小林 寿夫

We have measured the specific heat of FeS, Fe0.875X, and Co0.875X (X = S, Se) compounds with a NiAs-like structure over the temperature range from 2 to 30 K. It was found that the ground state of FeS has an insulating character. The opening up of a gap in the density of states is probably caused by electron correlation. Two types of superstructure are known to occur due to ordering of vacancies at iron sites in Fe0.875X; one is a 3c structure and the other a 4c structure. The gamma-values obtained for 3c structure are about 10% larger than those of 4c structures. Accordingly, this result indicates that the electronic structure of Fe0.875X should be sensitive to the ordered structure of the vacancies. Since the gamma-values obtained for non-stoichiometric compounds are larger than the calculated ones, there is electron effective-mass enhancement in non-stoichiometric compounds. Moreover comparison of the observed and calculated electronic contributions to the low-temperature specific heat suggests that the mass enhancement in Pauli-paramagnetic Co0.875X is larger than that in ferrimagnetic Fe0.875X.

The crystal structure of Sr3Fe2O7-delta With iron in the average oxidation state + 4 has been studied by time-of-flight neutron powder diffraction. The diffraction data were collected at more than 30 points between 18 and 300 K. A slight anomalous behavior in lattice parameters was observed at around 120 K. In addition, thermal expansion is anisotropic above 120 K whereas isotropic below this temperature. These phenomena are not caused by magnetic order which occurs at 100 K, Although this first finding is most likely related to the structural changes arising from the charge ordering, i.e. the disproportionation of Fe4+, structural models with two or more sites of Fe ions did not improve R-factors in Rietveld refinements. (C) 1999 Elsevier Science Ltd. All rights reserved.

We have measured the de Haas-van Alphen effect at 0.55 K in the field up to 9.5 T for the single crystal of Pauli paramagnetic NiAs. Six dHvA branches have been observed and they show complicated behavior as a function of the field direction which is applied in the symmetry planes (<11(2)over bar 0>), (<10(1)over bar 0>) and (0001). The corresponding cyclotron masses have been also determined from the temperature dependence of the dHvA amplitude. To discuss the experimental results we have performed the electronic hand calculation of NiAs for the non-magnetic state by using the full potential LAPW method. The total energy calculated as a function of the lattice parameter a, keeping the ratio cia as the observed value, becomes minimum at 3.60 Angstrom, which agrees very well with the observed one. The Fermi surface is found to consist of two kinds of hole surface around the Gamma A axis and one electron surface with six fold symmetry. For each Fermi surface we have calculated dHvA oscillation frequencies and succeeded to explain the field dependence of some of the observed frequencies. The calculated cyclotron masses are compared with the observed ones.

The de Haas-van Alphen (dHvA) effect was measured for the paramagnetic compound CrP with an orthorhombic MnP-type structure using a high quality single crystal. Ten branches of dHvA frequency below 9200T are obtained in the (010) plane at 0.6K. Four frequencies in a field along the symmetry axes agree qualitatively with the theoretical results calculated with an all-electron relativistic LAPW method. The cyclotron mass ratios in two branches along the [100] axis are 4.6 and 3.1, respectively.

The delay-time spectra of the nuclear resonant forward scattering for FeS were measured at ambient conditions with the sample not-enriched in Fe-57 and under high pressure using the sample with enriched about 10 at.% Fe-57. The quantum beat modulations in observed delay-time spectra under pressure change with two successive first-order phase transitions at 3.5 and 6.5 GPa. We have successfully analyzed the beat patterns in delay-time spectra using the maximum entropy method and extracted the frequencies in each phase. These frequencies are in good agreement with those expected form the hyperfine interactions which values were estimated by the analysis of Mossbauer absorption spectra. This is the first successful attempt to apply the maximum entropy method in this field.

