MAGNETIC DIPOLAR AND ELECTRIC QUADRUPOLE EFFECTS ON THE MÖSSBAUER SPECTRA OF MAGNETITE ABOVE THE VERWEY TRANSITION

Résumé
La magnétite Fe₃O₄ est étudiée par spectrométrie Mössbauer entre 120 et 880 K. En-dessous de T_c les spectres sont analysés en 3 sextuplets : 2 pour les sites octaédriques B, et 1 pour les sites tétraédriques A. Les champs magnétiques sont comparés aux mesures d'aimantation par diffraction des neutrons. Dans l'état paramagnétique, on observe une correlation intéressante entre les dépendences thermiques de la résistivité électrique et des déplacements isomériques.

Abstract
Mössbauer spectroscopic studies of powdered Fe₃O₄ have been undertaken between 120 K and 880 K. Below the magnetic transition temperature (T_c = 839,5 K) three six line patterns have been fitted to the experimental spectra. Two patterns are ascribed to the octahedral B-site and one pattern is ascribed to the tetrahedral A-site. It is shown that the broadening of the B-pattern is caused by the presence of two unresolved components. The corresponding Mössbauer patterns have intensity ratio 3 : l arising from the different magnetic dipolar and electric quadrupolar interactions on the B-site iron ions. It can therefore be concluded that line broadening due to electron hopping between B-sites ions is negligible. The temperature variation of the magnetic fields is found to be proportional to the sublattice magnetization as measured by neutron diffraction. The difference in the magnetic fields at the two nonequivalent B-sites is measured to be 1.1. T at 310 K in good agreement with values found from NMR studies. In the paramagnetic state the electric quadrupole splittings of iron at A and B-site are found to be constant, independent of temperature, having the values zero and 0.16 mm/s, respectively. The centroid shifts, on the other hand show above 700 K large deviations from the calculated second order Doppler shift. It is proposed that the deviations arise from a variation in band overlap. The unusual variation of the centroid shifts can be correlated with the observed maximum of the resistivity found for magnetic at about 750 K. The isomer shift variation is most pronounced for A-site which indicates that a model describing the high temperature conductivity in magnetic has to take into account not only the B-sites but also the A-sites.