Er2Ti2O7 has been suggested to be a realization of the frustrated 111 XY pyrochlore lattice antiferromagnet, for which theory predicts fluctuation-induced symmetry breaking in a highly degenerate ground state manifold. We present a theoretical analysis of the classical model compared to neutron scattering experiments on the real material, both below and above TN =1.173(2) K. The model correctly predicts the ordered magnetic structure, suggesting that the real system has order stabilized by zero-point quantum fluctuations that can be modelled by classical spin wave theory. However, the model fails to describe the excitations of the system, which show unusual features.PACS numbers: 28.20. Cz, 75.25.+z An important aspect of condensed matter is the separation of energy scales, such that the minimization of one set of interactions may result in the frustration of another. A paradigm is the frustrated antiferromagnet, in which the local magnetic couplings between ions are frustrated by the crystal symmetry that the ions adopt. However, a systematic study of the rare earth pyrochlore titanates R 2 Ti 2 O 7 has shown that local antiferromagnetic bond frustration is neither a necessary, nor a sufficient condition for magnetic frustration 1,2,3,4,5 . Rather, it arises from the interplay, in the context of the crystal symmetry, of the principal terms in the spin Hamiltonian. In the case of R 2 Ti 2 O 7 , the main terms are single-ion anisotropy, exchange and dipolar coupling. Depending on the balance of these factors, one observes spin ice behavior (R = Ho, Dy) 1,2,3 , spin liquid behavior (R = Tb) 4 , and dipole induced partial order (R = Gd)5 . Such behavior is best classified in terms of the dominant 111 single-ion anisotropy that arises from the trigonal crystal electric field (CEF) at the rare earth site. For example, whereas the Heisenberg antiferromagnet has a spin liquid ground state 6 , the 111 Ising (dipolar) ferromagnet has a spin ice ground state 1,3,7 . There is thus a clear motivation to study models based on other simple anisotropies and their realization in the titanate series. In this Letter we study one such model -the 111 XY model antiferromagnet 8 -and its realization Er 2 Ti 2 O 7 9,10,11,12 . We consider the Hamiltonian:where the classical spins, S i , populate a face centered cubic array of corner sharing tetrahedra: the pyrochlore lattice. The spins are confined to easy XY planes by a local d i = 111 anisotropy, D < 0, and are coupled antiferromagnetically by exchange J < 0. This model was first studied in Ref.8 , where a discrete, but macroscopically degenerate, set of ground states was identified. At finite temperature thermal fluctuations were found to select an ordered state by the mechanism that Villain called "order by disorder"13 and a first order phase transition was observed in numerical simulations. The propagation vector of the ordered state was found to be k = 0, 0, 0 (henceforth "k = 0"), but the basis vectors of the magnetic structure were not determined. We have recently discovere...

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CuGeO3 undergoes a transition from a spin-Peierls phase to an incommensurate phase at a critical field of H(c) approximately 12.5 T. In the high-field phase a lattice of solitons forms, with both structural and magnetic components, and these have been studied using neutron scattering techniques. Our results provide direct evidence for a long-ranged magnetic soliton structure which has both staggered and uniform magnetizations with amplitudes that are broadly in accord with theoretical estimates. The magnetic soliton width gamma(m) and the field dependence of the incommensurability deltak(SP) are found to agree well with theoretical predictions.

We report neutron diffraction measurements of the magnetic structures in two pyrochlore iridates, Yb2Ir2O7 and Lu2Ir2O7. Both samples exhibit the all-in-all-out magnetic structure on the Ir 4+ sites below TN 150 K, with a low temperature moment of around 0.45 µB/Ir. Below 2 K, the Yb moments in Yb2Ir2O7 begin to order ferromagnetically. However, even at 40 mK the ordered moment is only 0.57(3) µB/Yb, significantly smaller than the saturated moment of 2 µB/Yb deduced from magnetization measurements and from a refined model of the crystal field environment. The reduced moment on Yb is found to be a consequence of enhanced phase competition caused by the coupling to the all-in-all-out magnetic order on the Ir sublattice. arXiv:1909.07819v2 [cond-mat.str-el]