Piezoelectric and ferrimagnetic AFeO3 (A=Ga,Al) samples have been prepared by various annealing conditions and then their hyperfine structures have been investigated by x-ray diffraction and Mössbauer spectroscopy. From the analysis of the x-ray diffraction patterns by Rietveld refinement method, the crystal structure of samples was found to be an orthorhombic structure (Pc21n,Pna21) with four different cation sites which are labeled A1 and A2 (predominantly occupied by gallium and aluminum ions) and Fe1 and Fe2 (predominantly occupied by Fe ion). The crystal structure is not changed between the samples, but the occupancies of Fe ions in four cationic sites show slight difference. We notice that the occupancies of Fe ion in A1 tetrahedral site of the samples have an effect on the magnetic properties. From the x-ray diffraction results, the ratios of occupied Fe ions in A1 site were determined to be 9.0%, 9.5%, and 7.8% for slow-cooled GaFeO3, quenched GaFeO3, and AlFeO3, respectively, which accord with the result of Mössbauer spectroscopy. We found that the Néel temperature range decreases from 265to250K, with decreasing the Fe–O–Fe bond angles between GaFeO3 and AlFeO3. Also, external field dependence of magnetic moment curve shows a several-stepped shape which is similar with the exchange-spring magnet. It could be explained distinctly by an effect of Fe ion distribution in hyperfine structure.
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.
Polycrystalline samples of Fe1−xCr2S4(x=0.0, 0.04, 0.08) have been studied with x-ray and neutron powder diffraction, Mössbauer spectroscopy, magnetization, and magnetoresitance (MR) measurements. Neutron diffraction patterns were obtained at various temperature ranges from 10 K to room temperature. Neutron diffraction on FeCr2S4 above 10 K shows that there is no crystallographic distortion and reveals antiferromagnetic ordering, with the magnetic moment of Fe+2 (−3.52 μB) aligned antiparallel to Cr3+ (2.72 μB). Mössbauer spectra shows asymmetric line broadening in the temperature range from 13 to 170 K and it is considered to be dynamic Jahn–Teller stabilization. The charge states of the iron ions are ferrous in character. With increasing Fe deficiency, the peak of maximum magnetoresistance of x=0.0, 0.04, and 0.08, occurred at 171, 174, and 186 K, respectively. The increasing temperature of the MR peak position is interpreted as due to an enhancement of activation energy.
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