小林 寿夫

Understanding the strange metallic behavior that develops at the brink of localization in quantum materials requires probing the underlying electronic charge dynamics. Using synchrotron radiation-based Mossbauer spectroscopy, we studied the charge fluctuations of the strange metal phase of beta-YbAlB4 as a function of temperature and pressure. We found that the usual single absorption peak in the Fermi-liquid regime splits into two peaks upon entering the critical regime. We interpret this spectrum as a single nuclear transition, modulated by nearby electronic valence fluctuations whose long time scales are further enhanced by the formation of charged polarons. These critical charge fluctuations may prove to be a distinct signature of strange metals.

We report detailed valence fluctuation phenomena in the unique quantum-critical compound 0-YbAlB4 under pressures and at low temperatures. We directly observed the drastic change in the pressure dependence of the mean Yb valence at about 3 GPa with no pressure-induced structural transition. Below 3 GPa, the anomaly in the temperature dependences of the mean Yb valence was observed at about 55 K under ambient pressure, corresponding to a previously observed reduced Kondo coherence temperature, and this temperature rapidly decreases to 22 K at 2.2 GPa, in contrast to the monotonous decrease of the high Kondo temperature (similar to 250 K at ambient pressure). The present results reveal that this reduced Kondo coherence temperature is responsible for the unconventional quantum critical behaviors in beta-YbAlB4. Above 3 GPa, it was found that the temperature dependence of the mean Yb valence exhibits a discontinuous change at low temperature (10.5 and 9 K under 4.8 and 5.5 GPa, respectively).

We have studied Eu-151 Mossbauer effects on EuRh2Si2 at a high pressure of 1.0 GPa as well as at ambient pressure. The compound shows a temperature-induced valence transition from the Eu2+ state to the nearly Eu3+ state with decreasing temperature at P >= 1.0 GPa. The synchrotron-radiation-based Mossbauer spectroscopy is applied to the measurements under a high pressure. The Eu2+ ratio abruptly decreases at around 30K reflecting the temperature-induced valence transition. We also observed a valence transition due to magnetic field by Mossbauer spectroscopy for the first time. It was found that a considerable amount (similar to 45%) of Eu atoms do not participate in the valence transition. We discuss the origin of these Eu atoms by calculating the magnetization based on the Mossbauer spectroscopy data.

Magnetism in EuNiIn4 is studied using specific heat, magnetic susceptibility, magnetization, Eu-151 Mossbauer spectroscopy, and neutron diffraction experiments. The specific heat exhibited two magnetic transitions at T-N1 = 15.2 K and T-N2 = 14.3 K at zero magnetic field. An antiferromagnetic ground state of EuNiIn4 was observed to have a uniaxial magnetic anisotropy along the b axis using magnetic susceptibility and Eu-151 Mossbauer spectroscopy. Further, single crystal neutron diffraction experiments illustrated that this antiferromagnetic structure in the ground state is characterized by the commensurate propagation vector q = (1/2; 1/2; 1/2), which reveals no distinct anomaly at T-N2. The magnetization curve along the b axis at 2K was observed to exhibit four successive magnetic field-induced transitions up to 50 kOe, following which it increased linearly, resulting in saturated magnetization of 7 mu(B)/f.u. above 195.2 kOe, as expected of Eu2+ free ions. The magnetic phase diagram for H parallel to b in EuNiIn4 was found to have unique characteristics with five magnetic states in low magnetic fields.

© 2019, Springer Nature Switzerland AG. We successfully observed the synchrotron-radiation-based Mössbauer absorption spectra with 158Gd and 99Ru. Their nuclear resonant energies were 79.5 keV and 89.6 keV, respectively, and they are factually the highest energy which energy region synchrotron radiation covers with sufficient intensity as the incident X-rays for Mössbauer spectroscopy. Although the low recoilless fraction owing to these high resonant energy, Mössbauer energy spectra of GdPd3 to 158Gd2O3 and fcc-Ru nanoparticles to bulk hcp-99Ru metal were obtained with natural samples of the former compounds with sufficient amount, because of the high transparency of these high energy X-rays(to electronic scattering). In spite of large statistical errors, we can evaluate the hyperfine parameters when the spectrum includes simple 1-site profile. 99Ru and 158Gd SR-based Mössbauer absorption spectra of various complex materials including somewhat complex structures will be available with the improvements to the measurement system; More detector elements for larger solid angle subtended to the scatterer sample will yields more counting rates and improvement higher recoilless fraction by arranging more appropriate chemical specimen as the scatterer yields deeper absorption profile.

