Magnetic characterization of melt spun nanocomposite Nd4Fe77.5B18.5 alloys

Magnetic properties of nanoparticles in melt spun Nd4Fe77.5B18.5 alloys with low rare earth content and effect of thermal annealing have been investigated. The unannealed alloy samples show a high magnetization of 1.7 T with practically no coercivity which is indicative of the formation of metastable Fe23B6 phase and the absence of any hard magnetic phases in the unannealed sample. On annealing at 600°C, a symmetric rectangular type hysteresis loop with a saturation magnetization value of 1.44 T was obtained indicating the remanence enhancement due to exchange coupling between the nanocrystalline phases. The coercivity is relatively low at 0.17 T which is attributed to the nanocrystalline precipitation of soft magnetic grains having a dimension larger than twice the wall width of the hard phase and to the inhomogeneous grain structure. Weakening of exchange interaction due to the decomposition of Fe23B6 phase and coarsening of crystallite grains due to longer annealing (30 min) have been shown to be responsible for the subsequent deterioration in magnetic properties.