108 research outputs found
Novel Field-Induced Phases in HoMnO3 at Low Temperatures
The novel field-induced re-entrant phase in multiferroic hexagonal HoMnO3 is
investigated to lower temperatures by dc magnetization, ac susceptibility, and
specific heat measurements at various magnetic fields. Two new phases have been
unambiguously identified below the Neel transition temperature, TN=76 K, for
magnetic fields up to 50 kOe. The existence of an intermediate phase between
the P[6]_3[c]m and P[6]_3c[m] magnetic structures (previously predicted from
dielectric measurements) was confirmed and the magnetic properties of this
phase have been investigated. At low temperatures (T<5 K) a dome shaped phase
boundary characterized by a magnetization jump and a narrow heat capacity peak
was detected between the magnetic fields of 5 kOe and 18 kOe. The transition
across this phase boundary is of first order and the magnetization and entropy
jumps obey the magnetic analogue of the Clausius-Clapeyron relation. Four of
the five low-temperature phases coexist at a tetracritical point at 2 K and 18
kOe. The complex magnetic phase diagram so derived provides an informative
basis for unraveling the underlying driving forces for the occurrence of the
various phases and the coupling between the different orders.Comment: 14 pages, 14 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
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
Giant Magnetoelectric Effect in a Multiferroic Material with a High Ferroelectric Transition Temperature
We present a unique example of giant magnetoelectric effect in a conventional
multiferroic HoMnO3, where polarization is very large (~56 mC/m2) and the
ferroelectric transition temperature is higher than the magnetic ordering
temperature by an order. We attribute the uniqueness of the giant
magnetoelectric effect to the ferroelectricity induced entirely by the
off-center displacement of rare earth ions with large magnetic moments. This
finding suggests a new avenue to design multiferroics with large polarization
and higher ferroelectric transition temperature as well as large
magnetoelectric effects
A Low Temperature Nonlinear Optical Rotational Anisotropy Spectrometer for the Determination of Crystallographic and Electronic Symmetries
Nonlinear optical generation from a crystalline material can reveal the
symmetries of both its lattice structure and underlying ordered electronic
phases and can therefore be exploited as a complementary technique to
diffraction based scattering probes. Although this technique has been
successfully used to study the lattice and magnetic structures of systems such
as semiconductor surfaces, multiferroic crystals, magnetic thin films and
multilayers, challenging technical requirements have prevented its application
to the plethora of complex electronic phases found in strongly correlated
electron systems. These requirements include an ability to probe small bulk
single crystals at the micron length scale, a need for sensitivity to the
entire nonlinear optical susceptibility tensor, oblique light incidence
reflection geometry and incident light frequency tunability among others. These
measurements are further complicated by the need for extreme sample
environments such as ultra low temperatures, high magnetic fields or high
pressures. In this review we present a novel experimental construction using a
rotating light scattering plane that meets all the aforementioned requirements.
We demonstrate the efficacy of our scheme by making symmetry measurements on a
micron scale facet of a small bulk single crystal of SrIrO using
optical second and third harmonic generation.Comment: 8 pages, 5 figure
Novel Nonreciprocal Acoustic Effects in Antiferromagnets
The possible occurrence of nonreciprocal acoustic effects in antiferromagnets
in the absence of an external magnetic field is investigated using both (i) a
microscopic formulation of the magnetoelastic interaction between spins and
phonons and (ii) symmetry arguments. We predict for certain antiferromagnets
the existence of two new nonreciprocal (non-time invariant) effects:
A boundary-condition induced nonreciprocal effect and the occurrence of
transversal phonon modes propagating in opposite directions having different
velocities. Estimates are given and possible materials for these effects to be
observed are suggested.Comment: Euro. Phys. Lett. (in press
Advanced resistivity model for arbitrary magnetization orientation applied to a series of compressive- to tensile-strained (Ga,Mn)As layers
The longitudinal and transverse resistivities of differently strained
(Ga,Mn)As layers are theoretically and experimentally studied as a function of
the magnetization orientation. The strain in the series of (Ga,Mn)As layers is
gradually varied from compressive to tensile using (In,Ga)As templates with
different In concentrations. Analytical expressions for the resistivities are
derived from a series expansion of the resistivity tensor with respect to the
direction cosines of the magnetization. In order to quantitatively model the
experimental data, terms up to the fourth order have to be included. The
expressions derived are generally valid for any single-crystalline cubic and
tetragonal ferromagnet and apply to arbitrary surface orientations and current
directions. The model phenomenologically incorporates the longitudinal and
transverse anisotropic magnetoresistance as well as the anomalous Hall effect.
The resistivity parameters obtained from a comparison between experiment and
theory are found to systematically vary with the strain in the layer.Comment: 14 pages, 11 figures, submitted to Phys. Rev.
Theory of Non-Reciprocal Optical Effects in Antiferromagnets: The Case Cr_2O_3
A microscopic model of non-reciprocal optical effects in antiferromagnets is
developed by considering the case of Cr_2O_3 where such effects have been
observed. These effects are due to a direct coupling between light and the
antiferromagnetic order parameter. This coupling is mediated by the spin-orbit
interaction and involves an interplay between the breaking of inversion
symmetry due to the antiferromagnetic order parameter and the trigonal field
contribution to the ligand field at the magnetic ion. We evaluate the matrix
elements relevant for the non-reciprocal second harmonic generation and
gyrotropic birefringence.Comment: accepted for publication in Phys. Rev.
Symmetry Analysis of Second Harmonic Generation at Surfaces of Antiferromagnets
Using group theory we classify the nonlinear magneto-optical response at
low-index surfaces of fcc antiferromagnets, such as NiO. Structures consisting
of one atomic layer are discussed in detail. We find that optical second
harmonic generation is sensitive to surface antiferromagnetism in many cases.
We discuss the influence of a second type of magnetic atoms, and also of a
possible oxygen sublattice distortion on the output signal. Finally, our
symmetry analysis yields the possibility of antiferromagnetic surface domain
imaging even in the presence of magnetic unit-cell doubling.Comment: 23 pages, 10 figures incorporated. Accepted to Phys. Rev. B,
scheduled for July'9
- …