Magnetic susceptibility, magnetization process, specific heat, ultrasonic velocity, and inelastic neutron scattering experiments were performed on powdered and single-crystalline TmAu2 samples. TmAu2 with a simple body-centered tetragonal MoSi2-type structure undergoes a transition to an antiferromagnetic state at T-N of 3.2 K. An anomaly at 7.0 K is observed in the temperature dependence of specific heat and magnetic susceptibility. A very large softening beyond 50% from 300 down to 7.0 K is also observed in the temperature dependence of the elastic constant of the C-11-C-12 mode. The results reveal clearly that the anomaly at 7.0 K (=T-Q) originates from a ferroquadrupolar ordering with an orthorhombic gamma-symmetry-lowering mode. Separation between the ground and the first excited singlet states of 4f levels is so small in the quadrupolar ordered phase and a transition probability between these states is so extremely large that there occurs a Van Vleck antiferromagnetic transition at 3.2 K by exchange-induced magnetic moments arising from the transition between the ground and the first excited singlet states.

Transport properties of P-andSb-doped Yb4As3 were measured. The electrical resistivity measurement ofYb4(As0.88Sb0.12)3 under various pressure up to 8 GPa was performed. These results were compared with that of pure Yb4As3 under high pressure. The substitution effect of Sb is very similar to the pressure effect, however, there is a big differencebetween them. This similarity strongly suggests that the mobility of the electron is much smaller compared to that of the hole, [pressure effect, Yb4As3, Yb4(As1-xSbx)3, Yb4(As1-x-Px)3. © 1998, The Japan Society of High Pressure Science and Technology. All rights reserved.

The Mössbauer spectra have been measured for Sr3Fe2O7.8 at ambient conditions and for Sr3Fe2O6.80 under pressure. The iron sites are subdivided into two electronically nonequivalent states, that is. one is a tetravalent state and the other a trivalent one. The concentration and the isomer shift of tetravalent site depend on not only the oxygen vacancies but also pressure. It is found that the value of center shift exhibits a discontinuous decrease with increasing pressure and the change of pressure dependence around 8.5 GPa. © 1998, The Japan Society of High Pressure Science and Technology. All rights reserved.

The specific heat at the temperature range from 1.7 to 30 K and the dHvA effect at 0.55 K in the field up to 9.5 T have been measured for Pauli paramagnetic NiAs. The electronic specific heat coefficient is 3.0 mJ/(K-2 mel) which is in good agreement with the value estimated from the calculated density of states. It is found that the dHvA oscillation in the field along [0001] consists of four frequencies which correspond to the extremal areas in the k-space of 0.06, 0.11, 0.16 and 0.18 Angstrom(-2).

The low-temperature specific heat and Mossbauer spectra have been measured for Sr3Fe2O7-y. The iron sites are subdivided into two electronically nonequivalent sites, that is, one is a tetravalent site and the other a trivalent one. The concentration and the isomer shift of tetravalent site depend on the oxygen vacancies. The electronic specific heat of Sr3Fe2O6.92 is smaller by one order than those of Sr3Fe2O7-y above y = 0.2. It is concluded that the electronic state of Sr3Fe2O7-y changes around y similar to 0.15.

The low-temperature specific heat and the X-ray diffraction have been measured for Ti1+xS2 (0<x<0.1) with the well-defined composition. The obtained lattice and internal parameters increase linearly with the Ti content. The observed electronic specific heat increases linearly with x(1/3). This linear relation changes its slope at x(1/3) = 0.26 and the value goes to zero at x = 0. It is concluded that the electronic structure of stoichiometric TiS2 has a band gap and two Ti-based conduction bands with a small energy difference.

Magnetic properties of Dy3Ni have been investigated by means of magnetization measurement and Dy-161 Mossbauer spectroscopy on the single-crystalline and powdered samples, respectively. Dy3Ni is a noncollinear antiferromagnet below T-N = 52 K. There occur two antiferromagnetic order-order transitions at T-t1 = 22 K and T-t2 = 34 K. The effective paramagnetic moment mu(eff) is 11.7 mu(B)/Dy whose large value is presumably attributable to the paramagnetic moment on the Ni atom. The metamagnetic transitions occur around 45, 78 and 51 kOe at 4.2 K along the a-, b- and c-axis, respectively. The Mossbauer spectroscopy has shown that the Dy3+ ions are in strong crystal fields of A(2)(0) similar to 800 K/a(0)(2) and have the full moment of 10 mu(B). There is not evidence for the Ni moment to participate in the magnetic ordering below T-N down to 4.2 K.

