Inverted hysteresis loops observed in a randomly distributed cobalt nanoparticle system

We observed abnormal magnetic behaviors (so-called inverted hysteresis loops or negative remanence) in a randomly distributed Co nanoparticle (NP) system, where the Co NPs are prepared using laser irradiation. The inverted hysteresis loops are observed when ferromagnetic and superparamagnetic phases coexist within the NP system. Normal hysteresis loops appear in samples having only one phase, either ferromagnetic or superparamagnetic. The ferromagnetic and superparamagnetic Co NPs, with average diameters of 25 and 4 nm, respectively, showed negative remanence over a temperature range of 92\char21{}300 K. We have proposed that these abnormal features are directly related to the dipolar interaction between the superparamagnetic Co NPs and a ferromagnetic Co NP.