Geun Young Ahn

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.

Atomic migration in MgFe2−xCrxO4

Mg–Cr ferrite has been studied with Mossbauer spectroscopy, x-ray diffraction, and vibrating sample magnetometer. The crystal structure for this system is spinel structure, 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 Curie temperature decreases linearly with Cr concentration, suggesting the superexchange interaction Fe(A)–O–Fe(B) link is stronger than that for the Fe(A)–O–Cr(B) link. Debye temperatures for the A and B sites of MgFe1.9Cr0.1O4 are found to be θA=515±5 K and θB=265±5 K, respectively. Atomic migration of MgFe1.9Cr0.1O4 starts near 350 K and increases rapidly with increasing temperature to such a degree that 50% of the ferric ions at the A sites have moved over to the B sites by 600 K. The temperature dependence of both the magnetic hyperfine field and magnetization of MgFe1.9Cr0.1O4 is explained by the Neel theory of ferrimagnetism using three superexc...

Interpretation of ferromagnetic Fe doped ZnO by Mössbauer spectroscopy

Single phase Zn0.95Fe0.05O sample was obtained by the sol-gel method with annealing at 650°C for 6h in H2 5%/Ar balance gas atmosphere. The crystalline structure of Zn0.95Fe0.05O is determined to be a P63mc hexagonal structure with lattice constants a0=3.255A and c0=5.207A at room temperature. The Mossbauer spectra were obtained at various temperatures ranging from 4.2to295K. The values of the isomer shifts (δ) show that for all temperature ranges, they are in the ferrous (Fe2+) state. The magnetic hyperfine field (Hhf) and electric quadrupole splitting (ΔEQ) in the weak ferromagnetic state at 4.2K have been analyzed, yielding the following results: Hhf=37.8kOe, θ=67.5°, φ=0°, η=0.75, ΔEQ=2.06mm∕s, and R=7.4, respectively. From the Mossbauer spectrum at 77K, the paramagnetic quadrupole phase is related to the temperature dependence of spin-lattice relaxation.

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.

Mössbauer studies of iron-doped La0.67Sr0.33Mn0.9957Fe0.01O3

The iron-doped perovskite La 0.67 Sr 0.33 Mn 0.99 57 Fe 0.01 O 3 compound has been studied by X-ray diffraction, Mossbauer spectroscopy, and vibrating sample magnetometry. The single phase of the polycrystalline La 0.67 Sr 0.33 Mn 0.99 57 Fe 0.01 O 3 powder has been prepared by a water-based sol-gel method. Crystalline La 0.67 Sr 0.33 Mn 0.99 57 Fe 0.01 O 3 was a rombohedral structure with lattice parameters a 0 = 5.480 A, α = 60.259°. Mossbauer spectra of La 0.67 Sr 0.33 Mn 0.99 57 Fe 0.01 O 3 have been taken at various temperatures ranging from 20 to 400 K. Analysis of 57 Fe Mossbauer spectrum has considered anisotropic hyperfine field fluctuation. Temperature dependence of anisotropy energy is calculated from the relaxation rate.

Magnetic and Electronic Properties of Reduced Rutile Ti 1-x MnxO 2-δ Thin Films

Magnetic and electronic properties of reduced rutile titanium dioxide (TiO 2-δ ) thin films doped by Mn have been investigated. The present sol-gel-grown semiconducting TiO 2-δ :Mn films exhibit a ferromagnetic behavior at room temperature for a limited range of Mn content. The Mn-doped films have p-type electrical conductivity with the carrier concentration near 1019 cm ?3 . The observed room-temperature ferromagnetism is believed to be intrinsic but not related to free carriers such as holes. Oxygen vacancies are likely to contribute to the room-temperature ferromagnetism?trapped carriers in oxygen vacancies can mediate a ferromagnetic coupling between neighboring Mn 3+ ions. The energy band-gap change due to the Mn doping measured by spectroscopic ellipsometry exhibits a red-shift compared to that of the undoped sample at low Mn content. It is explainable in terms of strong spin-exchange interactions between Mn ion and the carrier.

