The influence of a demagnetizing field on hysteresis losses in a dense assembly of superparamagnetic nanoparticles

The electrodynamic method is used to measure the hysteresis losses of a dense assembly of magnetite nanoparticles with an average diameter D = 25 nm in the frequency range f = 10-150 kHz and for magnetic field amplitudes H-0 = 100-300 Oe. It is found that the specific loss power is determined by a demagnetizing factor of a whole sample. It diminishes approximately 4.5 times when the sample aspect ratio decreases from L/d = 11.4 to L/d approximate to 1, where L and d are the sample length and diameter, respectively. For H-0 <= 300 Oe the maximal specific loss power 120 W/g is obtained for the sample with L/d = 11.4 at f = 120 kHz. For comparison, the assembly specific absorption rate has been determined also by means of direct measurement of the temperature difference between the inner and outer surfaces of a flat cuvette containing magnetic nanoparticles. For both methods of measurement close values for the specific absorption rate are obtained for samples with similar demagnetizing factors. (C) 2012 Elsevier B.V. All rights reserved.