Observation of linewidth asymmetry in the Mössbauer Zeeman spectrum of amorphous yttrium iron garnet

A measurement of the six-line Mössbauer Zeeman spectrum of ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ in amorphous yttrium iron garnet at 4.2 K reveals a linewidth asymmetry involving the three line pairs $L_i$ and $L_{7-i}$, $i=1, 2, 3$. The major source of this asymmetry is found to be a positive correlation between isomer shift and hyperfine field with a degree of linearity of 18%. A complete linewidth analysis is given which also leads to separate estimates of the widths of the hyperfine field, isomer shift, and electric-field-gradient distributions in the glass.