Using neutron diffraction, 170 Yb Mössbauer and muon spin relaxation spectroscopies, we have examined the pyrochlore Yb 2 Ti 2 O 7 , where the Yb 31 S 0 1͞2 ground state has planar anisotropy. Below ϳ0.24 K, the temperature of the known specific-heat l transition, there is no long range magnetic order. We show that the transition corresponds to a first-order change in the fluctuation rate of the Yb 31 spins. Above the transition temperature, the rate, in the GHz range, follows a thermal excitation law, whereas below, the rate, in the MHz range, is temperature independent, indicative of a quantum fluctuation regime. DOI: 10.1103/PhysRevLett.88.077204 PACS numbers: 75.40. -s, 75.25. +z, 76.75. +i, 76.80. +y Geometrically derived magnetic frustration arises when the spatial arrangement of the spins is such that it prevents the simultaneous minimization of all interaction energies [1 -4]. In the crystallographically ordered pyrochlore structure compounds R 2 Ti 2 O 7 , the rare earth ions ͑R͒ form a sublattice of corner sharing tetrahedra and a number of these compounds have been observed to exhibit frustration related behavior [5][6][7][8][9][10][11][12]. The low temperature magnetic behavior associated with a particular rare earth depends on the properties of the crystal field ground state and on the origin (exchange/dipole), size, and sign of the interionic interactions. For example, the recently identified spin-ice configuration [6,10,11] has been linked with an Ising-like anisotropy and a net ferromagnetic interaction. Most of the published work on the pyrochlores have concerned rare earth ions with Ising-like characteristics [6 -12] and there has also been some interest in the properties of weakly anisotropic Gd 31 [5]. Less attention has been paid to the case, considered here, where the ion has planar anisotropy.To date, in systems where geometrical frustration may be present, two different low temperature magnetic ground states have been considered. First, under the influence of the frustration the system does not experience a magnetic phase transition and remains in a collective paramagnetic state with the spin fluctuations persisting as T ! 0 [3,4,[6][7][8][9][10][11]13]. Second, a long range ordered state is reached through a phase transition which may be first order [5,14,15]. Our results for Yb 2 Ti 2 O 7 , obtained using neutron diffraction, 170 Yb Mössbauer spectroscopy, and muon spin relaxation (mSR), evidence a novel scenario: there is a first-order transition which does not correspond to a transition from a paramagnetic state to a (long or short range) magnetically ordered state. The transition chiefly concerns the time domain, and involves an abrupt slowing down of the dynamics of short range correlated spins; below the transition temperature, these spins continue to fluctuate at a temperature independent rate.We have established the background magnetic characteristics for Yb 2 Ti 2 O 7 in a separate study [16,17]. The Yb 31 ion crystal field ground state is a very well isolated Kramers doubl...

In the pyrochlore-structure compounds R2Ti2O7, the rare-earth (R) sublattice forms a network of corner-sharing tetrahedra such that the magnetic interactions may be geometrically frustrated. The low-temperature magnetic properties of these compounds are fashioned both by the frustration and by the intrinsic properties of the rare earth, that is, by the degeneracy and anisotropy of the rare-earth crystal-field ground state and by the nature, size and strength of the inter-ionic magnetic coupling. For Yb2Ti2O7, we combine 170Yb Mössbauer spectroscopy, 172Yb perturbed angular correlation, magnetization and susceptibility measurements to establish the Yb3+ crystal-field level scheme and to show that the crystal-field ground state is a well isolated Kramers doublet having a planar anisotropy. The main contribution to the Yb3+-Yb3+ coupling is the exchange interaction which is ferromagnetic. We describe the frustration-related low temperature (<1 K) properties of Yb2Ti2O7 in a separate publication.

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In the garnet structure compound Yb3Ga5O12, the Yb 3+ ions (ground state effective spin S ′ = 1/2) are situated on two interpenetrating corner sharing triangular sublattices such that frustrated magnetic interactions are possible. Previous specific heat measurements have evidenced the development of short range magnetic correlations below ∼ 0.5K and a λ-transition at 0.054K (Filippi et al. J. Phys. C: Solid State Physics 13 (1980) 1277). From 170 Yb Mössbauer spectroscopy measurements down to 36 mK, we find there is no static magnetic order at temperatures below that of the λ-transition. Below ∼ 0.3 K, the fluctuation frequency of the short range correlated Yb 3+ moments progressively slows down and as T → 0, it tends to a quasi-saturated value of 3 × 10 9 s −1 . We also examined the Yb 3+ paramagnetic relaxation rates up to 300 K using 172 Yb perturbed angular correlation measurements: they evidence phonon driven processes.