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

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 $\sim 100$ 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.

10 pages, 10 figures, submitted to Phys. Rev. B

References in corpus (2)

• Magnetic nanowires as permanent magnet materials
• Thermal relaxation of magnetic clusters in amorphous Hf_{57}Fe_{43} alloy

Cited by in corpus (7)

• Dipolar interactions in arrays of ferromagnetic nanowires: a micromagnetic study
• High temperature structural and magnetic properties of cobalt nanowires
• Dipolar interactions in magnetic nanowires aggregates
• Monte Carlo study of the exchange bias effect in Co/CoO core-shell nanowires
• Exchange bias effect in cylindrical nanowires with ferromagnetic core and polycrystalline antiferromagnetic shell
• Magnetic properties of nanowires with ferromagnetic core and antiferromagnetic shell
• Ferromagnetic resonance of coupled shifted chains of nano-elements