Magneto-volume effects of γ-Fe precipitates in Cu and CuAl matrices

We have determined the dependence of the magnetic transition temperature T_N and of the magnetic hyperfine field B_hf on particle volume and on matrix lattice parameter in coherent antiferromagnetic γ-Fe precipitates in Cu _{100- x}Al _x 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 T_N( d) and of B_hf( d) at 4.2 K for γ-Fe in Cu is in agreement with a suggested structural phase transition at T_N for d⪆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.