Exchange bias in Co/CoO core-shell nanowires: roles of antiferromagnetic grain size distribution and interfacial spin glass

Abstract

Exchange bias effect in aligned Co/CoO core-shell nanowires with high aspect ratio was studied. Large exchange bias fields of about 2.2 kOe and 1.0 kOe in easy axis and hard axis, respectively, were obtained after field cooling. We demonstrate that both antiferromagnetic grains and the interfacial spin glass make strong contribution to the observed exchange bias, which is demonstrated with both the cooling field dependence of the exchange bias field and the training effect at different temperatures. The difference in the thermal activation temperature for these two magnetic states leads to a bimodal distribution of the blocking temperature which, in combination with the interplay between the fluctuating spin ordered antiferromagnetic grains and the demagnetizing field of Co core, is likely to explain the anomalous drop of coercivity at the temperature in range from 200 to 110 K.