Giant perpendicular exchange bias with antiferromagnetic MnN

Abstract

We investigated an out-of-plane exchange bias system that is based on the antiferromagnet MnN. Polycrystalline, highly textured film stacks of Ta / MnN / CoFeB / MgO / Ta were grown on SiO$_x$ by (reactive) magnetron sputtering and studied by x-ray diffraction and Kerr magnetometry. Nontrivial modifications of the exchange bias and the perpendicular magnetic anisotropy were observed both as functions of film thicknesses as well as field cooling temperatures. In optimized film stacks, a giant perpendicular exchange bias of 3600 Oe and a coercive field of 350 Oe were observed at room temperature. The effective interfacial exchange energy is estimated to be $J_\mathrm{eff} = 0.24$ mJ/m$^2$ and the effective uniaxial anisotropy constant of the antiferromagnet is $K_\mathrm{eff} = 24$ kJ/m$^3$. The maximum effective perpendicular anisotropy field of the CoFeB layer is $H_\mathrm{ani} = 3400$ Oe. These values are larger than any previously reported values. These results possibly open a route to magnetically stable, exchange biased perpendicularly magnetized spin valves.