Platelike high-quality NaYbS2 rhombohedral single crystals with lateral dimensions of a few mm have been grown and investigated in great detail by bulk methods like magnetization and specific heat, but also by local probes like nuclear magnetic resonance (NMR), electron-spin resonance (ESR), muon-spin relaxation (µSR), and inelastic neutron scattering (INS) over a wide field and temperature range. Our single-crystal studies clearly evidence a strongly anisotropic quasi-2D magnetism and an emerging spin-orbit entangled S = 1/2 state of Yb towards low temperatures together with an absence of long-range magnetic order down to 260 mK. In particular, the clear and narrow Yb ESR lines together with narrow 23 Na NMR lines evidence an absence of inherent structural distortions in the system, which is in strong contrast to the related spin-liquid candidate YbMgGaO4 falling within the same space group R3m. This identifies NaYbS2 as a rather pure spin-1/2 triangular lattice magnet and a new putative quantum spin liquid.Introduction. -In low-dimensional quantum magnets, competing confined magnetic exchange interactions restrict the magnetic degrees of freedom, which leads to a strong frustration accompanied by enhanced quantum fluctuations. Ultimately this prevents the systems from longrange order, and the ground state is supposed to be a magnetic liquid. There are various types of such quantum spin liquids (QSL) depending on the lattice geometry (in 2D: square-, triangular-, kagome-, or honeycomb-type; in 3D: hyperkagome, hyperhoneycomb, or pyrochlore), the magnetic exchange (e.g. Heisenberg, Kitaev, or Dzyaloshinskii-Moriya type), and the magnetic ion itself [1][2][3][4]. Planar spin-1/2 triangular lattice magnets (TLMs) with antiferromagnetic exchange interactions are ideal QSL candidates as proposed by P. W. Anderson [5]. A few examples are found among the organic materials, such as K-(BEDT-TTF) 2 Cu 2 (CN) 3 [6] and EtnMe 4−n Sb[Pd(DMIT) 2 ] 2 [7], whereas among inorganic compounds such QSL model systems are very rare, e.g. Ba 3 CuSb 2 O 9 [8].

High-quality single crystals of NaYbSe2, which resembles a perfect triangular-lattice antiferromagnet without the intrinsic disorder, are investigated by magnetization and specific-heat, as well as the local probe techniques nuclear magnetic resonance (NMR) and electron spin resonance (ESR). The low-field measurements confirm the absence of any spin freezing or long-range magnetic order down to 50 mK, which suggests a quantum spin liquid ground (QSL) state with gapless excitations. Instability of the QSL state is observed upon applying magnetic fields. For the H⊥c direction, a field-induced magnetic phase transition is observed above 2 T from the Cp(T ) data, agreeing with a clear Ms 3 plateau of M (H), which is associated with an up-up-down (uud) spin arrangement. For the H c direction, a field-induced transition could be evidenced at a much higher field range (9 -21 T). The 23 Na NMR measurements provide microscopic evidence for field-induced ordering for both directions. A reentrant behaviour of TN, originating from the thermal and quantum spin fluctuations, is observed for both directions. The anisotropic exchange interactions J ⊥ 4.7 K and Jz 2.33 K are extracted from the modified bond-dependent XXZ model for the spin-1 2 triangular-lattice antiferromagnet. The absence of magnetic long-range order at zero fields is assigned to the effect of strong bond-frustration, arising from the complex spin-orbit entangled 4f ground state. Finally, we derive the highly anisotropic magnetic phase diagram, which is discussed in comparison with the existing theoretical models for spin-1 2 triangular-lattice antiferromagnets. :1911.12712v1 [cond-mat.str-el] arXiv

Polycrystalline samples of NaYbO2 are investigated by bulk magnetization and specific-heat measurements, as well as by nuclear magnetic resonance (NMR) and electron spin resonance (ESR) as local probes. No signatures of long-range magnetic order are found down to 0.3 K, evidencing a highly frustrated spin-liquid-like ground state in zero field. Above 2 T, signatures of magnetic order are observed in thermodynamic measurements, suggesting the possibility of a field-induced quantum phase transition. The 23 Na NMR relaxation rates reveal the absence of magnetic order and persistent fluctuations down to 0.3 K at very low fields and confirm the bulk magnetic order above 2 T. The H-T phase diagram is obtained and discussed along with the existing theoretical concepts for layered spin-1 2 triangular-lattice antiferromagnets.

