96 research outputs found
Coupled Negative magnetocapacitance and magnetic susceptibility in a Kagome staircase-like compound Co3V2O8
The dielectric constant of the Kagome staircase-like Co3V2O8 polycrystalline
compound has been measured as function of temperature and magnetic field up to
14T. It is found that the application of an external magnetic field suppresses
the anomaly for the dielectric constant beyond 6.1K. Furthermore, its magnetic
field dependence reveals a negative magnetocapacitance which is proportional to
the magnetic susceptibility, suggesting a common magnetostrictive origin for
the magnetic field dependence of the two quantities. This result is very
different from that obtained from the isostructural compound Ni3V2O8 that
presents a peak in the dielectric constant at the incommensurate magnetic phase
transition coupled to a sign change of the magnetocapacitance
Competing Magnetic Phases on a "Kagome Staircase"
We present thermodynamic and neutron data on Ni_3V_2O_8, a spin-1 system on a
kagome staircase. The extreme degeneracy of the kagome antiferromagnet is
lifted to produce two incommensurate phases at finite T - one amplitude
modulated, the other helical - plus a commensurate canted antiferromagnet for T
->0. The H-T phase diagram is described by a model of competing first and
second neighbor interactions with smaller anisotropic terms. Ni_3V_2O_8 thus
provides an elegant example of order from sub leading interactions in a highly
frustrated systemComment: 4 pages, 3 figure
Magnetically driven ferroelectric order in NiVO
We show that for NiVO long-range ferroelectric and incommensurate
magnetic order appear simultaneously in a single phase transition. The
temperature and magnetic field dependence of the spontaneous polarization show
a strong coupling between magnetic and ferroelectric orders. We determine the
magnetic symmetry of this system by constraining the data to be consistent with
Landau theory for continuous phase transitions. This phenomenological theory
explains our observation 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.Comment: 11 pages, 3 figure
Field dependent competing magnetic ordering in multiferroic Ni3V2O8
The geometrically frustrated magnet Ni3V2O8 undergoes a series of competing
magnetic ordering at low temperatures. Most importantly, one of the
incommensurate phases has been reported to develop a ferroelectric correlation
caused by spin frustration. Here we report an extensive thermodynamic,
dielectric and magnetic study on clean polycrystalline samples of this novel
multiferroic compound. Our low temperature specific heat data at high fields up
to 14 Tesla clearly identify the development of a new magnetic field induced
phase transition below 2 K that shows signatures of simultaneous electric
ordering. We also report temperature and field dependent dielectric constant
that enables us to quantitatively estimate the strength of magneto-electric
coupling in this improper ferroelectric material.Comment: 18 pages, 4 figures. Accepted for publication in Euro. Phys. Let
Fermi surface and quasiparticle dynamics of Na(x)CoO2 {x=0.7} investigated by Angle-Resolved Photoemission Spectroscopy
We present an angle-resolved photoemission study of Na0.7CoO2, the host
cobaltate of the NaxCoO2.yH2O series. Our results show a large hexagonal-like
hole-type Fermi surface, an extremely narrow strongly renormalized
quasiparticle band and a small Fermi velocity. Along the Gamma to M high
symmetry line, the quasiparticle band crosses the Fermi level from M toward
Gamma consistent with a negative sign of effective single-particle hopping (t
): t is estimated to be about 8 meV which is on the order of exchange coupling
J in this system. This suggests that t ~ J ~ 10 meV is an important energy
scale in the system. Quasiparticles are well defined only in the T-linear
resistivity regime. Small single particle hopping and unconventional
quasiparticle dynamics may have implications for understanding the unusual
behavior of this new class of compounds.Comment: Revised text, Added Figs, Submitted to PR
Field dependence of magnetic ordering in Kagome-staircase compound Ni3V2O8
We present powder and single-crystal neutron diffraction and bulk
measurements of the Kagome-staircase compound Ni3V2O8 (NVO) in fields up to
8.5T applied along the c-direction. (The Kagome plane is the a-c plane.) This
system contains two types of Ni ions, which we call "spine" and "cross-tie".
