8 research outputs found
Symmetry Conditions for Non-reciprocal Light Propagation in Magnetic Crystals
Recent studies demonstrated the violation of reciprocity in optical processes
in low-symmetry magnetic crystals. In these crystals the speed of light can be
different for counter-propagating beams. Correspondingly, they can show strong
directional anisotropies such as direction dependent absorption also called
directional dichroism[S. Bord\'acs et al., Nat. Phys. 8, 734 (2012); M. Saito
et al., J. Phys. Soc. Jpn. 77, 013705 (2008)]. Based on symmetry
considerations, we identify the magnetic point groups of materials which can
host such directional anisotropies and also provide a list of possible
candidate materials to observe these phenomena. In most of these cases, the
symmetry of the crystal allows directional anisotropy not only for optical
processes but also for the propagation of beams of particles and scalar waves.
We also predict new types of directional optical anisotropies -- besides the
optical magnetoelectric effect and the magnetochiral dichroism investigated so
far -- and specify the magnetic point groups of crystals where they can emerge.Comment: Overall revisio
Insulating improper ferroelectric domain walls as robust barrier layer capacitors
We report the dielectric properties of improper ferroelectric h-ErMnO.
From the bulk characterisation we observe a temperature and frequency range
with two distinct relaxation-like features, leading to high and even 'colossal'
values for the dielectric permittivity. One feature trivially originates from
the formation of a Schottky barrier at the electrode-sample interface, whereas
the second one relates to an internal barrier layer capacitance (BLC). The
calculated volume fraction of the internal BLC (of 8 %) is in good agreement
with the observed volume fraction of insulating domain walls (DWs). While it is
established that insulating DWs can give rise to high dielectric constants,
studies typically focused on proper ferroelectrics where electric fields can
remove the DWs. In h-ErMnO, by contrast, the insulating DWs are
topologically protected, facilitating operation under substantially higher
electric fields. Our findings provide the basis for a conceptually new approach
to engineer materials exhibiting colossal dielectric permittivities using
domain walls in improper ferroelecctrics with potential applications in
electroceramic capacitors.Comment: 7 pages, 4 figure
Disorder Promotes Ferromagnetism: Rounding of the Quantum Phase Transition in Sr₁₋ₓCaₓRuO₃
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
Macroscopic Manifestation of Domain-wall Magnetism and Magnetoelectric Effect in a N\'eel-type Skyrmion Host
We report a magnetic state in GaVSe which emerges exclusively in
samples with mesoscale polar domains and not in polar mono-domain crystals. Its
onset is accompanied with a sharp anomaly in the magnetic susceptibility and
the magnetic torque, distinct from other anomalies observed also in polar
mono-domain samples upon transitions between the cycloidal, the N\'eel-type
skyrmion lattice and the ferromagnetic states. We ascribe this additional
transition to the formation of magnetic textures localized at structural domain
walls, where the magnetic interactions change stepwise and spin textures with
different spiral planes, hosted by neighbouring domains, need to be matched. A
clear anomaly in the magneto-current indicates that the domain-wall-confined
magnetic states also have strong contributions to the magnetoelectric response.
We expect polar domain walls to commonly host such confined magnetic edge
states, especially in materials with long wavelength magnetic order
Magnetic order and sign of the Dzyaloshinskii-Moriya interaction in 2D antiferromagnet Ba2CoGe2O7 under applied magnetic field
The Dzyaloshinskii-Moriya interaction (DMI), that is the antisymmetric part of the exchange coupling tensor, favors the perpendicular arrangement of magnetic moment, thus, induces canting in otherwise collinear structures. The DMI is the prerequisite for the emergence of weak ferromagnetism in antiferromagnets, but can stabilize twisted magnetic textures, such as spin spirals, soliton lattices and magnetic skyrmions. While the magnitude of the DMI determines the canting angle of adjacent spins, its sign dictates the sense of the spin rotation. Based on a focused polarized neutron diffraction (PND) study, combined with symmetry analysis, we determine the sign of the DMI in the unconventional multiferroic Ba2CoGe2O7 and reveal its detailed magnetic structure in magnetic fields applied in the tetragonal plane. As PND gives unique access to the scattering contribution from the phase-sensitive nuclear-magnetic interference, it is a valuable tool for a straightforward DMI sign determination in bulk materials and allows to disclose even very weak magnetic moments. Remarkably, the sign of the DMI could be determined from the PND measurement of a single reflection which is demonstrated to be reliable for a large range of applied magnetic field directions and values