High temperature coercivity of Nb-containing HITPERM alloys: Effect of Cu addition

Hysteresis loops of nanocrystalline samples of Fe39Co39Nb6B16 − yCuy (y = 0, 1) HITPERM-type alloys were measured from room temperature up to 773K. The alloy with Cu shows a nearly constant value of coercivity (∼ 135A/m) in all the studied temperature range, whereas for the Cu-free alloy, the coercivity increases from 125A/m at 300K up to 190A/m at 773K. Room temperature Mössbauer spectra can be interpreted on the basis of the presence of three different regions: amorphous, crystalline and interface. The observed nanocrystalline microstructure is similar for the two alloys, although bigger crystallites are found in the alloy without Cu. The very small grain size ensures the averaging out of the magnetocrystalline anisotropy but, as the expected value of magnetostriction constant for the crystalline phase is ∼ 60ppm, the magnetoelastic anisotropy cannot be neglected.