小林 寿夫

We have carried out X-ray diffraction and Fe-57 nuclear forward scattering using synchrotron radiation to study structural and magnetic properties of Fe2P under high pressure at low temperature. The lattice parameter ratio changes at 1.8 and 7.0 GPa as a function of pressure at 24K without structural change, where the transition at 1.8 GPa corresponds to a ferromagnetic-to-antiferromagnetic one. We deduced the magnetic structures of Fe2P at 3K and 2.5 GPa from refined hyperfine interaction parameters extracted from Fe-57 nuclear forward scattering spectra. Without external magnetic field where Fe2P is in the antiferromagnetic state, the magnetic structure is a complex one with a propagation vector along < 100 >. The distinct change of Fe-57 nuclear forward scattering spectra with external magnetic field along [001] indicates that a metamagnetic-like transition occurs below 5 kOe at 3K and 2.5 GPa. With external magnetic field, the magnetic structures are not simple collinear ones. Furthermore, the magnetic-field-induced change in the magnetic structure occurs at around 40 kOe.

The spacer thickness dependence of the interlayer magnetic coupling (IMC) between Gd and Fe layers separated by Al spacers has been studied. Magnetization measurement and X-ray magnetic circular dicroism (XMCD) spectroscopy at the Gd-L-3 edge have been carried out on sputtered Fe (20 angstrom)/Al (R angstrom)/Gd (40 angstrom) multilayer films (MLFs) for R = 0, 5, 10, and 100 and on some reference films. Experimental data at low magnetic fields below 1 kOe are carefully investigated. A simple theoretical model is introduced to evaluate the strength of IMC. The energy of IMC is represented by the term JM(Fe) . M-Gd, in which M-Fe and M-Gd denote the average overall magnetic moments of Fe and Gd layers, respectively. It is found that J, which is evaluated from the magnetization curves and the Gd-XMCD signals, is 370 x 10(4) Oe.cm(2)/emu for MLF of R = 0, and can be reduced by one order of magnitude when R is changed from 0 to 10. Most of characteristic features of the experimental data are ascribed to the varing magnetization of Gd layers. It is found that 40 angstrom Gd layers, which are paramagnetic at 5K when isolated using 100 angstrom Al spacers, show magnetic hysteresis when sandwiched between Fe layers.

We report magnetic, calorimetric, and transport properties of an iron-based itinerant electron magnet Fe3Mo3N. 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 Fe-57-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 chi are enhanced compared with the values estimated from the theoretical density of states, suggesting a strong magnetic enhancement. The NFL behaviors indicate that Fe3Mo3N is one of the ideal systems located in the vicinity of the ferromagnetic quantum critical point.

Two-dimensional spin-uncompensated momentum density distributions, rho(2D)(s)(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 rho(2D)(s)(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.

We have carried out high-pressure x-ray diffraction experiments on Eu4As3 at room temperature, and individual atomic coordinates as well as lattice parameters were derived precisely from the diffraction data. We have confirmed that a trigonal structure with the R3c space group does not change up to 19 GPa. The results show that the volume of the EuAs6 octahedron and the valence state of the Eu ion are closely correlated, and pure trivalent and divalent Eu ions occupy two nonequivalent Eu sites, respectively, in the charge-ordered state at ambient pressure. We present clear evidence on the basis of the pressure dependence of these volumes at two nonequivalent Eu sites that a melting of the static charge order occurs at about 10 GPa and Eu4As3 is in a mixed-valence state above 10 GPa.

The porous Fe2O3 was synthesized chemically. The average size of the particle was similar to 85.0 nm, which was observed by scanning electron microscopy. The signature of porous structure was confirmed by a N-2 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 similar to 498.0 kOe at 4.2 K which is much smaller than that of the hyperfine field of crystalline alpha/gamma-Fe2O3 and consistent with the values of amorphous Fe2O3. The temperature dependence of zero-field cooled magnetization exhibits a peak at 18.0 K (T-f), where T-f follows the Almeida-Thouless relation as T-f alpha H-2/3. The ageing phenomenon of the magnetic relaxation below T-f 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 T-f.