© 2018 American Physical Society. The interplay between magnetism and superconductivity is one of the important subjects to investigate the pairing mechanism in novel superconductors. We have found a new coexistence between an antiferromagnetic order in the Fe sublattice and superconductivity of the FeAs-based EuFe2As2 superconductor in the pressure range from 2.4 to 3.0 GPa by Fe57 nuclear forward scattering (NFS) using a single-crystal sample. The magnetic state in the Fe sublattice changes to a new antiferromagnetic one with superconductivity from a stripe-type antiferromagnetic one observed in the normal conducting state at 2.7 GPa. Below the superconducting transition temperature, the temperature dependence of Fe57 NFS spectra reveals that the new antiferromagnetic order develops with the superconductivity. This nontrivial coupling of two ordered states in EuFe2As2 under pressure demonstrates a new and intriguing relationship between magnetism and superconductivity in Fe-based superconductors.

The structural properties and the Yb 4f electronic state of the valence fluctuating α-YbAlB4 have been investigated by powder X-ray diffraction under pressure and 174Yb Mössbauer spectroscopy with magnetic fields at low temperature, respectively, using synchrotron radiation. Powder X-ray diffraction patterns showed that the crystal structure does not change up to p ∼ 18 GPa at 8 K and the volume decreases smoothly. However, the pressure dependence of the difference in the structure factor between the (060) and (061) diffraction lines changes at ∼ 3.4 GPa, indicating the change of atomic coordination parameters. The 174Yb Mössbauer spectroscopy measurements at 2 K with 10 and 50 kOe suggest that the electrical quadrupole interaction changes by applied magnetic fields.

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 Mössbauer 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 Mössbauer 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.

We investigated transverse acoustic (TA) phonons in iron-bearing magnesium oxide (ferropericlase) up to 56 GPa using inelastic x-ray scattering (IXS). The results show that the energy of the TA phonon far from the Brillouin zone center suddenly increases with increasing pressure above the spin transition pressure of ferropericlase. Ab initio calculations revealed that the TA phonon energy far from the Brillouin zone center is higher in the low-spin state than in the high spin state; that the TA phonon energy depend weakly on pressure; and that the energy gap between the TA and the lowest-energy-optic phonons is much narrower in the low-spin state than in the high-spin state. This allows us to conclude that the anomalous behavior of the TA mode in the present experiments is the result of gap narrowing due to the spin transition and explains contradictory results in previous experimental studies.

The structural and electronic properties of high-quality K0.66(6)Fe1.75(10)S2 single crystals have been investigated by angle-resolved X-ray diffraction and Fe-57 nuclear forward scattering using synchrotron radiation under pressure at room temperature. The samples exhibit phase separation into antiferromagnetic ordered K2Fe4S5 and nonmagnetic KxFe2S2 phases. It was found that a pressure-induced phase transition occurs at pc = 5.9(4) GPa with simultaneous suppression of the antiferromagnetic and Fe vacancy orders. From the results of Fe-57 nuclear forward scattering, the refined magnetic hyperfine field remains unchanged with pressure below pc, suggesting that the Neel temperature does not decrease with pressure up to pc. Above pc, all Fe atoms in K0.66Fe1.75S2 are in the same nonmagnetic state. A discontinuous increase in the center shift was observed at pc, reflecting a change in the Fe electronic state in K0.66Fe1.75S2.

<p>ペロブスカイト型遷移金属酸化物であるRNiO_3_（R:希土類）は、温度変化や圧力印加によって金属－絶縁体転移を示し、その転移温度、圧力は希土類元素Rによって異なる。今回は室温でP_MI_～8 GPaであるYbNiO_3_と、室温常圧で金属的であるLaNiO_3_について、SPring-8のBL10XUにて高圧下での粉末X線回折測定を行った。その結果、YbNiO_3_はP～16GPaで圧縮率の異常が見られ、LaNiO_3_ではP～10GPaまで圧縮率の異常は観測されなかった。これらの結果と、SPring-8のBL12XUで行ったNiのK吸収端でのX線発光分光測定の結果について議論する。</p>

