Long-range order and spin-liquid states of polycrystalline Tb2+xTi2−xO

Abstract

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...

Anisotropic Propagating Excitations and Quadrupolar Effects inTb2Ti2O7

“…[35]. No magnetic Bragg peaks could be detected at (1=2, 1=2, 1=2), as reported in Tb 2þx Ti 2À2x Nb x O 7 [36] and Tb 2þx Ti 2Àx O 7þy [29]. This is consistent with the lattice parameter [a ¼ 10:1528ð5Þ # A], determined precisely using x-ray scattering, and which positions our Tb 2 Ti 2 O 7 sample in the spin liquid phase [29].…”

XY antiferromagnetic ground state in the effectiveS=12pyrochloreYb2Ge2O7Hallas1, Gaudet2, Wilson3 et al. 2016Phys. Rev. B383350View full textAdd to dashboardCite

“…Furthermore, the large ionic radii difference between Yb 3+ and Ge 4+ , which necessitates high-pressure synthesis, also significantly reduces the probability of site-mixing [3]. This is particularly attractive in light of the sensitivity that the magnetism in some pyrochlores has shown to subtle variations in stoichiometry and so-called "stuffing" [23,41,42].…”

Symmetry and light stuffing ofHo2Ti2O7, EBaroudi1, Gaulin2, Lapidus3 et al. 2015Phys. Rev. B384320View full textAdd to dashboardCite

“…Thus the observation of a Fd-3m-forbidden (002) peak at 11-BM would indicate that the space group of the rare earth titanate pyrochlores is not Fd-3m. Variations in the rare earth (R) to titanium ratio are possible in these compounds, leading to "stuffing' [14][15][16][17], and so several compounds with small differences in the R to Ti ratios were studied to determine whether the space group is sensitive to small deviations from a 2:2 metal atom stoichiometry. In no instance was the (002) reflection observed at room temperature in any of the materials.…”