Giant exchange bias and its angular dependence in Co/CoO core-shell nanowire assemblies

The exchange-bias field (HEB) and its angular dependence are systematically investigated in Co/CoO core-shell nanowire assemblies (∼15 nm in diameter and ∼200 nm in length) consisting of single-crystalline Co core and polycrystalline CoO shell. Giant exchange-bias field (HEB) up to 2.4 kOe is observed below a blocking temperature (TEB ∼150 K) in the aligned Co/CoO nanowire assemblies. It is also found that there is an angular dependence between the HEB and the applied magnetization direction. The HEB showed a peak at 30° between the applied field and the nanowire aligned direction, which may be attributed to the noncollinear spin orientations at the interface between the ferromagnetic core and the antiferromagnetic shell. This behavior is quantitatively supported by an analytical calculation based on Stoner-Wohlfarth model. This study underlines the importance of the competing magnetic anisotropies at the interface of Co/CoO core-shell nanowires.