Irreversible and reversible measurements off exchange anisotropy

Recently, several different experimental techniques have been used to measu re the unidirectional exchange coupling, or exchange anisotropy, in ferroma gnetic/antiferromagnetic bilayers. In particular, it has been found that re versible ac susceptibility measurements show significantly larger exchange anisotropy than the conventional irreversible hysteresis loop measurements. We have theoretically investigated the irreversible and reversible measure ments of exchange anisotropy and our analysis shows that when the magnetic degrees of freedom of the antiferromagnet are taken into account, different measurement techniques may indeed give different results for the exchange coupling. [S0163-1829(99)02545-X].