Evolution of magnetic and optical properties in spinel ferrite Fe/sub x/Co/sub 3-x/O/sub 4/ thin film

The spinel ferrite Fe/sub x/Co/sub 3-x/O/sub 4/ thin film was prepared on silicon substrate by sol-gel method. The evolution of structural, magnetic, and optical properties of Fe composition in the film was investigated by vibrating sample magnetometer (VSM), Mossbauer spectroscopy, X-ray photoelectron spectroscopy (XPS), and spectroscopic ellipsometry.

Diluted ferromagnetic properties in Fe- and Co-doped TiO/sub 2-/spl delta// thin films

Magnetic and electronic properties of Fe- and Co-doped TiO/sub 2-/spl delta// thin films are investigated by vibrating sample magnetometer (VSM), Mossbauer spectroscopy, and X-ray photoelectron spectroscopy (XPS). When the precursor films are annealed in vacuum, the resulting oxygen-deficient TiO/sub 2-/spl delta//:Fe and TiO/sub 2-/spl delta//:Co films are found to become semiconducting with p-type carriers in the 10/sup 18/ cm/sup -3/ range. The oxygen-deficient films go through conductivity transitions from n-type to p-type by Fe and Co doping. Results of VSM measurements show that ferromagnetism is exhibited at room temperature. XPS measurements on the thin films reveal that Fe ions have both Fe/sup 2+/ and Fe/sup 3+/ ionic valences while Co ions have Co/sup 2+/ mostly. Also, the density of Fe/sup 2+/ ions is found to decrease after annealing.

Low temperature hydrogen treatment of Fe doped ZnO ferromagnetic semiconductor

Ferromagnetism of Zn/sub 1-x/Fe/sub x/O (x=0.00, 0.01, 0.03, 0.05, 0.07, 0.10) powder at room temperature, fabricated by sol-gel method, is investigated. The dried powder is grounded and annealed at 650/spl deg/C for 6 h in H/sub 2/ 5%/Ar bal. gas atmosphere. Hydrogen treatment is employed to control phase separation. X-ray diffraction with CuK/spl alpha/ radiation of the samples reveal that the doping of iron does not change the wurzite structure of ZnO for doping concentrations below x = 0.07. Above x = 0.10, some secondary peaks appeared, which are defined as phase separation of FeO. Rietveld refinement shows that Zn/sub 0.99/Fe/sub 0.01/O has a hexagonal crystal structure at room temperature. Measured magnetic hysteresis loops determine a ferromagnetic behaviour of the samples with coercivity of 200/spl sim/220 Oe. Also, it is noteworthy that the magnetic moment per Fe atom increases until x=0.07 but decreases in the x=0.10. This phenomena can be explained by decrease of Fe ions which contribute to Zn(Fe)O, and it is closely related to the low antiferromagnetic Neel temperature for the second phase of FeO (/spl sim/210 K).

Structural, Magnetic, and Optical Studies on Normal to Inverse Spinel Phase Transition in Fe x Co 3-x O 4 Thin Films

Phase transition from normal-to inverse-spinel structure has been observed for Fe_xCo_(3-x)O₄ thin films as the Fe composition (x) increases from 0 to 2. The samples were fabricated as thin films by sol-gel method on Si(100) substrates. X-ray diffraction measurements revealed a coexistence of two phases, normal and inverse spinel, for 0.76≤x≤0.93. The normal-spinel phase is dominant for x≤0.55 while the inverse-spinel phase for x≥1.22. The cubic lattice constant of the inverse-spinel phase is larger than that of the normal-spinel phase. For both phases the lattice constant increases with increasing x. X-ray photoelectron spectroscopy measurements revealed that both Fe²+ and Fe³+ ions exist with similar strength in the x=0.93 sample. Conversion electron Mossbauer spectra measured on the same sample showed that Fe²+ ions prefer the octahedral Co³+ sites, indicating the formation of the inverse-spinel phase. Analysis on the measured optical absorption spectra for the samples by spectroscopic ellipsometry indicates the dominance of the normal spinel phase for low x in which Fe³+ ions tend to substitute the octahedral sites.