layered honeycomb magnet alpha-RuCl3 has attracted intense scrutiny as a prime candidate for realizing the Kitaev quantum spin liquid, yet a consensus on its microscopic Hamiltonian remains elusive due to the material's extreme sensitivity to structural details. Here, we report a comprehensive reexamination of the low-temperature crystallographic and magnetic structures of high-quality alpha-RuCl3 single crystals using unpolarized and polarized neutron diffraction. We confirm a sharp, first-order structural phase transition to the rhombohedral R3 & strns; space group with a pronounced thermal hysteresis. Crucially, using both spherical and longitudinal neutron polarization analysis, we determine the 3D orientation of the ordered magnetic moment without the ambiguity typically arising from domain distributions. We find that the Ru3+ magnetic moments in the zigzag phase are tilted by 15.7 degrees out of the hexagonal plane and, remarkably, exhibit an additional in-plane twist of -13.8 degrees. This "tilted and twisted" geometry differentiates the ground state from the previously reported models based on unpolarized neutron diffraction or resonant elastic X-ray scattering (REXS) analysis.

Inelastic neutron scattering experiments with a large set of comounted Na2IrO3 crystals reveal the low-energy magnon dispersion in this candidate material for Kitaev physics. The magnon gap amounts to 1.7(1) meV and can be interpreted similarly to the sister compound alpha-RuCl3 to stem from the zone boundaries in the antiferromagnetic zigzag structure. The neutron experiments find no evidence for low-energy excitations with ferromagnetic character, which contrasts with the findings in alpha-RuCl3. Our results are consistent with a recently proposed microscopic model that involves an antiferromagnetic Heisenberg nearest-neighbor exchange in Na2IrO3 in contrast to the ferromagnetic one considered for alpha-RuCl3. Although the magnetic response shows the signatures of bond-directional anisotropy in both materials the different relative signs of Kitaev and Heisenberg interaction result in different deviations from the initial Kitaev model. Low-energy ferromagnetic fluctuations cannot be considered as a fingerprint of ferromagnetic Kitaev interaction.

Polarized neutron scattering experiments reveal anisotropy of magnetic correlations in the candidate Kitaev material α-RuCl_{3}. The anisotropy of the inelastic response at the magnetic Bragg positions is opposite to the expectation for a simple Heisenberg model. Near the antiferromagnetic propagation vector, there are no low-energy transversal magnon modes and the response is always uniaxially polarized. These observations directly prove the fully anisotropic and bond-directional character of the magnetic interaction in α-RuCl_{3}. However, other findings disagree with a simple or strongly dominant Kitaev component.

The magnetic and structural phase transitions occurring in K2ReCl6 were studied by macroscopic and microscopic techniques. Structural phase transitions associated with rotations of the ReCl6 octahedra lower the symmetry from cubic to monoclinic, form ferroelastic domains, and are visible in susceptibility, specific heat, and thermal expansion measurements. In the antiferromagnetically ordered state slightly below TN = 12 K these domains can be rearranged by a magnetic field inducing a relative elongation of the polydomain crystal parallel to the field of 0.6%. At zero field the magnetic structure in K2ReCl6 does not exhibit a weak ferromagnetic component, but at large magnetic field a distinct magnetic structure with a finite weak ferromagnetic component is stabilized. High magnetic fields rearrange the domains in the crystal to align the weak ferromagnetic moment parallel to the field. The altered domain structure with the crystal elongation is abruptly suppressed at lower temperature but persists upon heating to well above TN. However, heating above the lowest structural phase transition and successive cooling restore the initial shape.

We present a comprehensive microscopic insight into the spin configurations within the bond-frustrated cubic spinel compound MnCr2S4 directly unveiled through extensive single-crystal neutron diffraction studies carried out in zero magnetic field and in fields up to 35 T. While our zero field results confirm the ferrimagnetic structure with an antiparallel arrangement of the magnetic Cr3+ and Mn2+ sublattices below T FiM approximate to 65 K, as well as the presence of the exotic Yafet-Kittel phase below T YK approximate to 5 K, our data measured in fields enable us to precisely determine the field-induced magnetic structures and their evolution across the phase transitions at mu 0 H 1 approximate to 11 T and mu 0 H 2 approximate to 25 T and beyond that towards mu 0 H 3 (approximate to 50 T). Additionally, combining our experimental findings with mean-field-theory calculations reveals a complex field dependence of the Mn-Mn and Mn-Cr exchange interactions across the different phases, highlighting the significant influence of spin-lattice coupling in this material.

