Co/CoO core–shell nanoparticles—Temperature-dependent magneto-optic studies

We present temperature-dependent magneto-optic hysteresis curves of “quasi-two-dimensional” arrays of 10 nm Co nanoparticles which are covered by a native CoO layer. The particles deposited on thin aluminum films become magnetically very hard at 10 K, i.e. the coercive field exceeds 1.6 T. Only in a narrow temperature range near 100 K an out-of-plane magnetization loop with considerable coercivity is observed. A shift of the magneto-optic hysteresis curves due to exchange bias was detected. Magnetization measurements on a frozen volume of nanoparticle solution reveal a strong exchange-bias effect of 0.8 T at 10 K. The exchange-bias effect is attributed to the rough interface and/or spin-disordered state between CoO shell and Co core.