Antiferromagnetic materials are currently emerging as a new paradigm for spintronics as they offer key advantages over ferromagnets: insensitivity to external magnetic fields, much faster spin dynamics (THz range), and higher density packing because of the absence of stray fields. For skyrmionics, where information is encoded in topological spin structures, antiferromagnetic skyrmions would also benefit from the absence of gyrotropic forces, giving rise to fast and straight motions along the driving forces. While complex topological objects were recently discovered in intrinsic antiferromagnets, mastering their nucleation, stabilization, and manipulation with energy-efficient means remain to be demonstrated. As antiferromagnets are insensitive to external magnetic fields, one must find alternative ways to control them. Using magnetoelectric antiferromagnetic multiferroics one could achieve a low energy electric-field control of antiferromagnetism. Here we take advantage of the room-temperature magnetoelectric coupling in epitaxial thin films of multiferroic BiFeO₃ to deterministically control antiferromagnetic spin textures via the ferroelectric domains. Playing with the film orientation and epitaxial strain enables us to tailor the ferroelectric domain structure and finely tune the corresponding antiferromagnetic textures. In submicron devices based on BiFeO₃ thin films, we stabilize center divergent or center convergent polar states using a radial electric field. For films grown under compressive strain, we show that such polar textures contain flux closures of antiferromagnetic spin cycloids, with distinct antiferromagnetic entities at their cores depending on the electric-field polarity. For films grown under tensile strain, the center polar states include quadrant of canted-antiferromagnetic domains. We further discuss the different purely ferroelectric, antiferromagnetic or multiferroic domain boundaries embedded in such topological polar states. These results open the way for electrically-reconfigurable antiferromagnetic topological objects.

Integration of piezoelectrics with semiconductors opens to tune the electronic transport properties by strain. CuFeS2 is a tetragonal thermoelectric, low-bandgap semiconductor and collinear antiferromagnet (AFM) with a Néel temperature of 823 K. Here, density functional theory (DFT) calculations, utilizing GGA+U formalism and taking account of spin-orbit coupling, is used to explore possible piezoelectricity and polar phases emerging from breaking the bulk tetragonal symmetry of CuFeS2 by strain-engineering. Biaxial strain does not directly induce a piezoelectric state. However, shear strain distorts the structure into a monoclinic phase and the emergence of a possible polar state. The effect of shear plane on the polar state will be presented. The magnetic structure of CuFeS2 is altered under the application of strain and the occurrence of spin polarized planes perpendicular to the plane of applied strain will also be discussed.

The emerging field of spintronics has significant potential to reduce power consumption of computation while increasing memory and processing ability. At the heart of many proposed spintronic devices is the ability to control magnetization and magnetic textures in ferromagnetic (FM) and/or antiferromagnetic (AFM) components. AFMs are especially appealing for application in spintronics and sensing, owing to their lack of net magnetic fields making them more robust to disruption by external magnetic fields. Beyond AFMs, multiferroic materials, which can host multiple co-existing ferroic orderings – such as BiFeO3, which exhibits both ferroelectricity and antiferromagnetism – are another class of materials with potential application in spintronics. While the lack of net magnetic fields makes AFMs particularly appealing for developing robust spintronic devices, it strongly complicates their characterization. In addition, the spatially-varying magnetization in AFMs and multiferroics is inherently tied to the atomic structure with magnetic ordering periodicities on the order of a handful of unit-cells, necessitating near-atomic resolution.

When a converged probe is scanned across a thin sample, it acquires phase-shifts due to electrostatic and magnetic interactions with the sample which scatter the incident wavefunction whose intensity is collected for every probe position on a far-field pixelated detector. Simultaneously reconstructing the phase shifts due to electrostatic scattering and the much weaker magnetic scattering from a set of phase-less diffraction intensities is an inverse scattering problem. Ptychography is an iterative phase-retrieval technique which attempts to solve this inverse problem using the redundant information in the partially-overlapping illuminated regions. We have recently proposed a novel ptychographic algorithm which uses multiple sets of diffraction intensity measurements to jointly reconstruct the two scattering sources with physical regularizations at high-resolution. Here, we present experimental results applying the technique on soft X-ray Magnetic Circular Dichroism (XMCD) measurements on thin ferrimagnetic FeGd films, as-well as proof-of-concept simulations of electron tomography experiments on anti-ferromagnetic NiO nanoparticles. Finally, we discuss how the challenges in aligning the multiple measurements can be lifted in multiferroic thin films by flipping the anti-ferromagnetic domains in-situ using external magnetic and electric fields.

The three-dimensional domain structure in ferroelectric materials determines many of their physical and technological properties, including their electrostatic stability, coercive field and surface charge. The domain structure can be particularly complex in improper ferroelectrics such as the hexagonal manganites since the polarization is a slave to a non-ferroelectric primary order parameter that drives the domain formation. In antiferromagnetic YMnO3, for example, this leads to an unusual hexagonal vortex domain pattern with topologically protected domain walls, which have been shown to exhibit electrical conductivity and a net magnetic dipole moment at the sample surface. Characterizing the three-dimensional structure of these domains and domain walls has been elusive, however, due to a lack of suitable imaging techniques.

Here, we present a multi-Bragg coherent x-ray diffraction imaging (BCDI) determination of the domain walls and domain types in a single YMnO3 nanocrystal. By reconstructing high-resolution, three-dimensional images of the structure and the full strain tensor field, we resolve two ferroelectric domains separated by a domain wall and confirm that the primary atomic displacements occur along the crystallographic c-axis throughout the nanocrystal. By correlating the BCDI experiment with atomistic simulation, we are able to verify the "Mexican hat" symmetry model of domain formation in the hexagonal manganites, and establish that the two domains correspond to adjacent minima in the Mexican hat with opposite ferroelectric polarization and adjacent trimerization domains. Finally, using a circular mean comparison we show that for this sample the two domains correspond to a clockwise winding around the brim of the hat. Our results highlight the potential of multi-Bragg CDI combined with atomistic simulations for revealing and identifying ferroelectric domain structures.