Yoshikazu Tabata

Observation of Magnetic Monopoles in Spin Ice

Excitations from a strongly frustrated system, the kagome ice state of the spin ice Dy 2 Ti 2 O 7 under magnetic fields along a [111] direction, have been studied. They are theoretically proposed to be regarded as magnetic monopoles. Neutron scattering measurements of spin correlations show that close to the critical point the monopoles are fluctuating between high- and low-density states, supporting that the magnetic Coulomb force acts between them. Specific heat measurements show that monopole-pair creation obeys an Arrhenius law, indicating that the density of monopoles can be controlled by temperature and magnetic field.

Canonical spin glass behavior in Ce2AgIn3

A new intermetallic compound Ce 2 AgIn 3 has been prepared and found to crystallize with the hexagonal CaIn 2 -type structure. We report the results of ac and dc magnetic susceptibility (χ ac and χ dc ), specific heat ( C ) and electrical resistivity (ρ) measurements on this compound. At around 1.8 K, the real part in χ ac shows a cusp and the nonlinear part in χ dc diverges, while there are no anomalies in both C and ρ. On the other hand, ρ increases slightly with decreasing temperature below 10 K and C / T at 0.5 K is large at 780 mJ/K 2 mol-Ce. These results indicate that Ce 2 AgIn 3 is a spin glass of good quality and also belongs to the heavy fermion system.

Kagomé Ice State in the Dipolar Spin Ice Dy2Ti2O7

We have investigated the kagomé ice behavior of the dipolar spin-ice compound Dy2Ti2O7 in a magnetic field along a [111] direction using neutron scattering and Monte Carlo simulations. The spin correlations show that the kagomé ice behavior predicted for the nearest-neighbor interacting model, where the field induces dimensional reduction and spins are frustrated in each two-dimensional kagomé lattice, occurs in the dipole interacting system. The spins freeze at low temperatures within the macroscopically degenerate ground states of the nearest-neighbor model.

Quantum critical point of an itinerant antiferromagnet in a heavy fermion

A quantum critical point of the heavy fermion Ce(Ru(1-x)Rh(x))2Si2, (x = 0,0.03) has been studied by single-crystalline neutron scattering. By accurately measuring the dynamical susceptibility at the antiferromagnetic wave vector k3 = 0.35c*, we have shown that the inverse energy width gamma(k3), i.e., the inverse correlation time, depends on temperature as gamma(k3) = c1 + c2T((3/2)+/-0.1), where c1 and c2 are x dependent constants, in a low temperature range. This critical exponent 3/2 +/- 0.1 proves that the quantum critical point is controlled by that of the itinerant antiferromagnet.

Restoring Higher Symmetric Crystal Structure with Magnetic Field in Triangular Lattice Antiferromagnet CuFeO2

Structural deformations associated with magnetic phase transitions have been studied by synchrotron radiation X-ray diffraction in high magnetic fields up to 14 T for the geometrically frustrated triangular lattice antiferromagnet CuFeO 2 . When a magnetic field is applied along the hexagonal c -axis, “scalene triangular” (ST) lattice distortion, which causes the three different exchange interaction paths in the basal plane, is successively relieved at the critical fields corresponding to field-induced magnetic phase transitions. In particular, the ST lattice is deformed into the higher symmetric “isosceles triangular” lattice at the second-field-induced magnetic phase transition (∼13.5 T). We thus conclude that the spontaneous lattice distortion in CuFeO 2 , which occurs without any orbital degrees of freedom, originates from a magnetoelastic coupling produced by spin frustration.

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.

Field-induced Incommensurate Lattice Modulations in the Delafossite CuFeO2

The results of synchrotron X-ray diffraction measurements on a single crystal of the triangular lattice antiferromagnet CuFeO 2 under zero and non-zero applied magnetic fields are reported. We find four satellite reflections at (0, 3+ q , 1/2), (0, 4- q , 1/2), (0, 4-2 q , 0), and (0, 3+2 q , 0) with the incommensurate wave number q ∼0.415 in the ferroelectric incommensurate (FEIC) phase which appears in the magnetic field, H , between 7 and 13 T at low temperatures. In the partially disordered (PD) phase which exists in the temperature, T , range between 11 and 14 K, we find two satellite reflections at (0, 4- q , 1/2) and (0, 4-2 q , 0) with the incommensurate wave number q ∼0.4. The T and H dependence of these satellite reflections are studied. We interpret that the reflections observed in the FEIC phase arise from incommensurate lattice modulations caused by a magnetoelastic coupling with the underlying magnetic structure. The observation of the reflection (0, 4- q , 1/2) in finite fields and the (0, ...

Quantum spin fluctuations in the spin-liquid state of Tb2Ti2O7

Neutron scattering experiments on a polycrystalline sample of the frustrated pyrochlore magnet Tb(2)Ti(2)O(7), which does not show any magnetic order down to 50 mK, have revealed that it shows condensation behavior below 0.4 K from a thermally fluctuating paramagnetic state to a spin-liquid ground state with quantum spin fluctuations. Energy spectra change from quasielastic scattering to a continuum with a double-peak structure at energies of 0 and 0.8 K in the spin-liquid state. Specific heat shows an anomaly at the crossover temperature.

Pressure-induced instability of magnetic order in Kondo-lattice system: Neutron diffraction study of the pseudo-binary alloy system Ce(Ru0.90Rh0.10)2(Si1-yGey)2

Neutron diffraction experiments have been carried out to study the nature of the magnetic order of the pseudo-binary alloy system Ce(Ru 0.90 Rh 0.10 ) 2 (Si 1- y Ge y ) 2 . Response of the ordered atomic magnetic moment, µ, the transition temperature, T N , and the magnitude of the magnetic modulation vector, q , to the chemical pressure and also to the applied hydrostatic pressure, P , were examined at low temperatures. When y changes, all of µ, T N and q show a sudden alteration of the manner of the y -dependence at around y ∼0.08. The P -dependence of q shows quite different features for different y s of 0.0, 0.2 and 0.25. On the basis of these observations the possibility of a pressure-induced alternation of the magnetic regime of the order is discussed.

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.