646 research outputs found
A single crystal high-temperature pyrochlore antiferromagnet
We report the magnetic characterization of the frustrated transition metal
pyrochlore NaCaCoF. This material has high spin Co in CoF
octahedra in a pyrochlore lattice, and disordered non-magnetic Na and Ca on the
large-atom sites in the structure. Large crystals grown by the floating zone
method were studied. The magnetic susceptibility is isotropic, the Co moment is
larger than the spin-only value, and in spite of the large Curie Weiss theta
(-140 K), freezing of the spin system, as characterized by peaks in the ac and
dc susceptibility and specific heat, does not occur until around 2.4 K. This
yields a frustration index of f = / 56, an
indication that the system is highly frustrated. The observed entropy loss at
the freezing transition is low, indicating that magnetic entropy remains
present in the system at 0.6 K. The compound may be the realization of a
frustrated pyrochlore antiferromagnet with weak bond disorder. The high
magnetic interaction strength, strong frustration, and the availability of
large single crystals makes NaCaCoF an interesting alternative to rare
earth oxide pyrochlores for the study of geometric magnetic frustration in
pyrochlore lattices.Comment: Submitted to PRL; 14 pages, 4 figures, 2 table
NaCaNi2F7: A frustrated high temperature pyrochlore antiferromagnet with S=1 Ni2+
NaCaNiF is an insulating, frustrated ABF pyrochlore with
magnetic S = 1 Ni on the pyrochlore B site. Non-magnetic Na and Ca are
disordered on the A-site. Magnetic susceptibility measurements made on an
oriented single crystal, grown in a floating zone furnace, show isotropic
behavior at temperatures between 5 and 300 K, with an effective moment of 3.7
/Ni. Despite displaying a large Curie-Weiss theta (-129 K),
spin-ordering-related features are not seen in the susceptibility or specific
heat until a spin glass transition at 3.6 K. This yields an empirical
frustration index of f = -/T 36. The spin glass
behavior is substantiated by a shift of the freezing temperature with frequency
in the AC susceptibility, bifurcation in the DC susceptibility, and by a broad
maximum in the magnetic specific heat. The observations as made on large single
crystals suggest that NaCaNiF is likely a realization of a frustrated
spin 1 pyrochlore antiferromagnet with weak bond disorder.Comment: 15 Pages, 5 Figures, 2 Table
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
Heat capacity anomaly at the quantum critical point of the Transverse Ising Magnet CoNb_2O_6
The transverse Ising magnet Hamiltonian describing the Ising chain in a
transverse magnetic field is the archetypal example of a system that undergoes
a transition at a quantum critical point (QCP). The columbite CoNbO is
the closest realization of the transverse Ising magnet found to date. At low
temperatures, neutron diffraction has observed a set of discrete collective
spin modes near the QCP. We ask if there are low-lying spin excitations
distinct from these relatively high energy modes. Using the heat capacity, we
show that a significant band of gapless spin excitations exists. At the QCP,
their spin entropy rises to a prominent peak that accounts for 30 of the
total spin degrees of freedom. In a narrow field interval below the QCP, the
gapless excitations display a fermion-like, temperature-linear heat capacity
below 1 K. These novel gapless modes are the main spin excitations
participating in, and affected, by the quantum transition.Comment: 14 pages total, 8 figure
- …