Dy-161 and Gd-155 Mossbauer spectroscopic studies on DyCoC2, DyNiC2, GdCoC2 and GdNiC2 which were performed in order to obtain microscopic information on magnetic properties are reviewed. The experiments were done using the standard Tb-161 and Eu-155 Mossbauer sources prepared by neutron irradiation at the Japan Material Testing Reactor. A simple ferromagnetic DyCoC2 is a good object to compare the magnetic hyperfine field with the magnetic moment determined precisely by the magnetization measurement of the single crystalline sample. DyNiC2 is an antiferromagnet with commensurate magnetic structure and incommesurate moment-modulation structure. The magnetic hyperfine held changes correspondently well with the both magnetic structures. It is exhibited that the Gd-155 Mossbauer spectroscopy is very useful to determine the moment direction in the Gd compounds which are rarely investigated by neutron diffraction because of large neutron absorption cross section of the natural Gd nuclei. The crystalline electric field parameter A(2)(0) determined from the quadrupole interaction acting on Gd is also useful to interpret the magnetic anisotropy of the other rare earth atoms in the isostructural compounds.

We have measured under pressure Fe-57 Mossbauer spectra of FeS and 3c-type Fe7S8 up to 16 GPa and x-ray diffraction patterns of Fe7S8 up to 11 GPa at room temperature. It is found for Fe7S8 that the compressibilities of the lattice parameters exhibit definite anomalies at around 4.5 GPa and that there is no change in the crystal structure up to 11 GPa. Magnetically ordered Mossbauer spectra are observed below 6.5 GPa for FeS and 4.5 GPa for Fe7S8, whereas the spectra above these pressures are typical of a paramagnetic ordering with a quadrupole splitting. A large reduction in the centre shift is observed at these pressures. It is found that there is a distinct steplike feature of the magnetic hyperfine field at 3.5 GPa for FeS. The electronic states of FeS and Fe7S8 are deduced from the volume dependences of the centre shift and the magnetic hyperfine field. Below 3.5 GPa for FeS, the electronic state has an insulating character and the electrons on the iron are well localized and thus contribute to the magnetic moment. In the intermediate-pressure range, from 3.5 to 6.5 GPa for FeS and below 4.5 GPa for Fe7S8, the electronic state is like a semimetallic one. Above 6.5 GPa for FeS and 4.5 GPa for Fe7S8, the electronic bandwidth is large enough to cause the state to become metallic and produces a collapse of the iron magnetic moment.

Magnetic properties of GdCoC2 and GdNiC2 are investigated by means of magnetization measurement of single crystals and Gd-155 Mossbauer spectroscopy of powdered samples. GdCoC2 is an antiferromagnet below T-N = 15.6 K and shows an order-order transition at T-t = 14.0 K. The measurements of Gd-155 Mossbauer effect and magnetization process reveal that the Gd moments align noncollinearly by an angle of 48 degrees from the a-axis in the c plane. GdNiC2 is an antiferromagnet below T-N = 20.0 K. The Gd moments align noncollinearly by angles of 18 degrees from the c-axis and 45 degrees from the a-axis in the c-plane. By the Gd-155 Mossbauer measurements, crystal field parameters are derived to be A(2)(0) = -498 K/a(0)(2) in GdCoC2 and A(2)(0) = 277 K/a(0)(2) in GdNiC2.

The magnetic phase transitions in DyMn2Ge2 were studied by neutron diffraction. The (111) antiferromagnetic order reflection due to Mn atoms is observed only above 40 K, and the intermediate superlattice (110)± line is observed in the temperature range between 35 and 40 K. It is concluded that the intermediate temperature phase is a magnetically single phase in which the magnetic stracture is described with a propagation vector k ≈ (0, 0, 0.65). © 1995.

