Discoveries of “Hidden” Topologies Occurring Intrinsically Within a Multiferroic Matrix

Single-phase multiferroics intertwine ferroelectric and ferromagnetic properties, providing novel ways to manipulate data and store information, as well as providing opportunities for exploring new chemistry and physics. In practice, due to the fundamental contra-indication between ferroelectricity (empty <i>d</i>⁰ electronic structures) and ferromagnetism (occupied <i>d</i>ⁿ electronic structures), single-phase multiferroics are exceedingly scarce. In recent years, we reported the design of such a novel room temperature magnetoelectric multiferroic material that could ideally be suited to future fabrication of revolutionary memory devices. To achieve this design, we took the approach of creating a new layered Aurivillius composition, Bi₆Ti_xFe_yMn_zO₁₈ (B6TFMO; x = 2.80 to 3.04; Y = 1.32 to 1.52; Z = 0.54 to 0.64), which combines differing types of <i>A</i>-site (Bi³⁺) and <i>B</i>-site (Ti⁴⁺, Fe³⁺, Mn^3+/4+) cations, to drive both ferroelectricity and ferromagnetism within the same structural phase.<br/>While this is a rare breakthrough on its own, in this presentation I will describe how we have lately uncovered remarkable polar topological structures close to regions where the B6TFMO layering is disrupted by naturally occurring structural defects. Here, the internal elastic strain and electrostatic energy gradients are altered to give rise to an inhomogeneous polarisation distribution of polar magnitude and polar rotation angle. Atomic resolution scanning transmission electron microscopy, in conjunction with polar displacement mapping, reveals nominally charged ferroelectric domain walls and non-trivial continuous rotations of ferroelectric polarisation into exotic polar vortex structures. These emergent topological structures hitherto lay “hidden”, previously undiscovered within multiferroic B6TFMO, however they occur intrinsically, without the need for an artificial interface and in the absence of external strain engineering.<br/>These distinctive types of 2<i>D</i> topologies are intriguing entities in matter, because they can offer spatially confined emergent functional properties, such as electrical conductivity within an otherwise insulating matrix. The internal characteristic length scales of these polar vortex domain walls (~5 nm) are much more compact than that of their magnetic counterparts (~10 to 100 nm), signifying potential to transcend the limits of classical data storage.