Anomalous variation of the Lamb–Mössbauer factor at the magnetic transition temperature in magnetoelectric GaFeO3

Abstract

Temperature-dependent 57Fe Mössbauer spectroscopy (5–723 K) and neutron diffraction (2–290 K) measurements are carried out on polycrystalline magnetoelectric GaFeO3. From the neutron diffraction data, evidence for the magnetostriction and increased disorder at Fe sites close to the ferrimagnetic Curie transition temperature (TC) is observed. From the Mössbauer data, it is observed that the Lamb–Mössbauer factor as a function of temperature f(T), which is related to the integral over the first Brillouin zone of the phonon spectrum, shows a unequivocal variation at the TC. The observations are discussed in terms of spin–phonon coupling. The observed average hyperfine fields from 57Fe Mössbauer spectra match with the bulk magnetization data. A critical exponent (β) of 0.38 ± 0.02 and a Debye temperature (θD) of ∼350 K is estimated from the 57Fe Mössbauer data.