Measurements of S(Q, omega) of the filled skutterudite SmRu4P12 using inelastic x-ray scattering show a dispersionless mode at 9 meV. Meanwhile, the Sm partial phonon density of states measured by nuclear resonant inelastic scattering shows a strong peak at the same energy. The temperature dependence of the Sm phonon energy suggests that the Sm modes are anharmonic. The comparison of the experimental results in SmRu4P12 with the calculated ones in LaRu4P12 indicates the absence of any significant lattice instability due to 4f electrons.

Experimental and theoretical investigations have been made for the first time on the three-dimensional momentum-density distribution of magnetic electrons in ferromagnetic Ni, rho(mag)(p), which is the difference in the electron momentum-density distribution between majority- and minority-spin electrons. A magnetic Compton-scattering technique is used to determine rho(mag)(p) by applying the direct Fourier-transform method to reconstruct rho(mag)(p) from magnetic Compton profiles. The corresponding theoretical rho(mag)(p) is calculated by the full-potential linearized augmented plane-wave method. The present theory accurately reproduces the overall structures of experimental rho(mag)(p). There are, however, slight quantitative discrepancies between the experimental and calculated values of rho(mag)(p) around vertical bar p vertical bar = 0 au and at the positions related to the X-point. The analysis of rho(mag)(p) for individual bands attributes negative rho(mag)(p) near the first Brillouin zone boundary (vertical bar p vertical bar similar to 1.0 au) to the superposition of highly negative spin-polarized momentum-density of the first band and highly positive spin-polarized momentum-density of the sixth band. The positively polarized momentum-density at vertical bar p vertical bar similar to 2.2 au is attributed to the d-like fifth band spin polarizations.

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.

We have carried out high-resolution inelastic X-ray scattering and Sm-149 nuclear resonant inelastic scattering to investigate the anharmonicity of the mode of the atoms enclosed in the cages of filled skutterudites. The temperature dependence of the phonon energy obtained by Sm-149 nuclear resonant inelastic scattering from SmRu4P12 suggests an anharmonic vibration of the Sm atoms. A similar decrease of the vibration energy of the La atoms in LsOs(4)Sb(12) with decreasing temperature, which suggests that this motion too is anharmonic, was observed by high resolution inelastic X-ray scattering.

Fe-57 nuclear resonant forward scattering experiment has been performed on a single crystalline Fe2P under multi-extreme conditions, which shows a pressure induced ferromagnteic-antiferromagetic transition and magnetic field induced metamagnetic transition. We present the clear experimental evidence that internal magnetic fields in two Fe sites at 3 K, 25 kOe and 2.5 GPa are comparable with those in a ferromagnetic state at ambient pressure. These internal magnetic fields, which are related to magnetic moments, are not aligned collinearly along the [001] axis by the external magnetic field.

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.

A new polymorph of FeS has been observed at pressures above 30 GPa at 1,300 K by in situ synchrotron X-ray diffraction measurements in a laser-heated diamond anvil cell. It is stable up to, at least, 170 GPa at 1,300 K. The new phase (here called FeS VI) has an orthorhombic unit cell with lattice parameters a = 4.8322 (17) angstrom, b = 3.0321 (6) angstrom, and c = 5.0209 (8) angstrom at 85 GPa and 300 K. Its topological framework is based on the NiAs-type structure as is the case for the other reported polymorphs (FeS I-V). The unit cell of FeS VI is, however, more distorted (contracted) along the [010] direction of the original NiAs-type cell. For example, the c/b axial ratio is similar to 1.66 at 85 GPa and 300 K, which is considerably smaller than that of orthorhombic FeS II (similar to 1.72) and NiAs-type hexagonal FeS V (=root 3 approximate to 1.73). The phase boundary between FeS IV and VI is expected to be located around 30 GPa at 1,300 K. The phase transition is accompanied by gradual and continuous changes in volume and axial ratios and may be second order. At room temperature, FeS VI becomes stable over FeS III at pressures above 36 GPa. It is, therefore, suggested that the phase boundary of FeS III-VI and/or FeS IV-VI has negative pressure dependence.

The magnetic state in 6H-type BaFeO3-delta 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 similar to 124 angstrom. 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.

