Selection of ferrite powder was carried out to realize a thermal coagulation technique in which tumors are locally heated by an application of alternating magnetic field from external coils. Magnesium ferrite (MgFe2O4) showed the largest increase in temperature (AT) under an alternating magnetic field in all the ferrites examined. For all the samples, AT value under alternating magnetic field was increased with an increase in frequency (200-500 kHz). The heating ability for the Mg-ferrite was ca. 1.4 W/g under alternating magnetic field of 4.0 kA/m (200 W, 370 kHz). The heating ability in alternating magnetic field was clearly depended on the magnitude of the hysteresis loss for the ferrite powder. (C) 2005 Springer Science + Business Media, Inc.