Static and dynamic XY-like short-range order in a frustrated magnet with exchange disorder

A single crystal of the Co2+ based pyrochlore NaCaCo2F7 was studied by inelastic neutron scattering. This frustrated magnet with quenched exchange disorder remains in a strongly correlated paramagnetic state down to one 60th of the Curie-Weiss temperature. Below T_f = 2.4 K, diffuse elastic scattering develops and comprises 30 +/- 10% of the total magnetic scattering, as expected for J_{eff} = 1/2 moments frozen on a time scale that exceeds \hbar/\delta E=3.8 ps. The diffuse scattering is consistent with short range XY antiferromagnetism with a correlation length of 16 \AA. The momentum (Q) dependence of the inelastic intensity indicates relaxing XY-like antiferromagnetic clusters at energies below ~ 5.5 meV, and collinear antiferromagnetic fluctuations above this energy. The relevant XY configurations form a continuous manifold of symmetry-related states. Contrary to well-known models that produce this continuous manifold, order-by-disorder does not select an ordered state in NaCaCo2F7 despite evidence for weak (~12 %) exchange disorder. Instead, NaCaCo2F7 freezes into short range ordered clusters that span this manifold.Comment: 9 pages, 9 figures. This updated version features modified figures and some new discussio

Antiferromagnetic and Orbital Ordering on a Diamond Lattice Near Quantum Criticality

We present neutron scattering measurements on powder samples of the spinel FeSc2S4 that reveal a previously unobserved magnetic ordering transition occurring at 11.8(2)~K. Magnetic ordering occurs subsequent to a subtle cubic-to-tetragonal structural transition which distorts Fe coordinating sulfur tetrahedra lifting the orbital degeneracy. The application of 1~GPa hydrostatic pressure appears to destabilize this N\'eel state, reducing the transition temperature to 8.6(8)~K and redistributing magnetic spectral weight to higher energies. The relative magnitudes of ordered $\langle m \rangle^2\!=\!3.1(2)$ and fluctuating moments $\langle \delta m \rangle^2\!=\!13(1)$ show that the magnetically ordered ground state of FeSc2S4 is drastically renormalized and in proximity to criticality.Comment: 16 pages, 12 figure

Excitations in the quantum paramagnetic phase of the quasi-one-dimensional Ising magnet CoNb2_2O6_6 in a transverse field: Geometric frustration and quantum renormalization effects

The quasi-one-dimensional (1D) Ising ferromagnet CoNb$_2$O$_6$ has recently been driven via applied transverse magnetic fields through a continuous quantum phase transition from spontaneous magnetic order to a quantum paramagnet, and dramatic changes were observed in the spin dynamics, characteristic of weakly perturbed 1D Ising quantum criticality. We report here extensive single-crystal inelastic neutron scattering measurements of the magnetic excitations throughout the three-dimensional (3D) Brillouin zone in the quantum paramagnetic phase just above the critical field to characterize the effects of the finite interchain couplings. In this phase, we observe that excitations have a sharp, resolution-limited line shape at low energies and over most of the dispersion bandwidth, as expected for spin-flip quasiparticles. We map the full bandwidth along the strongly dispersive chain direction and resolve clear modulations of the dispersions in the plane normal to the chains, characteristic of frustrated interchain couplings in an antiferromagnetic isosceles triangular lattice. The dispersions can be well parametrized using a linear spin-wave model that includes interchain couplings and further neighbor exchanges. The observed dispersion bandwidth along the chain direction is smaller than that predicted by a linear spin-wave model using exchange values determined at zero field, and this effect is attributed to quantum renormalization of the dispersion beyond the spin-wave approximation in fields slightly above the critical field, where quantum fluctuations are still significant.Comment: 11 pages, 6 figures. Updated references. Minor changes to text and figure