Temperature dependent hyperfine distributions in Fe-B and Fe-Ni-Si-B glasses

Mössbauer spectra of amorphous Fe_xNi_{80- x} Si₁₀B₁₀ (x=10,50) and Fe₈₀B₂₀ have been taken at 4.2-300K and in 0-6T applied magnetic fields. Detailed analysis shows that the standard deviation of the magnetic hyperfine field σ(B) varies with temperature depending on the iron concentration. For small x the variation is taken as evidence for a break-down of long-range magnetic order. Temperature induced changes in the fluctuations and correlations of the hyperfine parameters are reported. In all cases the isomer shift fluctuations are small compared to those of the first-order electric quadrupole splitting (u); neither are strongly temperature dependent. The correlation <ΔBΔu> is negative and increases with temperature. For Fe₈₀B₂₀, <ΔBΔδ> is positive and increases with temperature, whereas for Fe-Ni-Si-B it is small, negative, and temperature invariant.