Magnetic and hyperfine interactions in amorphous Fe-P alloys

Amorphous alloys Fe_xP_100-x have been fabricated by rf sputtering in the composition range 50≤x≤75. Magnetic ordering temperatures were determined using thermogravimetric analysis (for x=75) and by zero-velocity thermal scanning (for 50≤x≤71). Ordering temperatures are depressed from those of the corresponding crystalline phases and range between 550 K for a-Fe₇₅P₂₅ and 10 K for a-Fe₅₀P ₅₀. The Mössbauer spectra at 8 K of alloys with x≤71 all had a central unsplit line and broad, essentially featureless wings, indicating the coexistence of magnetically ordered and nonmagnetically ordered regions. The spectrum of a-Fe₇₅P₂₅ consisted of six broad lines, indicating that this composition is magnetically more homogeneous. Hyperfine-field distributions P(H) and mean isomer shifts (IS), determined using Window's method, show that mean hyperfine fields have an essentially linear dependence on concentration and, by extrapolation, vanish at x∼46. The IS values at 293 K, however, have only small variations with composition and a weak maximum at x=67, whereas at 8 K the composition dependence is larger and there is a minimum at x=67. This behavior suggests a structural alteration whose nature is, as yet, unclear.