We have investigated the charge ordering in Sm4Bi3 under pressure by magnetic susceptibility and x-ray diffraction measurements. We observed discontinuous anomalies in the temperature dependences of magnetic susceptibility and volume at the charge ordering transition owing to the first-order transition and then the estimated charge ordering transition temperature decreases linearly with increasing pressure. The volume ratio between the SmBi6 octahedra at two nonequivalent Sm sites indicates the partial charge ordering of Sm ions in the charge ordered phase. The evidence of the partial charge ordering in Sm4Bi3 is consistent with the smaller electronic component in the entropy change at the charge ordering transition.

The structural properties of beta-YbAlB4 with an orthorhombic Cmmm symmetry have been investigated by powder X-ray diffraction analysis using synchrotron radiation under high pressure (up to similar to 20 GPa) at 7K. Although the refined lattice parameters exhibit no discontinuity and the pressure dependence of the volume was well reproduced by the Murnaghan's equation of state up to similar to 20 GPa, detailed analyses of the observed X-ray diffraction patterns reveal local structure changes in B layers at 3.5(2) and 5.8(1) GPa. The changes in the local structure strongly affect the bonding structures in the B layers among the conduction electrons of beta-YbAlB4 under pressure at 7K.

The Fe-57 Mossbauer spectra of the single crystalline and the finely ground Sr1-xLaxFe12-yCoyO19 (x = 0 : y = 0, x = 0.192 : y = 0.152 and x = 0.456 : y = 0.225) samples have been measured to investigate the La-Co substitution effects. All observed spectra at 150 K were well fitted using the five subspectra which correspond to the five crystallographical nonequivalent Fe sites in the M-type hexaferrite, indicating that the valence changes to Fe2+ ions in the Fe3+ ions were not observed in our Sr1-xLaxFe12-yCoyO19 samples. In SrFe12O19, the relative absorption intensities in the five subspectra show the large anisotropies in the recoilless fractions at the five Fe sites whereas these anisotropies were not observed in Sr0.544La0.456Fe11.775Co0.225O19. These results indicate the chemical compositional dependence on the anisotropies of the recoilless fractions at the five Fe sites. The substitution of a Co2+ ion for the Fe3+ ion changes the center shifts of the Fe3+ ions near the Co2+ ion by the perturbation of the Fe-O-Co hybridizations. Therefore, the Co2+ ions occupy the 4f(1) and the 4f(2) sites due to the chemical compositional dependences of the refined magnetic hyperfine field and center shifts of the Fe3+ ions.

We have investigated the physical properties of single-crystalline KxFe2-yS2 and KxFe2-ySe2 samples using Fe-57 Mossbauer spectroscopy. The observed Fe-57 Mossbauer spectra were reconstructed using a major antiferromagnetic ordered K2Fe4Se5 phase and a minor paramagnetic phase down to 5 K, despite being superconducting below 32.2 K in KxFe2-ySe2. The analysis of Fe-57 Mossbauer spectrum for KxFe2-yS2 at 290 K confirms the presence of a major antiferromagnetic ordered K2Fe4S5 phase and a minor paramagnetic phase in the KxFe2-yS2 single crystal. The derived hyperfine interaction parameters of the paramagnetic phase in KxFe2-yS2 suggest that the microstructure of this phase in KxFe2-yS2 is similar to that of the superconducting phase in KxFe2-ySe2 although the KxFe2-yS2 single crystals exhibit no superconductivity down to 5 K.

<p>YbNiO_3_は常圧下で金属絶縁体転移を示し、絶縁相での単斜晶P2_1_/cから金属相Pbnmへの構造相転移を伴う。圧力印加により、室温で8GPa近傍で金属絶縁体転移を示すが、この転移は常圧のそれと異なり構造相転移を伴わないことが分かっている。この圧力誘起される金属相について、Niの電子状態を調べるために圧力下X線発光分光を行い、その結果について議論する。</p>

