Setsu Morimoto

Mössbauer study on the high pressure phase of Fe2O3

Abstract Mossbauer spectroscopy is applied to study the electronic state of iron ions in Fe2O3 under high pressure up to 60 GPa using a diamond anvil cell. Spectra of the high pressure phase are found to consist of at least two components, a doublet and a sextet, indicating that the simple high-spin low-spin transition mechanism should be reconsidered.

Mössbauer and Neutron Scattering Studies of Magnetic Properties of Random Mixtures with Competing Spin Anisotropies: FexCo1-xTiO3

The physics of random mixtures with competing spin anisotropies is found to be much more complex than ever considered. According to our neutron scattering study of Fe x Co 1- x TiO 3 , orderings of the orthogonal spin components S // and \(\vec{\textbf{\itshape S}}_{\bot}\) seem to be decoupled with each other though not complete; two antiferromagnetic (AF) phases and a mixed ordering phase appear on the concentration vs temperature phase diagram. However, according to our Mossbauer study, S // and \(\vec{S}_{\bot}\) seem to be closely coupled in the AF phases; the boundary between the AF and the mixed ordering phase is not found. This apparently contradictory result is interpreted as follows. In the \(S_{//}(\vec{\textbf{\itshape S}}_{\bot})\) ordered AF phase, orientational distribution of \(\vec{\textbf{\itshape S}}_{\bot}(S_{//})\)s is such that it does not produce coherent scatterings of neutrons, but each \(\vec{\textbf{\itshape S}}_{\bot}(S_{//})\) stays statically at least in the time scale of th...

Behavior of the Transverse Spin Component in the Short-Range Ising Spin-Glass Fe0.5Mn0.5TiO3

The spin-glass Fe 0.5 Mn 0.5 TiO 3 ( T SG =21.5 K) has been studied by low-field dc-magnetization, neutron scattering and Mossbauer measurements. The results obtained by the magnetization and neutron scattering measurements indicate that only the longitudinal spin component freezes below T SG . This fact shows that Fe 0.5 Mn 0.5 TiO 3 behaves as a typical Ising spin-glass in both of the measurements. However, the hyperfine fields H hf at Fe nuclei observed by the Mossbauer measurement point to directions other than the hexagonal c -axis in the spin-glass state. This indicates that the transverse spin component contributes to H hf as does the longitudinal one. Combining the results obtained by the three methods, it is concluded that the transverse spin components fluctuate rather incoherently with one another with characteristic times longer than our Mossbauer time scale of ∼10 -7 s.

Mössbauer and neutron diffraction studies of competing magnetic orderings in random mixtures: Co1−xFexTiO3

Combined Mossbauer and neutron diffraction studies of Co1−xFexTiO3 show that the physics of random mixtures with competing orthogonal spin anisotropies is much more complex than ever considered. These studies give evidence that the orthogonal spin components are closely coupled with each other even in the antiferromagnetic phases. It is also shown that the transition temperature from the antiferromagnetic to the mixed-ordering phase is characteristic for the neutron diffraction measurement but not for the Mossbauer measurement.

Mössbauer Study of Reentrant Spin-Glass Pd1-x-yFexMny: Evidence for Coexistence of Spin-Glass and Ferromagnetic Orderings

The Mossbauer study has been made on the reentrant spin-glass state of Pd 0.881 Fe 0.027 Mn 0.092 and Pd 0.930 Fe 0.015 Mn 0.055 . The anomalous increase of the average hyperfine field \bar H hf , found in some alloys with the reentrant spin-glass transition, was clearly observed below the spin-glass transition temperature T g . This can be explained as freezing of the transverse spin component at T g . From the variation of the Mossbauer absorption intensity ratios with applied field, it has been shown that Fe spins behave partly ferromagnetic and partly like a spin-glass. We consider that this fact is an evidence for the coexistence of the spin-glass and ferromagnetic orderings. A model to explain our results has been proposed. It has been also shown that the spin-glass state is microscopically not much affected by magnetic fields up to 5 kOe, the maximum value examined.

