Toshio Takada

Charge disproportionation in CaFeO3 studied with the Mössbauer effect

Abstract The Mossbauer effect spectrum of stoichiometric CaFeO 3 at 4 K consisting of two magnetic hyperfine patterns with nearly the same intensities is explained assuming a charge disproportionation 2Fe 4+ →Fe 3+ + Fe 5+ . The disproportionation is supposed to set in at or in the vicinity of the Neel temperature. The single magnetic hyperfine pattern for SrFeO 3 at 4 K, on the other hand, indicates a rapid electron exchange between Fe 3+ and Fe 5+ ions, for the center shift and the hyperfine field coincide approximately with the average values of the corresponding parameters for CaFeO 3 .

Preparation and characterization of stoichiometric CaFeO3

Abstract Stoichiometric CaFeO 3 was prepared and was characterized by crystallographic, magnetic and electrical measurements. A slight tetragonal distortion from the ideal cubic perovskite structure was found. The tetramolecular unit cell has a = 5.325 (3) A and c = 7.579 (5) A. The susceptibility showed a maximum at about 115 K and the temperature dependence of electrical resistivity changed from metallic to semiconductive in the vicinity of the magnetic transition temperature. These indicate a phase transition from the metallic-paramagnetic (the high-temperature phase) to the semiconductive-antiferromagnetic phase. The Mossbauer spectra indicated that a charge disproportionation 2Fe 4+ → Fe 3+ + Fe 5+ associates with the transition.

Influence of substrate temperature and film thickness on the structure of reactively evaporated In2O3 films

Abstract Indium oxide films 25–550 A thick were reactively evaporated at an oxygen pressure of about 0.27 Pa and at a substrate temperature between room temperature and 400°C. The dependence of the structure of the films on the substrate temperature and on the film thickness was studied using transmission electron microscopy and electron diffraction. It was found that thick films (about 550 A) were amorphous at room temperature, partially crystallized at 50–125°C and crystalline at 150–400°C. The crystallinity of the films deposited at 150–250°C also depended markedly on the film thickness. Very thin films about 25 A thick were quasi-amorphous, but with increasing film thickness the amorphous phase transformed into a crystalline phase. The thermal transformation of the amorphous films after deposition was also studied. Amorphous films about 550 A thick deposited at room temperature and 100°C crystallized at 230°C and 210°C respectively.

Magnetic Properties of Fe-V Multilayered Films with Artificial Superstructures

Multilayered films with artificial superstructures were synthesized by alternate deposition of Fe and V in ultrahigh vacuum. X-ray diffraction confirmed the establishment of artificial periodicities with very short wavelengths, e.g. Fe(4 A)-V(8 A) of the minimum case, and bcc [110] texture. Magnetic properties as a function of Fe layer thickness were studied from Mossbauer and SQUID measurements. Local magnetization at the Fe interface layer contacting with V is discussed from Mossbauer results on surface-selectively enriched samples and also from polarized neutron diffraction experiments.

Surface magnetism of α-Fe2O3 by Mössbauer spectroscopy

Abstract α-Fe 2 O 3 particles were prepared from pure 56 Fe and the surface was very thinly coated with 57 Fe. Mossbauer results elucidated that the surface hyperfine field rapidly decreased with increase of temperature but the Morin transition took place at the same transition temperature as the bulk crystal.

Crystal growth of vanadium oxides by chemical transport

Single crystals of VO2, V2O3 and the intermediate vanadium oxides between VO2 and V2O3 with the general formula VnO2-n-1 (n = 3, 4, 5, 6, 7, 8,) were grown by the chemical transport using TeCl4 as a transport agent. The crystallography of VnO2n-1 and the electrical properties of the grown crystals are studied. Some related topics are also reviewed.

Preparation by reactive deposition and some physical properties of amorphous tin oxide films and crystalline SnO2 films

Tin oxide films were reactively deposited at oxygen pressures of (0.3–5) × 10-3 Torr and at substrate temperatures of 60–420 °C. The dependence of the crystalline phase of the films on the deposition conditions was studied by X-ray diffraction. It was found that, depending on the substrate temperature, amorphous films or crystalline SnO2 films were deposited at pressures above about 10-3 Torr. The amorphous films were non-conducting but their resistivity could be reduced to less than 1 Ω cm by heat treatment in air at 250 or 300 °C. This decrease in resistivity was accompanied by an increase in the light transmission to about 90%. The crystalline SnO2 films were conducting, had a resistivity of the order of 10-2 Ω cm and were highly transparent with an average light transmission of about 90%.

Mössbauer effect of FeOCl-pyridine complex

Abstract A Mossbauer effect study has been conducted on FeOCl-pyridine complex in the temperature range between 4.2° and 298°K. The isomer shift relative to Fe metal and the quadrupole splitting at 298°K are 0.36 ± 0.01 and 0.92 ± 0.01 mm/sec, respectively. An antiferromagnetic ordering occurs at 65 ± 3°K. The internal magnetic field is 435 ± 10 kOe at 4.2°K. The z axis of the electric field gradient tensor (|Vzz|> |Vxx| ⩾ |Vyy|, η being small) is shown to be parallel to the direction of the internal magnetic field and perpendicular to the crystalline b axis.

NMR Determination of Metal Ion Distribution in Manganese Ferrite Prepared from Aqueous Solution

In order to determine the metal ion distribution in a new type manganese ferrite (MnFe 2 O 4 ), which was prepared by precipitation from aqueous solution, the nuclear magnetic resonance (NMR) of Mn 55 was studied. In this ferrite, two separate resonance signals were observed, which were identified as associated with Mn 2+ ions located at the tetrahedral ( A -) and the octahedral ( B -) sites respectively. By integrating those NMR spectra, it was decided that about 10% of Mn 2+ ions are present at B -sites. Using the ratio of the amount of the Mn 2+ ions located at A -sites to that at B -sites and the value of saturation moment (4.2µ B ), the metal ion distribution in this manganese ferrite was determined to be Mn 0.48 2+ Fe 0.52 2+ (Mn 0.06 2+ Mn 0.46 3+ Fe 1.02 3+ Fe 0.46 2+ )O 4 . From the temperature dependence of the resonance frequency, the Curie temperature was estimated to be 770°K. The high Curie temperature compared with that of ordinary manganese ferrite (573°K) is explainable by the above metal...

Reactive Condensation and Magnetic Properties of Iron Oxide Films

Films of iron oxides such α-Fe3O3 and Fe3O4 were prepared by evaporating iron in a low pressure atmosphere of oxygen gas and investigated by X-ray and electron diffraction and Mossbauer effect measurements. The formation range of iron oxides was determined as functions of the substrate temperature and the deposition rate at an oxygen pressure of 4×10-4 Torr. The oxide film consisted of fine grains, and the grain size of Fe3O4 increased with increase of the film thickness and the substrate temperature. Magnetic properties were investigated. The coercivity was found to increase up to 1000 Oe with the oxidation of the as-deposited Fe3O4 films. The origin of such high coercivity was briefly discussed.