Controlling the exchange interaction using the spin-flip transition of antiferromagnetic spins in Ni81Fe19∕α-Fe2O3

We have studied exchange bias and coercivity in ferromagnetic (FM) Ni81Fe19 layers coupled with antiferromagnetic (AF) (0001), (112¯0), and (11¯02) α-Fe2O3 layers. We show that the exchange interaction between FM and AF layers is controllable by a magnetic-field annealing (MFA) through a spin reorientation temperature of an AF material, similar to a MFA through a Nèel temperature in normal exchange-biased systems. For (112¯0) and (11¯02) α-Fe2O3, the temperature-dependent exchange bias displayed a sudden increase below a spin-flip temperature of α-Fe2O3 (Tm) and the coercivity showed a clear variation near Tm.