Observation of second-order quadrupole shift in Mössbauer spectrum of amorphous YIG (yttrium iron garnet)

A previously unsuspected quadrupole shift of the nuclear Zeeman lines in amorphous magnets has been observed by ⁵⁷Fe M\"ossbauer spectroscopy in speromagnetic amorphous yttrium iron garnet at 4.2 K. The distinctive shift pattern is shown to arise theoretically as a second-order perturbation of the Zeeman levels by the distribution of electric field gradients in the amorphous state. It is observed to have a form which agrees quantitatively with theoretical expectation if the relative orientation of the electric-field-gradient axes and the hyperfine-field direction at each iron site is random.