We have measured Sn-119 Mossbauer spectra of the CePtSn and (Ce0.9La0.1)NiSn compounds in the range from 1.5 to 293 K. In CePtSn, the spectra observed above 8 K are well explained by an electric quadrupole interaction. The spectral shape changes below 8 K due to the presence of a magnetic hyperfine field produced by the ordering of the Ce magnetic moments. We have analyzed these spectra assuming an incommensurate magnetic structure. The temperature dependence of the magnetic hyperfine field matches with an S = 1/2 mean field curve with a step at 5 K. In (Ce0.9La0.1)NiSn, no magnetic order exists down to 1.5 K.

Magnetic properties of DyMn2Si2, DyMn2Ge2 and their mixed compounds DyMn2(Si1-xGex)2, which display a variety of interesting magnetic behaviors originating in competing magnetic interactions and anisotropy, have been investigated systematically by magnetization mesurements, Dy-161 Mossbauer spectroscopy and neutron diffraction experiments. This report presents a review of the results mainly obtained by the magnetization measurements.

161Dy Mössbauer effect in DyMn2(Si1-xGex)2 (0 < x < 1) has been measured at temperatures between 4.2 and 45 K. All the compounds which have antiferromagnetic ordering of Mn moments with TN = 430-523 K transform it furthermore into other magnetic structures accompanied by ordering of Dy moments at low temperatures of 25-37 K. Magnitude of the magnetic hyperfine field and its temperature dependence have clarified that the magnetic structure of low-temperature phase is canted-ferrimagnetic in the compounds with 0.1 ≤ x ≤ 0.9, where the Mn moments tilt about 60° from the c-axis. The compounds with x = 0.8 and 0.9 have an intermediate-temperature phase between 26 and 37 K. In this phase, the Dy ions have two kinds of magnetic state, which are observed as a static Mössbauer subspectrum and a relaxation one, respectively. It is concluded that the magnetic structure of the intermediate-temperature phase is a collinear ferrimagnetic one with a long periodic arrangement. This structure can be described as a stacking of Mn moment layers along the c-axis with a + + - + + - sequence. © 1993.

Between 1 K and 33 mK CeNiSn behaves as a paramagnet, approaching magnetic order, but no transition is observed. A strong reduction in the moment participating in the magnetic correlations is likely. The La-doped compounds give no magnetic response down to 1.5 K. CePtSn shows unusual spin freezing just above T(N) almost-equal-to 8 K and a sharp transition into a more complex spin order around 5.2 K.

Magnetic measurements have been performed in order to investigate magnetic properties of pseudo-ternary compounds DyMn2(Si1-xGex), (0 less-than-or-equal-to x less-than-or-equal-to 1). As x increases, lattice parameters increase from a = 3.904 angstrom and c = 10.41 angstrom at x = 0 to a = 3.975 angstrom and c = 10.79 angstrom at x = 1, and the Neel temperature decreases from 523 to 430 K. In all the compounds, magnetic transitions are observed at low temperatures where Dy moments start to order magnetically. DyMn2Si2 has three magnetic phases below 38 K whose magnetic structures are long-periodic, and it is supposed that there occurs a magnetic short-range order of Dy moments between 38 and 83 K. The compounds with 0.1 less-than-or-equal-to x less-than-or-equal-to 0.6 have a magnetic phase below about 25 K with a canted-ferrimagnetic structure. This phase exists up to x = 0.9, although DyMn2Ge2 has a collinear-ferrimagnetic phase. For 0.7 less-than-or-equal-to x less-than-or-equal-to 1.0, there exists an intermediate-temperature phase which may have a ferrimagnetic structure with a long-periodic arrangement.

