Takeshi Waki

Temperature-induced metal-insulator transition in BixV8O16

From the resistivity and magnetic susceptibility measurements of polycrystalline Bi x V 8 O 16 with 1.6< x <1.8, it was revealed for the first time that Bi x V 8 O 16 with 1.72< x <1.8 exhibits a t...

Observation of Bose-Einstein Condensation of Triplons in Quasi 1D Spin-Gap System Pb2V3O9

We observed a field-induced magnetic ordering in the quasi one-dimensional antiferromagnetic alternating chain system Pb 2 V 3 O 9 for the first time. The characteristic temperature dependence of t...

Incomplete Devil's Staircase in the Magnetization Curve of SrCu2(BO3)(2)

We report on NMR and torque measurements on the frustrated quasi-two-dimensional spin-dimer system SrCu(2)(BO(3))(2) in magnetic fields up to 34 T that reveal a sequence of magnetization plateaus at 1/8, 2/15, 1/6, and 1/4 of the saturation and two incommensurate phases below and above the 1/6 plateau. The magnetic structures determined by NMR involve a stripe order of triplets in all plateaus, suggesting that the incommensurate phases originate from proliferation of domain walls. We propose that the magnetization process of SrCu(2)(BO(3))(2) is best described as an incomplete devils staircase.

Existence of a Phase Transition under Finite Magnetic Field in the Long-Range RKKY Ising Spin Glass DyxY1-xRu2Si2

A phase transition of a model compound of the long-range Ising spin glass (SG) Dy x Y 1- x Ru 2 Si 2 , where spins interact via the Ruderman–Kittel–Kasuya–Yoshida (RKKY) interaction, has been investigated. The static and dynamic scaling analyses reveal that the SG phase transition in the model magnet belongs to the mean-field universality class. Moreover, the characteristic relaxation time in finite magnetic fields, as well as in zero field, exhibits a critical divergent behavior, indicating the stability of the SG phase in finite fields. The presence of the SG phase transition in the field in this model magnet strongly suggests that the replica symmetry is broken in the long-range Ising SG.

Interplay between quantum criticality and geometric frustration in Fe3Mo3N with stella quadrangula lattice

We propose the η-carbide–type correlated-electron metal Fe3Mo3N as a new candidate for testing geometric frustration in itinerant-electron magnets. In this compound, ferromagnetism is abruptly induced from a non-magnetic non–Fermi-liquid ground state either when a magnetic field (~14 T) applied to it or when it is doped with a slight amount of impurity (~5% Co). We observed a peak in the paramagnetic neutron scattering intensity at finite wave vectors, revealing the presence of the antiferromagnetic correlation hidden in the magnetic measurements. It causes a new type of geometric frustration in the stella quadrangula (SQ) lattice of the Fe sublattice, which is a corner-shared lattice of two nested regular tetrahedrons. Although the SQ lattice is one of pyrochlore-derived lattices, we anticipate the unique nature of the frustration due to the competition between first- and second-neighbour interactions, J1 and J2, possibly of direct exchange type. We propose that the frustrated antiferromagnetic correlation suppresses the ferromagnetic correlation to its marginal point and is therefore responsible for the origin of the ferromagnetic quantum critical behaviour in pure Fe3Mo3N.

Metal-insulator transition in BixV8O16: 51V NMR study

The 51 V nuclear magnetic resonance and relaxation (NMR) are measured for Bi x V 8 O 16 in the temperature range from 4.2 K to 280 K. The Bi x V 8 O 16 system with 1.60 ≤ x ≤1.80 is known to crystallize in the Hollandite structure with a [V 8 O 16 ] framework which is composed of double chains formed by edge-shared VO 6 octahedra. In this system, we observed temperature-induced Metal–Insulator transition (MIT) in the composition range of 1.72 ≤ x ≤1.80. In order to reveal the mechanism of MIT in this system, 51 V NMR measurements have been performed on Bi 1.77 V 8 O 16 as well as on the simple Pauli paramagnetic compound Bi 1.60 V 8 O 16 for a comparative study: the Knight shift K and the spin–lattice relaxation rate 1/ T 1 of 51 V NMR have been measured to investigate the magnetic and electric properties. By applying the random phase approximation (RPA) method the absolute values of 1/ T 1 T are analyzed in the Pauli paramagnetic regime of Bi 1.77 V 8 O 16 above T MIT and Bi 1.60 V 8 O 16 for whole tempe...