The magnetic and structural phase transitions occurring in K$_2$ReCl$_6$ were studied by macroscopic and microscopic techniques. Structural phase transitions associated with rotations of the ReCl$_6$ octahedra lower the symmetry from cubic to monoclinic, form ferroelastic domains, and are visible in susceptibility, specific heat and thermal expansion measurements. In the antiferromagnetically ordered state slightly below $T_{\rm N}$=12\,K these domains can be rearranged by a magnetic field inducing a relative elongation of the polydomain crystal parallel to the field of 0.6\%. At zero field the magnetic structure in K$_2$ReCl$_6$ does not exhibit a weak ferromagnetic component, but at large magnetic field a distinct magnetic structure with a finite weak ferromagnetic component is stabilized. High magnetic fields rearrange the domains in the crystal to align the weak ferromagnetic moment parallel to the field. The altered domain structure with the crystal elongation is abruptly suppressed at lower temperature but persists upon heating to well above $T_{\rm N}$. However, heating above the lowest structural phase transition and successive cooling restore the initial shape.

M. Skoulatos,1,2 F. Rucker,3 G. J. Nilsen,4,5 A. Bertin,6 E. Pomjakushina,7 J. Ollivier,4 A. Schneidewind,8 R. Georgii,1 O. Zaharko,2 L. Keller,2 Ch. Rüegg,9,10 C. Pfleiderer,3 B. Schmidt,11 N. Shannon,12 A. Kriele,13 A. Senyshyn,1 and A. Smerald14 1Heinz Maier-Leibnitz Zentrum (MLZ) and Physics Department E21, Technische Universität München, D-85748 Garching, Germany 2Laboratory for Neutron Scattering and Imaging, Paul Scherrer Institute, CH-5232 Villigen, Switzerland 3Physics Department E51, Technische Universität München, D-85748 Garching, Germany 4Institut Laue-Langevin, 6 rue Jules Horowitz, 38042 Grenoble, France 5ISIS Neutron and Muon Facility, Rutherford Appleton Laboratory, Didcot OX11 0QX, United Kingdom 6Institut für Festkörperphysik, TU Dresden, D-01062 Dresden, Germany 7Laboratory for Multiscale Materials Experiments, Paul Scherrer Institute, CH-5232 Villigen, Switzerland 8Jülich Centre for Neutron Science JCNS at Heinz Maier-Leibnitz Zentrum (MLZ), D-85748 Garching, Germany 9Neutrons and Muons Research Division, Paul Scherrer Institute, CH-5232 Villigen-PSI, Switzerland 10Department of Quantum Matter Physics, University of Geneva, 24, Quai Ernest Ansermet, CH-1211 Genève, Switzerland 11Max Planck Institute for the Chemical Physics of Solids, 01187 Dresden, Germany 12Okinawa Institute of Science and Technology Graduate University, Onna-son, Okinawa 904-0495, Japan 13German Engineering Materials Science Centre at MLZ, Helmholtz-Zentrum Geesthacht, Lichtenbergstrasse 1, D-85748 Garching, Germany 14Max Planck Institut für Festkörperforschung, Quantum Materials Unit, D-70569 Stuttgart, Germany

dc-magnetization data measured down to 40 mK speak against conventional freezing and reinstate YbMgGaO_{4} as a triangular spin-liquid candidate. Magnetic susceptibility measured parallel and perpendicular to the c axis reaches constant values below 0.1 and 0.2 K, respectively, thus indicating the presence of gapless low-energy spin excitations. We elucidate their nature in the triple-axis inelastic neutron scattering experiment that pinpoints the low-energy (E≤J_{0}∼0.2 meV) part of the excitation continuum present at low temperatures (TJ_{0} that is rooted in the breaking of nearest-neighbor valence bonds and persists to temperatures well above J_{0}/k_{B}, the low-energy one originates from the rearrangement of the valence bonds and thus from the propagation of unpaired spins. We further extend this picture to herbertsmithite, the spin-liquid candidate on the kagome lattice, and argue that such a hierarchy of magnetic excitations may be a universal feature of quantum spin liquids.

