Satoshi Higashitaniguchi

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...

Electronic and magnetic phase diagram of superconductors, SmFeAsO1?xFx

A crystallographic and magnetic phase diagram of SmFeAsO1−xFx is determined as a function of x in terms of temperature based on electrical transport and magnetization, synchrotron powder x-ray diffraction, 57Fe Mossbauer spectra (MS), and 149Sm nuclear resonant forward scattering (NRFS) measurements. MS revealed that the magnetic moments of Fe were aligned antiferromagnetically at ~144 K (TN(Fe)). The magnetic moment of Fe (MFe) is estimated to be 0.34 μB/Fe at 4.2 K for undoped SmFeAsO; MFe is quenched in superconducting F-doped SmFeAsO. 149Sm NRFS spectra revealed that the magnetic moments of Sm start to order antiferromagnetically at 5.6 K (undoped) and 4.4 K (TN(Sm)) (x=0.069). Results clearly indicate that the antiferromagnetic (AF) Sm sublattice coexists with the superconducting phase in SmFeAsO1−xFx below TN(Sm), while the AF Fe sublattice does not coexist with the superconducting phase.

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.

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.

Development of 151Eu Time-Domain Interferometry and Its Application for the Study of Slow Dynamics in Ionic Liquids

Time-domain interferometry (TDI) employing 151Eu nuclear resonant scattering was developed for the study of slow dynamics. We measured the relaxation times of the density correlation in super-cooled ionic liquid 1-butyl-3-methylimidazolium iodide (BmimI) by the developed TDI. We found that the temperature dependence of the relaxation times follows the Vogel–Fulcher–Tammann law and observed fragile behavior of BmimI. Furthermore, we discussed the potential of TDI for the study of slow dynamics.

Development of neV-Resolution Spectroscopy Using Synchrotron-Based 57Fe Mössbauer Radiation

A synchrotron-radiation-based neV-resolution spectrometer has been developed using single-line pure nuclear Bragg reflection. The developed optical system consists of two components: a 57FeBO3 single crystal near the Neel temperature for producing 57Fe Mossbauer radiation from synchrotron radiation, and a single-line Mossbauer absorber (57Fe-enriched stainless steel foil) for analyzing the energy distribution. As a feasibility study, we have performed the measurement of the Mossbauer radiation diffracted by a Si crystal under ultrasound vibration with the optical system and have successfully observed the neV-range energy modulation due to the ultrasound vibration.

A Spectrometer for Rayleigh Scattering of Mössbauer Radiation Using Synchrotron Radiation

A neV-resolution spectrometer using 14.4 keV synchrotron-based 57Fe Mossbauer radiation has been developed to observe the quasi-elastic scattering by condensed matter. It consists of a 57Fe2O3 nuclear resonant Bragg monochromator and a 57Fe stainless steel nuclear analyzer. As a feasibility study, the quasi-elastic scattering by supercooled glycerol near its melting point has been observed. The instrumental function of the spectrometer was 42.2 neV. Energy broadening of 11 neV due to the quasi-elastic scattering was observed.

High-Energy-Resolution Monochromator for Nuclear Resonant Scattering of Synchrotron Radiation by Te-125 at 35.49 keV

We have developed a high-resolution monochromator (HRM) for the measurement of nuclear resonant scattering (NRS) of synchrotron radiation by Te-125 at 35.49 keV using the backscattering of sapphire (9 1 -10 68). HRMs for nuclei with excitation energies less than 30 keV have been successfully developed using high angle diffractions by silicon crystals. Nearly perfect silicon crystal, however, is not suitable for high efficient HRMs at higher energy regions because the symmetry of the crystal structure is high and the Debye-temperature is low. Therefore, we used high quality synthetic sapphire crystal, which has low symmetry of crystal structure and high Debye-temperature. The temperature of the crystal was precisely controlled around 218 K to diffract synchrotron radiation with a Bragg angle of π/2 - 0.52 mrad. Energy was tuned by changing the crystal temperature under the condition of constant diffraction angle. Energy resolution was measured by detecting nuclear forward scattering by Te-125 in enriched TeO2. The relative energy resolution of 2.1×10-7 is achieved, that is 7.5 meV in energy bandwidth. This HRM opens studies on element-specific dynamics and electronic state of substances containing Te-125.

Ultrahigh-Pressure Measurement in the Multimegabar Range by Energy-Domain Synchrotron Radiation 57Fe-Mössbauer Spectroscopy Using Focused X-rays

Energy-domain synchrotron radiation Mossbauer spectroscopy was performed by a single-line Mossbauer filtering technique and focusing X-ray optics. Pure nuclear Bragg reflection from a heated 57FeBO3 single crystal was used for a neV-order-bandwidth ultrahigh-energy-resolution X-ray analyzer. As an example of small-target research, the Mossbauer transmission spectrum of polycrystalline iron metal was observed using a diamond anvil cell at multimegabar pressures (>250 GPa) for the first time.

Mössbauer Spectroscopy of La0.87Ca0.13FePO and LaFeAsO0.93F0.07 under External Magnetic Field and Nuclear Resonant Inelastic Scattering of La0.87Ca0.13FePO

An 57 Fe Mossbauer spectroscopic study of iron-based layered superconductors La 0.87 Ca 0.13 FePO and LaFeAsO 0.93 F 0.07 under external magnetic fields of up to 14 T was performed. Our results suggest that hyperfine fields induced by local magnetic moments are, if any, small and that the magnetic feature of Fe in La 0.87 Ca 0.13 FePO and LaFeAsO 0.93 F 0.07 is most likely to be paramagnetism with itinerant electrons. This feature is confirmed both above and below the transition temperature for LaFeAsO 0.93 F 0.07 . Furthermore, the temperature dependence of the iron-specific phonon density of states in La 0.87 Ca 0.13 FePO was measured by using the nuclear resonant inelastic scattering of synchrotron radiation. We have found that the temperature-dependent change in the iron-specific phonon density of states of La 0.87 Ca 0.13 FePO was smaller than that of LaFeAsO 0.89 F 0.11 .