Unlike conventional magnets where the magnetic moments are partially or completely static in the ground state, in a quantum spin liquid they remain in collective motion down to the lowest temperatures. The importance of this state is that it is coherent and highly entangled without breaking local symmetries. Such phenomena is usually sought in simple lattices where antiferromagnetic interactions and/or anisotropies that favor specific alignments of the magnetic moments are frustrated by lattice geometries incompatible with such order e.g. triangular structures. Despite an extensive search among such compounds, experimental realizations remain very few. Here we describe the investigation of a novel, unexplored magnetic system consisting of strong ferromagnetic and weaker antiferromagnetic isotropic interactions as realized by the compound Ca10Cr7O28. Despite its exotic structure we show both experimentally and theoretically that it displays all the features expected of a quantum spin liquid including coherent spin dynamics in the ground state and the complete absence of static magnetism.A quantum spin liquid is a macroscopic lattice of interacting magnetic ions with quantum spin number S=½, whose ground state has no static long-range magnetic order, instead the magnetic moments fluctuate coherently down to the lowest temperatures [1, 2]. It contrasts with the static long-range magnetically ordered ground states usually observed, and also with spin glass states where the spins are frozen into static short-range ordered configurations [3]. The excitations are believed to be spinons which have fractional quantum spin number S=½, and are very different from spin-waves or magnons that possess quantum spin number S=1 and are the characteristic excitations of conventional magnets. Spin liquids exist in one-dimensional magnets and the chain of spin-½ magnetic ions coupled by nearestneighbor, Heisenberg (isotropic), antiferromagnetic interactions is a well-established example [4]. This system has no static long-range magnetic order and the excitations are spinons. There is no energetic reason for the spinons to bind together, indeed if a spin-1 excitation is created e.g. by reversing a spin in the chain, it fractionalizes into two spin-½ spinons [5][6][7][8][9][10][11][12].The existence of spin liquids in dimensions greater than one is much less established. While static order does not occur in one-dimensional magnets, conventional two-and three-dimensional magnets order at temperatures at or above zero Kelvin [13]. This order can be suppressed by introducing competition (known as frustration) between the interactions that couple the magnetic ions. Geometrical frustration is achieved when the magnetic ions are located on lattices constructed from triangular motifs and are coupled by antiferromagnetic interactions. The antiferromagnetic coupling favors antiparallel spin alignment between nearest neighbor spins which can never be fully satisfied since the number of spins around the triangle is odd. This typically leads to h...

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Neutron inelastic scattering has been used to measure the magnetic excitations in powdered NiPS3, a quasi-two dimensional antiferromagnet with spin S = 1 on a honeycomb lattice. The spectra show clear, dispersive magnons with a ∼ 7 meV gap at the Brillouin zone center. The data were fitted using a Heisenberg Hamiltonian with a single-ion anisotropy assuming no magnetic exchange between the honeycomb planes. Magnetic exchange interactions up to the third intraplanar nearest-neighbour were required. The fits show robustly that NiPS3 has an easy axis anisotropy with ∆ = 0.3 meV and that the third nearest-neighbour has a strong antiferromagnetic exchange of J3 = −6.90 meV. The data can be fitted reasonably well with either J1 < 0 or J1 > 0, however the best quantitative agreement with high-resolution data indicate that the nearestneighbour interaction is ferromagnetic with J1 = 1.9 meV and that the second nearest-neighbour exchange is small and antiferromagnetic with J2 = −0.1 meV. The dispersion has a minimum in the Brillouin zone corner that is slightly larger than that at the Brillouin zone center, indicating that the magnetic structure of NiPS3 is close to being unstable.

We present thermodynamic and neutron scattering measurements on the quantum spin ice candidate Nd2Zr2O7. The parameterization of the anisotropic exchange Hamiltonian is refined based on high-energyresolution inelastic neutron scattering data together with thermodynamic data using linear spin wave theory and numerical linked cluster expansion. Magnetic phase diagrams are calculated using classical Monte Carlo simulations with fields along [100], [110] and [111] crystallographic directions which agree qualitatively with the experiment. Large hysteresis and irreversibility for [111] is reproduced and the microscopic mechanism is revealed by mean field calculations to be the existence of metastable states and domain inversion. Our results shed light on the explanations of the recently observed dynamical kagome ice in Nd2Zr2O7 in [111] fields.Although the measurements and analyses on Nd 2 Zr 2 O 7 are quite comprehensive, a qualitative understanding of the observations based on a spin Hamiltonian is still lacking [23,24,27]. The recently determined exchange parameters still cannot describe the INS data in several aspects (e.g. the transition field and spin excitation energies in fields). In this paper, we first refined the spin Hamiltonian by a comprehensive analysis of the high-energy-resolution INS data and thermodynamic properties of Nd 2 Zr 2 O 7 using linear spin wave theory, high temperature series expansion and numerical linked cluster expansion (NLCE). Second, we calculated the phase diagrams for external fields along [100], [110] and [111] directions using classical Monte Carlo simulations finding qualitative agreement with experiment. Third, we found that the large hysteresis for the [111] field is caused by an irreversibility of the magnetisation or domain inversion due to the existence of a metastable state based on mean field calculations which is critical for explaining the observed dynamical kagome modes in [111] fields [27]. arXiv:1904.07819v2 [cond-mat.mtrl-sci]

We discuss a cost-effective approach to understand magnetic relaxation in the new generation of rare-earth single-molecule magnets. It combines ab initio calculations of the crystal field parameters, of the magneto-elastic coupling with local modes, and of the phonon density of states with fitting of only three microscopic parameters. Although much less demanding than a fully ab initio approach, the method gives important physical insights into the origin of the observed relaxation. By applying it to high-anisotropy compounds with very different relaxation, we demonstrate the power of the approach and pinpoint ingredients for improving the performance of single-molecule magnets.