Non-Fermi-Liquid Behavior on an Iron-Based Itinerant Electron Magnet Fe3Mo3N

We report magnetic, calorimetric, and transport properties of an iron-based itinerant electron magnet Fe 3 Mo 3 N. Magnetic susceptibility shows a Curie–Weiss behavior at high temperatures and takes a broad maximum at around 75 K. The absence of magnetic long range order was confirmed by 57 Fe-Mossbauer and neutron diffraction measurements. C / T shows a divergent behavior following -log T at low temperatures and reaches 128 mJ/(f.u.mol K 2 ) at 0.5 K. The deviation from the T 2 power law of resistivity also suggests a non-Fermi-liquid (NFL) behavior. The observed C / T and χ are enhanced compared with the values estimated from the theoretical density of states, suggesting a strong magnetic enhancement. The NFL behaviors indicate that Fe 3 Mo 3 N is one of the ideal systems located in the vicinity of the ferromagnetic quantum critical point.

Ru9Zn7Sb8: a structure with a 2 × 2 × 2 supercell of the half-Heusler phase.

The title compound Ru(9)Zn(7)Sb(8) was synthesized via a high-temperature reaction from the elements in a stoichiometric ratio, and its structure was solved by a single-crystal X-ray diffraction method. The structure [cubic, space group Fm3m, Pearson symbol cF96, a = 11.9062(14) Å (293 K), and Z = 4] adopts a unique 2a(hh) × 2a(hh) × 2a(hh) supercell of a normal half-Heusler phase and shows abnormal features of atomic coordination against the Pauling rule. The formation of this superstructure was discussed in light of the valence electron concentration per unit cell. It is a metallic conductor [ρ(300 K) = 16 μΩ·m], and differential scanning calorimetry revealed that Ru(9)Zn(7)Sb(8) undergoes a transformation at 1356(1) K and melts, by all indications, congruently at 1386 K. At room temperature, its thermal conductivity is about 3 W/m·K, which is only one-quarter of that of most normal half-Heusler phases. Ru(9)Zn(7)Sb(8) as well as its analogues of iron-, cobalt-, rhodium-, and iridium-containing compounds are expected to serve as a new structure type for exploring new thermoelectric materials.

Novel Ordered Phases in the Orthogonal Dimer Spin System SrCu2(BO3)2

The magnetic properties of SrCu 2 (BO 3 ) 2 , a model spin system on the two-dimensional (2D) Shastry–Sutherland lattice, are reviewed with particular emphasis on the nuclear magnetic resonance (NMR) experiments. SrCu 2 (BO 3 ) 2 has a dimer singlet ground state at zero magnetic field and shows a sequence of magnetization plateaux at high magnetic fields. The plateaux are attributed to the localization of triplets into a superlattice due to mutual repulsion. Such a superstructure has been indeed observed at the 1/8 plateau by NMR experiments. Furthermore, the superstructure persists above the 1/8 plateau, where new phases have been identified by the torque and NMR measurements. SrCu 2 (BO 3 ) 2 also has the anisotropic Dzyaloshinski–Moriya interactions, which significantly change the ground state in magnetic fields. A recent development is the discovery of a new magnetic phase transition under high pressure. At 2.4 GPa, the 11 B NMR spectrum exhibits line splitting as a function of temperature in two step...

Magnetic susceptibility of η-carbide-type molybdenum and tungsten carbides and nitrides

We synthesized a series of molybdenum and tungsten η-carbide-type compounds and measured their magnetic susceptibility χ. Although the enhancement of χ was observed in several compounds, apparent magnetic order was found only in Fe3W3N (ferromagnetic at 110 K). The broad peak in χ of Fe6W6C is possibly due to antiferromagnetic order. Itinerant electron metamagnetic transitions were observed in Fe3W3C, Fe6W6C, Fe6W6N and Co6W6C. The magnetic enhancement depends on the 3d element in the order Fe > Co > Ni corresponding to DOS at theFermi energy in η-12 systems, whereas no clear trend has been observed in η-6 systems.