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Mössbauer Studies of Superexchange Interaction in Tetragonal CuFe2O4

CuFe 2 O 4 has been studied by Mossbauer spectroscopy and X-ray diffraction. The crystal is found to have a tetragonal spinel structure with the lattice constants a 0 =(5.805±0.005) A and C 0 = (8.669 ± 0.005) A. The iron ions are in ferric Fe 3+ states. The temperature dependence of the magnetic hyperfine fields at 57 Fe nuclei at the tetrahedral (A) and octahedral (B) sites is analyzed by the Neel theory of ferrimagnetism. The intersublattice superexchange interaction is found to be antiferromagnetic with its strength of J A-B = -19.5k B while intrasublattice superexchange interactions are ferromagnetic with their strengths of J A-A = 3.1k B and J B-B = 4.4k B .

Magnetic properties of Cr/sup 3+/ substituted BaFe/sub 12/O/sub 19/ powders grown by a sol-gel method

Cr/sup 3+/ substituted Ba-hexaferrite was fabricated by a sol-gel method. The crystallographic and magnetic properties of BaFe/sub 12-x/Cr/sub x/O/sub 19/ (0/spl les/x/spl les/7) were investigated XRD, Rutherford backscattering spectrometry, vibrating sample magnetometry and Mossbauer spectroscopy. The crystal structure was found to be magnetoplumbite, typical of M-type hexagonal ferrite. By substituting Fe/sup 3+/ in BaFe/sub 12/O/sub 19/ by Cr/sup 3+/, we have been able to attribute the Mossbauer parameters to the 5 crystallographic sites of the structure. Only the octahedral sublattices were occupied by Cr ions. The isomer shifts indicate that the valence state of the Fe ions was Fe/sup 3+/. The Curie temperatures of BaFe/sub 12-x/Cr/sub x/O/sub 19/ decreased linearly increasing Cr-substitution, at a rate of 55 K/Cr atom.

Mössbauer study of

has been studied by means of M?ssbauer spectroscopy and x-ray diffraction. The crystal is found to have a cubic spinel structure with the lattice constant . The iron ions are in ferric states. The temperature dependences of the magnetic hyperfine fields at the nuclei at the tetrahedral (A) and octahedral (B) sites are analysed using the N?el theory of ferrimagnetism. The inter-sublattice superexchange interaction is found to be antiferromagnetic with a strength of while the intra-sublattice superexchange interactions are ferromagnetic with strengths of and . The Debye temperatures of the tetrahedral and octahedral sites are determined to be and , respectively.

Superparamagnetic relaxation in Cu5FeS4

Abstract Cu5FeS4 has been investigated over a temperature range from 4 to 300 K using Mossbauer technique and X-ray diffraction. At room temperature the structure is orthorhombic with lattice constants a 0 = 10.967 A , b 0 =21.920 A , and c 0 = 10.967 A . The iron ions are found to be in ferric states. As the temperature increases toward the Neel temperature, line broadening and a pronounced central peak appear, suggesting superparamagnetic relaxation. Temperature dependence of anisotropy energy is calculated from the relaxation rate.

Mössbauer study of Zn0.4Cu0.6Fe1.2Cr0.8O4

Abstract Zn 0.4 Cu 0.6 Fe 1.2 Cr 0.8 O 4 has been studied by Mossbauer spectroscopy, SQUID magnetometry, and X-ray diffraction. The crystal is found to have a cubic spinel structure with the lattice constant a 0 =8.3667 A . The iron ions are in ferric states and occupy both the tetrahedral (A) and octahedral (B) sites; the fractions of the iron ions at the A-sites and B-sites are 0.52 and 0.34, respectively. While spin orderings are collinear at higher temperatures, spin canting begins to appear around 25 K and increases with decreasing temperature; the canting angle at 4.7 K reaches up to 27°. Debye temperatures of the tetrahedral and octahedral sites are determined to be 339 and 335 K, respectively.

