Time effects in exchange anisotropy-affected metal-evaporated tapes

The time-dependent magnetic behavior in metal evaporated tapes for high-density magnetic recording is studied at low temperatures, where the coupling between the ferromagnetic Co and the antiferromagnetic CoO, in the obliquely evaporated CoNiO film, induces a unidirectional exchange anisotropy, and it is compared with the effects obtained at room temperature, where no coupling occurs. At low temperature a contraction of the hysteresis loop as a function of the number of repeated cycles N is observed, with a remarkable decrease of the measured coercive field. No contraction occurs at room temperature. The time dependence of the magnetization is larger at low temperatures, with increase of magnetic viscosity and thermal fluctuation field, than at room temperature. The observed anomalous increase of thermal instability of magnetization and coercivity with the decrease of the temperature is imputed to the presence of exchange coupling between the ferromagnetic and antiferromagnetic phases at low temperature and to the effect that such coupling can exert on the reversal of the magnetization in the ferromagnetic grains. © 1996 American Institute of Physics.