Hisao Kobayashi

The effect of pressure on the electronic states of FeS and studied by Mössbauer spectroscopy

We have measured under pressure Mossbauer spectra of FeS and 3c-type up to 16 GPa and x-ray diffraction patterns of up to 11 GPa at room temperature. It is found for 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 , 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 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 , the electronic state is like a semimetallic one. Above 6.5 GPa for FeS and 4.5 GPa for , 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 single crystal GdMn2Ge2 in high magnetic field

Abstract The magnetic properties of GdMn 2 Ge 2 with a layer structure have been studied by measuring magnetization of the single crystal in high fields up to 150 kOe. The four magnetically ordered phases have been observed. In low magnetic fields, collinear antiferromagnetic properties are observed in the range from 96.5 to 365 K where the Mn moments couple ferromagnetically on the same Mn layer and the ferromagnetic Mn in layers couple antiferromagnetically with Mn on the next layers. The collinear ferrimagnetic structure becomes stable below 96.5 K where the magnetic moments of the Gd and Mn atoms are antiparallel to each other. As the magnetic field along the c -axis increases, the magnetic structure of the Mn sublattice changes from antiferromagnetic to ferromagnetic one above the critical field and then the ordering of the Gd moments is induced by the exchange field from the Mn sublattice moments. Below 96.5 K, the transition from the collinear to a canted ferrimagnet occurs in the applied magnetic field perpendicular to the c -axis. The Gd moments in this canted ferrimagnet lie in the c -plane and the Mn moments cant from the c -plane.

Magnetic properties of the tetragonal compounds CeB2C2 and PrB2C2

Abstract 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 TN=7.3xa0K with a moment direction in the tetragonal [0xa00xa01]-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 A20 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.

The low-temperature specific heat of FeS and M0.875X (M = Fe, Co; X = S, Se) with a NiAs-like structure

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 -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 -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.

Local structure of BaFeO3−δ studied by neutron scattering

Abstract In order to clarify local structure of 6H-type BaFeO3−δ, an atomic pair distribution function (PDF) was determined by neutron scattering experiments. Though Fe4+ concentration in BaFeO3−δ 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.

Electronic specific heat of Ti1+xS2 (0 < x < 0.1)

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 x13. This linear relation changes its slope at x13 = 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.

Antiferromagnetism of GdCoC2 and GdNiC2 intermetallics studied by magnetization measurement and 155Gd Mössbauer spectroscopy

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

Electronic state of Sr3Fe2O7−y studied by specific heat and Mössbauer spectroscopy

Abstract The low-temperature specific heat and Mossbauer spectra have been measured for Sr 3 Fe 2 O 7− 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 Sr 3 Fe 2 O 6.92 is smaller by one order than those of Sr 3 Fe 2 O 7− y above y = 0.2. It is concluded that the electronic state of Sr 3 Fe 2 O 7− y changes around y ∼ 0.15.

Magnetic properties of the single crystalline Dy3Ni studied by magnetization measurement and 161Dy Mössbauer spectroscopy

Abstract Magnetic properties of Dy3Ni have been investigated by means of magnetization measurement and 161Dy Mossbauer spectroscopy on the single-crystalline and powdered samples, respectively. Dy3Ni is a noncollinear antiferromagnet below TN = 52 K. There occur two antiferromagnetic order—order transitions at Tt1 = 22 K and Tt2 = 34 K. The effective paramagnetic moment μeff is 11.7 μB/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 A20 ∼ 800 K/a02 and have the full moment 10 μB. There is no evidence for the Ni moment to participate in the magnetic ordering below TN down to 4.2 K.

A statistical method of component identification of average evoked potentials

The polarity histogram was described for a method of statistically identifying AEP components. The technique is less sensitive to artifacts and is inexpensive.