Exchange-bias stabilization of the magnetic nanoparticles in a granular alloy grown by reactive sputtering

The superparamagnetic blocking temperature of a series of granular $Co22Ag78$ alloys grown by reactive sputtering under oxygen atmosphere exhibits a minimum when plotted as a function of the oxygen pressure. The magnetic stabilization observed above this minimum is found to arise from the exchange-bias between the Co core and a CoO shell, which appears to be spin disordered. The initial decrease in the blocking temperature at low oxygen pressures is mainly attributed to the inhibition of RKKY-like interactions by the formation of an insulating oxide layer, too thin to give rise to exchange bias, around the ferromagnetic cores.