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

TL;DR

This study investigates how superparamagnetic fluctuations in the antiferromagnetic CoO shell influence exchange bias and coercivity in Co/CoO nanowires, revealing geometry-specific magnetic behaviors not seen in other structures.

Contribution

It demonstrates the significant role of AFM superparamagnetic fluctuations in exchange bias mechanisms within 1D nanowire systems, supported by experimental and micromagnetic simulation data.

Findings

Large exchange bias fields of ~0.1 T below 100 K.

Coercivity drops by 0.2 T at the exchange bias onset.

Superparamagnetic fluctuations influence magnetization flipping.

Abstract

The magnetic properties of Co (<D>=15 nm, <L>=130nm) nanowires are reported. In oxidized wires, we measure large exchange bias fields of the order of 0.1 T below T ~ 100 K. The onset of the exchange bias, between the ferromagnetic core and the anti-ferromagnetic CoO shell, is accompanied by a coercivity drop of 0.2 T which leads to a minimum in coercivity at $\sim100$ 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 anti-ferromagnetic 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 1D system which possesses a large shape anisotropy and was not previously observed in 0D (spheres) or 2D (thin films) systems which have a high degree of symmetry and low coercivities. This study underlines the importance of the AFM super-paramagnetic fluctuations in the exchange bias mechanism.