Takaya Mitsui

Spin Ordering in LaFeAsO and Its Suppression in Superconductor LaFeAsO0.89F0.11 Probed by Mössbauer Spectroscopy

57 Fe Mossbauer spectroscopy was applied to an iron-based layered superconductor LaFeAsO 0.89 F 0.11 with a transition temperature of 26 K and to its parent material LaFeAsO. Throughout the temperature range from 4.2 to 298 K, a singlet pattern with no magnetic splitting was observed in the Mossbauer spectrum of the F-doped superconductor. Furthermore, no additional internal magnetic field was observed for the spectrum measured at 4.2 K under a magnetic field of 7 T. On the other hand, magnetically split spectra were observed in the parent LaFeAsO below 140 K, and this temperature is slightly lower than that of a structural phase transition from tetragonal to orthorhombic phase, which accompanies the electrical resistivity anomaly at around 150 K. The magnetic moment is estimated to be ∼0.35 µ B /Fe from the internal magnetic field of 5.3 T at 4.2 K in the orthorhombic phase, and the spin disorder appears to remain in the magnetically ordered state even at 4.2 K. The lack of a magnetic transition in LaFeA...

Development of an energy-domain 57Fe-Mössbauer spectrometer using synchrotron radiation and its application to ultrahigh-pressure studies with a diamond anvil cell.

An energy-domain (57)Fe-Mössbauer spectrometer using synchrotron radiation (SR) with a diamond anvil cell (DAC) has been developed for ultrahigh-pressure measurements. The main optical system consists of a single-line pure nuclear Bragg reflection from an oscillating (57)FeBO(3) single crystal near the Néel temperature and an X-ray focusing device. The developed spectrometer can filter the Doppler-shifted single-line (57)Fe-Mössbauer radiation with a narrow bandwidth of neV order from a broadband SR source. The focused incident X-rays make it easy to measure a small specimen in the DAC. The present paper introduces the design and performance of the SR (57)Fe-Mössbauer spectrometer and its demonstrative applications including the newly discovered result of a pressure-induced magnetic phase transition of polycrystalline (57)Fe(3)BO(6) and an unknown high-pressure phase of Gd(57)Fe(2) alloy placed in a DAC under high pressures up to 302 GPa. The achievement of Mössbauer spectroscopy in the multimegabar range is of particular interest to researchers studying the nature of the Earths core.

Generation and Application of Ultrahigh Monochromatic X-ray Using High-Quality 57FeBO3 Single Crystal

Ultrahigh monochromatic 14.4 keV X-rays with a narrow bandwidth of 15.4 neV were generated successfully with a high counting rate of 12,000 counts/s at the undulator beamline (BL11XU) of SPring-8. It was achieved by combining an intense X-ray from the third generation synchrotron radiation facility SPring-8 and pure nuclear Bragg scattering of a very high-quality 57FeBO3 perfect single crystal at the Neel temperature. We describe the detailed study of the beam characteristics and some performance test experiments of energy-domain synchrotron radiation Mossbauer spectroscopy, including a high-pressure experiment using a diamond anvil cel.

High-pressure radiative conductivity of dense silicate glasses with potential implications for dark magmas

The possible presence of dense magmas at Earths core-mantle boundary is expected to substantially affect the dynamics and thermal evolution of Earths interior. However, the thermal transport properties of silicate melts under relevant high-pressure conditions are poorly understood. Here we report in situ high-pressure optical absorption and synchrotron Mössbauer spectroscopic measurements of iron-enriched dense silicate glasses, as laboratory analogues for dense magmas, up to pressures of 85 GPa. Our results reveal a significant increase in absorption coefficients, by almost one order of magnitude with increasing pressure to ~50 GPa, most likely owing to gradual changes in electronic structure. This suggests that the radiative thermal conductivity of dense silicate melts may decrease with pressure and so may be significantly smaller than previously expected under core-mantle boundary conditions. Such dark magmas heterogeneously distributed in the lower mantle would result in significant lateral heterogeneity of heat flux through the core-mantle boundary.

Synchrotron radiation-based Mössbauer spectra of 174Yb measured with internal conversion electrons

A detection system for synchrotron-radiation (SR)-based Mossbauer spectroscopy was developed to enhance the nuclear resonant scattering counting rate and thus increase the available nuclides. In the system, a windowless avalanche photodiode (APD) detector was combined with a vacuum cryostat to detect the internal conversion (IC) electrons and fluorescent X-rays accompanied by nuclear de-excitation. As a feasibility study, the SR-based Mossbauer spectrum using the 76.5 keV level of 174Yb was observed without 174Yb enrichment of the samples. The counting rate was five times higher than that of our previous system, and the spectrum was obtained within 10 h. This result shows that nuclear resonance events can be more efficiently detected by counting IC electrons for nuclides with high IC coefficients. Furthermore, the windowless detection system enables us to place the sample closer to the APD elements and is advantageous for nuclear resonant inelastic scattering measurements. Therefore, this detection system...

Mössbauer spectroscopy in the energy domain using synchrotron radiation

We have developed a Mossbauer spectroscopic method that yields absorption-type spectra by using synchrotron radiation. Owing to the energy selectivity of synchrotron radiation, this method can be applied to almost all Mossbauer nuclides including those that are difficult to prepare their parent radioactive sources. This method offers the flexibility of measuring the sample in a transmission or in a scattering configuration depending on the sample condition. We have investigated the modulation and narrowing of the spectral shapes caused by the time-window effect, and we have confirmed that the previously developed theory reproduces the measured spectra well.

Variable-Frequency Nuclear Monochromator Using Single-Line Pure Nuclear Bragg Reflection of Oscillating 57FeBO3 Single Crystal

A variable-frequency 57Fe nuclear monochromator has been developed using a pure nuclear Bragg reflection from an oscillating 57FeBO3 single crystal at the Neel temperature. The Doppler-shifted single-line 57Fe Mossbauer radiation at a fixed beam position was successfully filtered using the third-generation synchrotron radiation (SR) of SPring-8. We describe the principle of the variable-frequency Mossbauer filtering technique, the study on the beam characteristics, and some demonstrative experiments on energy-domain SR Mossbauer spectroscopy.

Ultrahigh-pressure study on the magnetic state of iron hydride using an energy domain synchrotron radiation 57 Fe Mössbauer spectrometer

Pressure induced magnetic phase transition of iron hydride was investigated with an in-situ Mossbauer spectrometer using synchrotron radiation (SR). The spectrometer is composed of a high resolution monochromator, an X-ray focusing device, a variable frequency nuclear monochromator and a diamond anvil cell. The optical system, advantages of the spectrometer and the observed high pressure magnetic phases of iron hydride are described.

Nuclear Resonant Quasielastic Scattering from Fe Cations in Nafion Membranes: Effect of Dynamics in a Short Time Range

The effect of the diffusive motion in a short time range on the nuclear resonant quasielastic scattering spectrum is presented. The solution of the Langevin equation, which describes the motion of ...

Nuclear Resonant Scattering of Synchrotron Radiation by 121Sb and 149Sm

The first observation of the nuclear resonant scattering from the 37.1-keV level of 121 Sb and the 22.5-keV level of 149 Sm excited by synchrotron radiation is reported. Delayed emission of fluorescent K or L X-rays following the internal conversion process is mainly used to observe this resonance. The evaluated energy of each excited level was 37.147±0.023 keV for 121 Sb and 22.496±0.022 keV for 149 Sm. Each half-life of the excited level was evaluated to be 3.47±0.39 ns for 121 Sb and 7.40±1.15 ns for 149 Sm. These values agree well with the reported values for each level.