Sam Jin Kim

Crystallographic and magnetic properties of Y3Fe5-xAlxO12

Al 3+ -substituted garnet Y 3 Fe 5-x Al x O 12 (x = 0.0, 0.25, 0.5, 0.75, and 1.0) was fabricated by a sol-gel method. The crystal structure Y 3 Fe 5 O 12 is found to be a cubic with the lattice constant a 0 = 12.381±0.005 A. Mossbauer spectra of Y 3 Fe 5-x Al x O 12 were taken at various absorber temperatures ranging from 20 to 700 K. As the temperature increased toward T N a systematic line broadening effect in the Mossbauer spectra was observed and interpreted to originate from different temperature dependencies of the magnetic hyperfine fields at various iron sites. It results from the distribution ( 6 C n ) of Fe 3+ and Al 3+ at tetrahedral site. The isomer shifts indicated that the iron ions were ferric at the octahedral 16a site and the tetrahedral 24d sites. The quadrupole splitting showed that the orientation of the magnetic hyperfine field with respect to the principle axes of the electric field gradient was random.

Effects of cation distribution for AFeO3 (A=Ga,Al)

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 Mossbauer 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 th...

Preparation of Fe-doped ZnO ferromagnetic semiconductor by sol-gel method with hydrogen treatment

Zn/sub 1-x/Fe/sub x/O (x=0.00,0.01,0.03,0.05,0.07, and 0.10) compounds were fabricated by the sol-gel method. The crystal structure and magnetic properties were investigated as a function of doped Fe concentration. Specifically, we have used hydrogen treatment for the control of phase separation. The X-ray diffraction patterns show that the wurzite structure of ZnO does not change for the doping range below x=0.07. Furthermore, we could not find any Fe cluster or phase separation in the X-ray diffraction patterns. The Fe-doped ZnO indicate ferromagnetic behaviors with the Curie temperature higher than room temperature. Then, the magnetic moment per Fe atom increased with increasing Fe concentration.

Mossbauer studies of /sup 57/Fe-doped anatase TiO/sub 2/

In this paper, the magnetic properties of /sup 57/Fe-doped TiO/sub 2/ with varying /sup 57/Fe contents were studied. In order to determine magnetic behavior and ion state of doped transition metal (/sup 57/Fe) in TiO/sub 2/, Mossbauer studies were carried out.

Magnetic properties of NiZnCu ferrite powders and thin films prepared by a sol-gel method

Ultrafine Ni/sub 0.63/Zn/sub 0.17/Cu/sub 0.2/Fe/sub 2/O/sub 4/ powders and thin films were fabricated by a sol-gel method and their magnetic and structural properties were investigated with thermogravimetric and differential thermal analysis (TG-DTA), X-ray diffractometer (XRD), transmission electron microscope (TEM), Mossbauer spectrometer, atomic force microscope (AFM), and vibrating sample magnetometer (VSM). TG-DTA measurements showed an exothermic reaction peak at 306/spl deg/C with weight loss of 49%. NiZnCu ferrite powders which were fired at and above 450/spl deg/C had only a single phase spinel structure and behaved ferrimagnetically. Powders annealed at 250 and 350/spl deg/C had a typical spinel structure and were simultaneously paramagnetic and ferrimagnetic in nature. The magnetic behavior of NiZnCu ferrite powders fired at and above 550/spl deg/C showed that an increase of the annealing temperature yielded a decrease in the coercivity and an increase in the saturation magnetization. The maximum coercivity and the saturation magnetization of NiZnCu ferrite powders were H/sub c/=160 Oe and M/sub s/=64 emu/g, respectively. NiZnCu ferrite thin films annealed at 650/spl deg/C had a single phase spinel structure and there was no significance difference of their magnetic properties for external fields applied parallel and perpendicular to their planes. The microstructure of thin films annealed at 650/spl deg/C consisted of spherical grains with the average size of 120 nm and 5 nm in surface roughness (rms).

