88 research outputs found
Experimental band structure of the nearly half-metallic CuCrSe: An optical and magneto-optical study
Diagonal and off-diagonal optical conductivity spectra have been determined
form the measured reflectivity and magneto-optical Kerr effect (MOKE) over a
broad range of photon energy in the itinerant ferromagnetic phase of
CuCrSe at various temperatures down to T=10 K. Besides the low-energy
metallic contribution and the lower-lying charge transfer transition at
2 eV, a sharp and distinct optical transition was observed in the
mid-infrared region around 0.5 eV with huge magneto-optical activity.
This excitation is attributed to a parity allowed transition through the Se-Cr
hybridization-induced gap in the majority spin channel. The large off-diagonal
conductivity is explained by the high spin polarization in the vicinity of the
Fermi level and the strong spin-orbit interaction for the related charge
carriers. The results are discussed in connection with band structure
calculations
Magnetic field control of cycloidal domains and electric polarization in multiferroic BiFeO
The magnetic field induced rearrangement of the cycloidal spin structure in
ferroelectric mono-domain single crystals of the room-temperature multiferroic
BiFeO is studied using small-angle neutron scattering (SANS). The cycloid
propagation vectors are observed to rotate when magnetic fields applied
perpendicular to the rhombohedral (polar) axis exceed a pinning threshold value
of 5\,T. In light of these experimental results, a phenomenological model
is proposed that captures the rearrangement of the cycloidal domains, and we
revisit the microscopic origin of the magnetoelectric effect. A new coupling
between the magnetic anisotropy and the polarization is proposed that explains
the recently discovered magnetoelectric polarization to the rhombohedral axis
Unidirectional terahertz light absorption in the pyroelectric ferrimagnet CaBaCo4O7
Spin excitations were studied by absorption spectroscopy in CaBaCo4O7 which
is a type-I multiferroic compound with the largest magnetic-order induced
ferroelectric polarization ({\Delta}P=17mC/m2) reported, so far. We observed
two optical magnon branches: a solely electric dipole allowed one and a mixed
magnetoelectric resonance. The entangled magnetization and polarization
dynamics of the magnetoelectric resonance gives rise to unidirectional light
absorption, i.e. that magnon mode absorbs the electromagnetic radiation for one
propagation direction but not for the opposite direction. Our systematic study
of the magnetic field and temperature dependence of magnon modes provides
information about the energies and symmetries of spin excitations, which is
required to develop a microscopic spin model of CaBaCo4O7.Comment: 5 pages, 4 figure
New type of ellipsometry in infrared spectroscopy: The double-reference method
We have developed a conceptually new type of ellipsometry which allows the
determination of the complex refractive index by simultaneously measuring the
unpolarized normal-incidence reflectivity relative to the vacuum and to another
reference media. From these two quantities the complex optical response can be
directly obtained without Kramers-Kronig transformation. Due to its
transparency and large refractive index over a broad range of the spectrum,
from the far-infrared to the soft ultraviolet region, diamond can be ideally
used as a second reference. The experimental arrangement is rather simple
compared to other ellipsometric techniques.Comment: submitted to Appl. Phys. Let
Optical conductivity in multiferroic GaVS and GeVS: Phonons and electronic transitions
We report on optical spectroscopy on the lacunar spinels GaVS and
GeVS in the spectral range from 100 to 23000 cm and for
temperatures from 5 to 300 K. These multiferroic spinel systems reveal
Jahn-Teller driven ferroelectricity and complex magnetic order at low
temperatures. We study the infrared-active phonon modes and the low-lying
electronic excitations in the cubic high-temperature phase, as well as in the
orbitally and in the magnetically ordered low-temperature phases. We compare
the phonon modes in these two compounds, which undergo different
symmetry-lowering Jahn-Teller transitions into ferroelectric and orbitally
ordered phases, and exhibit different magnetic ground states. We follow the
splitting of the phonon modes at the structural phase transition and detect
additional splittings at the onset of antiferromagnetic order in GeVS.
We observe electronic transitions within the -derived bands of the V
clusters and document a significant influence of the structural and magnetic
phase transitions on the narrow electronic band gaps.Comment: 12 pages, 10 figure
Recommendations for riparian ecosystem management based on the general frame defined in EUFORGEN and results from EUROPOP
International audienc
Characteristics of ferroelectric-ferroelastic domains in N{\'e}el-type skyrmion host GaVS
GaVS is a multiferroic semiconductor hosting N{\'e}el-type magnetic
skyrmions dressed with electric polarization. At T = 42K, the compound
undergoes a structural phase transition of weakly first-order, from a
non-centrosymmetric cubic phase at high temperatures to a polar rhombohedral
structure at low temperatures. Below T, ferroelectric domains are formed
with the electric polarization pointing along any of the four axes. Although in this material the size and the shape of the
ferroelectric-ferroelastic domains may act as important limiting factors in the
formation of the N{\'e}el-type skyrmion lattice emerging below T=13\:K, the
characteristics of polar domains in GaVS have not been studied yet.
Here, we report on the inspection of the local-scale ferroelectric domain
distribution in rhombohedral GaVS using low-temperature piezoresponse
force microscopy. We observed mechanically and electrically compatible lamellar
domain patterns, where the lamellae are aligned parallel to the (100)-type
planes with a typical spacing between 100 nm-1.2 m. We expect that the
control of ferroelectric domain size in polar skyrmion hosts can be exploited
for the spatial confinement and manupulation of N{\'e}el-type skyrmions
Anomalous Hall effect in (In,Mn)Sb dilute magnetic semiconductor
High magnetic field study of Hall resistivity in the ferromagnetic phase of
(In,Mn)Sb allows one to separate its normal and anomalous components. We show
that the anomalous Hall term is not proportional to the magnetization, and that
it even changes sign as a function of magnetic field. We also show that the
application of pressure modifies the scattering process, but does not influence
the Hall effect. These observations suggest that the anomalous Hall effect in
(In,Mn)Sb is an intrinsic property and support the application of the Berry
phase theory for (III,Mn)V semiconductors. We propose a phenomenological
description of the anomalous Hall conductivity, based on a field-dependent
relative shift of the heavy- and light-hole valence bands and the split-off
band
Disorder promotes ferromagnetism: Rounding of the quantum phase transition in Sr_{1-x}Ca_xRuO_3
The subtle interplay of randomness and quantum fluctuations at low
temperatures gives rise to a plethora of unconventional phenomena in systems
ranging from quantum magnets and correlated electron materials to ultracold
atomic gases. Particularly strong disorder effects have been predicted to occur
at zero-temperature quantum phase transitions. Here, we demonstrate that the
composition-driven ferromagnetic-to-paramagnetic quantum phase transition in
Sr1-xCaxRuO3 is completely destroyed by the disorder introduced via the
different ionic radii of the randomly distributed Sr and Ca ions. Using a
magneto-optical technique, we map the magnetic phase diagram in the
composition-temperature space. We find that the ferromagnetic phase is
significantly extended by the disorder and develops a pronounced tail over a
broad range of the composition x. These findings are explained by a microscopic
model of smeared quantum phase transitions in itinerant magnets. Moreover, our
theoretical study implies that correlated disorder is even more powerful in
promoting ferromagnetism than random disorder.Comment: 15 pages, 4 figures, submitted to Phys. Rev. Let
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