NiMn/FeNi exchange biasing systems–magnetic and structural characteristics after short annealing close to the phase transition point of the AFM layer

Abstract

This work addresses the issue of acceleration of the long post-deposition annealing procedures typically used for obtaining the antiferromagnetic (AFM) state of the pinning film in NiMn-based spin-valve systems. It presents results on exchange biasing (EB) bi-layers NiMn(50 nm)/Ni 19Fe 81(5 nm) grown on a permalloy seed layer after annealing for a very short time at temperatures in the vicinity of the phase transition of the NiMn film. Both the magnetic and the structural characteristics measured after the short annealing procedure reveal clear differences between the samples annealed below and above a specific temperature T * which is lower than the phase transition temperature in the equilibrium NiMn phase diagram. The absence of an EB in the samples short annealed at T an< T * is related to the dominance of the face-centered cubic (FCC) paramagnetic NiMn phase. In contrast, the significant EB effect in the samples annealed at T an> T * and the saturation found in H EB for T an⩾375°C are connected with the existence of a dominant stable face centered tetragonal (FCT) AFM NiMn phase. This shows that for the investigated sputter-deposited NiMn pinning films a specific temperature T * exists above which a fast FCC⇒FCT (paramagnetic⇒AFM) phase transition occurs. The unidirectional EB effect surprisingly found in samples cooled without magnetic field can be explained with a two-step generation model: (i) the formation of specific magnetic anisotropy in the as-deposited permalloy films strongly depending on the phase composition of the NiMn pinning film, on its surface morphology and the stresses in the AFM/FM interface, respectively, and (ii) recording (“burning in”) of the existing magnetic anisotropy through the described fast paramagnetic⇒AFM phase transition.