Ryoichi Kajimoto

First Demonstration of Novel Method for Inelastic Neutron Scattering Measurement Utilizing Multiple Incident Energies

We succeeded in experimentally demonstrating that a series of two-dimensional maps of a dynamical structure factor in momentum–energy space with multiple incident energies can be simultaneously obt...

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.

Two-Dimensional Spin Density Wave State in LaFeAsO

The parent compound of the Fe oxypnictide superconductor, LaFeAsO, has been studied by pulsed neutron powder inelastic scattering measurement. The inelastic scattering intensity along the Q -axis at T =140 K exhibits a prominent asymmetric peak ascribed to the (π,π, l ) magnetic rod, suggesting the two-dimensionality of the spin density wave above the magnetic transition temperature. It persists even in the tetragonal phase up to room temperature, which is well above the magnetic transition temperature, corresponding to two antiferromagnetic sublattices or domains with a strong effective antiferromagnetic interaction J 2 between next-nearest-neighbour Fe magnetic moments in a single Fe square lattice.

Dimensional Crossover of Low-Energy Magnetic Excitation for Delafossite Oxide Cu1-xAgxCrO2 with a Spin-3/2 Antiferromagnetic Triangular Sublattice

Dimensional crossover from anisotropic-three-dimensional (3D) to 2D low-energy magnetic excitations was found in delafossite oxide Cu 1- x Ag x CrO 2 having a spin-3/2 antiferromagnetic (AF) triangular lattice. The crossover is evidenced by a decrease in AF transition temperature ( T N ), distinct 2D magnetic excitations above T N for Cu 0.85 Ag 0.15 CrO 2 , a disappearance of a peak of magnetic specific heat ( C mag ) around T N , and a successive change from T 3 to T 2 dependence of low- T C mag accompanying a low- T shift of magnetic entropy. These results may suggest a gradual change from AF to spin-liquid like states with an increase in x .

Effect of Spin Dilution on the Magnetic State of Delafossite CuCrO2 with an S = 3/2 Antiferromagnetic Triangular Sublattice

We investigated an effect of spin dilution on the magnetic and thermal properties of delafossite CuCrO 2 having an S = 3/2 antiferromagnetic triangular lattice (ATL) by the substitution of Al 3+ for Cr 3+ by measurements of magnetization, specific heat, and neutron scattering. Spin defects introduced into Cr ATL layers blur the antiferromagnetic transition and induce a dimensional crossover of low-energy magnetic excitation in the residual Cr ATL region due to the reduction of inter-layer magnetic interaction, while accompanying an enhancement of a spin-glass component. The observed T 2 contribution to the low- T C mag above y = 0.15, as well as in Cu 0.85 Ag 0.15 CrO 2 , comes mainly from a short-range 2D AF excitation originating in a nontrivial magnetic state such as spin-nematic or spin-liquid states.

Temperature and Ag Doping Effect on Magnetic Excitations in the Quasi-Two-Dimensional Triangular Lattice Antiferromagnet CuCrO2 Studied by Inelastic Neutron Scattering

We have studied magnetic excitations in quasi-two-dimensional triangular lattice antiferromagnets CuCrO 2 and Cu 0.85 Ag 0.15 CrO 2 by inelastic neutron scattering. CuCrO 2 has two types of magnetic excitations: one is highly dispersive, while the other is less dispersive and spreads over a wide Q range. With increasing temperature, the weight of the latter component transfers to a lower energy, and the spin dynamics becomes a mixture of a disordered spin wave and another diffuse component. Ag doping induces a similar spin dynamics even at 5 K. This manifests the impact of Ag doping on magnetic correlations and gives a clue to understanding the unconventional spin state in the Ag-doped CuCrO 2 .

R-dependence of spin exchange interactions in RMnO3 (R = rare-earth ions)

Magnon excitations have been investigated for PrMnO 3 showing an A -type antiferromagnetic ordering and TbMnO 3 forming a long-period spin ordering by inelastic neutron scattering. Combined with a ...

Switching of intra-orbital spin excitations in electron-doped iron pnictide superconductors

We investigate the doping dependence of the magnetic excitations in two-superconducting-dome-system LaFeAsO1-xDx. Using inelastic neutron scattering, spin fluctuations at different wavenumbers were observed under both superconducting domes around x = 0.1 and 0.4, but vanished at x = 0.2 corresponding to the Tc valley. Theoretical calculations indicate that the characteristic doping dependence of spin fluctuations is rationally explained as a consequence of the switching of the two intra-orbital nestings within Fe-3dYZ, ZX and 3dX2-Y2 by electron doping. The present results imply that the multi-orbital nature plays an important role in the doping and / or material dependence of the Tc of the iron pnictide superconductors.

Critical spin dynamics in the two-dimensional percolating Ising antiferromagnet, Rb2CocMg1-cF4

We performed inelastic neutron-scattering experiments with high energy resolution, in order to investigate the critical spin dynamics in the two-dimensional (2D) Ising antiferromagnet, Rb 2 Co c Mg 1- c F 4 , with c =0.6, 0.65, 0.7 and 1, above the percolation threshold c p ( c p =0.593 for a square lattice). First, we have confirmed the dynamical scaling for the homogeneous system ( c =1), and the observed dynamical critical exponent ( z =2.01±0.05) was very close to that obtained from recent numerical studies. Next, the observed spin dynamics in the dilute systems could be described by the temperature dependence expected by the standard scaling theory, the power of the reduced temperature, however, the values of the exponents deviate from those predicted by the 2D Ising model and increase as c approaches to c p from the homogeneous limit. Also, the observed spin dynamics for c close c p was in a quantitatively good agreement with that predicted by the model for critical spin dynamics in a percolating Is...

Development of Spin Correlations in the Geometrically Frustrated Triangular-Lattice Heisenberg Antiferromagnet CuCrO2

Magnetic excitations in the triangular-lattice Heisenberg antiferromagnet CuCrO2 were studied using single-crystal inelastic neutron scattering. A diffusive quasielastic component that persisted without developing a correlation length over a wide temperature range both below and above the ordering temperature was observed. Furthermore, characteristic momentum dependence was observed that was reproduced using minimum spin clusters. The robust spin clusters contrast with conventional magnetic ordering and may be universal in triangular-lattice Heisenberg antiferromagnets.