A Study of Exchange Anisotropy

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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.