Professor Paolo G. Radaelli OSI

Abstract:

In the quest for post-CMOS (complementary metal–oxide–semiconductor) technologies, driven by the need for improved efficiency and performance, topologically protected ferromagnetic ‘whirls’ such as skyrmions1,2,3,4,5,6,7,8 and their anti-particles have shown great promise as solitonic information carriers in racetrack memory-in-logic or neuromorphic devices1,9,10,11. However, the presence of dipolar fields in ferromagnets, which restricts the formation of ultrasmall topological textures3,6,8,9,12, and the deleterious skyrmion Hall effect, when skyrmions are driven by spin torques9,10,12, have thus far inhibited their practical implementation. Antiferromagnetic analogues, which are predicted to demonstrate relativistic dynamics, fast deflection-free motion and size scaling, have recently become the subject of intense focus9,13,14,15,16,17,18,19, but they have yet to be experimentally demonstrated in natural antiferromagnetic systems. Here we realize a family of topological antiferromagnetic spin textures in α-Fe2O3—an Earth-abundant oxide insulator—capped with a platinum overlayer. By exploiting a first-order analogue of the Kibble–Zurek mechanism20,21, we stabilize exotic merons and antimerons (half-skyrmions)8 and their pairs (bimerons)16,22, which can be erased by magnetic fields and regenerated by temperature cycling. These structures have characteristic sizes of the order of 100 nanometres and can be chemically controlled via precise tuning of the exchange and anisotropy, with pathways through which further scaling may be achieved. Driven by current-based spin torques from the heavy-metal overlayer, some of these antiferromagnetic textures could emerge as prime candidates for low-energy antiferromagnetic spintronics at room temperature1,9,10,11,23.

Abstract:

Mixed-halide perovskites offer bandgap tunability essential for multijunction solar cells; however, a detrimental halide segregation under light is often observed. Here we combine simultaneous in situ photoluminescence and X-ray diffraction measurements to demonstrate clear differences in compositional and optoelectronic changes associated with halide segregation in MAPb(Br_0.5I_0.5)₃ and FA_0.83Cs_0.17Pb(Br_0.4I_0.6)₃ films. We report evidence for low-barrier ionic pathways in MAPb(Br_0.5I_0.5)₃, which allow for the rearrangement of halide ions in localized volumes of perovskite without significant compositional changes to the bulk material. In contrast, FA_0.83Cs_0.17Pb(Br_0.4I_0.6)₃ lacks such low-barrier ionic pathways and is, consequently, more stable against halide segregation. However, under prolonged illumination, it exhibits a considerable ionic rearrangement throughout the bulk material, which may be triggered by an initial demixing of A-site cations, altering the composition of the bulk perovskite and reducing its stability against halide segregation. Our work elucidates links between composition, ionic pathways, and halide segregation, and it facilitates the future engineering of phase-stable mixed-halide perovskites.

Abstract:

The datasets included herein contain experimental results (X-ray absorption, dichroic photoemission electron microscopy, diffraction, magnetometry etc.) and related analysis for the investigation of antiferromagnetic topological textures. The processes used in the obtaining, reducing and analysing the datasets can be found in the Methods and Supplementary Information sections of the published manuscript.