MAGNETO-VOLUME EFFECTS OF GAMMA-FE PRECIPITATES IN CU AND CUAL MATRICES

We have determined the dependence of the magnetic transition temperature TN and of the magnetic hyperfine field Bhf on particle volume and on matrix lattice parameter in coherent antiferromagnetic γ-Fe precipitates in Cu100-xAlx matrices (0≤x≤14) by 57Fe Mössabuer spectroscopy and dc susceptibility measurements. The average particle diameter d was determined precisely by transmission electron microscopy (TEM). The behavior of TN(d) and of Bhf(d) at 4.2 K for γ-Fe in Cu is in agreement with a suggested structural phase transition at TN for d{greater-than or approximate}20 nm. Precipitates smaller than d≈15-20 nm show superparamagnetism. The saturation hyperfine-field (and consequently the atomic Fe moment) of γ-Fe was found to increase remarkably with increasing lattice expansion of the CuAl matrix. © 1989.