Mössbauer study of the high-temperature phase of Co-substituted magnetites,CoxFe3

Abstract

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.

The Center Shift in Mössbauer Spectra of Maghemite and Aluminum Maghemites

“…The spectra were fitted with two main asymmetric sextet components. The asymmetry is due to the combined effects of random orientation of the hyperfine field with respect to the electric-field-gradient's principal axis and a non-zero dipolar contribution to the hyperfine field (De Grave et aL, 1993). For all maghemites at 4.2 K a small hematite fraction (less than 3 mole %) could be detected in the external-field spectra (Bowen et aL, 1994).…”

Influence of Nonstoichiometry and the Presence of Maghemite on the Mössbauer Spectrum of Magnetite

“…The relative area ratio of the two spectral components at RT is close to 1" 1.88; it is not exactly 1:2, as would be expected by considering the chemical formula, due to different recoilless fractions (Sawatzky et al 1969). For completeness, it should be mentioned that several authors (H~iggstrtm et al 1978, De Grave et al 1993 considered two B-site patterns at RT, with a relative abundance 1:3. The explanation for that interpretation is that the magnetite moments are aligned along a domain's [ 100] axis, while the electric field gradient (EFG) has its principal axis along a local [111] direction, resulting in two distinct quadrupole shifts and dipolar-field contributions to Hhf.B.…”

“…The two models seem to be complimentary, and neither of the two is able to explain the whole of observed magnetic and electronic properties of magnetite. The inclusion of a certain concentration of impurities on B sites, be it vacancies and/or substitutional cations other than iron, seems to favor a more localized electronexchange process (Vandenberghe and De Grave 1989;De Grave et al 1993). Typical values for the hyperfine parameters ofweU-crystallized magnetite at room temperature are (Daniels and Rosencwaig 1969): Hhf, k = 490 kOe, ~A = 0.29 mm/s, Hhf.a = 460 kOe, 68 = 0.66 mm/s and ~Q = 0 for both sites, where Hhf is the hyperfine field, EQ the quadrupole shift and ~ the center shift quoted relative to metallic iron.…”

Nanogrowth twins and abnormal magnetic behavior in CoFe2O4 epitaxial thin films

“…[6][7][8][9] Several papers have previously been published on superexchange interactions in CFO. [10][11][12][13][14] The effect of heat treatment on superexchange interaction is known to be quite small, which means that the interaction is not so sensitive to whether Co occupies A or B sites. PLD, 19 and molecular beam epitaxy.…”