We have carried out inelastic X-ray scattering and Sm-149 nuclear resonant inelastic scattering of SmRu4P12 to investigate its metal-insulator transition. We have not observed any significant change in both inelastic X-ray scattering spectra and nuclear resonant inelastic scattering spectra at the transition temperature. In spite of the proposal of the Fermi surface nesting at q = (100) by the recent band calculation, the anomalies correlated with the metal-insulator transition were not found in the inelastic X-ray scattering spectrum at the longitudinal zone boundary along the [ 1 0 0] direction. The results of 149 Sm nuclear resonant inelastic scattering also suggest that the Sm vibration modes are not correlated with the metal-insulator transition. (c) 2006 Elsevier B.V. All rights reserved

High-pressure 149 Sm nuclear forward scattering experiments have been performed on a mixed valence Sm4Bi3 compound which shows a charge ordering at 260 K. A pressure-induced first- order structural phase transition takes place at 2.3 GPa and 200 K. We present the. first clear experimental evidence that magnetic ordering occurs just above 120K at 3.2 GPa. This magnetic ordering temperature is much higher than that at ambient pressure. The higher magnetic ordering temperature is consistent with the occurrence of a valence change of nonmagnetic Sm (2+) ion toward magnetic Sm (3+) ion at 2.3 GPa. (c) 2006 Elsevier B. V. All rights reserved.

We have carried out high resolution inelastic x-ray scattering and Sm-149 nuclear resonant inelastic scattering of a skutterudite SmRu4P12. The inelastic x-ray scattering spectra and dispersion show q-dependence and zone-dependence except for the modes lying at 9 meV. The dispersionless modes, which are zone-independent, are observed at 9 meV. The energy of this mode agrees with that obtained by Sm-149 nuclear resonant inelastic scattering. The dispersion suggests the presence of strong hybridization between low-lying optical mode at 9 meV and acoustic ones.

We have carried out the Sm-149 nuclear resonant inelastic scattering of Sm-based filled-skutterudite compounds at room temperature. The sharp and periodic phonon excitations are observed in SmFe4P12 and SmRu4P12, whereas the glass-like phonon spectrum is observed in SmFe4Sb12. The present results suggest that the phonon spectra associated with Sm atoms correlate with the size of pnicogen cages and the electronic states of these compounds. (c) 2006 Elsevier B.V. All rights reserved.

FeS is one of the 3d transition-metal compounds and undergoes a pressure-induced metal-insulator transition accompanied by the successive structural phase transition from a troilite (semiconductor) to a hexagonal structure (metallic) at around 3.5 GPa and finally to a monoclinic structure at 6.6 GPa at room temperature. In this paper, we report the recent results of the spectromicroscopic study on Fes under pressure in the far-infrared region to observe the change in the electronic states. (c) 2006 Published by Elsevier B.V.

Eu-151 nuclear resonant inelastic scattering (NRIS) experiments have been performed on the charge ordering Eu4As3 compound from 301 to 406K and the Eu partial phonon densities of states were extracted from the observed spectra. We derived the thermodynamic parameters from inelastic components in the observed NRIS spectra using the sum rules. The mean recoilless fraction shows the softening in the lattice vibration of the Eu ion around the charge ordering temperature in spite of no anomaly in the temperature dependence of average sound velocity. We present by the temperature dependence of mean force constant that the hybridization between the 4f states and the conduction or valence bands in Eu4As3 plays a role around the charge ordering temperature and in the valence fluctuating state.

We have measured the Mossbauer spectra of magnetite (Fe3O4) under a pressure of up to 18 GPa at room temperature using nonenriched high-quality samples. An analysis of the observed Mossbauer spectra confirms that Fe3O4 does not undergo a magnetic or crystalline structural transition up to 18 GPa at room temperature. In the pressure dependences of the refined hyperfine interaction parameters, however, small but distinct anomalies are found at 7.0 GPa in the center shifts of two crystallographically nonequivalent Fe sites, which are caused by the discontinuous decrease of an oxygen internal coordinate in the cubic inverse spinel structure. We believe that the pressure-induced discontinuous change of the Fe-O bond length is related to the absence of a metal-insulator transition above 7.5 GPa.

We have carried out Mossbauer spectroscopy and nuclear resonant inelastic scattering to elucidate the lattice dynamics in filled-Skutterudite compounds, especially phosphides. The second-order Doppler shift obeys the Debye model in RFe(4)P(12). Nuclear quadrupole interaction reveals an unusual temperature dependence in these compounds. An anomaly is observed in (57)Fe nuclear resonant inelastic scattering of these compounds. The energy where the anomaly observed in SmFe(4)P(12) agrees with the phonon excitation energy observed by (149)Sm nuclear resonant inelastic scattering. We have also performed the (99)Ru Mossbauer measurements of SmRu(4)P(12).

