88 research outputs found
Polar and magnetic order in GaV4Se8
In the present work, we provide results from specific heat, magnetic
susceptibility, dielectric constant, ac conductivity, and electrical
polarization measurements performed on the lacunar spinel GaV4Se8. With
decreasing temperature, we observe a transition from the paraelectric and
paramagnetic cubic state into a polar, probably ferroelectric state at 42 K
followed by magnetic ordering at 18 K. The polar transition is likely driven by
the Jahn-Teller effect due to the degeneracy of the V4 cluster orbitals. The
excess polarization arising in the magnetic phase indicates considerable
magnetoelectric coupling. Overall, the behavior of GaV4Se8 in many respects is
similar to that of the skyrmion host GaV4S8, exhibiting a complex interplay of
orbital, spin, lattice, and polar degrees of freedom. However, its dielectric
behavior at the polar transition markedly differs from that of the Jahn-Teller
driven ferroelectric GeV4S8, which can be ascribed to the dissimilar electronic
structure of the Ge compound.Comment: 7 pages, 6 figures. Revised version according to suggestions of
referee
Structural, magnetic, electric, dielectric, and thermodynamic properties of multiferroic GeV4S8
The lacunar spinel GeV4S8 undergoes orbital and ferroelectric ordering at the
Jahn-Teller transition around 30 K and exhibits antiferromagnetic order below
about 14 K. In addition to this orbitally driven ferroelectricity, lacunar
spinels are an interesting material class, as the vanadium ions form V4
clusters representing stable molecular entities with a common electron
distribution and a well-defined level scheme of molecular states resulting in a
unique spin state per V4 molecule. Here we report detailed x-ray, magnetic
susceptibility, electrical resistivity, heat capacity, thermal expansion, and
dielectric results to characterize the structural, electric, dielectric,
magnetic, and thermodynamic properties of this interesting material, which also
exhibits strong electronic correlations. From the magnetic susceptibility, we
determine a negative Curie-Weiss temperature, indicative for antiferromagnetic
exchange and a paramagnetic moment close to a spin S = 1 of the V4 molecular
clusters. The low-temperature heat capacity provides experimental evidence for
gapped magnon excitations. From the entropy release, we conclude about strong
correlations between magnetic order and lattice distortions. In addition, the
observed anomalies at the phase transitions also indicate strong coupling
between structural and electronic degrees of freedom. Utilizing dielectric
spectroscopy, we find the onset of significant dispersion effects at the polar
Jahn-Teller transition. The dispersion becomes fully suppressed again with the
onset of spin order. In addition, the temperature dependencies of dielectric
constant and specific heat possibly indicate a sequential appearance of orbital
and polar order.Comment: 15 pages, 9 figure
Polar Dynamics at the Jahn-Teller Transition in Ferroelectric GaV4S8
We present a dielectric spectroscopy study of the polar dynamics linked to
the orbitally driven ferroelectric transition in the skyrmion host GaV4S8. By
combining THz and MHz-GHz spectroscopy techniques, we succeed in detecting the
relaxational dynamics arising from coupled orbital and polar fluctuations in
this material and traced its temperature dependence in the paraelectric as well
as in the ferroelectric phase. The relaxation time significantly increases when
approaching the critical temperature from both sides of the transition. It is
natural to assume that these polar fluctuations map the orbital dynamics at the
Jahn-Teller transition. Due to the first-order character of the
orbital-ordering transition, the relaxation time shows an enormous jump of
about five orders of magnitude at the polar and structural phase transition.Comment: 5 pages, 4 figure
Interplay of Spin and Lattice Degrees of Freedom in the Frustrated Antiferromagnet CdCr_2O_4: High-field and Temperature Induced Anomalies of the Elastic Constants
Temperature and magnetic field studies of the elastic constants of the
chromium spinel CdCr_2O_4 show pronounced anomalies related to strong
spin-phonon coupling in this frustrated antiferromagnet. A detailed comparison
of the longitudinal acoustic mode propagating along the [111] direction with
theory based on an exchange-striction mechanism leads to an estimate of the
strength of the magneto-elastic interaction. The derived spin-phonon coupling
constant is in good agreement with previous determinations based on infrared
absorption. Further insight is gained from intermediate and high magnetic field
experiments in the field regime of the magnetization plateau. The role of the
antisymmetric Dzyaloshinskii-Moriya interaction discussed and we compare the
spin-phonon coupling in CdCr_2O_4 in both the ordered and disordered states.Comment: 12 pages, 8 figures; Appendix added,To appear in Phys Rev.