Nuclear Magnetic Resonance of Fe 57 in Various Ferrimagnetic Oxides

Enhanced nuclear magnetic resonance of Fe 57 in yttrium iron garnet, nine substituted iron garnets, magnetite and hematite were observed. In garnets two resonances for two positions of Fe 3+ were measured. Resonance frequencies in rare earth substituted iron garnets do not so much differ from the resonance frequencies of Fe 57 in yttrium iron garnet. If one compares relatively accurate resonance frequencies observed in various garnets, it is clearly seen that the effective magnetic field at the nucleus is proportionally increased with their crystal lattice parameters. The fractional variation of this field over all garnets is within four percents on both sites. The resonance of Fe 57 in magnetite is also enhanced by domain wall excitation. Two resonance frequencies corresponding to octahedral and tetrahedral sites were observed at room temperature. Below the transition temperature only one broad resonance was confirmed. A resonance of Fe 57 corresponding for Fe 3+ in hematite was observed on sample contai...Enhanced nuclear magnetic resonance of Fe/sup 57/ in yttrium iron garnet, substituted iron garnets, magnetite, and hematite were observed. In garnets, two resonances for two positions of Fe/sup 3+/ were measured. Resonance frequencies in rare-earth-substituted iron garnets differ little from the resonance frequencies of Fe/sup 57/ in yttrium iron garnet. Comparing relatively accurate resonance frequencies observed in various garnets, it is seen that the effective magnetic field at the nucleus is proportionally increased with the crystal lattice parameters. The fractional variation of this field over all garnets is within 4% on both sites. The resonance of Fe/sup 57/ in magnetite is also enhanced by domain-wall excitation. Two resonance frequencies corresponding to octahedral and tetrahedral sites were observed at room temperature. Below the transition temperature, only one broad resonance was confirmed. A resonance of Fe/sup 57/ corresponding for Fe/sup 3+/ in hematite was observed in a sample containing a small amount of magnetite.

Magnetic behavior of Fe0.2Mg0.8TiO3 and Fe0.3Mg0.7TiO3 probed by Mössbauer timescale

Magnetization and Mössbauer measurements have been made on a diluted antiferromagnets FexMg1−xTiO3 with x=0.2, 0.3 and 0.7. It has been demonstrated that this system exhibits a spin glass or a reentrant spin glass like behavior in the samples with x around the percolation concenrration xc≈0.25. It has been shown that various sizes of magnetic clusters are formed well above the transition temperatures determined by the magnetization measurements in the samples with x near xc. The behavior of Fe0.2Mg0.8TiO3 is discussed as a typical cluster glass.

Nuclear Magnetic Resonance of Fe57 in Lithium Ferrite and Magnetite

Nuclear magnetic resonance of Fe57 in lithium ferrite a frequency modulated spectrometer operated at a low power level. Second derivative of absorption versus frequency exhibited two strong lines and two weak lines. A sample quenched from high temperature exhibited resonances although the intensity was decreased. The x-ray diffraction of this sample showed weak superstructure lines which indicates a residual ordering. (W.D.M.)

Studies on spin-glass freezing and antiferromagnetic long-range order in an Ising spin-glass system Fe x Mn1−x TiO3

A Mössbauer time window has been positively used for studying the behavior of spins in FexMn1−xTiO3 withx=0.50, 0.60 and 0.65 which are of typical Ising spin-glass systems. Spin dynamics aboveTSG andTN (RSG) (definition is given in the text) are discussed. In the RSG samples, the existence of contributions of the transverse spin component to the hyperfine fields is pointed out.

Mössbauer Study on Fe-Rich Amorphous Fe–La Alloys

Mossbauer spectra have been measured for two Fe-rich amorphous alloys, a -Fe 91.5 La 8.5 with spin glass behavior and a -Fe 82.5 La 17.5 with reentrant spin glass behavior, in the temperature range between 4.2 K and room temperature. These spectra are analyzed using the Hesse and Rubertsch method to obtain the distribution of hyperfine fields. The distribution of hyperfine fields at 4.2 K consists of a main component ranging from 200 kOe to 400 kOe and a sub-component around 150 kOe. The average Fe-moment is estimated to be 1.96µ B for a -Fe 91.5 La 8.5 and 2.00µ B for a -Fe 82.5 La 17.5 . Hyperfine fields in a -Fe 91.5 La 8.5 appear progressively below 150 K, which is apparently higher than the spin glass temperature of 124 K determined by the ac susceptibility measurement. On the other hand, hyperfine fields in a -Fe 82.5 La 17.5 appear just below the ferromagnetic ordering temperature of 267.5 K, but increase anomalously below 70 K. This anomaly can be explained in terms of the freezing of transverse c...