Exchange bias in Co/CoO core-shell nanowires: Role of antiferromagnetic superparamagnetic fluctuations

Abstract

The magnetic properties of $\text{Co}\phantom{\rule{0.3em}{0ex}}(⟨D⟩=15\text{ }\text{nm},\text{ }⟨L⟩=130\text{ }\text{nm})$ nanowires are reported. In oxidized wires, we measure large exchange-bias fields of the order of 0.1 T below $T\ensuremath{\sim}100\text{ }\text{K}$. The onset of the exchange bias, between the ferromagnetic core and the antiferromagnetic (AFM) CoO shell, is accompanied by a coercivity drop of 0.2 T which leads to a minimum in coercivity at $\ensuremath{\sim}100\text{ }\text{K}$. Magnetization relaxation measurements show a temperature dependence of the magnetic viscosity $S$ which is consistent with a volume distribution of the CoO grains at the surface. We propose that the superparamagnetic fluctuations of the antiferromagnetic CoO shell play a key role in the flipping of the nanowire magnetization and explain the coercivity drop. This is supported by micromagnetic simulations. This behavior is specific to the geometry of a one-dimensional system which possesses a large shape anisotropy and was not previously observed in zero-dimensional (spheres) or two-dimensional (thin films) systems which have a high degree of symmetry and low coercivities. This study underlines the importance of the AFM superparamagnetic fluctuations in the exchange-bias mechanism.