Effect of particle growth on heat generation ability in AC magnetic field for nano-sized magnetic Y<sub>3</sub>Fe<sub>5</sub>O<sub>12</sub> powder prepared by bead milling

A superparamagnetic magnetic Y3Fe5O12 ferrite of 20.5 nm in particle size was prepared by bead milling using 0.05 mmφ beads for 10 h. The heat generation ability in an AC magnetic field (370 kHz, 1.77 kA·m−1) was 0.34 W·g−1 for the bead-milled sample and was improved by calcination at low temperature. The main reason for this heat generation property of the milled samples was ascribed to a Néel relaxation of the superparamagnetic material. The maximum ability of 0.46 W·g−1 in an AC magnetic field (370 kHz, 1.77 kA·m−1) was obtained for the sample (36.8 nm in particle size) calcined at 700°C. The heat generation ability was decreased with particle growth when the calcination temperature was higher than 700°C. For the sample calcined at 600°C, the heat generation ability (W·g−1) was proportional to the square of the magnetic field (H/kA·m−1). In the case of the calcination at 700°C, the heat generation ability (W·g−1) depended on the cube of the magnetic field. The heat generation mechanism would change from superparamagnetic to ferrimagnetic due to the pariticle growth at 700°C.