We report neutron-scattering and ac magnetic susceptibility measurements of the two-dimensional spin-1/2 frustrated magnet BaCdVO(PO4)(2). At temperatures well below T-N approximate to 1 K, we show that only 34% of the spin moment orders in an up-up-down-down stripe structure. Dominant magnetic diffuse scattering and comparison to published muon-spin-rotation measurements indicates that the remaining 66% is fluctuating. This demonstrates the presence of strong frustration, associated with competing ferromagnetic and antiferromagnetic interactions, and points to a subtle ordering mechanism driven by magnon interactions. On applying magnetic field, we find that at T = 0.1 K the magnetic order vanishes at 3.8 T, whereas magnetic saturation is reached only above 4.5 T. We argue that the putative high-field phase is a realization of the long-sought bond-spin-nematic state.

We report synchrotron radiation diffraction and muon spin rotation (μSR) measurements on the frustrated pyrochlore magnet Tb2Ti2O7. The powder diffraction study of a crushed crystal fragment does not reveal any structural change down to 4 K. The μSR measurements performed at 20 mK on a mosaic of single crystals with an external magnetic field applied along a three-fold axis are consistent with published a.c. magnetic-susceptibility measurements at 16 mK. While an inflection point could be present around an internal field intensity slightly above 0.3 T, the data barely support the presence of a magnetization plateau. The study of geometrically frustrated magnetic systems reveals a large variety of new magnetic phases. Among the frustrated materials, the rare-earth pyrochlore oxides which crystallize in a cubic structure (Fd3̄m space group) form a family for which different exotic ground states have been found [1]. Focusing on the insulators, we mention (i) the spin-ice ground state of Ho2Ti2O7 and Dy2Ti2O7 [2, 3], (ii) the spin-liquid ground state of Yb2Ti2O7 [4] characterized by a pronounced peak in the specific heat for powder samples [5, 6], although an exotic ordered magnetic state has been reported for a single crystal [7, 8], (iii) the unconventional dynamical ground state of Tb2Sn2O7 for which magnetic Bragg reflections are observed by neutron diffraction [9] while no spontaneous magnetic field is found by the zero-field positive muon spin relaxation (μSR) technique [10, 11], (iv) the persistent spin dynamics detected in the ordered states of Gd2Sn2O7, Gd2Ti2O7 and Er2Ti2O7 [12, 13, 14, 15, 16], and (v) the splayed ferromagnet Yb2Sn2O7, i.e. essentially a ferromagnetic compound [17, 18, 19], with an emergent gauge field [20]. The most mysterious compound of the rare-earth pyrochlore oxide family might be Tb2Ti2O7 for which no long-range magnetic order is detected down to 20 mK, far below the absolute value of its Curie-Weiss temperature ΘCW = −19 K [21, 22]. Two theoretical ground states have been suggested: a quantum spin ice ground state proposed in Ref. [23] and a Jahn-Teller like distorted ground state based on specific heat measurements [24]. In analogy with the spin-ice systems, a magnetization plateau is expected at low temperature for an external magnetic field Bext applied along a [111] crystal direction if the former ground state is reached [25]. While static magnetization measurements have not found any signature of the predicted plateau down to 43 mK [26, 27, 28], the presence of a weak magnetization plateau below about 0.05 K has been proposed from a.c. susceptibility experiments for 0.06 < Bext < 0.6 T [29]. Pointing out to the complexity of the physics involved, an anomaly in the static magnetic response has been observed around 0.15 K [30, 22, 26, 27] which strangely enough corresponds to a minimum rather than a maximum in the specific heat [22]. Pinch points in the neutron scattering intensity have been observed [31] with dispersive excitations emerging from them [32, 33], suggesting a strong magnetoelastic coupling in the Coulomb phase of Tb2Ti2O7. In addition, a neutron scattering intensity measured at the (

P. Dalmas de Réotier, 2 A. Yaouanc, 2 A. Maisuradze, A. Bertin, 2 P. J. Baker, A. D. Hillier, and A. Forget Université Grenoble Alpes, INAC-PHELIQS, F-38000 Grenoble, France CEA, INAC-PHELIQS, F-38000 Grenoble, France Department of Physics, Tbilisi State University, Chavchavadze 3, GE-0128 Tbilisi, Georgia ISIS Facility, STFC Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX, UK SPEC, CEA, CNRS, Université Paris-Saclay, CEA Saclay, 91191 Gif-sur-Yvette Cedex, France (Dated: November 15, 2018)