The perovskite antiferromagnetic (TN∼220K) insulator EuNiO3 undergoes at ambient pressure a metal-to-insulator transition at TMI=460K which is associated with a simultaneous orthorhombic-to-monoclinic distortion, leading to charge disproportionation. We have investigated the change of the structural and magnetic properties of EuNiO3 with pressure (up to ∼20GPa) across its quantum critical point (QCP) using low-temperature synchrotron angle-resolved x-ray diffraction and Eu151 nuclear forward scattering of synchrotron radiation, respectively. With increasing pressure, we find that after a small increase of TN (p≤2GPa) and the induced magnetic hyperfine field Bhf at the Eu151 nucleus (p≤9.7GPa), both TN and Bhf are strongly reduced and finally disappear at pc≅10.5GPa, indicating a magnetic QCP at pc. The analysis of the structural parameters up to 10.5 GPa reveals no change of the lattice symmetry within the experimental resolution. Since the pressure-induced insulator-to-metal transition occurs at pIM≅6GPa, this result implies the existence of an antiferromagnetic metallic state between 6 and 10.5 GPa. We further show from the analysis of the reported high-pressure electrical resistance data on EuNiO3 at low temperatures that in the vicinity of the QCP the system behaves as non-Fermi-liquid, with the resistance changing as Tn, with n=1.6, whereas it becomes a normal Fermi liquid, n=2, for pressures above ∼15GPa. On the basis of the obtained data, a magnetic phase diagram in the (p, T) space is suggested.

The perovskite antiferromagnetic (T-N similar to 220 K) insulator EuNiO3 undergoes at ambient pressure a metal-to-insulator transition at T-MI = 460 K which is associated with a simultaneous orthorhombic-to-monoclinic distortion, leading to charge disproportionation. We have investigated the change of the structural and magnetic properties of EuNiO3 with pressure (up to similar to 20GPa) across its quantum critical point (QCP) using low-temperature synchrotron angle-resolved x-ray diffraction and Eu-151 nuclear forward scattering of synchrotron radiation, respectively. With increasing pressure, we find that after a small increase of T-N (p &lt;= 2 GPa) and the induced magnetic hyperfine field B-hf at the Eu-151 nucleus (p &lt;= 9.7 GPa), both T-N and B-hf are strongly reduced and finally disappear at pc congruent to 10.5 GPa, indicating a magnetic QCP at pc. The analysis of the structural parameters up to 10.5 GPa reveals no change of the lattice symmetry within the experimental resolution. Since the pressure-induced insulator-to-metal transition occurs at p(IM) congruent to 6 GPa, this result implies the existence of an antiferromagnetic metallic state between 6 and 10.5 GPa. We further show from the analysis of the reported high-pressure electrical resistance data on EuNiO3 at low temperatures that in the vicinity of the QCP the system behaves as non-Fermi-liquid, with the resistance changing as T-n, with n = 1.6, whereas it becomes a normal Fermi liquid, n = 2, for pressures above similar to 15 GPa. On the basis of the obtained data, a magnetic phase diagram in the (p, T) space is suggested.

We have investigated the electronic states of single-crystal CaFe2As2 under hydrostatic pressure using Fe-57 M ssbauer spectroscopy and magnetization measurements. The center shift and the quadrupole splitting were refined from observed Fe-57 M ssbauer 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 similar to 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 AFe(2)As(2).

The displacement modulation due to the charge density wave (CDW) in Mn3Si, which occurs as second harmonics of the spin density wave (SDW), has been investigated by synchrotron x-ray diffraction and the existence of CDW below Neel temperature of 23 K has been confirmed. High resolution measurement revealed the splitting of the satellite peaks deviating very little from the [111] axis toward [110] direction, suggesting the possibility of a single or double q CDW state. The temperature dependence of the satellites indicates the existence of the phase transition at T-t = 8.5 K, in agreement with the neutron diffraction result [Phys. Rev. B 73, 224416 (2006)]. The analysis of reflection intensities using a single q model, which is a limiting case of no splitting, shows that in both phases Si and Mn-II have larger displacement than that of Mn-I. The formation of single and double q SDWs and their nesting mechanism are discussed.

Sm-149 nuclear resonant inelastic scattering was carried out in a charge density wave compound SmNiC2. We have investigated temperature dependences of the Sm partial phonon density of states and recoil-free fraction at the Sm site and the average sound velocity estimated from the Sm partial density of states. The Sm partial density of states exhibits temperature dependence, suggesting that the phonon modes between 20 and 25 meV may correlate with the charge density wave. Temperature dependence of the recoil-free fraction is difficult to prove the correlation with either the charge density wave or ferromagnetic ordering. The average sound velocity obtained by the Sm partial phonon density of states exhibits temperature dependence, agreeing qualitatively with very recent elastic constant measurements.