The crystallographic, magnetic and thermal properties of polycrystalline BiMn2PO6 and its nonmagnetic analogue BiZn2PO6 were investigated by x-ray diffraction, magnetization M , magnetic susceptibility χ, heat capacity Cp, and 31 P nuclear magnetic resonance (NMR) measurements versus applied magnetic field H and temperature T as well as by density-functional band-theory and molecular field calculations. Both compounds show a strong monotonic lattice softening on cooling, where the Debye temperature decreases by a factor of two from ΘD ∼ 650 K at T = 300 K to ΘD ∼ 300 K at T = 2 K. The χ(T ) data for BiMn2PO6 above 150 K follow a Curie-Weiss law with a Curie constant consistent with a Mn +2 spin S = 5/2 with g-factor g = 2 and an antiferromagnetic (AFM) Weiss temperature θCW −78 K. The χ data indicate long-range AFM ordering below TN 30 K, confirmed by a sharp λ-shaped peak in Cp(T ) at 28.8 K. The magnetic entropy at 100 K extracted from the Cp(T ) data is consistent with spin S = 5/2 for the Mn +2 cations. The band-theory calculations indicate that BiMn2PO6 is an AFM compound with dominant interactions J1/kB 6.7 K and J3/kB 5.6 K along the legs and rungs of a Mn two-leg spin-ladder, respectively. However, sizable and partially frustrating interladder couplings lead to an anisotropic three-dimensional magnetic behavior with long-range AFM ordering at TN 30 K observed in the χ, Cp and NMR measurements. A second magnetic transition at ≈ 10 K is observed from the χ and NMR measurements but is not evident in the Cp data. The Cp data at low T suggest a significant contribution from AFM spin waves moving in three dimensions and the absence of a spin-wave gap. A detailed analysis of the NMR spectra indicates commensurate magnetic order between 10 K and 30 K, while below 10 K additional features appear that may arise from an incommensurate modulation and/or spin canting. The commensurate order is consistent with microscopic density functional calculations that yield a collinear Néel-type AFM spin arrangement both within and between the ladders, despite the presence of multiple weak interactions frustrating this magnetic structure of the Mn spins. Frustration for AFM ordering and the 1D spatial anisotropy of the 3D spin interactions are manifested in the frustration ratio f = |θCW|/TN 2.6, indicating a suppression of TN from 68 K in the absence of these effects to the observed value of about 30 K in BiMn2PO6.

We report thermodynamic properties, magnetic ground state, and microscopic magnetic model of the spin-1 frustrated antiferromaget Li2NiW2O8 showing successive transitions at TN1 18 K and TN2 12.5 K in zero field. Nuclear magnetic resonance and neutron diffraction reveal collinear and commensurate magnetic order with the propagation vector k = ( 1 2 , 0, 1 2 ) below TN2. The ordered moment of 1.8 µB at 1.5 K is directed along [0.89(9), −0.10(5), −0.49 (6)] and matches the magnetic easy axis of spin-1 Ni 2+ ions, which is determined by the scissor-like distortion of the NiO6 octahedra. Incommensurate magnetic order, presumably of spin-density-wave type, is observed in the region between TN2 and TN1. Density-functional band-structure calculations put forward a three-dimensional spin lattice with spin-1 chains running along the [011] direction and stacked on a spatially anisotropic triangular lattice in the ab plane. We show that the collinear magnetic order in Li2NiW2O8 is incompatible with the triangular lattice geometry and thus driven by a pronounced easy-axis single-ion anisotropy of Ni 2+ . a O3 O2 O1 O2 ' c b J 011 J 100 x z J 010 J 110