Mössbauer Studies on Exchange Interactions in CoFe2O4

Two polycrystalline samples of CoFe2O4 were prepared by slow cooling and quenching and studied using Mossbauer 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 a0=8.381 A and the quenched sample being a0=8.391 A. The temperature dependence of the magnetic hyperfine field in 57Fe nuclei at the tetrahedral (A) and octahedral (B) sites was analyzed based on the Neel 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 JA–B=-25.0kB and JA–A=-18.9kB, respectively, while the intrasublattice B–B superexchange interaction was ferromagnetic with a strength of JB–B=3.9kB. In the quenched sample, however, their strengths were JA–B=-22.6kB, JA–A=-17.6kB, and JB–B=3.9kB, respectively.

Neutron diffraction and Mössbauer studies on Fe1−xCr2S4(x=0.0, 0.04, 0.08)

Polycrystalline samples of Fe1−xCr2S4(x=0.0, 0.04, 0.08) have been studied with x-ray and neutron powder diffraction, Mossbauer 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). Mossbauer 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.

Mössbauer studies of superexchange interactions and atomic migration in CoFe2O4

The Co ferrite, CoFe 2 O 4 , has been investigated by X-ray diffraction and Mossbauer spectroscopy. The crystal structure is found to be an inverse cubic spinel with the lattice constants a 0 = 8.381 ± 0.005 A and 8.391 ± 0.005 A for slowly cooled and quenched CoFe 2 O 4 , respectively. The iron ions are in ferric Fe 3+ states. The temperature dependence of the magnetic hyperfine fields of 57 Fe at the tetrahedral (A) and octahedral (B) sites is analyzed by the Neel theory of ferrimagnetism For the slowly cooled sample, the A-B intersublattice superexchange interaction is found to be antiferromagnetic with a strength of J A-B = -24.4k B . while A-A and B-B intrasublattice superexchange interactions are antiferromagnetic and ferromagnetic with J A-A = - 18.2k B and J B-B = 3.9k B , respectively. For the quenched sample J A-B = -23.6k B J A-A = - 17.8k B . and J B-B = 3.9k B are found. The decrease of the Mossbauer absorption area ratio of A to B patterns above 400 K is explained in terms of migrating iron ions from A to B sites.

Crystallographic and magnetic properties in CoAl/sub 0.2/Fe/sub 1.8/O/sub 4/ thin films prepared by a sol-gel method

Thin films of Al-substituted cobalt ferrite layers on thermally oxidized silicon wafers were fabricated via the sol-gel method with various annealing temperatures. Structural and magnetic properties of the films were investigated with thermogravimetric and differential thermal analysis (TG-DTA), an x-ray diffractometer, vibrating sample magnetometer (VSM), and atomic force microscopy (AFM). TG-DTA measurements showed exothermic reaction peak at 285/spl deg/C. CoAl/sub 0.2/Fe/sub 1.8/O/sub 4/ thin films that fired at and above 400/spl deg/C had a single cubic spinel structure without any preferred crystallite orientation. Lattice constants monotonically decreased from 0.8381 to 0.8354 nm with increasing annealing temperature from 400 to 800/spl deg/C. As annealing temperature increased from 400 up to 800/spl deg/C, grain size increased from 4.6 to 25.4 nm, whereas the surface roughness was minimized at 700/spl deg/C with a value of 2.0 nm. Parallel and perpendicular coercivity at room temperature showed maximum values of 1980 and 2490 Oe, respectively, in the sample annealed at 700/spl deg/C. Coercivity was shown to be strongly dependent not only on annealing temperature but also on surface roughness.

Mössbauer studies of dynamic Jahn-Teller relaxation on the Cu-substituted sulfur spinel

Samples of Fe1−xCuxCr2S4 (x=0.0, 0.1, 0.3, and 0.5) have been studied with Mossbauer spectroscopy, x-ray diffraction, magnetization, and magnetoresistance. A cusp-like anomaly is observed for the sample x=0.1 in the both field-cooled and zero-field-cooled magnetization curves near 130 K under an applied field H=100 Oe. The charge state of iron ions are ferrous for the samples x=0.0 and x=0.1, whereas they are ferric for the samples x=0.3 and x=0.5. The Mossbauer spectra for the sample x=0.1 show asymmetric line broadening, and it is considered to be dynamic Jahn-Teller relaxation. The unusual reduction of magnetic hyperfine field below 110 K can be interpreted in terms of cancellation effect between the mutually opposite orbital current field HL and Fermi contact field HC.