Improved thermal stability in doped MnN/CoFe exchange bias systems

Abstract

We have investigated the influence of doping the antiferromagnetic MnN component in the polycrystalline MnN/CoFe exchange bias system, which shows high exchange bias of up to 1800 Oe at room temperature. The thermal stability of this system is limited by nitrogen diffusion that occurs during annealing. In order to predict improved thermal stability, defect energies of potential dopant elements which substitute for Mn were calculated via density functional theory. Elements calculated to have negative defect energies will bind nitrogen more strongly to the lattice, limiting diffusion. We prepared exchange bias stacks with dopant concentrations of a few percent by (reactive) co-sputtering, testing dopant elements with defect energies ranging from highly negative to slightly positive. We show that doping with elements calculated to have negative defect energies indeed improves the thermal stability. Y-doped MnN layers with dopant concentrations below 2% result in exchange bias fields higher than 1000 Oe for annealing temperatures up to 485°C.