Yusuke Nambu

We measure the mode-resolved direction of the precessional motion of the magnetic order, i.e., magnon polarization, via the chiral term of inelastic polarized neutron scattering spectra. The magnon polarization is a unique and unambiguous signature of magnets and is important in spintronics, affecting thermodynamic properties such as the magnitude and sign of the spin Seebeck effect. However, it has never been directly measured in any material until this work. The observation of both signs of ma

Magnetism in the insulating BaFe${}_{2}$Se${}_{3}$ was examined through susceptibility, specific heat, resistivity, and neutron diffraction measurements. After formation of a short-range magnetic correlation, a long-range ordering was observed below ${T}_{\mathrm{N}}\ensuremath{\sim}255$ K. The transition is obscured by bulk properties. Magnetic moments ($\ensuremath{\parallel}a$) are arranged to form a Fe${}_{4}$ ferromagnetic unit, and each Fe${}_{4}$ stacks antiferromagnetically. This block m

Nonmagnetic impurity effects of the spin disordered state in the triangular antiferromagnet NiGa$_2$S$_4$ was studied through magnetic and thermal measurements for Ni$_{1-x}$Zn$_x$Ga$_2$S$_4$ (0.0\le x\le 0.3). Only 1 % substitution is enough to strongly suppress the coherence observed in the spin disordered state. However, the suppression is not complete and the robust feature of the T^2 dependent specific heat and its scaling behavior with the Weiss temperature indicate the existence of a cohe

Impurity effects on the triangular antiferromagnets Ni1-xMxGa2S4 (M=Mn, Fe, Co and Zn) are studied. The 2D frozen spin-disordered state of NiGa2S4 is stable against the substitution of Zn2+ (S=0) and Heisenberg Fe2+ (S=2) spins, and exhibits a T2-dependent magnetic specific heat, scaled by the Weiss temperature. In contrast, the substitutions with Co2+ (S=3/2) spin with Ising-like anisotropy and Heisenberg Mn2+ (S=5/2) spin induce a conventional spin glass phase below 1 K. From these comparisons

The temporal magnetic correlations of the triangular-lattice antiferromagnet NiGa_{2}S_{4} are examined through 13 decades (10^{-13}-1 sec) using ultrahigh-resolution inelastic neutron scattering, muon spin relaxation, and ac and nonlinear susceptibility measurements. Unlike the short-ranged spatial correlations, the temperature dependence of the temporal correlations show distinct anomalies. The spin fluctuation rate decreases precipitously upon cooling towards T^{*}=8.5 K, but fluctuations on

Magnetism in the orthorhombic metal CaFe(4)As(3) was examined through neutron diffraction for powder and single crystalline samples. Incommensurate [q(m) ≈ (0.37-0.39) × b*] and predominantly longitudinally (|| b) modulated order develops through a 2nd order phase transition at TN = 89.63(6) K with a 3D Heisenberg-like critical exponent β = 0.365(6). A 1st order transition at T2 = 25.6(9) K is associated with the development of a transverse component, locking q(m) to 0.375(2)b*, and increasing t

Polycrystalline samples of ${\text{NiGa}}_{2}{\text{S}}_{4+\ensuremath{\delta}}$ $(\ensuremath{-}0.2\ensuremath{\le}\ensuremath{\delta}\ensuremath{\le}+0.2)$ have been synthesized by solid-state reaction. X-ray powder-diffraction measurements indicate that these samples are single phase with a trigonal lattice with a nominal concentration $\ensuremath{\delta}$ at least between $\ensuremath{-}0.12$ and $+0.16$. Rietveld analyses for time-of-flight neutron-diffraction data have revealed that there

The triangular antiferromagnet is one of the most fundamental systems of geometrically frustrated magnets. NiGa(2)S(4) is a layered chalcogenide compound with an equilateral triangular lattice, and it is a prime candidate for an S = 1 triangular antiferromagnet. Here we focus on low temperature magnetism in NiGa(2)S(4), where quasi-static spins develop a spin-wave-like mode without forming any long-range ordering. We have studied low temperature magnetism of both polycrystalline samples and sing

Obtaining structural information has always been of primary importance in condensed matter physics, chemistry, and materials science. To refine crystal and magnetic structures, neutron powder diffraction is widely employed owing to comparable scattering cross-sections of neutrons for nuclear and magnetic scattering. The neutron powder diffractometer HERMES stationed at the Japan Research Reactor has served for structural studies, and modifications and improvements were made after the Great East