Irreversible and reversible measurements of exchange anisotropy

Recently, several different experimental techniques have been used to measure the unidirectional exchange coupling, or exchange anisotropy, in ferromagnetic/antiferromagnetic bilayers. In particular, it has been found that reversible ac susceptibility measurements show significantly larger exchange anisotropy than the conventional irreversible hysteresis loop measurements. We have theoretically investigated the irreversible and reversible measurements 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.