Magnetic effects in Co_3−x O₄ defect films from the data of emission Mössbauer spectroscopy

Abstract

The emission Mössbauer spectra of Co_3−x O₄ defect films are measured in external magnetic fields at strengths of 0.6–3.5 T. It is shown that the memory effect is observed in the spectra of Co_3−x O₄ defect films after exposure to an external magnetic field. At temperatures above the Néel temperature T _N=26 K, the memory effect manifests itself in an increase in the relative contribution from the spectral line of the tetrahedral A sublattice in the spinel structure of Co_3−x O₄ crystallites ([Co ²⁺_0.83 ]_tetr[Co ³⁺_2.20 ]_octO₄ and [Co ²⁺_0.95 ]_tetr[Co ³⁺_2.10 ]_octO₄ prior to and after the magnetic field treatment, respectively). The isomer shifts δ and the quadrupole splitting ΔE of the spectral lines for both A and B sublattices also change from δ_A=−0.19 mm/s, δ_B=−0.31 mm/s, and ΔE _B =0.83 mm/s to the values δ_A=−0.24 mm/s, δ_B=−0.33 mm/s, and ΔE _B =0.60 mm/s, which are close to δ and ΔE for stoichiometric Co₃O₄ oxide. In the low-temperature spectra (T<T _N), the memory effect additionally shows itself as a decrease in the hyperfine magnetic field H _hf of the spectral component for the A sublattice as compared to that in the spectra measured prior to the magnetic field treatment. It is assumed that the concentration of cation vacancies decreases (and, correspondingly, the fraction of reduced Co²⁺ cations increases) in the Co_3−x O₄ defect films under the action of an external magnetic field. A possible mechanism of this process is proposed.