High field irreversibility in NiFe2O4 nanoparticles (abstract)

Several anomalous magnetic properties of organic coated NiFe₂O₄ nanoparticles have been reported previously by Berkowitz et al. These properties included low magnetization with a large differential susceptibility at high fields, shifted hysteresis loops after field cooling below 50 K, and Mössbauer spectra indicating a fraction of the material being magnetically ordered but not responsive to applied fields. It was suggested that a ``strongly anisotropic phase'' was present on the particles surface, and that the organic coating was responsible for producing high local anisotropy fields on the surface atoms. The present study extends the original work in several ways. We find that the lack of saturation in high fields is accompanied by irreversibility up to 20 T, in some cases. We have confirmed the previously reported behavior, in addition to observing similar behavior in samples prepared without the organic surfactant. This implies that we are observing a finite size effect. We have recently reported time dependence of the remanent magnetization which persists down to 0.4 K and does not appear to follow a thermally activated law. Instead, the viscosity becomes temperature independent below 2 K. The model we propose for the high field irreversibility as well as the previously reported behaviors, is that there is a layer of spins at the surface which are spin glass-like, and that these spins can be reoriented irreversibly by a field or by thermal activation. The effect of surface spin interactions on magnetization relaxation will be discussed.