Toyotaka Osakabe

AMATERAS: A Cold-Neutron Disk Chopper Spectrometer

AMATERAS is a new disk-chopper-type spectrometer installed at Materials and Life Science Experimental Facility (MLF) of J-PARC. AMATERAS is equipped with an extra chopper for pulse shaping at the upstream position, in addition to a monochromating chopper, which conventional chopper spectrometers at pulsed source have. Owing to the use of these choppers and the high peak intensity from a coupled moderator source at MLF, the AMATERAS design realizes high-intensity and high-energy-resolution measurements in quasielastic and inelastic neutron scattering experiments. The spectrometer had the first neutron beam in May 2009. During the course of commissioning, the performance of the spectrometer was confirmed by conducting test experiments. AMATERAS is now open to users and is producing scientific outputs.

Neutron Diffraction Study of the Pressure-Induced Magnetic Ordering in the Spin Gap System TlCuCl3

Neutron elastic scattering measurements have been performed under a hydrostatic pressure in order to investigate the spin structure of the pressure-induced magnetic ordering in the spin gap system TlCuCl 3 . Below the ordering temperature T N =16.9 K for the hydrostatic pressure P =1.48 GPa, magnetic Bragg reflections were observed at reciprocal lattice points Q =( h , 0, l ) with integer h and odd l , which are equivalent to those points with the lowest magnetic excitation energy at ambient pressure. This indicates that the spin gap closes due to the applied pressure. The spin structure of the pressure-induced magnetic ordered state for P =1.48 GPa was determined.

Magnetic Orders in CeP

The magnetic orders in CeP under applied magnetic field of up to 70 T are studied based on the layered-type Γ 8 magnetic polaron lattice model studied in the previous paper. Entropy changes, total induced moments and results of neutron scattering in various phases are explained consistently. Fermi surface nesting is important to keep the period constant.

Charge order and one-dimensional properties of Yb4As3

Abstract A polarized neutron diffraction experiment gives direct evidence for a charge order in the extremely low-carrier system Yb4As3. Inelastic neutron scattering experiments on Yb4As3 revealed the existence of low-energy spin excitations that are characteristic of a one-dimensional antiferromagnetic coupling at low temperatures. The results indicate that the one-dimensional arrangement of Yb3+ ions caused by the charge order plays a main role for the heavy-electron behavior in Yb4As3.

Feasibility Tests on Pressure-Transmitting Media for Single-Crystal Magnetic Neutron Diffraction under High Pressure

The hydrostaticity of pressure-transmitting media, such as graphite powder, a mixture of Fluorinert, Fomblin oil, silicone oil, Daphne7373 oil and glycerin, was investigated by high-pressure neutron diffraction experiments on the mosaic spread of NaCl single crystals. The width of the rocking curve of the NaCl single crystal in the medium other than glycerin increases rapidly with increasing pressure below 2.5 GPa, indicating that the medium solidifies below 2.5 GPa. On the other hand, glycerin transmits nearly hydrostatic pressure at least up to 7 GPa. The diffraction profiles of the media were also measured to evaluate the background intensities. Daphne7373 oil and glycerin generate low and flat diffraction profiles particularly at approximately 25°, where the principal magnetic diffraction signals are often present. Thus, glycerin is the most suitable pressure medium for single-crystal magnetic neutron scattering experiments beyond 3 GPa.

Novel magnetic structures of the low-carrier system CeP under high pressure

Abstract The results of neutron scattering experiments on the low-carrier system CeP under high pressure up to 1.7 GPa are presented. The magnetic structures derived from the scattering patterns at various pressures at low temperatures are similar to those of CeP under magnetic field, forming the periodic stacking of the double Γ8 Ce (0 0 1) planes with sandwiching the Γ7 Ce (0 0 1) planes. The difference is a wide and systematic change of the period of the stacking of Γ8 Ce (0 0 1) planes with changing pressure. These results support strongly the magnetic polaron model for the unusual properties of CeP.

Incommensurate Magnetic Structure of the Heavy Fermion Antiferromagnet Ce7Ni3.

Magnetic structures of the heavy fermion antiferromagnet Ce 7 Ni 3 have been studied by neutron diffraction using a single crystal. It undergoes the two successive phase transitions at T N1 = 1.8 K and T N2 = 0.7 K in zero field. At T N1 ordering of a single- k type magnetic structure with an incommensurate modulation vector k IC ≃0.22 c * takes place. This structure is approximately a sinusoidally modulated structure of c -axis moments, which belongs to the Γ 2 and Γ 3 irreducible representations of the space group P 6 3 m c . Root mean squares of the moments at T = 1.4 K are 0.46±0.07, 0.70±0.03 and 0.10±0.05 µ B for the Ce1, Ce2 and Ce3 sites, respectively. Below T N2 the incommensurate modulation vector becomes temperature independent, and a commensurate structure with \({\mib k}_{\rm C} = \frac{1}{4} {\mib c}^{\ast}\) appears in addition to the incommensurate structure. The metamagnetic transition under the magnetic field of H ∥ c = 0.16 T at T = 1.5 K is a first order transition to a ferromagnetic s...

Exchange Fields in Gd(Fe11Ti) and Gd(Fe11Ti)N1-δ Investigated by High Energy Inelastic Neutron Scattering

Inelastic neutron scattering experiments on polycrystalline Gd(Fe 11 Ti) and its isostructural nitride Gd(Fe 11 Ti)N 1-δ yield a direct measurement of the exchange field ( B ex ) acting at the unique Gd site in the ThMn 12 -type structure. B ex is 255(8) T in the parent compound but nitrogen occupation of the interstitial sites above and below the rare-earth along the c -axis reduces B ex in the nitride to 220(8) T. These values are in agreement with those deduced from the Curie-temperature using molecular field theory and they are consistent with inelastic neutron scattering data on other Gd 1- x Fe x intermetallic compounds. \makeatletter \makeatother

Crystal electric field and quadrupole ordering in semiconducting PrPtBi

We prepared PrPtBi crystal by tungsten crucible and carried out inelastic neutron and thermal expansion experiments together with transport properties. The crystals show semiconducting properties both in electrical resistivity and Hall constant. As for crystal electric field (CEF), the results show that the CEF level scheme consists of non-Kramers doublet Γ3 ground state, a first excited state Γ5 with an energy of 75 K, a second excited one Γ4 with an energy of 116 K. Precise experiment of thermal expansion under the magnetic field suggests that the phase transition at 1.35 K is of a ferro-quadrupolar-type.

Pressure Effects on Quadrupolar and Magnetic Ordering in HoB2C2 Observed by Single-Crystal Neutron Diffraction

High-pressure single-crystal neutron diffraction experiments were carried out on a tetragonal HoB 2 C 2 where the antiferroquadrupolar (AFQ) order coexists with the antiferromagnetic (AFM) order competitively. The transition temperature T N increases monotonically with increasing pressure up to 9.8 GPa, whereas T Q decreases and vanishes at approximately 4 GPa as pressure increases, indicating that applying pressure enhances the AFM interaction and simultaneously suppresses the AFQ order. We presume that the volume shrinkage by applying pressure constrains the local atomic displacement of (B–C) layers and leads to the suppression of the AFQ order. We additionally found pressure-induced phases where the ordered state was considered to be antiferromagnetic.