The 161Dy Mössbauer effect in DyMn2(Si0.1Ge0.9)2 has been measured at temperatures between 4.2 and 42.2 K. The magnetic behavior of the compound bears some resemblance to that of DyMn2Ge2, that is, the compound exhibits three magnetic transitions at TN = 448 K, T1 = 26.0 K and T2 = 35.0 K. The magnitude and the temperature dependency of hyperfine parameters have deduced the conclusion that the magnetic structure is canted-ferrimagnetic below T1 while that of DyMn2Ge2 is collinear-ferri-magnetic. The magnetic structure between T1 and T2 is a collinear-ferrimagnetic one with a long-periodic arrangement which is just the same with that observed in DyMn2Ge2. © 1993.

We have measured Fe-57 Mossbauer spectra of (CaV0.99Fe0.01O3)-Fe-57 in the region from room temperature to liquid helium temperature for a detailed research of magnetic properties of CaVO3. The magnetic susceptibility of this oxygen stoichiometric sample indicates a small antiferromagnetic-like kink around 170 K but the Mossbauer spectra have two paramagnetic contributions in the entire temperature region. One is a strong single line attributed to Fe3+ and the other is a weak doublet with a large eqQ splitting attributed to Fe2+. The results suggest that the magnetic susceptibility anomaly at 170 K is not related to a magnetic transition but to some kind of structural transition.

We have measured the Fe-57 Mossbauer spectra for the hexagonal BaFeO3-y system with different oxygen contents. The Fe sites are subdivided into two electronically nonequivalent sites, that is, one is a tetravalent site and the other a trivalent one. The Fe4+ concentration decreases not only with increase of oxygen vacancies but also with temperature. In BaFeO2.88, the magnetic order occurs at 130 K and the temperature dependence of Fe4+ concentration changes around 170 K.

Conversion electron Mossbauer effect measurements have been performed on the (111)-oriented (Bi, Y)3Fe5O12 single-crystal films at room temperature. The dependence of the isomer shift (delta) and magnetic hyperfine field (H(hf)) of the 24d site on Bi concentration shows that the degree of covalence of the bond decreases with Bi concentration and the change of H(hf) is not directly proportional to that of the 24 d sublattice magnetization. The values of delta and H(hf) of the 16a site are kept at almost the same value. The magnetic structure of the (Bi, Y)3Fe5O12 films is collinear ferrimagnetic and the direction of the net magnetization of the Bi3Fe5O12 film is along the [110] axis.

The Dy-161 Mossbauer effect has been studied on DyMn2Si2. Magnetization measurements have revealed that there occur intricate magnetic transitions far below T(N) = 511 K. Three cusps at T1 = 14.5 K. T2 = 22.4 K and T3 = 36.7 K and weak ferromagnetic transition at T4 = 83.4 K have been observed in the magnetization vs. temperature curves. Consideration on the de Gennes rule leads us to the conclusion that T3 is the magnetic ordering temperature of Dy moments. Below T2, the spectra consists of a static subspectrum and a relaxation one. Both the hyperfine field and the quadrupole splitting of the relaxation subspectrum increase anomalously as the temperature increases from T1 to T2. This behavior is interpreted by a local structure model with the arrangement of the canted Mn moments where the Dy-Mn magnetic interactions frustrate the easy magnetization directions of the Dy moments. The facts that the analyses of the spectra require at least three subspectra between T2 and T3 and two subspectra above T3 imply that the magnetic structures are not simple.

The electronic ground state of the Dy ion and the magnetic structure in DyMn2Ge2 have been studied by Dy-161 Mossbauer spectroscopy. Three different types of spectra are observed corresponding to three magnetic phases. Below 33 K, where the magnetic structure is collinearly ferrimagnetic, the hyperfine parameters derived from the static single-site hyperfine spectra indicate the fact that the electronic ground state of the Dy ion is formed purely with the \ - 15/2&gt; state. Above 37.5 K, where the Dy moments remain disordered, the relaxation spectra are observed. From refined hyperfine parameters, the electronic ground state of the Dy ion is mainly formed with the \ +/- 15/2&gt; states, and the effect of other states mixed through the crystalline electric field is recognizable. Two hyperfine subspectra for the crystallographically equivalent Dy sites are observed in a temperature range between 33 and 37.5 K, and the intensity ratio between the subspectra is about 1:1, which indicates that there are two kinds of Dy ions with different electronic ground states. From this result, we have deduced a magnetic structure; that is, about 25% abundant ferromagnetic Mn layers couple antiferromagnetically with adjacent ferromagnetic Mn layers, and those Mn layers with antiparallel magnetization distribute randomly along the c axis. The net magnetization of the Mn sublattice aligns antiparallel to that of the Dy sublattice like a ferrimagnetic state.

