Nano-sized superparamagnetic Y
_{3}
Fe
_{5}
O
_{12}
ferrite having a high heat generation ability in an AC magnetic field was prepared by physical bead milling. The highest heat ability in the AC magnetic field was for the fine Y
_{3}
Fe
_{5}
O
_{12}
powder with a 16-nm crystallite size (the samples were milled for 4 h using 0.1 mm� beads). The main reason for the high heat generation property of the milled samples was ascribed to an increase in the Néel relaxation of the superparamagnetic material. For the samples milled for 4 h using 0.1 mm� beads, the heat generation ability (W·g
^{-1}
) was estimated using a 3.58 × 10
^{-4}
· f· H
^{2}
frequency (f/kHz) and the magnetic field(H/kA·m
^{-1}
), which is the highest reported value of superparamagnetic materials. Furthermore, Y
_{3}
Fe
_{5}
O
_{12}
microspheres having a 20–32 μm diameter range were prepared by a spray dryer using the bead-milled nano-sized particles.
