Ferromagnetic/antiferromagnetic exchange coupling in SrFe12O19/CoO composites

The surface of SrFe12O19 coated with a CoO layer reveals a strong exchange bias characterized by magnetic hysteresis loops. The low-temperature coercivity, H-C, and the squareness, M-R/M-S, of a permanent magnet of SrFe12O19/CoO powder prepared by the sol-gel method are enhanced after field cooling through the Neel temperature (T-N=290 K) when compared to those after zero-field cooling. The existence of loop shifts and the enhancement of H-C indicate that exchange-bias effects, which are induced by the ferromagnetic/antiferromagnetic (FM/AFM) exchange-coupling interactions, are responsible for these behaviors. According to our experimental results, some of the factors controlling the exchange bias, such as FM/AFM interfaces and the CoO amount of the antiferromagnetic layer, are discussed.