Competition between magnetic interactions in multilayers stabilizes complex, vertically twisted topological spin textures.

Antiferromagnetic thin films are currently generating considerable excitement for low dissipation magnonics and spintronics. However, while tuneable antiferromagnetic textures form the backbone of functional devices, they are virtually unknown at the submicron scale. Here we image a wide variety of antiferromagnetic spin textures in multiferroic BiFeO₃ thin films that can be tuned by strain and manipulated by electric fields through room-temperature magnetoelectric coupling. Using piezoresponse force microscopy and scanning NV magnetometry in self-organized ferroelectric patterns of BiFeO₃, we reveal how strain stabilizes different types of non-collinear antiferromagnetic states (bulk-like and exotic spin cycloids) as well as collinear antiferromagnetic textures. Beyond these local-scale observations, resonant elastic X-ray scattering confirms the existence of both types of spin cycloids. Finally, we show that electric-field control of the ferroelectric landscape induces transitions either between collinear and non-collinear states or between different cycloids, offering perspectives for the design of reconfigurable antiferromagnetic spin textures on demand.

Ti $K$ edge x-ray absorption near-edge structure (XANES) measurements across the pressure induced ferroelectric–paraelectric phase transition in $\mathrm{Pb}\mathrm{Ti}{\mathrm{O}}_3$, a prototypical ferroelectric perovskite, are reported. A quantitative analysis of the XANES allows us to obtain the relative local displacement, directly related to the local electric polarization, of Ti and Pb atoms under pressure. In particular, we found that above the critical pressure, in the so-called cubic-paraelectric phase, the Ti atoms remain locally displaced from their cubic site, indicating that this phase transition has, in a general way, an essential order-disorder component. However, the magnitude of these local displacements is strongly affected by the applied pressure as opposed to what happens as a function of temperature. These conclusions have important consequences in the long standing controversial debate on the interpretation of the ferroelectric phase transition of $\mathrm{Pb}\mathrm{Ti}{\mathrm{O}}_3$ and related family compounds.

Interactions between electrons and lattice vibrations are responsible for a wide range of material properties and applications. Recently, there has been considerable interest in the development of resonant inelastic x-ray scattering (RIXS) as a tool for measuring electron-phonon ($e$-ph) interactions. Here, we demonstrate the ability of RIXS to probe the interaction between phonons and specific electronic states both near to, and away from, the Fermi level. We perform carbon $K$-edge RIXS measurements on graphite, tuning the incident x-ray energy to separately probe the interactions of the ${\pi }^{*}$ and ${\sigma }^{*}$ electronic states. Our high-resolution data reveal detailed structure in the multiphonon RIXS features that directly encodes the momentum dependence of the $e$-ph interaction strength. We develop a Green’s-function method to model this structure, which naturally accounts for the phonon and interaction-strength dispersions, as well as the mixing of phonon momenta in the intermediate state. This model shows that the differences between the spectra can be fully explained by contrasting trends of the $e$-ph interaction through the Brillouin zone, being concentrated at the $\mathrm{\Gamma }$ and $K$ points for the ${\pi }^{*}$ states while being significant at all momenta for the ${\sigma }^{*}$ states. Our results advance the interpretation of phonon excitations in RIXS and extend its applicability as a probe of $e$-ph interactions to a new range of out-of-equilibrium situations.