To investigate thermal decay of ferro/antiferromagnetic coupling, spin valve films with Co/(Cr0.4Mn0.6)90Pt10 exchange coupling layers were heat treated at various temperatures under a magnetic field whose direction was opposite to the pinning direction, and the pinning fields were evaluated at 35°C. The pinning field decayed as the treatment time increased. During the first 10 min of the heat treatment, the pinning field decreased rapidly and then decreased gradually. The initial decrease in the pinning field became larger and the rate of the change of the pinning field after 10 min became larger at the higher treatment temperatures. A thermal fluctuation model, which assumes coherent rotations and grain size distributions in the antiferromagnets, was used to obtain the activation energy and relaxation times for reversal of the antiferromagnetic moment. Smaller antiferromagnetic grains had lower activation energies and shorter relaxation times. As a result, the smaller grains reversed their moments at an earlier time stage, which gave rise to the rapid decay of the pinning field during the first 10 min of the heat treatment. The model explains the thermal decay of the pinning field when the temperature is lower than 130°C. However, a narrower distribution of the activation energy than can be expected from the model is required to explain the thermal decay at 150°C, which suggests that a incoherent magnetic rotation occurs in antiferromagnets above 150°C.
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