Hisao Kobayashi

New high-pressure B2 phase of FeS above 180 GPa

Abstract FeS exhibits extensive polymorphism at high pressure and temperature. All with NiAs-type (B8) or closely related structures. Here we report a new phase transition from FeS VI to CsCl-type (B2) phase (FeS VII) above 180 GPa based on the synchrotron X-ray diffraction (XRD) measurements. A significant volume reduction by 3.0% was observed at the phase transition, due to an increase in the coordination number from six to eight. Present results suggest that a substantial amount of sulfur may be incorporated into an Fe-Ni alloy with bcc structure in the Earth’s inner core.

Spin-glass like behaviour in the nanoporous Fe2O3with amorphous structure

The porous Fe2O3 was synthesized chemically. The average size of the particle was ~85.0 nm, which was observed by scanning electron microscopy. The signature of porous structure was confirmed by a N2 adsorption/desorption isotherm and intense x-ray powder diffraction peak at low angle. The x-ray diffraction pattern at high angle indicates the amorphous structure. Mossbauer investigations show that the value of the hyperfine field is ~498.0 kOe at 4.2 K which is much smaller than that of the hyperfine field of crystalline α/γ-Fe2O3 and consistent with the values of amorphous Fe2O3. The temperature dependence of zero-field cooled magnetization exhibits a peak at 18.0 K (Tf), where Tf follows the Almeida–Thouless relation as . The ageing phenomenon of the magnetic relaxation below Tf and the memory effect in the field-cooled magnetization indicate the typical features of the classical spin-glass compounds below the spin freezing temperature at Tf.

Mixed magnetic phase in 6H-type BaFeO3−δ

The magnetic state in 6H-type BaFeO3−δ at low temperature was studied using small-angle neutron scattering, positive-muon spin relaxation and magnetization measurements. These experiments demonstrate the appearance of two different types of magnetic states: an antiferromagnetic ordering with a long-range correlation and magnetic domains with a short-range correlation. The antiferromagnetic Fe spin arrangement occurs below 130 K. In contrast, the magnetic domains are formed below 170 K and the average size of the magnetic domains was estimated as ~124 A. These results explain the discrepancy of the Neel temperatures between three measurement techniques: magnetization, neutron powder diffraction and Mossbauer measurements. Furthermore, it was found that the magnetic domains coexist with the long-range antiferromagnetic ordering below 130 K.

Observation of a pressure-induced As–As hybridization associated with a change in the electronic state of Fe in the tetragonal phase of EuFe2As2

The electronic and vibrational properties of EuFe(2)As(2) in the tetragonal phase between 0 and 5 GPa have been investigated using (57)Fe Mössbauer spectroscopy and (57)Fe nuclear resonance inelastic scattering, respectively. We find a discontinuous increase of the center shift around 2.3 GPa, reflecting a change of the electronic state of Fe, and above 2.5 GPa a softening of the optical phonon modes associated with an increase of the relative volume of the FeAs(4) tetrahedron in the unit cell. Our findings reveal that an effective As-As hybridization along the c axis appears at approximately 2.3 GPa in the tetragonal phase of EuFe(2)As(2), along with a change in the electronic state of Fe, causing bulk superconductivity to appear at a low temperature. Consequently, the change in the electronic state of the Fe atom and the effective As-As hybridization play key roles in the pressure-induced superconductivity in the tetragonal phase of AFe(2)As(2).

Structural Evidence for the Charge Disproportionation of Fe4+ in BaFeO3-δ

A weak distortion of a [FeO 6 ] octahedron was observed in hexagonal BaFeO 2.91 at around 180 K by neutron powder diffraction. The occurrence of a weak distortion is consistent with the results of Mossbauer measurements, which indicated a decrease in the Fe 4+ concentration below 170 K. We strongly suggest that the weak distortion of a [FeO 6 ] octahedron results from a charge-disproportionation phenomenon, 2Fe 4+ → Fe + + Fe + , where 0 at 50 K.

