Effect of the coupling between Co and CoO in metal evaporated tapes

In metal evaporated (ME) tapes the interaction between the ferromagnetic Co grains and the antiferromagnetic CoO layer may affect the magnetic behaviour at low temperatures. In fact, when the tapes are cooled from room temperature in the remanence state or in the presence of a strong magnetic field, a shift in the hysteresis cycle is observed. This is an effect of the presence of unidirectional exchange anisotropy, which originates in the exchange coupling between the ferromagnetic and antiferromagnetic phases, and also affects the magnetic behaviour in the presence of a rotating magnetic field. The torque curves have been measured at 80 K. For tapes cooled in the remanence state or in the presence of a strong magnetic field, they show a superposition of sin 2 theta and sin theta torque functions, which indicates the simultaneous presence of the uniaxial anisotropy of the ferromagnetic phase, and of the unidirectional exchange anisotropy. The rotational hysteresis energy loss at 80 K, deduced from the area enclosed in the torque curves, shows anomalous behaviour, with no decrease with increasing field, and also finite values at applied fields larger than the anisotropy field of the ME tapes. This is due to the drag of the ferromagnetic Co moments by the antiferromagnetic CoO exchange-coupled spins during the rotation of the field direction.