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Six Co-substituted magnetites Co"Fe3 "0&, with x ~0.04 have been studied by Fe Mossbauer spectroscopy at temperatures T between their respective Verwey transition and =900 K. The Curie temperature T& decreases from 868 K for x =0 to 859 K for x =0.03. All spectra were decomposed into tetrahedral (A-site) and octahedral (B-site) components. The obtained results are consistent with the fact that at low temperatures above the Verwey transition, the spins lie along a domain s [100] axis, but fiip to a [111]axis at a temperature strongly increasing with increasing x. The temperature variations of the hyperfine parameters (center shift 6, quadrupole shift c. &, magnetic hyperfine field Hhf and dipolar field Hd;~) have been determined. The 5 and Hhf values for the octahedral Fe are consistent with the well-known fast electron exchange between the involved Fe + and Fe + species. From the temperature dependence of the B-site linewidth it is concluded that the nature of the exchange process at relatively low temperatures, depending on x, is different from that at higher temperatures. The c& vs T and Hd'p vs T curves for the B sites of the x =0, 0.005, and 0.01 samples (both mentioned parameters for the A sites are zero) were interpreted on the basis of a common crystalline-field model, yielding, as argued, unrealistic values for certain involved physical quantities. The behavior of the center shifts implies a slight temperature variation for the intrinsic isomer shift. Unlike the A-site hyperfine fields, the temperature dependence of the B-site fields could not be reproduced adequately by the usual formulas based on Heisenberg-type exchange. Instead, a non-localized-electron model led to an excellent description of the experimental curves. The bandwidth, which appears in that model, turned out to be high (0.82 eV for x =0 at 300 K) and is suggested to be responsible for the failure of the crystalline-field model to explain the temperature variation of e qg/2 and H~;~. For all compositions e& and H~;~a t selected temperatures have also been determined from spectra recorded in external magnetic fields. These spectra further indicate that a field-induced contribution to the total hyperfine field on the B sites (not on A sites) is to be taken into account. Its origin is discussed briefly.

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