Non-Fermi-Liquid Behavior on an Iron-Based Itinerant Electron Magnet Fe3Mo3N

We report magnetic, calorimetric, and transport properties of an iron-based itinerant electron magnet Fe 3 Mo 3 N. Magnetic susceptibility shows a Curie–Weiss behavior at high temperatures and takes a broad maximum at around 75 K. The absence of magnetic long range order was confirmed by 57 Fe-Mossbauer and neutron diffraction measurements. C / T shows a divergent behavior following -log T at low temperatures and reaches 128 mJ/(f.u.mol K 2 ) at 0.5 K. The deviation from the T 2 power law of resistivity also suggests a non-Fermi-liquid (NFL) behavior. The observed C / T and χ are enhanced compared with the values estimated from the theoretical density of states, suggesting a strong magnetic enhancement. The NFL behaviors indicate that Fe 3 Mo 3 N is one of the ideal systems located in the vicinity of the ferromagnetic quantum critical point.

Pressure-induced amorphization of Cu Fe S 2 studied by Fe 57 nuclear resonant inelastic scattering

Static and dynamic structural properties of CuFeS2 under high pressure were investigated by x-ray diffraction and Fe-57 nuclear resonant inelastic scattering. Using x-ray diffraction, we obtained evidence that a pressure-induced amorphization occurs at about 6.3 GPa, which represents a metal-insulator transition. Although partial phonon densities of states extracted from Fe-57 nuclear resonant inelastic scattering spectra are similar in the two phases, the derived thermodynamical parameters show anomaly at the transition. The results of extracted partial phonon densities of states and ab initio phonon calculations indicate that this pressure-induced amorphization is caused by anomalous pressure dependences of the lower optical phonon branches and the initial slopes of the transverse acoustic phonon branches.

Direct observation of localization of the minority-spin-band electrons in magnetite below the Verwey temperature

Two-dimensional spin-uncompensated momentum density distributions, ?s2D(p)s, were reconstructed in magnetite at 12 and 300 K from several measured directional magnetic Compton profiles. Mechanical detwinning was used to overcome severe twinning in the single-crystal sample below the Verwey transition. The reconstructed ?s2D(p) in the first Brillouin zone changes from being negative at 300 K to positive at 12 K. This result provides the first clear evidence that electrons with low momenta in the minority-spin bands in magnetite are localized below the Verwey transition temperature.

Pressure-induced change of the electronic state in the tetragonal phase of CaFe2As2

We have investigated the electronic states of single-crystal CaFe2As2 under hydrostatic pressure using (57)Fe Mössbauer spectroscopy and magnetization measurements. The center shift and the quadrupole splitting were refined from observed (57)Fe Mössbauer spectra using the single-crystalline sample under pressure at room temperature. A discontinuous decrease in the pressure dependence of the refined center shift was observed at 0.33 GPa without any anomaly in the pressure dependence of the refined quadrupole splitting, indicating a purely electronic state change in CaFe2As2 with a tetragonal structure. Such a change is shown to be reflected in the peak-like anomalies observed in the pressure dependences of the magnetic susceptibility at 0.26 GPa above 150 K. Our results reveal that this pressure-induced electronic state change suppresses the tetragonal-to-orthorhombic structural phase transition accompanied by an antiferromagnetic ordering. We further observed superconductivity in CaFe2As2 below ∼ 8 K around 0.33 GPa although our sample was not in a single phase at this pressure. These findings suggest that the electronic state change observed in CaFe2As2 with the tetragonal structure is relevant to the appearance of the pressure-induced superconductivity in AFe2As2.

57Fe Mössbauer and Co K β x-ray emission spectroscopic investigations of La-Co and La substituted strontium hexaferrite

In the ferrite magnets, the magnetoplumbite-type strontium hexaferrite (SrFe12O19) is an important base material with a relatively high Curie temperature. The La-Co substitution in the Sr hexaferrites ((Sr,La)(Fe,Co)12O19) significantly improves magnetic coercivity. We have investigated the electronic states of Sr1−xLaxFe12−yCoyO19 by 57Fe Mossbauer and Co K β x-ray emission spectroscopies using the single-crystalline samples. The Co K β x-ray emission spectrum shows that all Co2+ ions are in the high-spin state. The hyperfine parameters extracted from the observed 57Fe Mossbauer spectra clearly show the Co and La substitution dependences at two Fe sites of the five crystallographically nonequivalent Fe sites, revealing that most Co2+ ions occupy the 4f1 and 2a Fe sites in Sr1−xLaxFe12−yCoyO19. These Co site occupations are consistent with the higher magnetic coercivity without a decrease in remanent magnetization in the La-Co substituted Sr hexaferrites.