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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]

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The magnetic ground state of the Eu 2+ moments in a series of Eu(Fe1−xCox)2As2 single crystals grown from the Sn flux has been investigated in detail by neutron diffraction measurements. Combined with the results from the macroscopic properties (resistivity, magnetic susceptibility and specific heat) measurements, a phase diagram describing how the Eu magnetic order evolves with Co doping in Eu(Fe1−xCox)2As2 is established. The ground-state magnetic structure of the Eu 2+ spins is found to develop from the A-type antiferromagnetic (AFM) order in the parent compound, via the A-type canted AFM structure with some net ferromagnetic (FM) moment component along the crystallographic c direction at intermediate Co doping levels, finally to the pure FM order at relatively high Co doping levels. The ordering temperature of Eu declines linearly at first, reaches the minimum value of 16.5(2) K around x = 0.100(4), and then reverses upwards with further Co doping. The doping-induced modification of the indirect Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between the Eu 2+ moments, which is mediated by the conduction d electrons on the (Fe,Co)As layers, as well as the change of the strength of the direct interaction between the Eu 2+ and Fe 2+ moments, might be responsible for the change of the magnetic ground state and the ordering temperature of the Eu sublattice. In addition, for Eu(Fe1−xCox)2As2 single crystals with 0.10 x 0.18, strong ferromagnetism from the Eu sublattice is well developed in the superconducting state, where a spontaneous vortex state is expected to account for the compromise between the two competing phenomena.