Magnetic hysteresis loss and the resulting local heating in implanted thermoseed (magnetic biocompatible material) can be used to selectively destroy cancerous tissue. We have prepared magnetic bioglass ceramics (MBC) by annealing 41CaO.44 SiO₂. 4 P₂O₅.8 Fe₂O₃.3 Na₂O glass at temperatures (T_A ) ranging from 750°C to 1150°C for 1 h. X-ray diffraction patterns confirm the presence of bone mineral (hydroxyapatite and wollastonite) and magnetite (Fe O ) phases in nanocrystalline form in theMBCs. Average crystallite size (d_av ) of all the phases increased with increasing T_A . Magnetization ( M-H ) curves of all MBCs were recorded up to 20 kOe at 300 K to understand the evolution of magnetism in the nonmagnetic glass upon heat treatment. Magnetic properties correlated with d_{av(magnetite)} for samples annealed up to 1050°C. Electron paramagnetic resonance study was carried out to understand the behavior of sample annealed at 1150°C and the evolution of magnetic properties in the MBCs. This study also reveals a methodology to control the magnetic properties of this MBC and its potential application in hyperthermia treatment of bone cancer.