16 research outputs found
Ferroelectric Switching Pathways and Domain Structure of SrBi(Ta,Nb)O from First Principles
Several families of layered perovskite oxide ferroelectrics exhibit a
coupling between polarization and structural order parameters, such as
octahedral rotation distortions. This coupling provides opportunities for novel
electric field-based manipulation of material properties, and also stabilizes
complex domain patterns and domain wall vortices. Amongst layered perovskites
with such coupled orders, the Aurivillius-phase oxides SrBiO
(=Ta, Nb) are well-known for their excellent room temperature ferroelectric
performance. This work combines group theoretic analysis with density
functional theory calculations to examine the ferroelectric switching processes
of SrBiO. Low-energy two-step ferroelectric switching paths are
identified, with polarization reversal facilitated by structural order
parameter rotations. Analysis of the domain structure reveals how the relative
energetics of the coupled order parameters translates into a network of several
distinct domain wall types linked by domain wall vortex structures. Comparisons
are made between the ferroelectric switching and domain structure of
SrBiO and those of the layered =2 Ruddlesden-Popper hybrid
improper ferroelectrics. The results provide new insight into how ferroelectric
properties may be optimized by engineering the complex crystal structures of
Aurivllius-phase oxides
Charge order textures induced by non-linear lattice coupling in a half-doped manganite
The self-organization of strongly interacting electrons into superlattice
structures underlies the properties of many quantum materials. How these
electrons arrange within the superlattice dictates what symmetries are broken
and what ground states are stabilized. Here we show that cryogenic scanning
transmission electron microscopy enables direct mapping of local symmetries and
order at the intra-unit-cell level in the model charge-ordered system
NdSrMnO. In addition to imaging the prototypical
site-centered charge order, we discover the nanoscale coexistence of an exotic
intermediate state which mixes site and bond order and breaks inversion
symmetry. We further show that nonlinear coupling of distinct lattice modes
controls the selection between competing ground states. The results demonstrate
the importance of lattice coupling for understanding and manipulating the
character of electronic self-organization and highlight a novel method for
probing local order in a broad range of strongly correlated systems
Manipulation of spin orientation via ferroelectric switching in Fe-doped Bi2WO6 from first principles
Atomic-scale control of spins by electric fields is highly desirable for future technological applications. Magnetically doped Aurivillius-phase oxides present one route to achieve this, with magnetic ions substituted into the ferroelectric structure at dilute concentrations, resulting in spin-charge coupling. However, there has been minimal exploration of the ferroelectric switching pathways in this materials class, limiting predictions of the influence of an electric field on magnetic spins in the structure. Here, we determine the ferroelectric switching pathways of the end member of the Aurivillius phase family, Bi2WO6, using a combination of group theoretic analysis and density functional theory calculations. We find that in the ground state P21ab phase, a two-step switching pathway via C2 and Cm intermediate phases provides the lowest energy barrier. Considering iron substitutions on the W site in Bi2WO6, we determine the spin easy axis. By tracking the change in spin directionality during ferroelectric switching, we find that a 90∘ switch in the polarization direction leads to a 112° reorientation of the spin easy axis. The low-symmetry crystal-field environment of Bi2WO6 and magnetoelastic coupling on the magnetic dopant provide a route to spin control via an applied electric field