335 research outputs found
Low temperature dielectric anomalies in HoMnO_3: The complex phase diagram
The dielectric constant of multiferroic hexagonal HoMnO_3 exhibits an
unprecedented diversity of anomalies at low temperatures (1.8 K< T <10 K) and
under external magnetic fields related to magnetic phase transitions in the
coupled system of Ho moments, Mn spins, and ferroelectric polarization. The
derived phase diagram is far more complex than previously assumed including
reentrant phases, phase transitions with distinct thermal and field hysteresis,
as well as several multicritical points. Magnetoelastic interactions introduce
lattice anomalies at the magnetic phase transitions. The re-evaluation of the
T-H phase diagram of HoMnO_3 is demanded.Comment: 12 pages, 3 figure
Strong spin-lattice coupling in multiferroic HoMnO: Thermal expansion anomalies and pressure effect
Evidence for a strong spin-lattice coupling in multiferroic HoMnO_3 is
derived from thermal expansion measurements along a- and c-axis. The
magnetoelastic effect results in sizable anomalies of the thermal expansivities
at the antiferromagnetic (T_N) and the spin rotation (T_{SR}) transition
temperatures as well as in a negative c-axis expansivity below room
temperature. The coupling between magnetic orders and dielectric properties
below T_N is explained by the lattice strain induced by the magnetoelastic
effect. At T_{SR} various physical quantities show discontinuities that are
thermodynamically consistent with a first order phase transition
Pressure-Temperature Phase Diagram of Multiferroic
The pressure-temperature phase diagram of multiferroic is
investigated for hydrostatic pressures up to 2 GPa. The stability range of the
ferroelectric phase associated with the incommensurate helical spin order is
reduced by pressure and ferroelectricity is completely suppressed at the
critical pressure of 1.64 GPa at 6.2 K. Thermal expansion measurements at
ambient pressure show strong step-like anomalies of the lattice parameters
associated with the lock-in transition into the commensurate paraelectric
phase. The expansion anomalies are highly anisotropic, the related volume
change is consistent with the high-pressure phase diagram
Structural Anomalies at the Magnetic and Ferroelectric Transitions in (R=Tb, Dy, Ho)
Strong anomalies of the thermal expansion coefficients at the magnetic and
ferroelectric transitions have been detected in multiferroic . Their
correlation with anomalies of the specific heat and the dielectric constant is
discussed. The results provide evidence for the magnetic origin of the
ferroelectricity mediated by strong spin-lattice coupling in the compounds.
Neutron scattering data for indicate a spin reorientation at the
two low-temperature phase transitions
The effect of transverse magnetic correlations on a coupled order parameter: shifted transition temperatures and thermal hysteresis
We use a Green's function method with Random Phase Approximation to show how
magnetic correlations may affect electric polarization in multiferroic
materials with magnetic-exchange-type magnetoelectric coupling. We use a model
spin 1/2 ferromagnetic ferroelectric system but our results are expected to
apply to multiferroic materials with more complex magnetic structures. In
particular, we find that transverse magnetic correlations result in a change in
the free energy of the ferroelectric solutions leading to the possibility for
thermal hysteresis of the electric polarization above the magnetic Curie
temperature. Although we are motivated by multiferroic materials, this problem
represents a more general calculation of the effect of fluctuations on coupled
order parameters
Robust Ferroelectric State in Multiferroic MnZnWO
We report the remarkably robust ferroelectric state in the multiferroic
compound MnZnWO. The substitution of the magnetic Mn
with nonmagnetic Zn reduces the magnetic exchange and provides control
of the various magnetic and multiferroic states of MnWO. Only 5 % of Zn
substitution results in a complete suppression of the frustrated collinear
(paraelectric) low temperature phase. The helical magnetic and ferroelectric
phase develops as the ground state. The multiferroic state is stable up to a
high level of substitution of more than 50 %. The magnetic, thermodynamic, and
dielectric properties as well as the ferroelectric polarization of single
crystals of MnZnWO are studied for different substitutions up
to x=0.5. The magnetic phases have been identified in single crystal neutron
scattering experiments. The ferroelectric polarization scales with the neutron
intensity of the incommensurate peak of the helical phase.Comment: 6 pages, 8 figure
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