First-Order Transition in the Spin Dynamics of Geometrically FrustratedYb2Ti2O7

Abstract

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...

A novel type of splayed ferromagnetic order observed in Yb2Ti2O7

“…[22] were similar. A single phase polycrystalline sample was prepared by heating the constituent oxides up to 1400 • C with four intermediate grindings.…”

“…Remarkably, the Ising character of the exchange interaction is in contrast to the marked planar anisotropy of the Yb 3+ ground state Kramers doublet [19,20]. The measured entropy provides evidence for a ground state doublet well isolated from the excited doublets [21,22,23].…”

“…Apart from the magnetic order described here, measurements on polycrystalline samples have revealed § While, in Yb 2 Sn 2 O 7 , a single angle of 65 • between the magnetic moment and the local three-fold axis was deduced from Mössbauer spectroscopy, this angle cannot be directly measured in Yb 2 Ti 2 O 7 . This is due to a vanishing quadrupolar interaction in the latter compound [22]. a sharp change in the spin dynamics at T c in zero field [36].…”

“…Alternatively the coherent magnetic order could be so disrupted by crystal defects that any magnetic Bragg reflections would be too broad to be observable [26]. Due to the long-standing controversy as to the presence of magnetic Bragg reflections [22,27,28,29,30,31,32], the magnetic nature of the ground state of Yb 2 Ti 2 O 7 at low temperature is still uncertain. Magnetic Bragg reflections are reported only in Refs.…”

“…[27,31] which describe data measured on the same crystal. The diversity of the specific heat [33,34,31,35] and muon spin relaxation (µSR) results [22,36,25,23] adds to the difficulty in understanding the ground state. In addition to the sample quality issue first noticed in Ref.…”