Infrared and Raman studies of the Verwey transition in magnetite

We present infrared and Raman measurements of magnetite (Fe₃O₄). This material is known to undergo a metal-insulator and a structural transition (Verwey transition) at Tₕ=120 K. The structural aspect of the Verwey transition is disclosed by the appearance of additional infrared-active and Raman-active phonons. The frequencies of the infrared-active phonons show no significant singularities at the transition whereas their linewidths increase. The frequency and linewidth of the Raman-active phonon at 670 cm^-1 change abruptly at the transition. For T<Tₕ, we observe fine structure in the infrared and Raman spectra which may indicate strong anharmonicity of the system below the transition. The effective mass of the mobile carriers is estimated to be m^*100m, where m is the electronic mass.