We have measured the 57 Fe M�ssbauer spectra of Srx-1LaxFe12-xNixO19 (x=0, 0.1 and 0.2) at room temperature using the powder and the single crystalline samples. All observed spectra were well fitted using the five subspectra, which correspond to the five nonequivalent Fe sites. In Srx-1LaxFe12-xNixO19 (x=0.1 and 0.2), the substitutions of Ni2+ ions for Fe3+ ions are required by the electric charge balance. For SrFe12O19, the relative absorption intensities in the spectrum observed using the single crystalline sample are much different from those observed using the powder one. This result shows large anisotropies of the recoilless fractions at the Fe sites. The refined hyperfine interaction parameters of the 4f1, 2a, and 2b sites decrease linearly with increasing x. The substitution of a Ni2+ ion for a Fe3+ ion changes hyperfine interaction parameters by the perturbation of the Fe-O-Ni hybridizations. Accordingly, the Fe ions in the vicinity of the 4f1, 2a, and 2b sites are replaced by the Ni2+ ions.

The nuclear resonant scattering and Mössbauer spectroscopy are going to be the experimental technique to investigate the element-specific electronic and vibrational properties in materials under multi-extreme conditions by using synchrotron radiation as an excitation source for a Mössbauer isotope. In this article, I introduced the basic principles of nuclear resonant scattering and Mössbauer spectroscopy and reviewed these current statuses in material science. Finally, I described future prospects of these methods using shorter-pulsed structure and higher-brilliance Xrays from new-generation synchrotron radiation facilities.<br>

The magnetic properties of high-quality similar to 25 at.% Fe doped alpha-YbAlB4 with an orthorhombic Pbam structure were investigated by using magnetization measurements and Fe-57 Mossbauer spectroscopy. The temperature dependence of the magnetization along the a-axis showed anomalies at T (N2) similar to 7.7 K and T (N1) similar to 10.4 K. Broadenings of the peaks in the Mossbauer spectrum were observed at temperatures below 6 K, which were due to the magnetic hyperfine fields transferred from the Yb magnetic moments. The evaluated transferred magnetic hyperfine field almost disappears around T (N2). The anomalous temperature dependence of the transferred magnetic hyperfine field reveals that the magnetic structure and the ordered magnetic moments of the Yb ions show complex temperature dependences at temperatures below T (N1).

Two modifications have been made to a miniature ceramic anvil high pressure cell (mCAC) designed for magnetic measurements at pressures up to 12.6 GPa in a commercial superconducting quantum interference (SQUID) magnetometer [N. Tateiwa et al., Rev. Sci. Instrum. 82, 053906 (2011); 83, 053906 (2012)]. Replacing the Cu-Be piston in the former mCAC with a composite piston composed of the Cu-Be and ceramic cylinders reduces the background magnetization significantly smaller at low temperatures, enabling more precise magnetic measurements at low temperatures. A second modification to the mCAC is the utilization of a ceramic anvil with a hollow in the center of the culet surface. High pressures up to 5 GPa were generated with the "cupped ceramic anvil" with the culet size of 1.0 mm. (C) 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4802832]

The electronic and vibrational properties of EuFe2As2 in the tetragonal phase between 0 and 5 GPa have been investigated using Fe-57 Mossbauer spectroscopy and Fe-57 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 FeAs4 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 EuFe2As2, 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-nduced superconductivity in the tetragonal phase of AFe(2)As(2).

We have studied the electronic state of Fe atoms in SrFe2As2 under pressure at room temperature by Fe-57 Mossbauer spectroscopy using a single crystalline sample. A center shift delta(c.s.) and an electric quadrupole interaction parameter e(2)qQ/2 show the discontinuous increase and decrease at around 7 GPa, respectively, implying the pressure-induced structural phase transition. Furthermore, delta(c.s.) deviates from the linear pressure dependence above 4 GPa without any anomaly in the pressure dependence of e(2)qQ/2. The anomaly corresponds to a change of the pressure dependence of lattice parameter ratio c/a, where volume of the unit cell decreases monotonously with increasing pressure.