Relaxation dynamics and colossal magnetocapacitive effect in CdCr2S4
A thorough investigation of the relaxational dynamics in the recently
discovered multiferroic CdCr2S4 showing a colossal magnetocapacitive effect has
been performed. Broadband dielectric measurements without and with external
magnetic fields up to 10 T provide clear evidence that the observed
magnetocapacitive effect stems from enormous changes of the relaxation dynamics
induced by the development of magnetic order.Comment: 4 pages, 4 figure
Nanoscale layering of antiferromagnetic and superconducting phases in Rb2Fe4Se5
We studied phase separation in a single-crystalline antiferromagnetic
superconductor Rb2Fe4Se5 (RFS) using a combination of scattering-type scanning
near-field optical microscopy (s-SNOM) and low-energy muon spin rotation
(LE-\mu SR). We demonstrate that the antiferromagnetic and superconducting
phases segregate into nanometer-thick layers perpendicular to the iron-selenide
planes, while the characteristic in-plane size of the metallic domains reaches
10 \mu m. By means of LE-\mu SR we further show that in a 40-nm thick surface
layer the ordered antiferromagnetic moment is drastically reduced, while the
volume fraction of the paramagnetic phase is significantly enhanced over its
bulk value. Self-organization into a quasiregular heterostructure indicates an
intimate connection between the modulated superconducting and antiferromagnetic
phases.Comment: 5 pages, 2 figures. Updated version published in Phys. Rev. Lett. on
5 July 201
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
Evidence for local lattice distortions in giant magnetocapacitive CdCr2S4
Raman scattering experiments on CdCr2S4 single crystals show pronounced
anomalies in intensity and frequency of optical phonon modes with an onset
temperature T*=130 K that coincides with the regime of giant magnetocapacitive
effects. A loss of inversion symmetry and Cr off-centering are deduced from the
observation of longitudinal optical and formerly infrared active modes for
T<T_c=84 K. The intensity anomalies are attributed to the enhanced electronic
polarizability of displacements that modulate the Cr-S distance and respective
hybridization. Photo doping leads to an annihilation of the symmetry reduction.
Our scenario of multiferroic effects is based on the near degeneracy of polar
and nonpolar modes and the additional low energy scale due to hybridization.Comment: 4 pages, 6 figure
Multiferroicity and skyrmions carrying electric polarization in GaV4S8
Skyrmions are whirl like topological spin objects with high potential for
future magnetic data storage. It is a fundamental question, relevant for both
basic research and application, if a ferroelectric (FE) polarization can be
associated with their magnetic texture and if these objects can be manipulated
by electric fields. Here, we study the interplay between magnetism and electric
polarization in the lacunar spinel GaV4S8, which undergoes a structural
transition associated with orbital ordering at 44 K and reveals a complex
magnetic phase diagram below 13 K, including a ferromagnetic (FM), cycloidal,
and N\'eel-type skyrmion lattice (SkL) phase. We found that the orbitally
ordered phase of GaV4S8 is FE with a sizable polarization of ~1 {\mu}C/cm2.
Moreover, we observed spin-driven excess polarizations in all magnetic phases
and, hence, GaV4S8 hosts three different multiferroic phases with coexisting
polar and magnetic order. These include the SkL phase where we predict a strong
spatial modulation of the FE polarization close to the skyrmion cores. By
taking into account the crystal symmetry and spin patterns of the magnetically
ordered phases, we identify the exchange striction as the main microscopic
mechanism behind the spin-driven FE polarization in each multiferroic phase.
Since GaV4S8 is unique among the known SkL host materials due to its polar
crystal structure and the observed strong magnetoelectric effect, this study is
an important step towards the non-dissipative electric-field control of
skyrmions.Comment: 17 pages, 7 figures + 2 pages, 4 figures in Supplementary Materials.
Revised version as accepted for publication in Science Advance
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