TEM Study of Nd-Fe-B Melt-Spun Permanent Magnets with Fe3B As Main Phase

Permanent magnet materials with isotropic hard magnetic properties have been obtained by annealing amorphous melt spun flakes with Nd4Fe78B18 composition. This yields a two-phase permanent magnet consisting of the metastable Fe3B in tetragonal modification, and of the hard magnetic Nd2Fe14B for 15% of the alloy.The microstructure of the flakes is dependent on the annealing treatment. The amorphous as-spun flakes were first heated to 730°C with a rate of 200 K/min . Fig. 1 shows that further isothermal annealing during 0, 15 and 60 min yields crystallite diameters of approximately 10, 30 and 50 nm, respectively.Optimal coercivity (μoHc = 0.4 T ) is obtained for 15 min of annealing, in which case the grain size is of the order of the domain wall widthAnalytical TEM with EDX and parallel EELS confirms the two-phase nature of these magnets . For the sample with an annealing time of 60 min ( fig. lc ), parallel EELS is carried out on a CM12 at 120 kV in nanoprobe mode so that a local area of ∼ 10 nm can be analysed.