Order-disordered structure and magnetic properties of Li0.5Fe2.5−xRhxO4

Li0.5Fe2.5−xRhxO4 (x=0.25–1.50) has been studied by Mossbauer spectroscopy, superconducting quantum interference device magnetometry, and x-ray diffraction. The crystals are found to be a cubic spinel structure and have been classified into two different sets by crystallographic symmetry, the space group Fd3m for x=0.25–1.25 and the space group F4¯3m for x=1.50, respectively. The migration of Li ion has been confirmed by x-ray patterns and the results of Mossbauer analysis. The saturated magnetic moment measured at 4.2K and Mossbauer spectra taken at various temperatures with 6.0T applied field show that the spin structure of Li0.5Fe2.5−xRhxO4 has the collinear Neel model.

Interface Sensitive Photocurrent Spectrum of ZnSe/GaAs Heterostructure with Laser Illumination

We have performed new modulation photocurrent (PC) and current-voltage (I-V) measurements on 1 μm thick ZnSe epilayers grown on a (100) GaAs substrate, which are sensitive to interfacial properties. When the sample was modulated by the pump beam whose energy is between that of the ZnSe and GaAs bandgaps, the intensity of the PC signals only above the ZnSe absorption edge increased. Through the analysis on the modulation mechanism we showed the existence of trap states at the interface and its strong influence on the current flow in the epilayers. In addition, we observed states localized at the interface by the modulated PC spectrum. The possible mechanisms of the modulation were also discussed.

Mssbauer Study of Fe7Se8?xSx

Effects of interatomic separation on superexchange interactions, interatomic binding, and spin-rotation transition in Fe7Se8—xSx have been studied by Mossbauer spectroscopy and X-ray diffraction. The crystal structure is found to be “3c” hexagonal superstructure of the NiAs structure, and lattice constants A and C decrease linearly with increasing sulphur concentration. As interatomic distance decreases, Debye temperature, Ne�el temperature, and magnetic hyperfine field increase monotonically. When the number of sulfur ions replacing selenium ions increases from 1 to 7, the interatomic distance decreases by 4.4%, and Debye temperature, Neel temperature, and magnetic hyperfine field at 15 K increase by 12%, 10%, and 6%, respectively. While the spin rotation near 130 K persists for Fe7Se7S, it disappears in Fe7Se8—xSx (x ≥ 2). The iron ions at all three sites are found to be in a highly covalent ferrous state.

Superparamagnetic Relaxation in Cu0.5Fe0.5Cr0.5Rh1.5S4

Cu 0.5 Fe 0.5 Cr 0.5 Rh 1.5 S 4 has been studied by Mossbauer spectroscopy, X-ray diffraction and magnetic measurements. The crystal structure is found to be a cubic spinel with lattice constant ao = 9.884 A. The iron ions are in ferric states and occupy the tetrahedral sites. As the temperature increases toward the Neel temperature, line broadening and a pronounced central peak appear, suggesting superparamagnetic relaxation. As the temperature increases, the relaxation rate increases rapidly with a tenth power of the absolute temperature.

Mössbauer study of Cu0.5Fe0.5Rh2SexS4-x

Abstract Effects of interatomic separation on superexchange interactions and interatomic binding in Cu 0.5 Fe 0.5 Rh 2 Se x S 4 − x (0 ⩽ x ⩽2) have been studied by Mossbauer, X-ray and magnetic-susceptibility techniques. The crystal structure is found to be a cubic spinel, and lattice constant a 0 increases linearly with increasing selenium concentration. Temperature dependence of the magnetic hyperfine field at each site follows the Brillouin function for S = 5 2 . The longer-range superexchange interaction between second-nearest Fe neighbors is weakened much more rapidly by increase of interatomic separation than that between first-nearest Fe neighbors. As interatomic distance increases, both Neel and Debye temperatures decrease exponentially while magnetic hyperfine field decreases linearly.