Jae Yun Park

Magnetic properties of CoFe2O4 thin films prepared by a sol-gel method

Thin films with cobalt ferrite layers on thermally oxidized silicon wafers were fabricated by a sol-gel method. Magnetic and structural properties of the films were investigated with an x-ray diffractometer, a vibrating sample magnetometer and atomic force microscopy. The crystallization temperature for Co ferrite thin films was determined by using Mossbauer spectroscopy. Co ferrite films annealed at and above 450 °C have only a single phase spinel structure without any preferred crystallite orientation. Their rms surface roughness is less than 3 nm and the size of grains is about 30 nm for annealing temperatures greater than 650 °C. Films fired at and above 550 °C have moderate saturation magnetization and there is no significant difference of their magnetic properties for external fields applied parallel and perpendicular to their planes. The coercivity shows a strong dependence on the annealing temperature.

Growth of ultra-fine cobalt ferrite particles by a sol-gel method and their magnetic properties

Ultra-fine CoFe2O4 particles are fabricated by a sol–gel method and magnetic and structural properties of powders are investigated. Cobalt ferrite powders fired at and above 450 °C have only a single-phase spinel structure and behave ferrimagnetically. Powders annealed at 350 °C have a typical spinel structure and are of the paramagnetic and ferrimagnetic nature, simultaneously. With X-ray diffraction and Mossbauer spectroscopy measurements, the formation of nano-crystallized particles is confirmed when cobalt ferrite is annealed at 200 °C. In addition, the transition from the paramagnetic to the ferrimagnetic state is observed in samples fired at 200 °C as the measuring temperature decreases from the room to liquid nitrogen temperature. The magnetic behaviour of CoFe2O4 powders fired at and above 350 °C shows that an increase of the annealing temperature yields a decrease in the coercivity and, in contrast, an increase in the saturation magnetization. The maximum coercivity and the saturation magnetization of cobalt ferrite powders prepared by the sol–gel method are 2020 Oe and 76.5 e.m.u. g−1, respectively.

Atomic migration in Ni–Co ferrite

Ni–Co ferrite has been studied with Mossbauer spectroscopy and x‐ray diffraction. The crystal structure for this system is spinel, and the lattice constant is in accord with Vegard’s law. The Mossbauer spectra consist of two six‐line patterns corresponding to Fe3+ at the tetrahedral (A) and octahedral (B) sites. The Neel temperature increases linearly with Ni concentration, suggesting the superexchange interacion for the Ni–O–Fe link is stronger than that for the Co–O–Fe link. It is found that Debye temperatures for the A and B sites of CoFe2O4 and NiFe2O4 are found to be θA=734 K, θB=248 K, and θA=378 K, θB=357 K, respectively. The intensity ratio of the A to B patterns is found to increase at low temperatures with increasing temperature due to the large difference of Debye temperatures of the two sites and to decrease at high temperatures due to migration of Fe3+ ions from A to B sites. Atomic migration of CoFe2O4 starts near 400 K and increases rapidly with increasing temperature to such a degree that ...

Magnetic properties of the monoclinic FeRh2Se4

FeRh2Se4 has been studied by x‐rays, Mossbauer spectroscopy, and superconducting quantum interference device (SQUID) magnetometry. Mossbauer spectra of FeRh2Se4 have been taken at various temperatures ranging from 4.2 to 550 K. Magnetic hyperfine and quadrupole interactions at 4.2 K have been fitted, yielding the following results: H=161.6 kOe, 1/2e2qQ(1+1/3η2)1/2=−0.93 mm/s, θ=61°, φ=0°, and η=0.1. The temperature dependence of the quadrupole splitting, calculated in terms of direct lattice‐sum calculations using x‐ray crystallographic data, is in good agreement with experimental values up to 550 K. The axial field parameter Δ1=2.0‖λ‖, the rhombic field parameter Δ2=3.3‖λ‖, and the covalency factor α2=0.65 were obtained. Magnetic susceptibility measurements by dc SQUID magnetometry show that superexchange interactions between Fe2+ ions are antiferromagnetic.

Crystallographic and magnetic properties of CoxFe1−xCr2S4

The mixed series CoxFe1−xCr2S4 has been studied by x‐ray, Mossbauer spectroscopy, and superconducting quantum‐interference device (SQUID) magnetometry. The crystal structure is found to be a cubic spinel, and the lattice constant a0 decreases linearly with increasing cobalt concentration. Mossbauer spectra of CoxFe1−xCr2S4 have been taken at various temperatures ranging from 4.2 to 300 K. The iron ions are ferrous and occupy the tetrahedral sites. The Curie temperature increases linearly with cobalt concentration, suggesting that the superexchange interaction for the Co‐S‐Cr link is stronger than that for the Fe‐S‐Cr link. Magnetic hyperfine and quadrupole interactions in Co0.9Fe0.1Cr2S4 at 4.2 K have been studied, yielding the following results: Hhf=80.8 kOe, 1/2e2qQ(1+1/3η2)1/2=2.65 mm/s, θ=15°, φ=75°, and η=1.0. It is notable that, as the temperature decreases below the Curie temperature, quadrupole splitting increases with decreasing temperature, suggesting the presence of an electric field gradient a...

