Mössbauer Studies on Exchange Interactions in CoFe₂O₄

Abstract

Two polycrystalline samples of CoFe2O4 were prepared by slow cooling and quenching and studied using Mössbauer spectroscopy and X-ray diffraction. The crystals were found to have a cubic spinel structure with the lattice constants of the slowly cooled sample being a 0=8.381 Å and the quenched sample being a 0=8.391 Å. The temperature dependence of the magnetic hyperfine field in 57Fe nuclei at the tetrahedral (A) and octahedral (B) sites was analyzed based on the Néel theory of ferrimagnetism. For the slowly cooled sample, the intersublattice A–B superexchange interaction and intrasublattice A–A superexchange interaction were antiferromagnetic with a strength of J A–B =-25.0k B and J A–A =-18.9k B, respectively, while the intrasublattice B–B superexchange interaction was ferromagnetic with a strength of J B–B =3.9k B. In the quenched sample, however, their strengths were J A–B =-22.6k B, J A–A =-17.6k B, and J B–B =3.9k B, respectively.

Mössbauer and Magnetic Properties of Coherently Mixed Magnetite-Cobalt Ferrite Grown by Infrared Pulsed-Laser Deposition

“…At room temperature (RT), a clear magnetic pattern is observed with no singlet or doublet contribution. Similar spectra have been previously reported for CoFe2O4, [16][17][18][19] confirming that cobalt ferrite is magnetically ordered at room temperature. The highest velocity peak, at 7 mm s -1 , shows a clear assymetry that strongly suggests that at least it involves two different components.…”

“…Such ratio is shown in Figure 3a for several values of the respective Debye temperatures, as well as the experimental data. This strong dependence of the recoil free fraction for cobalt ferrite has been previously reported by Kim et al [16,22] Another way to estimate the Debye temperatures is through the second order Doppler (SOD) shift [21] of the isomer shift. In this case, a temperature dependent isomer shift is assumed to arise from the mean square velocity of each atom,…”

“…As shown in Figure 3b the experimental data, although following the trend in temperature, do not fall on the predicted curves for either Kim et al [16] Debye temperatures, or the more appropriate ones that correspond to our room temperature data of (735 K / 190 K). A more detailed study would be interesting to understand the source of this lack of agreement, although it is worth mentioning that both estimates are unequally affected by the high-frequency lattice vibrations cut off in the Debye model.…”

“…21. The ratios between the recoil free fraction of the 3 tet Fe  components of cobalt ferrite and magnetite is assumed to be 1.3 as it arises from the respective Debye temperatures of 725 K [16] and 334 K. [21] Using all the constraints together provides a composition of each film and also the detailed cation distribution and oxygen deficiency of the cobalt ferrite phase, which given some of the assumptions should be taken as a rough indication of the relative importance of the magnetite component in the film rather than as a definite statement of each component composition. We note that, due to their isostructural nature, most material science characterization techniques will be relatively blind to this kind of coherent mixture of magnetite and cobalt ferrite, highlighting the need for Mössbauer spectroscopy characterization in this context.…”

Spinel ferrite nanocrystals embedded inside ZnO: Magnetic, electronic, and magnetotransport properties

“…The hyperfine parameters calculated according to the evaluations of the spectra are given in Table I. The outer sextet (S1) with a larger magnetic hyperfine field corresponds to octahedral sites, while the inner one (S2) with a smaller magnetic hyperfine field to Fe 3+ at tetrahedral sites 47,48,49,50,51 . This feature of two sextets is a fingerprint that identifies ferrites.…”