We report a study of the geometrically frustrated magnetic material Tb 2 Sn 2 O 7 by the positive muonspin relaxation technique. No signature of a static magnetically ordered state is detected while neutron magnetic reflections are observed in agreement with a published report. This is explained by the dynamical nature of the ground state of Tb 2 Sn 2 O 7 : the Tb 3 magnetic moment characteristic fluctuation time is ' 10 ÿ10 s. The strong effect of the magnetic field on the muon-spin-lattice relaxation rate at low fields indicates a large field-induced increase of the magnetic density of states of the collective excitations at low energy. DOI: 10.1103/PhysRevLett.96.127202 PACS numbers: 75.40.ÿs, 75.25.+z, 76.75.+i Magnetic materials with antiferromagnetically coupled spins located on triangular motifs exhibit geometrical magnetic frustration because their spatial arrangement is such that it prevents the simultaneous minimization of all the interaction energies [1]. The frustration, which leads to a highly degenerate ground state, forbids magnetic order to occur. Perturbations to the nearest-neighbor exchange interaction, such as exchange interactions extending beyond nearest-neighbor magnetic atoms, dipole coupling, or magnetic anisotropy, are believed to be responsible for the magnetic order observed in some compounds [2]. Typical examples are given by the spinel structure oxide [6]. A prerequisite for understanding the unanticipated behavior of these latter systems is a careful characterization of their dynamical properties.Here we show that positive muon-spin relaxation ( SR) and ND results in the ordered phase of Tb 2 Sn 2 O 7 can be simultaneously accounted for only if the Tb 3 moments are strongly dynamical. An independent and consistent time scale is obtained from a careful analysis of the neutron data. In addition, the initial strong and counterintuitive increase of the muon relaxation rate when a magnetic field is applied indicates an increase of the density of magnetic excitations at very low energy.Tb 2 Sn 2 O 7 crystallizes with the cubic space group Fd 3m. Rietveld refinements of powder x-ray and ND patterns yield the lattice constant a 10:426 A and the free position parameter allowed by the space group for the 48f site occupied by oxygen, x 0:336 [6]. Magnetic measurements point to a magnetic transition at 0.87 K and to strong antiferromagnetic interactions as deduced from the large and negative Curie-Weiss constant CW ÿ12 K [13]. Powder ND indicates a structure with both ferromagnetic and antiferromagnetic components below T sr 1:3 1 K where short-range magnetic correlations which are not liquidlike appear [6]. A steep increase of the Tb 3 magnetic moment Tb and correlation length L c is observed around T lr 0:87 K, where a peak is seen in the temperature dependence of the specific heat C p T . We present below (i) C p T data recorded using a dynamic adiabatic technique, (ii) ND measurements carried out at the cold neutron powder diffractometer DMC of the SINQ facility at the Paul Scherrer I...

From magnetic, specific heat, 170 Yb Mössbauer effect, neutron diffraction, and muon spin relaxation measurements on polycrystalline Yb 2 Sn 2 O 7 , we show that below the first order transition at 0.15 K all of the Yb 3þ ions are long-range magnetically ordered and each has a moment of 1:1 B which lies at ' 10 to a common fourfold cubic axis. The four sublattice moments have four different directions away from this axis and are therefore noncoplanar. We term this arrangement splayed ferromagnetism. This ground state has a dynamical component with a fluctuation rate in the megahertz range. The net ferromagnetic exchange interaction has an anisotropy that favors the local threefold axis. We discuss our results in terms of the phase diagram proposed by Savary and Balents [Phys. Rev. Lett. 108, 037202 (2012) There is much interest in the pyrochlore lattice compounds R 2 M 2 O 7 , where R is a rare earth and M is a transition or sp metal. The chief motivation is that the R ions form corner-sharing tetrahedra such that the interionic interactions are prone to geometrical frustration. A number of different situations have been encountered depending on the form, sign, size, and anisotropy of the various possible interionic interactions [1]. The quest for the spin liquid state, namely a ground state where the spins are strongly correlated while they show no long-range order and are dynamical, has motivated important theoretical and experimental efforts. For example, pyrochlores like Ho 2 Ti 2 O 7 , where the R ¼ Ho ion has a large axial anisotropy and where there is a net ferromagnetic interaction, evidence magnetic frustration which shows analogies with the positional fluctuations of the protons in ice. Such systems have been labeled ''spin ice '' [2]. Their behavior is essentially driven by the classical dipolar interaction between the spins [3]. More recently, the interest has focused on the quantum spin liquid model where exchange interactions determine the low temperature behavior [4-6] and the possibility has been raised that the pyrochlore Yb 2 Ti 2 O 7 is a physical realization of this model [7][8][9][10]. Even more striking, four phases are predicted from a gauge mean field theory depending on values of the symmetry dictated nearest neighbor Hamiltonian parameters: two Higgs phases either ferromagnetic (FM) or antiferromagnetic (AFM), the quantum spin liquid phase, and a new exotic phase, the Coulombic ferromagnetic (CFM) phase [7]. Three types of magnetic excitations which are either gapped or not can be observed: they are respectively associated with an effective electric charge (gapped), an effective magnetic charge (gapped), and an artificial photon (gapless) [9]. Fractional spin excitations are present in the CFM phase.The Higgs FM phase has recently been proposed for Yb 2 Ti 2 O 7 at low temperature [11]. However a longstanding controversy exists as to the intrinsic presence of magnetic Bragg reflections in this compound [11][12][13][14][15][16][17]. In this respect, recent studies show the variability of...

We report measurements performed on a polycrystalline sample of the pyrochlore compound Nd 2 Sn 2 O 7 . It undergoes a second order magnetic phase transition at T c ≈ 0.91 K to a noncoplanar all-in-all-out magnetic structure of the Nd 3+ magnetic moments. The thermal behavior of the low temperature specific heat fingerprints excitations with linear dispersion in a three-dimensional lattice. The temperature independent spin-lattice relaxation rate measured below T c and the anomalously slow paramagnetic spin dynamics detected up to ≈30T c are suggested to be due to magnetic short-range correlations in unidimensional spin clusters, i.e., spin loops. The observation of a spontaneous field in muon spin relaxation measurements is associated with the absence of a divergence-free field for the ground state of an all-in-all-out pyrochlore magnet as predicted recently.

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Zero and longitudinal field SR measurements in Pb 2 VO͑PO 4 ͒ 2 and BaCdVO͑PO 4 ͒ 2 , two prototypes of the frustrated S =1/ 2 square lattice model with competing ferromagnetic and antiferromagnetic interactions, are presented. Both systems are observed to undergo a phase transition to a long-range magnetic order at T N Ӎ 3.46 K, for Pb 2 VO͑PO 4 ͒ 2 , and at T N Ӎ 0.99 K, for BaCdVO͑PO 4 ͒ 2 . In Pb 2 VO͑PO 4 ͒ 2 both the temperature dependence of the order parameter and the longitudinal relaxation rate above T N are consistent with a twodimensional XY model, as it is found for Sr 2 CuO 2 Cl 2 . On the other hand, for BaCdVO͑PO 4 ͒ 2 , which lies very close to the magnetically disordered region of the phase diagram where a bond-nematic order was predicted, a peculiar logarithmic increase in the relaxation is observed above T N . In both systems a rather broad distribution of internal fields at the muon sites is noticed below T N . The origin of this distribution is discussed in the light of the SR experiments already performed on S =1/ 2 frustrated antiferromagnets on a square lattice.