Vortices, occurring whenever a flow field 'whirls' around a one-dimensional core, are among the simplest topological structures, ubiquitous to many branches of physics. In the crystalline state, vortex formation is rare, since it is generally hampered by long-range interactions: in ferroic materials (ferromagnetic and ferroelectric), vortices are observed only when the effects of the dipole-dipole interaction are modified by confinement at the nanoscale, or when the parameter associated with the vorticity does not couple directly with strain . Here, we observe an unprecedented form of vortices in antiferromagnetic haematite (α-FeO) epitaxial films, in which the primary whirling parameter is the staggered magnetization. Remarkably, ferromagnetic topological objects with the same vorticity and winding number as the α-FeO vortices are imprinted onto an ultra-thin Co ferromagnetic over-layer by interfacial exchange. Our data suggest that the ferromagnetic vortices may be merons (half-skyrmions, carrying an out-of plane core magnetization), and indicate that the vortex/meron pairs can be manipulated by the application of an in-plane magnetic field, giving rise to large-scale vortex-antivortex annihilation.

The physical properties of epitaxial films can fundamentally differ from those of bulk single crystals even above the critical thickness. By a combination of non-resonant x-ray magnetic scattering, neutron diffraction and vector-mapped x-ray magnetic linear dichroism photoemission electron microscopy, we show that epitaxial (111)-BiFeO 3 films support sub-micron antiferromagnetic domains, which are magneto-elastically coupled to a coherent crystallographic monoclinic twin structure. This unique texture, which is absent in bulk single crystals, should enable control of magnetism in BiFeO 3 film devices via epitaxial strain.PACS numbers: 77.55. Nv, 68.37.Yz, 75.60.Ch, Electrical manipulation of spins in insulators is a promising route to a new generation of fast, low consumption electronics [1][2][3]. Although direct electrical control of ferromagnets is challenging, much progress has been made towards electrical switching of antiferromagnetic spins [4]. Multiferroic BiFeO 3 (BFO) is one of the most promising materials: at room temperature, BFO is both ferroelectric and antiferromagnetic, and its spins can be rotated by switching the direction of the electrical polarisation [5,6]. Thus far, a fundamental limitation towards practical BFO devices has been the lack of understanding of the interplay between ferroelectricity, ferromagnetism and lattice distortions (ferroelasticity). Using a combination of non-resonant x-ray magnetic scattering (NXMS), neutron diffraction and vectormapped x-ray magnetic linear dichroism photoemission electron microscopy (XMLD-PEEM), we show that the antiferromagnetic domain structure of 1 µm thick, epitaxial (111)-oriented BFO films displays a ≈100 nm-scale texture, dramatically different from the mm-size features in bulk single crystals. We also demonstrate that this magnetic texture is coherent (having matching topography and symmetry elements) with a pattern of monoclinic domains at the nanoscale. This texture is reminiscent of the dense polydomain states that are thermodynamically stable in ferroelectric perovskites such as PbTiO 3 in the presence of strain misfit [7]. This strongly suggests that the relaxed (111)-oriented BFO structure is not trigonal, but is a texture of coherent monoclinic micro twins. Besides providing a new pathway towards strainengineering multiferroic domains in BFO, our approach yielded a detailed picture of the interplay between magnetism and lattice over 5 orders of magnitude in length * p.g.radaelli@physics.ox.ac.uk scales, and could be applied to many classes of functional magnetic oxide devices.Below its ferroelectric Curie temperature of T C = 1103 K, bulk BFO is generally believed to possess a rhombohedrally-distorted perovskite structure with space group R3c (pictured in fig. 1(a) and (b)) [8,9], although very recent high-resolution synchrotron measurements have suggested a small monoclinic distortion [10]. The Bi 3+ and Fe 3+ cations are displaced away from their centrosymmetric positions along the (111) (pseudocubic setting) axis [11], producing a ...

The presence of domains in ferroic materials can negatively affect their macroscopic properties and hence their usefulness in device applications. From an experimental perspective, measuring materials comprising multiple domains can complicate the interpretation of material properties and their underlying mechanisms. In general, BiFeO3 films tend to grow with multiple magnetic domains and often contain multiple ferroelectric and ferroelastic domain variants. By growing (111)-oriented BiFeO3 films on an orthorhombic TbScO3 substrate, we are able to overcome this, and, by exploiting the magnetoelastic coupling between the magnetic and crystal structures, bias the growth of a given magnetic-, ferroelectric-, and structural-domain film. We further demonstrate the coupling of the magnetic structure to the ferroelectric polarization by showing the magnetic polarity in this domain is inverted upon 180 • ferroelectric switching.

Using micromagnetic simulations, we have modelled the formation of imprinted merons and antimerons in cobalt overlayers of different thickness (1-8 nm), stabilised by interfacial exchange with antiferromagnetic vortices in α-Fe2O3. Structures similar to those observed experimentally could be obtained with reasonable exchange parameters, also in the presence of surface roughness. We produce simulated meron/antimeron images by magnetic force microscopy (MFM) and nitrogen-vacancy (N-V) centre microscopy, and established signatures of these topological structures in different experimental configurations. arXiv:1912.07511v3 [cond-mat.str-el]