Ichiro Tamura

Mössbauer Studies on Particle Volume Distribution of α-FeOOH in Rust Formed on Weathering Steel

Rust formed on weathering steel and that on mild steel were studied by Mossbauer spectroscopy at various temperatures from room temperature to 12 K. The observed spectra were decomposed into the contribution of fine antiferromagnetic particles of α-FeOOH (85 at.%, TN=403 K) and that of γ-FeOOH (15 at.%, TN=73 K). Volume distributions of α-FeOOH particles obtained by analyzing the superparamagnetic relaxation effect on Mossbauer spectra show the following distinct characteristics: While 83 vol% of α-FeOOH particles in the rust on mild steel are larger than 7.5×10-24 m3 and the rest of them (17 vol%) are smaller than 3.0×10-24 m3, the rust on weathering steel shows a continuous and wide volume distribution down to the ultrafine particle region (smaller than 3.0×10-24 m3) without a missing region, and 53 vol% of particles are smaller than 7.5×10-24 m3. It is understood that this continuous volume distribution of α-FeOOH particles in the rust on weathering steel is favorable for accommodating a closely packed structure in the protective rust layer on weathering steel.

Mössbauer effect in the oxide surface layer on iron microcrystals and an interpretation of the spectrum

Abstract Iron microcrystals with a slowly oxidized surface have been studied by the Mossbauer technique. The spectrum of the oxide surface layer is a hyperfine split pattern at low temperatures and changes into a single very broad absorption band at the room temperature. The temperature change of the spectrum can be explained by the model in which an asymmetrical uniaxial anisotropy is assumed for each of the oxide crystallites constituting the surface layer. The anisotropy results from the combination of a symmetrical uniaxial anisotropy of the crystallite and a smaller unidirectional anisotropy due to the interaction with the contiguous metallic core.

Magnetic interactions among closely packed γ-Fe2O3 microcrystals studied by Mössbauer spectroscopy

Abstract The Mossbauer effect in chemically synthesized microcrystalline γ-Fe 2 O 3 which were closely stacked was studied and the magnetic interactions among the crystallites were discussed by comparing the results with a model based on Southerns effective-field theory of disordered spin systems (B.W. Southern, 1976). The model is a generalization of Morups super-spin-glass and super-ferromagnet model (S. Morup et al., 1983); moreover, it assumes a divisibility of the system into quasihomogeneous microsubsystems, each of which can be treated by Southerns theory applied to infinite spins. A fairly good fit is obtained between the theoretical and experimental spectra which are dependent on temperature and applied field. The results indicate that the systems are in a super-spin-glass phase at low temperatures.

Origin of Weak Ferromagnetism in YbxFe4Sb12, Relationship between Weak Ferromagnetism and Filling Ratio x

The magnetization and specific heat of single-crystalline samples of filled-skutterudite compounds Yb x Fe 4 Sb 12 were measured. Six single crystals with the filling ratio of Yb, x , from 0.875 to 0.910 were prepared. The weak ferromagnetism of Yb x Fe 4 Sb 12 appearing at low temperatures was found to strongly correlate with x ; with increasing x from 0.875 to 0.910, the transition temperature, T C , decreases from 17 to 5 K and the weak-ferromagnetic moment, m 0 , decreases from 0.066 to 0.005 µ B /Fe. From these trends, both T C and m 0 were expected to reach zero at the critical concentration, x c =0.93. In the temperature dependence of the specific heat, no anomaly at T C was observed, reflecting that m 0 is extremely small compared with the effective Bohr magneton number. With the application of pressure, the T C of the sample with x =0.882 increases rapidly, with the ratio of d T C /d P =+5.27 K/GPa. This weak ferromagnetism of Yb x Fe 4 Sb 12 originates from the itinerant 3d electrons of Fe in an...

Mössbauer Effect and Magnetization Studies of YbFe4Sb12 and LaFe4Sb12

We prepared two YbFe 4 Sb 12 samples (#1 and #2) and a LaFe 4 Sb 12 sample. Mossbauer and magnetization measurements were performed on the samples at various temperatures and in applied magnetic fields. The experimental results indicate that #1 is in a weakly ferromagnetic state below 13 K, while #2 is in an itinerant paramagnetic state even at 2 K, and that quadrupole splitting for #1 at each temperature is slightly less than that for #2. Therefore, we conclude that YbFe 4 Sb 12 is an intermediate material between a nearly ferromagnetic material and a weakly ferromagnetic material.

Antiferromagnetic transition of fayalite under high pressure studied by Mssbauer spectroscopy

Néel temperature (TmNof α-Fe2SiO4 (fayalite) was measured as a function of pressure by means of Mössbauer spectroscopy in the pressure range 0–16 Gpa. High pressure was generated using a clamp-type miniature diamond anvil cell which was inserted into a cryostat. The Néel temperature increased linearly with increasing pressure at a rate of dTN/dp=2.2±0.2 K/GPa. The result is discussed on the basis of the model proposed for the magnetic structure of fayalite by Santoro et al. (1966). The observed dTN/dp suggests that the superexchange interactions vary as the −10/3 power of the volume while the volume dependence of the direct exchange interactions is positive and small.

Mössbauer effect study of the internal magnetic field in small iron particles

Abstract A temperature and size dependent enhancement of the internal magnetic field was observed for small iron particles produced by the gas evaporation technique and was explained by the demagnetization field and the dipole interaction of the particles. Thermal fluctuation of the magnetization and fusion between the particles have important effects.

Mössbauer Effect Study of Lattice Vibration of the Interface Layer between the Oxidized Surface and the Metallic Core of Small Iron Particles

The lattice vibration of the metallic interface layer between the oxidized surface and the metallic core of small iron particles was studied by Mossbauer spectroscopy. From the temperature dependence of the absorption area of the six-line component due to the interface layer, a Debye temperature of 184 K was deduced, indicating a marked softening of lattice vibration. The average thickness of the interface layer was estimated to be of one atomic dimension.

Explanation for magnetic properties of interacting iron oxide nanocrystals

To explain the results of magnetization measurements and Mossbauer measurements on interacting iron oxide nanocrystals which have been produced by leaving iron nanoparticles in air of 1 atm for 10 years, we propose the following model. Several tens of the oxide nanocrystals randomly cohere to form a cluster. As a result, we assume that each nanocrystal within a cluster has random unidirectional magnetic anisotropy, and that the clusters are weakly interacting each other at random. The super-spin-glass model is used for analyzing the random interaction among the clusters.

Mössbauer effect in oxidized iron fine particles and explanation of the spectra by magnetic fluctuation

Abstract Mossbauer measurements were made on iron fine particles of various degrees of oxidation, and the spectra due to the oxidized part were studied with varying temperature (78 K - room temperature) and applied magnetic field (0–1.05 T). All samples had similar dependence of the spectral shape on temperature below 200 K. The spectra for less oxidized samples were only slightly affected by an applied magnetic field of 0.71 T at room temperature. For largely oxidized samples, strong dependence on temperature and applied field was observed above 200 K and below 0.36 T. The dependence of the spectral shape on temperature and applied field was discussed by fitting a magnetic fluctuation model to the experimental spectra. The result showed that there was a thermal fluctuation of the magnetization vector of the microcrystals constituting the oxide surface layer under the influence of various magnetic interactions, and that the magnetic fluctuation of the cluster as a whole on a much longer time scale was responsible for the rapid change of the spectra with temperature and applied field above 200 K and below 0.36 T observed for largely oxidized samples.