66 research outputs found
Direct transition from a disordered to a multiferroic phase on a triangular lattice
Competing interactions and geometric frustration provide favourable
conditions for exotic states of matter. Such competition often causes multiple
phase transitions as a function of temperature and can lead to magnetic
structures that break inversion symmetry, thereby inducing ferroelectricity
[1-4]. Although this phenomenon is understood phenomenologically [3-4], it is
of great interest to have a conceptually simpler system in which
ferroelectricity appears coincident with a single magnetic phase transition.
Here we report the first such direct transition from a paramagnetic and
paraelectric phase to an incommensurate multiferroic in the triangular lattice
antiferromagnet RbFe(MoO4)2 (RFMO). A magnetic field extinguishes the electric
polarization when the symmetry of the magnetic order changes and
ferroelectricity is only observed when the magnetic structure has chirality and
breaks inversion symmetry. Multiferroic behaviour in RFMO provides a
theoretically tractable example of ferroelectricity from competing spin
interactions. A Landau expansion of symmetry-allowed terms in the free energy
demonstrates that the chiral magnetic order of the triangular lattice
antiferromagnet gives rise to a pseudoelectric field, whose temperature
dependence agrees with that observed experimentally.Comment: 16 pages pdf including 3 figure
Successive Magnetic Transitions of the Kagome Staircase Compound Co3V2O8 Studied in Various Magnetic Fields
For the spin-3/2 kagome staircase system Co3V2O8, magnetic field
(H)-temperature (T) phase diagrams have been constructed for the fields along
three principal directions up to 5 T, using results of various macroscopic
measurements on single crystal samples and also using neutron diffraction data
taken on both powder and single crystal samples under H along c. In zero
magnetic field, the system exhibits three transitions at temperatures Tc1~11.2
K, Tc2~8.8 K and Tc3~ (6.0-7.0) K. The single crystal data present clear
evidence for the noncollinear nature of the magnetic structures in all
magnetically ordered phases below Tc1. The sinusoidal nature of the
incommensurate modulation of the ordered moment reported in the former work has
been confirmed between Tc1 and Tc2, that is, no higher harmonics of the
modulation have been detected even for the present large single crystal. Even
in the phase of commensurate modulation between Tc2 and Tc3, we have not
detected any higher harmonics of the modulation. The phase diagrams show that
the magnetically ordered phases sensitively change to other phases with H,
indicating that the geometrical frustration inherent in this system is
important for the determination of the phase diagram. No evidence for
ferroelectric transitions has been observed in the measurements of the
dielectric constant applying the electric fields along three crystallographic
axes, a, b and c. Only small dielectric anomalies closely connected with the
magnetic phase transitions have been found.Comment: 5 pages, 10 figures, submitted to JPS
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 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
Complex magnetic order in the kagome staircase compound Co3V2O8
Co3V2O8 (CVO) has a geometrically frustrated magnetic lattice, a Kagome
staircase. The crystal structure consists of two inequivalent Co sites,
one-dimensional chains of Co(2) spine sites, linked by Co(1) cross-tie sites.
Neutron powder diffraction has been used to solve the basic magnetic and
crystal structures of this system, while polarized and unpolarized single
crystal diffraction measurements have been used to reveal a variety of
incommensurate phases, interspersed with lock-in transitions to commensurate
phases. CVO initially orders magnetically at 11.3 K into an incommensurate,
transversely polarized, spin density wave state, with wave vector k=(0,delta,0)
with delta=0.55 and the spin direction along the a axis. Delta is found to
decrease monotonically with decreasing temperature, and then it locks into a
commensurate antiferromagnetic structure with delta=0.5 for 6.9<T<8.6 K. Below
6.9 K the magnetic structure becomes incommensurate again. Delta continues to
decrease with decreasing temperature, and locks-in again at delta=1/3 over a
narrow temperature range (6.2<T<6.5 K). The system then undergoes a strongly
first order transition to the ferromagnetic ground state (delta=0) at Tc=6.2 K.
A dielectric anomaly is observed around the ferromagnetic transition
temperature of 6.2 K, demonstrating a significant spin-charge coupling present
in CVO. A theory based on group theory analysis and a minimal Ising model with
competing exchange interactions can explain the basic features of the magnetic
ordering
Relationship between Magnetic Structure and Ferroelectricity of LiVCuO4
Neutron scattering studies and measurements of the dielectric susceptibility
and ferroelectric polarization P have been carried out in various magnetic
fields H for single-crystal samples of the multiferroic system LiVCuO4 with
quasi one-dimensional spin 1/2 Cu2+ chains formed of edge-sharing CuO4 square
planes, and the relationship between the magnetic structure and
ferroelectricity has been studied. The ferroelectric polarization is
significantly suppressed by the magnetic field H above 2 T applied along a and
b axes. The helical magnetic structure with the helical axis parallel to c has
been confirmed in H=0, and for H//a, the spin flop transition takes place at
H=2 T with increasing H, where the helical axis changes to the direction
parallel to H. The ferroelectric polarization along a at H=0 is found to be
proportional to the neutron magnetic scattering intensity, indicating that the
magnetic order is closely related to the appearance of the ferroelectricity.
The relationship between the magnetic structure and ferroelectricity of LiVCuO4
is discussed by considering the existing theories.Comment: 4 pages (5 figures), submitted to J. Phys. Soc. Jp
Magnetically driven ferroelectric order in Ni3V2O8
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
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