Epitaxial crystallization of deposited amorphous GaAs layers on GaAs(100) up to the surface by bombardments with 400 keV Ar and 400 keV Kr has been successfully performed at a temperature range between 125 and 200-degrees-C. Properties of crystal growth were investigated as a function of ion species (Ar and Kr), energy (400 and 800 keV), ion dose, dose rate and substrate temperature by RBS channeling experiments. The growth rate has shown a nearly linear dependence on ion fluence. Ion bombardments below 100-degrees-C have induced further amorphization beyond the initial crystal/amorphous interface. On the scale of nuclear energy deposition density, bombardments with higher electronic excitation efficiency give a small increase of the growth rate. Ar bombardments have shown a strong dependence of the growth rate on dose rate, whereas Kr bombardments have revealed a weak dependence. An apparent activation energy of 0.13 +/- 0.06 eV for the crystal growth was observed.

The dynamics of the irradiation-induced martensitic transformation in a single crystal of 17/13 austenitic stainless steel has been studied using the RBS channelling technique, conversion electron Mossbauer spectroscopy (CEMS), and glancing incidence X-ray diffraction (GXRD). 300 keV xenon ions were used to induce the martensitic alpha phase in the austenitic steel. It is shown that in the single-crystalline steel the martensitic phase occurs abruptly and covers almost all, i.e. the region near the surface of the specimen in a very short fluence increase, i.e. from 4 x 10(16) to 5 x 10(16) ions/cm2, which suggest an introduction of a critical transition, in contrast with the influence dependence in polycrystalline 17/7 steel specimens; in the latter case the induced phase increases gradually to a saturated amount around a fluence of (5-7) x 10(16) ions/cm2.

Structural distortion of polycrystalline AiN thin films irradiated with He and Ar ions was investigated by means of glancing angle X-ray diffraction experiments. Samples were implanted with He ions with energies from 40 to 220 keV to make a homogeneous implantantion profile and were also bombarded with 800 keV Ar ions to produce the same extent of the radiation damage due to direct atomic displacements. No phase transformation was observed throughout the whole implantation process. On the scale of the deposited energy into nuclear collisions approximately the same increase of the lattice parameter along the c-axis was observed in He- and Ar-implanted samples. On the other hand, the lattice expansion along the a-axis showed 60% larger values at the maximum fluence in the He-implanted samples than the Ar-implanted samples. The compressive strain along the a-axis also showed the large increase in the He-implanted samples. These results suggest a preferential configuration in clustering of He atoms in the AIN crystalline lattice.

Magnetic properties of single-crystal DyMn2Ge2 have been studied by magnetization measurement in high fields up to 150 kOe and by neutron diffraction. Four magnetically ordered phases have been observed. In low magnetic fields parallel to the [001] direction, collinear antiferromagnetism is observed in the temperature range from 37.5 to 431 K with the Mn moments ferromagnetically coupled on the (001) layers, and with antiferromagnetic coupling between the adjacent (001) Mn layers. Below 33 K, there is a collinear ferrimagnetic structure with the Dy and Mn moments antiparallel to each other along the c axis. A new magnetic phase is observed in the temperature range between 33 and 37.5 K, where its magnetic structure is not uniquely determined but the averaged moments at the Dy and the Mn atoms are evaluated to be about 6-mu-B and 1-mu-B, respectively. Another new magnetic phase is observed in high magnetic fields along the ]001[ directions below 65 K. The magnetic structure of this phase, which has been deduced from the magnetization measurement, is a canted ferrimagnetic structure, with the Dy moments oriented along the c axis, and the Mn moments canted away from the c axis.