The insulating pyrochlore compound Nd2Sn2O7 has been shown to undergo a second order magnetic phase transition at T-c approximate to 0.91 K to a noncoplanar all-in-all-out magnetic structure of the Nd3+ magnetic moments. An anomalously slow paramagnetic spin dynamics has been evidenced from neutron backscattering and muon spin relaxation (mu SR). In the case of mu SR this has been revealed through the strong effect of a 50 mT longitudinal field on the spin-lattice relaxation rate. Here, motivated by a recent successful work performed for Yb2Ti2O7 and Yb2Sn2O7, analyzing the shape of the mu SR longitudinal polarization function, we substantiate the existence of extremely slow paramagnetic spin dynamics in the microsecond time range for Nd2Sn2O7. Between 1.7 and 7 K, this time scale is temperature independent. This suggests a double spin-flip tunneling relaxation mechanism to be at play, probably involving spin substructures such as tetrahedra. Unexpectedly, the standard deviation of the field distribution at the muon site increases as the system is cooled. This exotic spin dynamics is in sharp contrast with the dynamics above 100 K which is driven by the Orbach relaxation mechanism involving single Nd3+ magnetic moments.

Magnetic systems with spins sitting on a lattice of corner sharing regular tetrahedra have been particularly prolific for the discovery of newmagnetic states for the last two decades. The pyrochlore compounds have offered the playground for these studies, while little attention has been comparatively devoted to other compounds where the rare earth R occupies the same sublattice, e.g., the spinel chalcogenides CdR2X4 (X = S or Se). Here, we report measurements performed on powder samples of this series with R = Yb using specific heat, magnetic susceptibility, neutron diffraction, and muon-spin-relaxation measurements. The two compounds are found to be magnetically similar. They long-range order into structures described by the Gamma(5) irreducible representation. The magnitude of the magnetic moment at low temperature is 0.77 (1) and 0.62 (1) mu(B) for X = S and Se, respectively. Persistent spin dynamics is present in the ordered states. The spontaneous field at the muon site is anomalously small, suggesting magnetic moment fragmentation. A double spin-flip tunneling relaxation mechanism is suggested in the cooperative paramagnetic state up to 10 K. The magnetic space groups into which magnetic moments of systems of corner-sharing regular tetrahedra order are provided for a number of insulating compounds characterized by null propagation wave vectors.

The first experimental characterization of a multiple energy analysis wide angle backend for a cold triple-axis spectrometer is reported. The multi-analyzer module MultiFLEXX employs 155 detection channels which simultaneously probe an extensive range in wavevector and energy transfer. Successful mapping of magnetic excitations in MnF2 and Ho demonstrate order of magnitude gains in data collection efficiency using this novel type backend. MultiFLEXX is competitive to standard triple-axis spectroscopy in terms of energy resolution and signal-to-noise ratio. A minority of the detector channels is affected by spurious signals inherent to this multiplexing concept. The characteristic signature of these spurious signals easily allows for their discrimination. The instrument concept focuses on detection efficiency in the horizontal scattering plane which makes it an ideal technique for fast mapping and parametric studies including extreme sample environment.

We report specific heat, magnetic, and muon spin relaxation measurements performed on a polycrystalline sample of the normal spinel CdHo2S4. The rare-earth ions sit on a lattice of corner-sharing regular tetrahedra as in pyrochlore compounds. Magnetic ordering is detected at Tc 0.87 K. From spin-lattice relaxation rate measurements on both sides of Tc we uncover similar magnetic excitation modes driving the so-called persistent spin dynamics at T < Tc. Unidimensional excitations are argued to be at its origin. Often observed spin loop structures are suggested to support these excitations. The possibility of a generic mechanism for their existence is discussed.

We report measurements performed on a polycrystalline sample of the pyrochlore compound Nd2Sn2O7. It undergoes a second order magnetic phase transition at T-c approximate to 0.91 K to a noncoplanar all-in-all-out magnetic structure of the Nd3+ 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 approximate 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.

We report synchrotron radiation diffraction and muon spin rotation (mu SR) measurements on the frustrated pyrochlore magnet Tb2Ti2O7. The powder diffraction study of a crushed crystal fragment does not reveal any structural change down to 4 K. The mu SR measurements performed at 20 mK on a mosaic of single crystals with an external magnetic field applied along a three-fold axis are consistent with published a.c. magnetic-susceptibility measurements at 16 mK. While an inflection point could be present around an internal field intensity slightly above 0.3 T, the data barely support the presence of a magnetization plateau.

Using a simple scaling law, a unique set of crystal-field parameters for the rare-earth ions R in the R(2)Ti(2)O(7) pyrochlore series is shown to provide a proper description of the crystal-field excitations previously observed by neutron scattering. The two spectroscopic g factors are given for the compounds with doublet ground states, as well as the ground-state wavefunctions for all the compounds.