Thermal relaxation in exchange coupled ferromagnet/antiferromagnet bilayers

Summary form only given. Stability of exchange-coupled F/AF bilayers is a very important issue for magnetoresistive (MR) read head for high density magnetic recording. Many different effects, such as training effect, sweeping-rate dependent coercivity, thermally-assisted switching of AF spins and annealing effect through atomic diffusion at F/AF interface, are involved in this issue. Strong thermal effect was observed in Py(10nm)/IrMn(xnm) bilayer system, for example, in sample with x=5, the easy-axis hysteresis loop changed drastically after putting the sample at a hard-direction remanent state for several months at room temperature. It implies that the pinned directions of some parts of the sample reversed during the waiting period. This can be ascribed to thermally-assisted switching of the AF surface spins driven by the interface F/AF exchange coupling in the regions (magnetic domains) with the F moment antiparallel to the initial pinned direction. Dependence of switching field on the waiting time at the saturation state was measured to probe the switching mechanism of the AF surface spins. The switching field first shows a linear dependence on the logarithm of the waiting time and then tends to saturate. The linear part, extending three decades, indicates a wide distribution of activation energy. Thermal dynamics of the AF spins was studied by measuring the samples with different AF layer thickness at different temperatures. The domain structures at different magnetizing states were observed using magnetic force microscope.