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, HC, and the squareness, MR/MS, of a permanent magnet of SrFe12O19/CoO powder prepared by the sol–gel method are enhanced after field cooling through the Neel temperature (TN=290 K) when compared to those after zero-field cooling. The existence of loop shifts and the enhancement of HC 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.