Investigation of the magnetic fluctuations in Tb2_2Sn2_2O7_7 ordered spin ice by high resolution energy-resolved neutron scattering

We have studied magnetically frustrated Tb$_2$Sn$_2$O$_7$ by neutron diffraction and high resolution energy-resolved neutron scattering. At 0.1 K, we observe short range magnetic correlations with a typical scale of 4 \AA, close to the near neighbor distance between Tb$^{3+}$ ions. %(3.686 \AA), This short range order coexists with ferromagnetic correlations and long range spin ice order at the scales of 18 and 190 \AA, respectively. Spin dynamics was investigated at a time scale down to 10$^{-9}$s, by energy-resolved experiments on a backscattering spectrometer. We observe a freezing of the spin dynamics for all length-scales, with a strong slowing down of the spin fluctuations when long range order settles in. We discuss the spin fluctuations remaining in the ground state in comparison with previous data obtained by muon spectroscopy.Comment: 6 pages, 5 figure

Quantum dynamics of the Neel vector in the antiferromagnetic molecular wheel CsFe8

The inelastic neutron scattering (INS) spectrum is studied for the antiferromagnetic molecular wheel CsFe8, in the temperature range 2 - 60 K, and for transfer energies up 3.6 meV. A qualitative analysis shows that the observed peaks correspond to the transitions between the L-band states, from the ground state up to the S = 5 multiplet. For a quantitative analysis, the wheel is described by a microscopic spin Hamiltonian (SH), which includes the nearest-neighbor Heisenberg exchange interactions and uniaxial easy-axis single-ion anisotropy, characterized by the constants J and D, respectively. For a best-fit determination of J and D, the L band is modeled by an effective SH, and the effective SH concept extended such as to facilitate an accurate calculation of INS scattering intensities, overcoming difficulties with the dimension of the Hilbert space. The low-energy magnetism in CsFe8 is excellently described by the generic SH used. The two lowest states are characterized by a tunneling of the Neel vector, as found previously, while the higher-lying states are well described as rotational modes of the Neel vector.Comment: 12 pages, 10 figures, REVTEX4, to appear in PR

Inelastic neutron scattering study and Hubbard model description of the antiferromagnetic tetrahedral molecule Ni4Mo12

The tetrameric Ni(II) spin cluster Ni4Mo12 has been studied by INS. The data were analyzed extensively in terms of a very general spin Hamiltonian, which includes antiferromagnetic Heisenberg interactions, biquadratic 2-spin and 3-spin interactions, a single-ion magnetic anisotropy, and Dzyaloshinsky-Moriya interactions. Some of the experimentally observed features in the INS spectra could be reproduced, however, one feature at 1.65 meV resisted all efforts. This supports the conclusion that the spin Hamiltonian approach is not adequate to describe the magnetism in Ni4Mo12. The isotropic terms in the spin Hamiltonian can be obtained in a strong-coupling expansion of the Hubbard model at half-filling. Therefore detailed theoretical studies of the Hubbard model were undertaken, using analytical as well as numerical techniques. We carefully analyzed its abilities and restrictions in applications to molecular spin clusters. As a main result it was found that the Hubbard model is also unable to appropriately explain the magnetism in Ni4Mo12. Extensions of the model are also discussed.Comment: 12 pages, 12 figure

Quantum Phase Interference and Neel-Vector Tunneling in Antiferromagnetic Molecular Wheels

The antiferromagnetic molecular wheel Fe18 of eighteen exchange-coupled Fe(III) ions has been studied by measurements of the magnetic torque, the magnetization, and the inelastic neutron scattering spectra. The combined data show that the low-temperature magnetism of Fe18 is very accurately described by the Neel-vector tunneling (NVT) scenario, as unfolded by semiclassical theory. In addition, the magnetic torque as a function of applied field exhibits oscillations that reflect the oscillations in the NVT tunnel splitting with field due to quantum phase interference.Comment: 5 pages, 4 figures, REVTEX4, to appear in PR

Dispersionless spin waves and underlying field-induced magnetic order in gadolinium gallium garnet

We report the results of neutron diffraction and inelastic neutron scattering on a powder sample of Gd3Ga5O12 at high magnetic fields. Analysis of the diffraction data shows that in high fields (B ≳ 1.8 T) the spins are not fully aligned, but are canted slightly as a result of the dipolar interaction. The magnetic phase for fields ≲1.8 T is characterized by antiferromagnetic peaks at (210) and an incommensurate wave vector. The dominant contribution to inelastic scattering at large momentum transfers is from a band of almost dispersionless excitations. We show that these correspond to the spin waves localized on ten site rings, expected on the basis of nearest neighbor exchange interaction, and that the spectrum at high fields B ≳ 1.8 T is well described by a spin wave theory

Diffusive energy transport in the S=1 Haldane chain compound AgVP2S6

We present the results of measurements of the thermal conductivity $\kappa$ of the spin S=1 chain compound AgVP_2S_6 in the temperature range between 2 and 300 K and with the heat flow directed either along or perpendicular to the chain direction. The analysis of the anisotropy of the heat transport allowed for the identification of a small but non-negligible magnon contribution $\kappa_m$ along the chains, superimposed on the dominant phonon contribution $\kappa_ph$. At temperatures above about 100 K the energy diffusion constant D_E(T), calculated from the $\kappa_m(T)$ data, exhibits similar features as the spin diffusion constant D_S(T), previously measured by NMR. In this regime, the behaviour of both transport parameters is consistent with a diffusion process that is caused by interactions inherent to one-dimensional S=1 spin systems.Comment: 6 pages, 4 figure

Quantum statistics of interacting dimer spin systems

The compound TlCuCl3 represents a model system of dimerized quantum spins with strong interdimer interactions. We investigate the triplet dispersion as a function of temperature by inelastic neutron scattering experiments on single crystals. By comparison with a number of theoretical approaches we demonstrate that the description of Troyer, Tsunetsugu, and Wurtz [Phys. Rev. B 50, 13 515 (1994)] provides an appropriate quantum statistical model for dimer spin systems at finite temperatures, where many-body correlations become particularly important