Magnetic interactions on single-crystal AFe(2)As(2) (A: Eu and Sr) were studied by Fe-57 Mossbauer spectroscopy. An abrupt appearance of the effective magnetic hyperfine field (B-eff) is observed at the spin density wave transition temperature (similar to 190K for Eu and similar to 200K for Sr) in both samples. The value of B-eff in EuFe2As2 shows a small but significant increase around the Neel temperature T-N(Eu) (=19 K) of the Eu2+ sublattice, while B-eff in SrFe2As2 almost saturates at lower temperatures. Consistent with this, the isomer shift of EuFe2As2 indicates an anomalous temperature dependence below approximately T-N(Eu). These results provide direct evidence of the coupling between the localized Eu2+ moments and the conduction electrons from the FeAs layers and suggest that this coupling may affect the observed superconductivity in EuFe2As2 under pressure.

Sm-149 and Fe-57 nuclear resonant inelastic scattering (NRIS) measurements were carried out on SmFe4P12, SmFe4As12 and SmFe4Sb12. A clear dip structure of the Fe-57 NRIS spectrum was found in SmFe4P12, which was not clearly observed for SmFe4As12 and SmFe4Sb12. On the other hand, the line width of the phonon excitation in the Sm-149 NRIS spectrum increases with increasing the ionic radius of the pnictogen. These findings imply that the hybridization between the Sm and Fe phonon modes is correlated to changes in the ionic radius of the pnictogen.

An Upgrade of the nuclear resonant scattering beamline, BL09XU in SPring-8 has been conducted. A liquid nitrogen cooled high-heat load monochromator was installed and a 2nd experimental hutch was constructed. The instruments installed in the hutch allow for a variety of sample conditions. Newly developed high-resolution monochromators with better stability including the back scattering geometry monochromator have opened up the easy access to more isotopes and more precise measurements.

The structural and magnetic phase transition in AFe(2)As(2) (A = Sr and Eu) was investigated from the viewpoint of the Fe-specific phonon density of states, g(Fe)(e)(E), utilizing Fe-57 nuclear resonant inelastic-scattering and ab initio phonon calculations. The c-axis-polarized Fe optical phonon branches in g(Fe)(e)(E) around 25 meV show no anomalies across the phase transition temperature, T-SDW, with decreasing temperature. Meanwhile, the peak intensity in g(Fe)(e)(E) around 32 meV decreases and increases at 1.25T(SDW) and T-SDW, respectively, which originates from five optical phonon branches involving Fe and As atomic displacements in the tetragonal structure. This decrease in peak intensity of g(Fe)(e)(E) at 1.25T(SDW) is related to the phonon branches involving atomic displacements in the ab plane. Furthermore, the average sound velocity derived from g(Fe)(e)(E) below 8 meV shows no temperature dependence down to TSDW, suggesting softening in some of the sound waves. Our results clearly indicate the presence of orthorhombic fluctuations below 1.25T(SDW) in tetragonal AFe(2)As(2).

We have succeeded in synthesising iron-tungsten nitrides using the hot isostatic pressing (HIP) method and have measured their magnetic properties. Two eta-carbide-type iron-tungsten nitrides with lattice constants a = 11.043(1) and 10.937(2) angstrom were synthesised directly from metal elements under high-pressure nitrogen gas. Their compositions are expected to be Fe3W3N and Fe6W6N in analogy with other eta-carbide-type compounds. Fe3W3N is a ferromagnet with a Curie temperature T-C = 110 K and a saturation moment PS = 0.78 mu(B)/Fe, whereas Fe6W6N is an antiferromagnet with a Neel temperature T-N = 75 K and shows a metamagnetic transition at around 25 T. (C) 2011 Elsevier B.V. All rights reserved.

Magnetic Compton scattering at high pressure is presented for the first time. To study the pressure-induced magnetic transition in Laves-phase ErCo2, Er 4f and Co 3d spin magnetic moments, together with the conduction electron component, were separately and quantitatively estimated at pressures up to 1.84 GPa. The Co 3d spin moments show a remarkable reduction with increasing pressure, which is in contrast to the weak pressure variation of the Er 4f spin moment. The relationship between the magnetic transition and instability of the Co 3d moment at high pressure is discussed from the viewpoint of lattice contraction and the molecular field of Er 4f moments.