Low-temperature states of polycrystalline samples of a frustrated pyrochlore oxide Tb2+xTi2−xO7+y have been investigated by specific heat, magnetic susceptibility, and neutron scattering experiments. We have found that this system can be tuned by a minute change of x from a spin-liquid state (x < xc) to a partly ordered state with a small antiferromagnetic ordering of the order of 0.1µB. Specific heat shows a sharp peak at a phase transition at Tc = 0.5 K for x = 0.005. Magnetic excitation spectra for this sample change from a quasielastic to a gapped type through Tc. The possibility of a Jahn-Teller transition is discussed.PACS numbers: 75.10. Kt, 75.40.Cx, 75.70.Tj, 78.70.Nx Magnetic systems with geometric frustration, a prototype of which is antiferromagnetically coupled Ising spins on a triangle, have been intensively studied experimentally and theoretically for decades 1 . Spin systems on networks of triangles or tetrahedra, such as triangular 2 , kagomé 3 , and pyrochlore 4 lattices, play major roles in these studies. Subjects that have fascinated many investigators in recent years are classical and quantum spin-liquid states 5-8 , where conventional long-range order (LRO) is suppressed to very low temperatures. Quantum spin-liquids 6,7 in particular have been challenging both theoretically and experimentally since the proposal of the resonating valence-bond state 9 . The spin ice materials R 2 Ti 2 O 7 (R = Dy, Ho) are the well-known classical examples 5 , while other experimental candidates found recently have been studied 10-14 .Among frustrated pyrochlore oxides 4 , Tb 2 Ti 2 O 7 has attracted much attention because it does not show any conventional LRO down to 50 mK and remains in a dynamic spin-liquid state [15][16][17] . Theoretical considerations of the crystal-field (CF) states of Tb 3+ and exchange and dipolar interactions of the system [18][19][20] showed that it should undergo a transition into a magnetic LRO state at about 1.8 K within a random phase approximation 20 . The puzzling origin of the spin-liquid state of Tb 2 Ti 2 O 7 is a subject of hot debate 4,21-28 . An interesting scenario for the spin-liquid state is the theoretical proposal of a quantum spin-ice state 22 . More recently, another scenario of a two-singlet spin-liquid state was proposed to explain why inelastic neutron spectra in a low energy range are observed despite the fact that Tb 3+ is a nonKramers ion 23,24 .Several experimental puzzles of Tb 2 Ti 2 O 7 originate from the difficulty of controlling the quality of single crystalline samples, resulting in strongly sampledependent specific-heat anomalies at temperatures below 2 K 18,26,[29][30][31][32][33] . In contrast, experimental results on polycrystalline samples are more consistent 15,16,26 . Among the experimental results reported to date, an important clue to solve the puzzles of Tb 2 Ti 2 O 7 seems to be a change of state at about 0.4 K suggested by specific heat 26 , inelastic neutron scattering 26 , and neutron spin echo 16 measurements on polycrystalline samp...

The low-temperature crystal structure of Fe1.13Te, which exhibits an anomalous two-step magnetic transition, was clarified by the systematic x-ray diffraction measurements. It was found that two-step structural phase transition, tetragonal-orthorhombic-monoclinic, occurred correspondingly to the two-step magnetic transition. The detailed analysis of the profile at 5 K indicated the coexistence of the minor orthorhombic area inside the major monoclinic lattice, which could explain the lower-shift (suppression) of the antiferromagnetic transition temperature in Fe1.13Te and suggest a possibility of superconductivity at the domain boundary.

Experimental data of the frequency-dependent ac susceptibility [χac(ω)] for the dipolar spin ice Dy2Ti2O7 has been analyzed by Monte Carlo simulations on the basis of the single-spin-flip Metropolis algorithm. We have directly evaluated χac(ω) by applying an ac magnetic field. We found that this simulation reasonably reproduces the experimental behavior of χac(ω) in the temperature range from 0.6 to 1.0 K, where dilute magnetic monopoles diffusively move. The conversion factor from simulation time to real time, i.e., the rate of hopping of monopoles to nearest-neighbor sites, strongly depends on temperature.

The cubic symmetry of pyrochlore iridium oxides R 2 Ir 2 O 7 (R = Nd, Eu, and Pr) has been investigated by high resolution x-ray diffraction experiments down to 4 K, in order to clarify the relationship between the metal-insulator transition (MIT) and the small structural phase transition suggested by Raman scattering experiments in these compounds. We have found that a small negative thermal expansion of the order of 10 −3Å appears only in Nd 2 Ir 2 O 7 below the MIT, T MIT = 34 K, ascribable to the magnetovolume effect of the long-range order of Ir moments. However, any breaking of the cubic symmetry of three iridates has not been observed as appearance of superlattice reflections nor splittings of cubic reflections below T MIT . These results imply that lowering of the cubic symmetry plays a minor role for the change in the electronic state of these compounds, while a magnetic order of Ir moments plays a major role for the MIT.

The spin dynamics of the dipolar spin-ice Dy 2 Ti 2 O 7 under a dc magnetic field (B dc ) along the [111] direction has been investigated using ac susceptibility ( ac ), which enables us to study two-dimensional monopole motion in an ac magnetic field perpendicular to B dc . We have observed the slowing down of monopole dynamics in the kagomé spinice regime (B dc $ 0:4 T) and the extreme speeding up under B dc close to the critical point (B dc $ 0:93 T). An analysis using a Monte Carlo simulation suggests that the dynamics changes from a random walk of monopoles to a collective motion as B dc increases.