Study of the 123-K phase transition of magnetite by critical neutron scattering

Abstract

A neutron-scattering study has been carried out on magnetite at temperatures just above the Verwey transition point of $T_V=123\mathrm{}$ K. No critical scattering was observed at the (0,0,2) reciprocal-lattice point where the magnetic scattering appeared below $T_V$ due to the Verwey ordering as previously reported by Hamilton. On the other hand, well-defined critical scattering was observed at the points corresponding to the (4,0,½)-type satellites which had been found to result mainly from atomic displacements by Samuelsen et al. These results indicate that atomic displacements due to internal lattice modes play an important part in the critical fluctuation of this phase transition. The intensity calculated on the basis of a ${\Delta }_5$ phonon mode with the wave vector of $\overrightarrow{\mathrm{k}}=(0, 0, \frac{1}{2})$ qualitatively explains the observed intensity distribution of the critical scattering. The mode of the charge-density fluctuation on the octahedral Fe sites which couples with this phonon mode is quite different from the scheme of the well-known Verwey ordering.