7 research outputs found
Unconventional order-disorder phase transition in improper ferroelectric hexagonal manganites
The improper ferroelectricity in YMnO and other related multiferroic
hexagonal manganites are known to cause topologically protected ferroelectric
domains that give rise to rich and diverse physical phenomena. The local
structure and structural coherence across the ferroelectric transition,
however, were previously not well understood. Here we reveal the evolution of
the local structure with temperature in YMnO using neutron total scattering
techniques, and interpret them with the help of first-principles calculations.
The results show that, at room temperature, the local and average structures
are consistent with the established ferroelectric symmetry. On
heating, both local and average structural analyses show striking anomalies
from K up to the Curie temperature consistent with increasing
fluctuations of the order parameter angle. These fluctuations result in an
unusual local symmetry lowering into a \textit{continuum of structures} on
heating. This local symmetry breaking persists into the high-symmetry non-polar
phase, constituting an unconventional type of order-disorder transition.Comment: 10 pages, 5 figure
Charged domain walls in improper ferroelectric hexagonal manganites and gallates
Ferroelectric domain walls are attracting broad attention as atomic-scale
switches, diodes and mobile wires for next-generation nanoelectronics. Charged
domain walls in improper ferroelectrics are particularly interesting as they
offer multifunctional properties and an inherent stability not found in proper
ferroelectrics. Here we study the energetics and structure of charged walls in
improper ferroelectric YMnO, InMnO and YGaO by first principles
calculations and phenomenological modeling. Positively and negatively charged
walls are asymmetric in terms of local structure and width, reflecting that
polarization is not the driving force for domain formation. The wall width
scales with the amplitude of the primary structural order parameter and the
coupling strength to the polarization. We introduce general rules for how to
engineer - and -type domain wall conductivity based on the domain size,
polarization and electronic band gap. This opens the possibility of fine-tuning
the local transport properties and design --junctions for domain
wall-based nano-circuitry.Comment: 10 pages, 6 figures, Supp. Info. available on reques
Electrical half-wave rectification at ferroelectric domain walls
Ferroelectric domain walls represent multifunctional 2D-elements with great
potential for novel device paradigms at the nanoscale. Improper ferroelectrics
display particularly promising types of domain walls, which, due to their
unique robustness, are the ideal template for imposing specific electronic
behavior. Chemical doping, for instance, induces p- or n-type characteristics
and electric fields reversibly switch between resistive and conductive
domain-wall states. Here, we demonstrate diode-like conversion of
alternating-current (AC) into direct-current (DC) output based on neutral
180 domain walls in improper ferroelectric ErMnO. By combining
scanning probe and dielectric spectroscopy, we show that the rectification
occurs for frequencies at which the domain walls are fixed to their equilibrium
position. The practical frequency regime and magnitude of the output is
controlled by the bulk conductivity. Using density functional theory we
attribute the transport behavior at the neutral walls to an accumulation of
oxygen defects. Our study reveals domain walls acting as 2D half-wave
rectifiers, extending domain-wall-based nanoelectronic applications into the
realm of AC technology
Direct observation of spin correlations in an artificial triangular lattice Ising spin system with grazing-incidence small-angle neutron scattering
The triangular lattice with Ising magnetic moments is an archetypical example of geometric frustration. In the case of dipolar-coupled out-of-plane moments, the geometric frustration results in a disordered classical spin-liquid state at higher temperatures while the system is predicted to transition to an anti-ferromagnetic stripe ground state at low temperatures. In this work we fabricate artificial triangular Ising spin systems without and with uniaxial in-plane compression to tune the nature and temperature of the correlations. We probe the energy scale and nature of magnetic correlations by grazing-incidence small-angle neutron scattering. In particular, we apply a newly-developed empirical structure-factor model to describe the measured short-range correlated spin-liquid state, and find good agreement with theoretical predictions. We demonstrate that grazing-incidence neutron scattering on our high-quality samples, in conjunction with detailed modeling of the scattering using the Distorted Wave Born Approximation, can be used to experimentally quantify the spin-liquid-like correlations in highly-frustrated artificial spin systems.ISSN:2055-6756ISSN:2055-676