Giant uncompensated magnetization and exchange bias in FexNi1-xF2/Co bilayers

FexNi1-xF2 is an antiferromagnet with a magnetic anisotropy that can be varied continuously by changing the concentration x. In studying the exchange bias of a polycrystalline Co grown on FexNi1-xF2 (110) epitaxial films, a giant uncompensated magnetization, of up to 16% of the Co magnetization, was observed in the hysteresis loops, whose sign was correlated with the sign of the exchange bias field H-E. The exchange bias was measured as a function of temperature after cooling the samples in a field of 2 kOe from above the Neel temperature. The uncompensated magnetization had a large coercivity and an exchange bias field H-EU of its own. The uncompensated magnetization of the x=0.05 and x=0.21 samples had a negative H-EU while that of the x=0.49 sample was positive. The uncompensated magnetization could be reversed by applying a large field at low temperatures, which also reversed the sign of the exchange bias.