Effect of bead milling on heat generation ability in AC magnetic field of FeFe2O4 powder

Nano-sized FeFe2O4 ferrite powder having a heat generation ability in an AC magnetic field was prepared by bead milling for a thermal coagulation therapy application. A commercial powder sample (non-milled sample) of ca. 2.0 mu m in particle size showed a temperature enhancement (Delta T) of 3 degrees C in an AC magnetic field (powder weight 1.0g, 370 kHz, 1.77 kA m(-1)) in ambient air. The heat generation ability in the AC magnetic field improved with the milling time, i.e., due to a decrease in the average crystallite size for all the examined ferrites. The highest heat ability (Delta T = 26 degrees C) in the AC magnetic field in ambient air was for the fine FeFe2O4 powder with a 4.7 nm crystallite size (the samples were milled for 6 h using 0.1 mm phi beads). However, the heat generation ability decreased for the excessively milled FeFe2O4 samples having average crystallite sizes of less than ca. 4.0 nm. The heat generation of the samples showed some dependence on the hysteresis loss for the B-H magnetic property. The reasons for the high heat generation properties of the milled samples would be ascribed to an increase in the Neel relaxation of the superparamagnetic material. The hysteresis loss in the B-H magnetic curve would be generated as the magnetic moment rotates (Neel relaxation) within the crystal. The heat generation ability (Wg(-1)) can be estimated using a 1.07 x 10(-4) fH(2) frequency (f, kHz) and the magnetic field (H, kA m(-1)) for the samples milled for 6 h using 0.1 mm phi beads. Moreover, an improvement in the heating ability was obtained by calcination of the bead-milled sample at low temperature. The maximum heat generation (Delta T = 59 degrees C) ability in the AC magnetic field in ambient air was obtained at ca. 5.6 nm for the sample calcined at 500 degrees C. The heat generation ability (Wg(-1)) for this heat treated sample was 2.54 x 10(-4) fH(2). (C) 2011 Elsevier B.V. All rights reserved.