Norio Achiwa

High-Pressure Cell for Neutron Scattering

We have developed a high-pressure cell for neutron scattering. The cell uses a barrel-shaped support cylinder made of sintered ceramics. Samples 6 mm in diameter and 10 mm long can be accommodated. The cell is suitable for use at low and high temperatures, after a pressure generated at room temperature is locked. The change of pressure on cooling to cryogenic temperatures is estimated by measuring the lattice parameter of NaCl. Besides the pressure locking technique, continuous pressurization can be achieved at room temperature by using a built-in mini-press while the cell is mounted on a goniometer of neutron spectrometer.

Neutron Reflectivities of Ni-Mn and Ni-Ti Multilayers for Monochromators and Supermirrors

Multilayer films for neutron mirror were made by vacuum depositing thin films of Ni and Mn or Ni and Ti on a glass plate alternately. Their neutron reflectivities were measured as a function of neutron wavelength by the time-of-flight technique. For Ni-Mn multilayer monochromators, the numbers of layers necessary to give a nearly perfect reflection (≥90%) at the main Bragg peak, were found to be 7, 11 and 21 for layer thicknesses of 163, 112 and 83 A, respectively. The reflectivities obtained experimentally were compared with those calculated with the method applying the conventional optics, and their agreements were quantitatively reasonable. Moreover, for supermirrors, the numbers of layers and the distributions of layer thicknesses were examined in order to get a reflectivity of near unity down to the wavelength much shorter than the critical wavelength of a totally reflecting mirror of Ni. The reflectivity of a supermirror consisting of 160 layers of Ni and Ti was observed to behave like that of a tot...

Metal-ion distribution and magnetic structure of Fe-substituted cobaltite spinel: FeCo2O4

Abstract The metal ion distribution of FeCo2O4 (cubic spinel) which was quenched from 900°C in air was determined to be ( Fe 3+0.18 Co 2+0.82) [ Fe 3+0.82 Co 2+0.18 CoIII+1.0] O4 with the neutron diffraction method. The oxygen spacial parameter u was determined to be 0.382 ± 0.002. The magnetic structure was explained as a Neel type ferrimagnet and the origin of the spontaneous magnetization was attributed to the uncompensation of the spins between A- and B-site of the spinel lattice. The magnetic moment per molecule was estimated to be 0.70 μB.

Neutron optics and neutron forward-scattering using a transverse neutron-spin-echo instrument

Abstract A new principle of neutron-spin-echo (NSE) method is proposed for neutron optics and neutron forward-scattering experiments. The essential of this method is to set a sample in one of the Larmor precession fields. This sample geometry gives new physical information, in particular the refractive index of neutrons for a Si single crystal which was determined to be 1 − (1.85 ± 1.16) × 10 −5 at 5.7 A.

Magnetic structure of hcp HoCe and ErCe alloys

Abstract We have established the magnetic phase diagram of both hcp Houe5f8Ce and Erue5f8Ce alloys by means of neutron diffraction. The alloys with 84% Ho or more exhibit a helix → cone transition, which is similar to the case of pure Ho. The 80% Ho alloy shows only a helical structure. The Erue5f8Ce alloys with 95% and 90% Erexhibit a longitudinal sinusoidal oscillation → c-axis ferromagnet → cone transition and the 80% Er alloy shows only the c-axis ferromagnet, which does not occur in pure Er. The tendency of stabilizing the ferromagnetic phase exists in hcp Houe5f8Co and Erue5f8Ce alloys. Neel temperatures and initial turn angles are lowered with the deGennes factor. The RKKY interaction in hcp Houe5f8Ce and Erue5f8Ce alloys is strongly affected to have a smaller range by alloying with light rare earth Ce.

Direct evidence of helifan structures in holmium by single crystal neutron diffraction

Abstract We give direct evidence of the existence of helifan structures, an intermediate phase between helix and fan, in holmium at 50xa0K in a magnetic field by single-crystal neutron-diffraction experiments. Two types of helifan structures, helifan(3/2) and helifan(2) have been observed at 1.4 and 2.0xa0T, respectively. The magnetic structure successively changes with increasing magnetic field, from a normal helix at zero field to a helifan(3/2) structure, to a helifan(2) structure, to a fan structure and, finally, to an induced ferromagnetic structure.

Development of a modified neutron spin echo spectrometer using multilayer spin splitters

Abstract A multilayer spin splitter (MSS) is a device which gives the same effect as a magnetic field to the Larmor-precessing neutron. Since MSS is a single mirror, a neutron spin echo (NSE) can be much smaller (less than 1 m along the neutron beam) when MSSs successfully replace the Larmor precession magnetic field. The aim of our research is to replace the precession magnetic fields in NSE spectrometer by a set of MSSs and realize a very compact neutron spin echo spectrometer. In the present report, we will present the results of the preliminary experiment of the compact NSE spectrometer, and discuss its applicability to the pulsed neutron sources.

Magnetic structure of PrCo2Si2 under high pressure

Abstract The effects of high pressure on the magnetic structures of PrCo 2 Si 2 , which exhibits three types of long-range order with peculiar long-period modulations, have been studied by neutron diffraction using a clamp-type high pressure cell. The results are presented as a pressure—temperature phase diagram up to 2.25 GPa and down to 10 K. Pressure causes a linear increase of the magnetic-ordering temperatures for the low- and intermediate-temperature phases, while for the hightemperature phase the Neel temperature shows a nonlinear increase with pressure. No new phases or drastic changes were found in the present pressure—temperature range.

Martensitic Transformation and Magnetic Structures Between the Sm-Type and the HCP Structures in Tb-Light Rare Earth Alloys

An interesting sequence of close packed structures such as hcp→Sm-type→double hcp→fcc, occurs in the heavy and the light intra-rare earth binary alloys with increasing the composition of lighter elements(1,2). Some alloys located at the boundary of two crystal phases in the intra-rare earth phase diagram, exhibit a temperature induced martensitic transformation between the two close packed structures (3,4). In the previous paper (4), we have reported the preliminary neutron diffraction studies of crystal and magnetic structures of Tb0.8-La0.2, Tb0.8-Pr0.2 and Tb0.75-Nd0.25 using single crystals. Below 30 K, such a martensitic transformation from the Sm-type to the hcp structure was observed for the latter two crystals. The Sm-type crystals order in a sinusoidal magnetic structure (o++o−−o++..) along the c-axis and zeroes correspond to the cubic site, while the hcp crystals have ferromagnetic structure. In the present paper, neutron diffraction studies on Tb0.803-Pr0.197 and Tb0.798-Pr0.202 single crystals were carried out extensively in both phases of Sm-type and hcp structures at the various stages of cooling and heating cycles that were accompanied by the martensitic transformations between the two phases. The martensitic crystals of hcp and Sm-type structures have stacking faults that increase with increasing number of martensitic cycles.