17 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
Multi-Q mesoscale magnetism in CeAuSb
We report the discovery of a field driven transition from a striped to woven
Spin Density Wave (SDW) in the tetragonal heavy fermion compound CeAuSb.
Polarized along , the sinusoidal SDW amplitude is 1.8(2) /Ce for
=6.25(10) K with wavevector (). For , harmonics appearing at
evidence a striped magnetic texture below T. Above , these are replaced by woven harmonics at
until T, where satellites vanish and magnetization non-linearly
approaches saturation at 1.64(2) /Ce for T.Comment: 5 pages, 4 figure
Ga-NMR local susceptibility of the kagome-based magnet SrCr_9pGa_(12-9p)O_19. A high temperature study
We report a high- Ga-NMR study in the kagome-based antiferromagnetic
compound SrCrGaO (), and present a
refined mean-field analysis of the high T local NMR susceptibility of Cr
frustrated moments. We find that the intralayer kagome coupling is K,
and the interlayer coupling through non-kagome Cr moments is K. The ratio confirms the common belief that
the frustrated entity is a pyrochlore slab.Comment: 8 pages, 4 figures Conference paper: Highly Frustrated Magnetism
2000, Waterloo (Canada) Submitted to Canadian Journal of Physic
Magnetic Inversion Symmetry Breaking and Ferroelectricity in TbMnO\u3csub\u3e3\u3c/sub\u3e
TbMnO3 is an orthorhombic insulator where incommensurate spin order for temperature TN\u3c41  K is accompanied by ferroelectric order for T\u3c28  K. To understand this, we establish the magnetic structure above and below the ferroelectric transition using neutron diffraction. In the paraelectric phase, the spin structure is incommensurate and longitudinally modulated. In the ferroelectric phase, however, there is a transverse incommensurate spiral. We show that the spiral breaks spatial inversion symmetry and can account for magnetoelectricity in TbMnO3
Quantum paramagnetism in a non-Kramers rare-earth oxide: Monoclinic
Little is so far known about the magnetism of the monoclinic
layered perovskites that replace the spin-ice supporting pyrochlore structure
for . We show that high quality monoclinic PrTiO
single crystals with a three-dimensional network of non-Kramers Pr ions
that interact through edge-sharing super-exchange interactions, form a singlet
ground state quantum paramagnet that does not undergo any magnetic phase
transitions down to at least 1.8 K. The chemical phase stability, structure,
and magnetic properties of the layered perovskite PrTiO were
investigated using x-ray diffraction, transmission electron microscopy, and
magnetization measurements. Synthesis of polycrystalline samples with the
nominal compositions of PrTiO () showed
that deviations from the PrTiO stoichiometry lead to secondary
phases of related, structures including the perovskite phase PrTiO
and the orthorhombic phases PrTiO and PrTiO. No
indications of site disordering (stuffing and anti-stuffing) or vacancy defects
were observed in the PrTiO majority phase. A procedure for growth
of high-structural-quality, stoichiometric single crystals of PrTiO
by the traveling solvent floating zone (TSFZ) method is reported.
Thermo-magnetic measurements of single-crystalline PrTiO reveal an
isolated singlet ground state that we associate with the low symmetry crystal
electric field environments that split the -fold degenerate
spin-orbital multiplets of the four differently coordinated Pr ions into
36 isolated singlets resulting in an anisotropic temperature independent
van-Vleck susceptibility at low . A small isotropic Curie term is associated
with 0.96(2)\% non-interacting Pr impurities.Comment: 18 pages, 7 figures, 6 table
Magnetically Driven Ferroelectric Order in Ni\u3csub\u3e3\u3c/sub\u3eV\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e8\u3c/sub\u3e
We show that long-range ferroelectric and incommensurate magnetic order appear simultaneously in a single phase transition in Ni3V2O8. The temperature and magnetic-field dependence of the spontaneous polarization show a strong coupling between magnetic and ferroelectric orders. We determine the magnetic symmetry using Landau theory for continuous phase transitions, which shows that the spin structure alone can break spatial inversion symmetry leading to ferroelectric order. This phenomenological theory explains our experimental observation that the spontaneous polarization is restricted to lie along the crystal b axis and predicts that the magnitude should be proportional to a magnetic order parameter
Incommensurate Magnetism in FeAs Strips: Neutron Scattering from CaFeAs
Magnetism in the orthorhombic metal CaFeAs was examined through
neutron diffraction for powder and single crystalline samples. Incommensurate
() and predominantly
longitudinally () modulated order develops through a 2nd order
phase transition at K with a 3D Heisenberg-like critical
exponent . A 1st order transition at K is
associated with the development of a transverse component, locking to , and increasing the moments from 2.1(1)
to 2.2(3) for Fe and from 1.3(3) to 2.4(4)
for Fe. The {\it ab-initio} Fermi surface is consistent with a nesting
instability in cross-linked FeAs strips.Comment: 5 pages, 5 figures, accepted for publication in Phys. Rev. Let
Ga NMR study of the local susceptibility in SrCr8Ga4O19: pseudogap and paramagnetic defects
We present the first Ga(4f) NMR study of the Cr susceptibility in the
archetype of Kagome based frustrated antiferromagnets,
SrCrGaO. Our major finding is that the susceptibility of the
frustrated lattice goes through a maximum around 50 K. Our data also supports
the existence of paramagnetic ``clusters'' of spins, responsible for the Curie
behavior observed in the macroscopic susceptibility at low T. These results set
novel features for the constantly debated physics of geometrically frustrated
magnets.Comment: 4 pages, 5 figures Submitted to Phys. Rev. Let