Magnetic field orientation dependent dynamic susceptibility and Brownian relaxation time of magnetic nanoparticles

Abstract

This paper investigates the dynamic ac susceptibility (ACS) and the Brownian relaxation time of magnetic nanoparticles (MNPs) in dc magnetic fields with arbitrary orientations with respect to the ac magnetic field. A CoFe2O4 MNP sample, dominated by Brownian relaxation, is used to perform ACS measurements in an ac magnetic field with a constant amplitude of 0.2 mT (from 2 to 3000 Hz) and a superposed dc magnetic field with amplitudes ranging from 0 to 5 mT. Experimental results indicate that the ACS and Brownian relaxation time are significantly affected not only by the strength but also by the orientation of the dc magnetic field. Moreover, a mathematical model is proposed to analyze the ACS and Brownian relaxation time in dependence of the orientation of the dc magnetic field, which extends the established models parallel or perpendicular to arbitrary-oriented dc magnetic fields. Experimental results indicate that the good fitting between the experimental data (ACS and Brownian relaxation time) and the proposed models demonstrates the feasibility of the proposed model for the description of ACS and Brownian relaxation time in arbitrary-orientated ac and dc magnetic fields.