3,120 research outputs found
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
and fluctuating moments 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 CoNbO in a transverse field: Geometric frustration and quantum renormalization effects
The quasi-one-dimensional (1D) Ising ferromagnet CoNbO 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
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