Our neutron measurements can be described with the paramagnetic space group
Cmca for T < 15K and each observed magnetically ordered phase is characterized
by the appropriate irreducible representation(s). Our zero-field measurements
show that at T_PH=9.1K NVO undergoes a transition to an incommensurate order
which is dominated by a longitudinally-modulated structure with the spine spins
mainly parallel to the a-axis. Upon further cooling, a transition is induced at
T_HL=6.3K to an elliptically polarized incommensurate structure with both spine
and cross-tie moments in the a-b plane. At T_LC=4K the system undergoes a
first-order phase transition, below which the magnetic structure is a
commensurate antiferromagnet with the staggered magnetization primarily along
the a-axis and a weak ferromagnetic moment along the c-axis. A specific heat
peak at T_CC'=2.3K indicates an additional transition, which we were however
not able to relate to a change of the magnetic structure. Neutron, specific
heat, and magnetization measurements produce a comprehensive temperature-field
phase diagram. The symmetries of the two incommensurate magnetic phases are
consistent with the observation that only one phase has a spontaneous
ferroelectric polarization. All the observed magnetic structures are explained
theoretically using a simplified model Hamiltonian, involving competing
nearest- and next-nearest-neighbor exchange interactions, spin anisotropy,
Dzyaloshinskii-Moriya and pseudo-dipolar interactions.Comment: 25 pages, 19 figure
The Kagome-staircase lattice: Magnetic ordering in Ni3V2O8 and Co3V2O8
Ni3V2O8 and Co3V2O8 have spin-1 and spin-3/2 magnetic lattices that are a new
anisotropic variant of the Kagome net, wherein edge-sharing MO6 octahedra form
the rises and rungs of a "Kagome staircase". The anisotropy largely relieves
the geometric frustration, but results in rich magnetic behavior.
Characterization of the magnetization of polycrystalline samples reveals that
the compounds are ferrimagnetic in character. Heat capacity measurements show
the presence of four magnetic phase transitions below 9 K for Ni3V2O8 and two
below 11 K for Co3V2O8. Comparison to the low temperature heat capacity of
isostructural nonmagnetic Zn3V2O8 provides an estimate of the magnetic entropy
involved with the phase transitions. The results suggest that Co3V2O8 may
display magnetic transitions below 2 K.Comment: 9 pages, 5 figure
Spin entropy as the likely source of enhanced thermopower in $\rm\bf Na_xCo_2O_4
In an electric field, the flow of electrons in a solid produces an entropy
current in addition to the familiar charge current. This Peltier effect
underlies all thermoelectric refrigerators. The upsurge in thermoelectric
cooling applications has led to a search for more efficient Peltier materials
and to renewed theoretical interest in how electron-electron interaction may
enhance the thermopower of materials such as the transition-metal oxides
\cite{Mahan,Beni,Kotliar,Chaikin}. An important factor in this enhancement is
the electronic spin entropy, which is predicted \cite{Chaikin,Kwak,KwakChaikin}
to dominate the entropy current. Here we report evidence for such suppression
in the layered oxide , and present evidence that it is a
strong-correlation effect.Comment: 5 pages, 5 figures, already publishe
beta-Cu3V2O8: Magnetic ordering in a spin-1/2 kagome-staircase lattice
The spin-1/2 Cu2+ ions in beta-Cu3V2O8 occupy the sites of a Kagome-staircase
lattice, an anisotropic variant of the Kagome net: buckled layers and imbedded
plaquettes of three edge-shared CuO4 squares break the ideal Kagome symmetry.
Susceptibility and heat capacity measurements show the onset of short-range
ordering at approximately 75 K, and a magnetic phase transition with the
characteristics of antiferromagnetism at ~29 K. Comparison to the Curie Weiss
theta (theta,CW = -135 K) indicates that the geometric frustration is largely
relieved by the anisotropy. A ferromagnetic contribution to the magnetization
below the ordering temperature and negative magnetization in zero-field cooled
measurements at low fields are attributed to uncompensated spins at grain
boundaries or defects.Comment: 9 pages, 5 figure
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