130 research outputs found
Asymmetric isolated skyrmions in polar magnets with easy-plane anisotropy
We introduce a new class of isolated magnetic skyrmions emerging within
tilted ferromagnetic phases of polar magnets with easy-plane anisotropy. The
asymmetric magnetic structure of these skyrmions is associated with an
intricate pattern of the energy density, which exhibits positive and negative
asymptotics with respect to the surrounding state with a ferromagnetic moment
tilted away from the polar axis. Correspondingly, the skyrmion-skyrmion
interaction has an anisotropic character and can be either attractive or
repulsive depending on the relative orientation of the skyrmion pair. We
investigate the stability of these novel asymmetric skyrmions against the
elliptical cone state and follow their transformation into axisymmetric
skyrmions, when the tilted ferromagnetic moment of the host phase is reduced.
Our theory gives clear directions for experimental studies of isolated
asymmetric skyrmions and their clusters embedded in tilted ferromagnetic
phases
Skyrmion robustness in non-centrosymmetric magnets with axial symmetry: The role of anisotropy and tilted magnetic fields
We investigate the stability of N\'eel skyrmions against tilted magnetic
fields, in polar magnets with uniaxial anisotropy ranging from easy-plane to
easy-axis type. We construct the corresponding phase diagrams and investigate
the internal structure of skewed skyrmions with displaced cores. We find that
moderate easy-plane anisotropy increases the stability range of N\'eel
skyrmions for fields along the symmetry axis, while moderate easy-axis
anisotropy enhances their robustness against tilted magnetic fields. We stress
that the direction, along which the skyrmion cores are shifted, depends on the
symmetry of the underlying crystal lattice. The cores of N\'eel skyrmions,
realized in polar magnets with C symmetry, are displaced either along or
opposite to the off-axis (in-plane) component of the magnetic field depending
on the rotation sense of the magnetization, dictated by the sign of the
Dzyaloshinskii constant. The core shift of antiskyrmions, present in chiral
magnets with D symmetry, depends on the in-plane orientation of the
magnetic field and can be parallel, anti-parallel, or perpendicular to it. We
argue that the role of anisotropy in magnets with axially symmetric crystal
structure is different from that in cubic helimagnets. Our results can be
applied to address recent experiments on polar magnets with C symmetry,
GaVS and GaVSe
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
Orbital-Order Driven Ferroelectricity and Dipolar Relaxation Dynamics in Multiferroic GaMoS
We present the results of broadband dielectric spectroscopy of GaMoS,
a lacunar spinel system that recently was shown to exhibit non-canonical,
orbitally-driven ferroelectricity. Our study reveals complex relaxation
dynamics of this multiferroic material, both above and below its Jahn-Teller
transition at T K. Above T, two types of
coupled dipolar-orbital dynamics seem to compete: relaxations within
cluster-like regions with short-range polar order like in relaxor
ferroelectrics and critical fluctuations of only weakly interacting dipoles,
the latter resembling the typical dynamics of order-disorder type
ferroelectrics. Below the Jahn-Teller transition, the onset of orbital order
drives the system into long-range ferroelectric order and dipolar dynamics
within the ferroelectric domains is observed. The coupled dipolar and orbital
relaxation behavior of GaMoS above the Jahn-Teller transition markedly
differs from that of the skyrmion host GaVS, which seems to be linked
to differences in the structural distortions of the two systems on the
unit-cell level.Comment: 6 pages, 3 figures + Supplemental Material (2 pages, 2 figures
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
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
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
Magnetic-field-induced transition in BaVS3
The metal-insulator transition (MIT) of BaVS3 is suppressed under pressure
and above the critical pressure of p~2GPa the metallic phase is stabilized. We
present the results of detailed magnetoresistivity measurements carried out at
pressures near the critical value, in magnetic fields up to B=12T. We found
that slightly below the critical pressure the structural tetramerization --
which drives the MIT -- is combined with the onset of magnetic correlations. If
the zero-field transition temperature is suppressed to a sufficiently low value
(T_MI<15K), the system can be driven into the metallic state by application of
magnetic field. The main effect is not the reduction of T_MI with increasing B,
but rather the broadening of the transition due to the applied magnetic field.
We tentatively ascribe this phenomenon to the influence on the magnetic
structure coupled to the bond-order of the tetramers.Comment: 5 pages, 5 figure
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
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