(La1-xYx)Mn4Al8 (0 <= x <= 1) is a very unique itinerant electron system, in which the spin pseudogap can be controlled continuously and nearly uniformly in a wide range of the gap width from Delta similar or equal to 250 to 500 K by the anisotropic volume shrinkage induced by the chemical pressure. The hydrostatic pressure on LaMn4Al8 also increases Delta but tends to fill the gap. The strong and anisotropic volume dependence of Delta strongly supports the idea that the origin of the gap formation is associated with the one-dimensional geometry of the Mn spin arrangement.

Interlayer magnetic coupling (IMC) between Fe and Gd layers through Al spacer has been experimentally studied on [Al(R angstrom)/Gd(40 angstrom)/Al(R angstrom)/Fe(20 angstrom)]xN/Al(5 angstrom) magnetic multilayers, where R = 0, 5, 10, and 20 as nominal figures. Magnetization curves and the temperature dependence of magnetization are reported. Magnetic Compton-profile technique has been used to determine the amounts and the signs of spin magnetizations of Fe and Gd layers for R = 5 and 20. A line-shape analysis of the magnetic Compton profile on the multilayer of R = 5 has verified that the IMC is antiferromagnetic, and the negative small amount of magnetization of Gd layer exists even at room temperature under 1 kOe, despite the absence of ferromagnetic magnetization of a A1(5 angstrom)/Gd(40 angstrom) multilayer at room temperature. The antiferro magnetic IMC has been confirmed in the case of R = 0, 5 and 10, but the IMC is not observed in the case of R = 20.

We have measured the Dy-161 nuclear resonant inelastic X-ray scattering on single crystalline DyB6 sample at low temperatures along [1 0 0] and [1 1 1]. Partial phonon densities of states were extracted from the inelastic component in the observed spectra. These extracted partial phonon densities of states are not strongly modified by the two-phase transitions at 25 and 30 K in spite of the structural change. (c) 2005 Elsevier B.V. All rights reserved.

The pressure versus temperature phase diagram of the mixed-valence EU(4)AS(3) compound was determined by means of x-ray diffraction measurements using synchrotron radiation. The structural phase transition occurs at 343 K under ambient pressure with volume reduction and the transition temperature increases linearly with pressure up to about 5 GPa. The entropy change at this transition is estimated to be 41 J mol(-1) K-1 from the Clausius-Clapeyron relation. The value is twice larger than the configuration entropy which is expected from a random arrangement of Eu (3+) and Eu2+ ions in the thermally driven valence fluctuating state of EU4AS3. Moreover, a new structural phase was observed above 20 GPa at room temperature.

We have systematically investigated the structural and electrical properties of stoichiometric FeS compound under pressure at low temperature. It is found that two successive first-order pressure-induced structural phase transitions occur at 5.0(2) and 7.0(5) GPa (at 17 K). Furthermore, the pressure dependence of volume changes at around 2.3 GPa. The temperatures dependence of resistivity above 3.0 GPa clearly shows a metallic behavior below 100 K. Consequently, the new isostructural nonmetal-metal transition occurs at around 2.5 GPa in FeS. The transition is almost continuous at 17 K because the volume reduction is less than 0.3% at the transition. In the vicinity of the phase transition at the metallic phase, the pressure dependences of coefficient of quadratic temperature dependence in resistivity and residual resistivity show the critically divergent behaviors.

We report the partial phonon densities of states (DOS) of iron sulfide, a possible component of the rocky planet's core, measured by the Fe-57 nuclear resonant inelastic x-ray scattering and calculate the total phonon DOS under pressure. From the phonon DOS, we drive thermodynamic parameters. A comparison of the observed and estimated compressibilities makes it clear that there is a large pure electronic contribution in the observed compressibility in the metallic state. Our results present the observation of thermodynamic parameters of iron sulfide with the low-spin state of an Fe2+ ion at the high density, which is similar to the condition of the Martian core.

