Exchange coupling in the paramagnetic state

Exchange coupling has been observed in a ferromagnet (FM)/antiferromagnet (AF) bilayer with Curie temperature ${(T}_C)\ll$Néel temperature ${(T}_N)$ in the regime of $T<\mathrm{}{T}_N,$ even when the FM layer is in the paramagnetic state. The exchange field $H_E$ and coercivity $H_C$ of the coupled bilayer show completely different temperature dependence from conventional FM/AF bilayers with $T_C>T_N.$ With increasing temperature, $H_C$ vanishes at $T_C,$ whereas $H_E$ persists to $T_N.$ The results show that the exchange coupling can be established in FM/AF bilayers regardless of the relative values of $T_C$ and $T_N.$