotheses about the origins of the interfacial polarization of the antiferromagnet. One invokes unbalanced spins at the interface, and the other a canting of the antiferromagnetic spin structure at the interface. The unbalanced spin picture implies a dependence of the polarization upon the domain structure of the antiferromagnet. Experimental evidence exists supporting both hypotheses. It may in fact be the case that both occur but under different circumstances. In order to understand the phenomenon of exchange anisotropy it is necessary to be able to observe (1) the interfacial polarization of the antiferromagnet, (2) the domain structure in the antiferromagnet, and (3) the magnetic anisotropy of the anti ferromagnet. Unfortunately, until recently, only the third parameter has been directly measurable. They have realized that X-ray magnetic dichroism can be used to measure directly the other "hidden" parameters. X-ray magnetic circular dichroism, which is element-specific and has monolayer sensitivity, can be used to measure directly the interfacial polarization. X-ray magnetic linear dichroism determines the axis of an ordered system but not the net magnetization. It can therefore measure the spin system orientation in both antiferromagnets and ferromagnetism. They propose to use the linear dichroism to observe both antiferromagnetic domain structure and the relative orientation of the ferro- and antiferro-magnetic films. The antiferromagnet they propose to use initially in their studies is cubic NiO because it has a very simple crystal structure and a known simple spin structure. They have developed a technique for MBE deposition of NiO and of the magnetic metals needed for their samples. The MBE technique has both fine control on the film thickness and produces high quality epitaxial films.