RELAXATION EFFECTS IN MAGNETITE (Fe3O4) USING SELECTIVE EXCITATION / DOUBLE MÖSSBAUER (SEDM) PROCEDURES

— We have applied the SEDM technique to study the electron hopping time at the octahedral (B) sites in magnetite. We have found that this relaxation time is either zero or less than 10" s at room temperature and 125 K. At 112 K the SEDM results for the octahedral (B) sites are quite different from those at room temperature and 125 K. Relaxation may be occurring, but in the absence of a theoretical model no time estimate can be made. The most surprising result was the observation of relaxation at the A sites at all three temperatures. Although magnetite, Fe 3 0 4 , has been studied for quite some time, there are still some unresolved problems. Magnetite crystallizes in a cubic inverse spinel structure above the Verwey transition (Tv — 119 K). In magnetite both ferrous and ferric ions are present at the octahedral (B) sites, while only ferric ions are on the tetrahedral (A) sites. At room temperature the electrical conductivity of Fe 3 0 4 is unusually high (250 £1 1 cm 1 ) . The explanation of this fact is thought to be a rapid electron exchange between the ferrous and ferric ions on the octahedral (B) sites. The electrical conductivity results suggest that the electron exchange relaxation time is about 10" 1 s at room temperature. However, previous Mossbauer absorption measurements [1] on magnetite at room temperature have found the electron exchange relaxation time to be 10 ~ s. The Mossbauer result was obtained by examining the line broadening in the hyperfine pattern associated with the octahedral (B) sites. It was the discrepancy between the electrical conductivity, and Mossbauer results that prompted our interest in magnetite. Conventional Mossbauer absorption spectroscopy has some disadvantages when applied to the study of relaxation problems. Thickness, inhomogeneous, and/ or small field effects will produce line broadening which can be confused with relaxation in certain cases. Recently, selective excitation double Mossbauer (*) Supported by the National Science Foundation Grant No. DMR 73-07665A03. (f) Present address, National Institutes of Health, Bethesda, Maryland. (SEDM) procedures [2] have been developed which can detect relaxation processes in cases where it is difficult to observe otherwise. A complete, general theory which can be applied quantitatively to SEDM experiments has not yet been formulated, although calculations neglecting thickness considerations and Rayleigh scattering have recently been published [3, 4]. In the meantime, we can still perform experiments and compare the results with the rather well established SEDM theory in the absence of relaxation [2]. In figure 1 we show a schematic diagram of an DRIVE FUNCTION ELECTRONICS 8ENERAT0R 1 I ' ' ' SYNCH PULSE DRIVE j 1 . • 2 M U L T I C A D CHANNEL ANALYZER 57 I C o (SOURCE) I • I