Crystallographic and magnetic properties of the spinel phase for NixFe1−xCr2S4

Ferrimagnetic NixFe1−xCr2S4 is found to crystallize with a pure spinel structure in the composition range 0≤x≤0.4. 57Fe Mossbauer spectra of NixFe1−xCr2S4 have been taken at various temperatures ranging from 13 K to room temperature. Analysis of Mossbauer spectra and x‐ray crystallographic data indicate that Ni ions occupy the tetrahedral site, and the lattice constant is in good agreement with Vegard’s law. The isomer shifts indicate that the valence states of the Fe ions have a ferrous character. The Neel temperature increases linearly with Ni concentration, suggesting that the superexchange interaction for Ni‐S‐Cr link is stronger than that for Fe‐S‐Cr link. The anomalous reduction of the magnetic hyperfine field below 70 K could be explained in terms of the cancellation effect between mutually opposite orbital current field HL and the Fermi contact field HD. It is notable that, as the temperature decreases below the Neel temperature, both quadrupole shift and asymmetrical line broadening appear and in...

Evolution of structural and magnetic properties and the electronic structure of spinel Fe/sub x/Co/sub 3-x/O/sub 4/ thin films

Evolution of structural, optical, and magnetic properties has been investigated for sol-gel-grown spinel Fe/sub x/Co/sub 3-x/O/sub 4/ thin films as the Fe composition (x) increases from 0 to 2. The crystal structure of Fe/sub x/Co/sub 3-x/O/sub 4/ is found to remain cubic with the lattice constant increasing with increasing x. Coexistence of two phases is observed by X-ray diffraction for 0.76/spl les/x/spl les/0.93, interpreted as due to normal spinel, dominant for x/spl les/0.55, and inverse spinel, dominant for x/spl ges/1.22. Analysis on the measured optical absorption spectra by spectroscopic ellipsometry for the samples indicates the dominance of the normal spinel phase for low x in which Fe/sup 3+/ ions mostly substitute octahedral Co/sup 3+/ sites. X-ray photoelectron spectroscopy measurements revealed that both Fe/sup 2+/ and Fe/sup 3+/ ions exist with similar strength in the x=0.93 sample. Vibrating sample magnetometry measurements revealed that the Fe/sub x/Co/sub 3-x/O/sub 4/ films start exhibiting magnetic hysteresis behaviors for x/spl ges/0.76 with the saturation magnetization increasing with increasing x. Conversion electron Mo/spl uml/ssbauer spectra measured on the x=0.93 sample showed that Fe/sup 2+/ ions prefer the octahedral sites, indicating the formation of the inverse spinel phase. The remarkable change of the lattice constant for x/spl ges/0.76 is derivable from the site preference of the Fe/sup 2+/ and Fe/sup 3+/ ions.

Effects of Cr doping on Magnetic Properties of Inverse Spinel CoFe₂O₄ Thin Films

Variation of magnetic properties through Cr substitution for Co in inverse-spinel CoFe₂O₄ has been investigated by vibrating-sample magnetometry (VSM) and conversion electron Mossbauer spectroscopy (CEMS). Cr x Co 1-x Fe₂O₄ samples were prepared as thin films by a sol-gel method. The lattice constant of the CrxCo1-x- Fe₂O₄ samples was found to remain unchanged, explainable in terms of a reduction of tetrahedral Fe 3+ ion to Fe 2+ due to substitution of Cr 3+ ion into octahedral Co 2+ site. The existence of the tetrahedral Fe 2+ ions in Cr x Co 1-x Fe₂O₄ was confirmed by CEMS analysis. Room-temperature magnetic hysteresis curves for the Cr x Co 1-x Fe₂O₄ films measured by VSM revealed that the saturation magnetization (M S ) increases by Cr doping. The MS is maximized when x = 0.1 and decreases for higher x but is still bigger than that of CoFe₂O₄. The increase of M S can be explained partly by the reduction of the tetrahedral Fe 3+ ion to Fe 2+ .

Ferromagnetic properties of anatase Ti1-xFexO2-δ thin films

The effects of Fe doping on the magnetic and electronic properties of reduced TiO2−δ thin films have been investigated. Anatase Ti1−xFexO2−δ films exhibit a ferromagnetic behavior at room temperature for a certain range of Fe doping. Conversion electron Mossbauer spectroscopy measurements indicate that Fe2+ and Fe3+ ions coexist in the Fe-doped films, substituting the octahedral Ti4+ sites. The contribution of possible Fe3O4 clusters to the observed ferromagnetism is not likely to happen. The Ti1−xFexO2−δ films exhibit a p-type character by Hall effect measurements but the observed ferromagnetism turns out to be independent of the hole concentration. The observed ferromagnetism in the Ti1−xFexO2−δ films can be explained in terms of a direct ferromagnetic coupling between two neighboring Fe3+ ions via an electron trapped in oxygen vacancy nearby.

Structure and magnetic properties of Gd/sub 1-x/Sr/sub x/FeO/sub 3-/spl delta//

The perovskite type Gd/sub 1-x/Sr/sub x/FeO/sub 3-/spl delta// (x=0.0, 0.25, 0.5, 0.75 and 1.0) system has been studied by X-rays, Mohrs salt analysis, and Mossbauer spectroscopy. From the results of X-ray diffraction measurement the structure of the Gd/sub 1-x/Sr/sub x/FeO/sub 3-/spl delta// system is orthorhombic with x values of 0.0, 0.25 and 0.5, and cubic with the x values of 0.75 and 1.0. Mossbauer spectra of Gd/sub 1-x/Sr/sub x/FeO/sub 3-/spl delta// have been taken at various temperatures ranging from 13 to 800 K. Analysis of Mossbauer spectra for this system demonstrated the existence of the mixed valence states of iron and the coordination state of Fe/sup 3+/ and Fe/sup 4+/ ions. The Neel temperature decreases linearly with Sr concentration, suggesting that the superexchange interaction for Gd-O-Fe link is stronger than that for Sr-O-Fe link. >