Apfim and Hrem Studies of Nanocomposite Soft and Hard Magnetic Materials

Fe or Co based alloys with nanocrystalline microstructure show excellent permeability because the net magnetocrystalline anisotropy is significantly reduced when the grain size becomes smaller than the ferromagnetic exchange length. Fe-Si-B-Nb-Cu alloy is the pioneering nanocrystalline soft magnetic material invented by Yoshizawa et al. in 1988 [1]. Our previous works [2,3] reported evidence for clustering of Cu prior to the onset of the crystallization reaction. However, in the previous studies, it was not confirmed that these Cu clusters work as heterogeneous nucleation sites for α-Fe primary crystals. By employing a three dimensional atom probe (3DAP), we have shown that Cu atoms form clusters by annealing well below the crystallization temperature (FIG. 1). 3DAP analysis of the partially crystallized alloy indicated that Cu particles are in intimate contact with the α-Fe particles, suggesting that Cu clusters provide heterogeneous nucleation sites directly for primary crystals. Of particular interest was that Si does not partition in the α-Fe primary particles in the early stage of crystallization (450 °C for 10 min.), while Si does partition into the α-Fe phase after optimum heat treatment (550 °C for 10 min.).