The specific heat and magnetic susceptibility have been measured on high-quality single crystals of NiSb. NiSb is found to be a typical Pauli paramagnetic compound with an NiAs-type structure. The electronic specific heat coefficient is 2.60 mJ/K(2)mol, which is in good agreement with that obtained from the calculated density of states. The strong dHvA oscillation was observed at 60 mK in the field along [0001], which consists of seven fundamental dHvA frequencies, and the largest frequency corresponds to an orbit with 0.82 A(-2). (C) 2003 Elsevier B.V. All rights reserved.

A weak distortion of a [FeO6] octahedron was observed in hexagonal BaFeO2.91 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 Fe4+ concentration below 170 K. We strongly suggest that the weak distortion of a [FeO6] octahedron results from a charge-disproportionation phenomenon, 2Fe(4+) --> Fe<4+tau>+ + Fe<4-tau>+, where 0 < tau less than or equal to 1. Below 130K, BaFeO2.91 orders antiferromagnetically and the propagation vector is <00 0.45c*> at 50K.

In order to clarify local structure of 6H-type BaFeO3-delta, an atomic pair distribution function (PDF) was determined by neutron scattering experiments. Though Fe4+ concentration in BaFeO3-delta decreases below 170 K, the PDF pattern does not change at all temperatures. It indicates that the local structure around Fe ion dose not change in the decrease of Fe4+ concentration. (C) 2003 Elsevier Science B.V. All rights reserved.

We have measured the magnetization of single crystalline Sr3Fe2O6.5 with a lot of oxygen deficiencies. It was observed that the spin direction is parallel to the c-axis below T-N. Moreover, we carried out neutron powder diffraction experiments with a Sr3Fe2O6.21 powder sample.. The magnetic structure with a lot of oxygen deficiencies (delta > 0.5) has a typical Ising antiferromagnet system with the magnetic unit cell root2a x root2a x c. On the other hand, it was found that the magnetic structure Of Sr3Fe2O6.94 with a few oxygen deficiencies is larger than that of 2a x 2a x 4c. The magnetic structure of Sr3Fe2O6.94 is obviously different from that of Sr3Fe2O7-delta (8 > 0.5). Such a difference in the magnetic structures is associated with a drastic change of the exchange interactions between Fe-spins due to the introduction of oxygen.

The Fe-57 Mossbauer spectra of satisfactory quality were measured with the sample CuFeS2 not-enriched in Fe-57 under pressure up to 13 GPa. The vanishing of the magnetic hyperfine field in the spectra was observed around 6.5 GPa and room temperature where the structural phase transition occurs. It was found by the analysis of observed spectra that there exists the significant anomaly in the pressure dependence of the center shift around 6.5 GPa. These results show a substantial change in the electronic structure of CuFeS2 at 6.5 GPa.

The delay-time spectra of Fe-57 nuclear resonant forward scattering in FeS which belongs to the boundary between the charge-transfer and Mott-Hubbard insulators have been observed under high pressure up to about 6 GPa at BL09XU in SPring-8. The observed beat pattens in delay-time spectra are distinguished two different types which correspond to the two different phases. We have successfully applied the maximum-entropy method to analyze the observed delay-time spectra. The frequencies of quantum beats which directly depend on the energy differences between two hyperfine splitting levels of Fe-57 nuclei were extracted by this analysis and then the hyperfine interaction parameters were determined from there extract-ed frequencies. Since the evaluated magnetic hyperfine field at; 0.1MPa is smaller than that at 2 GPa, the Neel temperature of FeS with a troilite structure increases with pressure.

The semiconductor-metal-semiconductor transitions in stoichiometric FeS compounds were observed under nearly hydrostatic pressure condition up to 8 GPa and a temperature down to 4 K. In the low-pressure semiconductor state, the energy gap caused by the electron correlation decreases with a volume reduction. On the other hand, FeS in a high-pressure semiconductor state is a band insulator and the energy gap increases with pressure when the gap is formed between the occupied nonbonding and unoccupied antibonding bands. In the metallic phase at intermediate pressure range FeS can be regarded as a strongly correlated metal because of the large quadratic temperature dependence of resistivity at low temperature.

