Dynamic remagnetisation of CoFe <sub>2</sub> O <sub>4</sub> nanoparticles: thermal fluctuational thawing of anisotropy
D. A. Balaev, I. S. Poperechny, A. A. Krasikov, S. Semenov, S. I. Popkov, Yu. V. Knyazev, V. L. Kirillov, S. S. Yakushkin, Oleg N. Martyanov, Yu. L. Raĭkher
Abstract
Abstract We report a study of the magnetodynamics of cobalt ferrite (CoFe 2 O 4 ) nanoparticles with an average diameter of ∼6 nm. Hysteresis loops were measured under quasi-static conditions and in pulse fields with amplitudes H 0 of up to 130 kOe and for durations τ P of 8 and 16 ms. The growth of coercivity H c observed with an increase in the magnetic field variation rate dH / dt (determined by the values of H 0 and τ P ) and the reduction of H c with temperature is ascribed to the superparamagnetic effect. The proposed theoretical model explains the observed dependences fairly well. Notably, the effective magnetic anisotropy constant obtained exceeds the value for bulk crystals and might be indicative of the contribution of surface magnetic anisotropy.