| Research to determine the nature of the exchange coupling between a ferromagnet and an antiferromagnet is presented. The long-range nature of the exchange coupling, the dependence on the crystal orientation of the antiferromagnet, the dependence on the magnetization of the ferromagnet, the temperature dependence, and the memory effect have been closely examined. This has led to a better understanding of the exchange coupling phenomenon.; While all the models have assumed a nearest-neighbor coupling at the interface, the author will show that a long-range coupling can also induce exchange coupling. By inserting a spacer layer between the ferromagnet and the antiferromagnet, the author has probed the nature of the coupling and shown that it can persist up to a spacerlayer thickness greater than 65 Å.; A comparison of the exchange coupling of single crystal CoO grown in different crystal orientations suggests that a perfectly compensated surface will not have any exchange bias, as suggested by several models. For the first time, it is shown that a net moment at the interface is essential for the establishment of exchange bias.; The author also shows conclusively that the coupling of the antiferromagnet to the magnetization of the ferromagnet is much stronger than the coupling to the external field. Using novel cooling techniques, the author was able to distinguish between the effects of the cooling field and the effects of the magnetization of the ferromagnet.; Extensive research on the temperature dependence and memory effect of the exchange coupling suggest the existence of a domain wall in the antiferromagnet with a temperature dependent width. While most models assumed that the exchange bias is established when crossing the Néel temperature of the antiferromagnet, the data shows that the coupling can be established over a wide range of temperatures. The results are similar for both poly-crystalline and single crystal samples, suggesting that the temperature dependence is intrinsic to the coupling and not related to materials issues. A model is proposed where the temperature dependence of the thickness of the domain wall leads to the observed memory effect and temperature dependence. |