The de Haas-van Alphen (dHvA) effect has been investigated on high-quality single crystals of FeP in a double helical magnetic state with a 5c period in the magnetic fields up to 14T and at temperatures down to 0.42 K. Single crystals were grown by the chemical transport technique using iodine as the transport agent. Nineteen branches were observed in three crystallographic planes (100), (010) and (001), and analyzed with simple analytic equations for the Fermi surfaces. Many of the dHvA branches have a characteristic angular dependence and are found to correspond to the orbits on surfaces beyond the Brillouin zone in the magnetic state which is reduced to 1/5 of the non-magnetic one with the MnP-type along the [001] direction. These branches originate from the orbits caused by a magnetic breakdown phenomenon under high fields. Cyclotron mass ratios were obtained to be up to 5.9, whereas the electronic specific heat coefficient is rather small. 2.80 mJK(-2) mol(-1).

Magnetic properties of CeB2C2 and PrB2C2 compounds have been investigated by means of magnetization, specific heat and elastic constant measurements on single-crystalline samples. CeB2C2 is an antiferromagnet of T-N = 7.3 K with a moment direction in the tetragonal [001]-plane. An order-order transition appears at 6.5 K, which is observed in the specific heat and the anomaly in the susceptibility is extremely small. The magnetization measurement confirms that PrB2C2 is a Van Vleck paramagnet. The 4f ground state of PrB2C2 is a singlet with a magnetic doublet as a first excited state. The analyses based on the second-order approximation of crystalline electric field (CEF) interaction give no systematic result. The second-order CEF parameter A(2)(0) is positive in PrB2C2, although that is negative in CeB2C2 which is similar to that in NdB2C2. It is supposed that the CEF interactions of higher order play important roles in the RB2C2 system. (C) 2000 Elsevier Science B.V. All rights reserved.

We have performed electronic band calculations for the paramagnetic NiAs and NiSb using the full potential LAPW method. The Fermi surface of each compound consists of two kinds of cylindrical-like hole surface around the Gamma A-axis and one electron surface. For NiSb one more Fermi surface is obtained. The dHvA oscillation frequencies expected from each Fermi surface are calculated. Recently, we have measured dHvA oscillation for the single crystal of NiAs at 0.55 K in a field of up to 9.5 T and observed six branches. The observed results are discussed in comparison with the calculated results. (C) 2000 Elsevier Science B.V. All rights reserved.

The 3d transition metal monopnictides show various interesting electric and magnetic properties. In this study, we have measured the dHvA effect and the transverse magnetoresistance in FeP which has the MnP-type structure and is antiferromagnetic below 125 a with the double helical spin structure. Ten dHvA branches were observed in the (001)and (100) planes and their cyclotron mass ratios at the [010] direction are 0.6-2.2. (C) 2000 Elsevier Science B.V. All rights reserved.

Dy-161 Mossbauer effects were measured on the tetragonal antiferroquadrupolar ordered compound DyB2C2. In the magnetic phase below T-C = 15.3 K, the Mossbauer spectra are interpreted well by a single set of hyperfine parameters which gives the Dy magnetic moment of 9.0 mu(B). The hyperfine field anomalously decreases below T-C as temperature decreases, which can bt interpreted as symmetry frustration between antiferromagnetic and antiferroquadrupolar interactions acting on Dy3+ ions. Reflecting the 4f electronic state in which two Kramers doublets locate with a low-energy separation, the spectra in the antiferroquadrupolar ordered phase and paramagnetic phase are complicated and cannot be well analyzed by the two-level relaxation model with a single set of hyperfine parameters, although it is supposed that the Dy electronic state is one type.

We have measured the Dy-161 Mossbauer effects on the cubic ferroquadrupolar ordering compound DyB6. In the antiferromagnetic phase, the Mossbauer spectra are interpreted well by a single set of hyperfine parameters which gives the Dy magnetic moment of 9.8 mu(B). The nuclear quadrupole splitting implies that the crystal field parameter A(2)(0) originating from the rhombohedral strain by the ferroquadrupoler ordering is positive. Based on these facts, we conclude that the antiferromagnetic structure is a collinear one where the Dy moment direction is nearly parallel to the strain axis (the cubic [1 1 1] axis). Reflecting the 4f electronic state that two or more Kramers doublers are quasi-degenerated, the spectra in the ferroquadrupolar ordered and paramagnetic phases can not be analyzed well by the two-level relaxation model.