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.

In the field of antiferromagnetic (AFM) spintronics, information about the Néel vector, AFM domain sizes, and spin-flop fields is a prerequisite for device applications but is not available easily. We have investigated AFM domains and spin-flop induced changes of domain patterns in Mn 2 Au(001) epitaxial thin films by X-ray magnetic linear dichroism photoemission electron microscopy (XMLD-PEEM) using magnetic fields up to 70 T. As-prepared Mn 2 Au films exhibit AFM domains with an average size ≤1 µm. Application of a 30 T field, exceeding the spin-flop field, along a magnetocrystalline easy axis dramatically increases the AFM domain size with Néel vectors perpendicular to the applied field direction. The width of Néel type domain walls (DW) is below the spatial resolution of the PEEM and therefore can only be estimated from an analysis of the DW profile to be smaller than 80 nm. Furthermore, using the values for the DW width and the spin-flop field, we evaluate an in-plane anisotropy constant ranging between 1 and 17 µeV/f.u.. arXiv:1803.03022v1 [cond-mat.mtrl-sci]

We report on the exchange coupling and magnetic properties of a strained ultrathin CoO/PtFe double-layer with perpendicular magnetic anisotropy. The cobalt oxide growth by reactive molecular beam epitaxy on a Pt-terminated PtFe/Pt(001) surface gives rise to a hexagonal surface and a monoclinic distorted CoO 3nm film at room temperature. This distorted ultrathin CoO layer couples with the PtFe(001) layer establishing a robust perpendicular exchange bias shift. Soft x-ray absorption spectroscopy provides a full description of the spin orientations in the CoO/PtFe doublelayer. The exchange bias shift is preserved up to the Néel antiferromagnetic ordering temperature of TN =293 K. This unique example of selfsame value for blocking and ordering temperatures, yet identical to the bulk ordering temperature, is likely related to the original strain induced distortion and strengthened interaction between the two well-ordered spin layers.PACS numbers: 75.50.Ee,75.25.+z, 78.70.Dm The conception and optimization of tuned devices for spintronic applications 1 stir up a great interest in the exchange coupling between antiferromagnetic (AFM) and ferromagnetic (FM) layered materials 2-5 and, particularly, in the unidirectional anisotropy effect known as exchange bias (EB) 6 . The AFM/FM exchange coupling relies on a variety of microscopic and atomic parameters, as crystallographic order, surface morphology, strain effects, spin orientation and competing anisotropies 5 . The EB effect is largely used to pin the FM magnetization along one orientation in a spin valve or magnetic tunnel junction [1][2][3][4][5] . It also provides greatest opportunities to explore phenomena interlinking the spin and charge degrees of freedom 2 and, more recently, to control the electronic transport in a tunneling anisotropic magnetoresistance device 3 . In the latter the tunneling resistance is strongly affected by the orientation of the magnetic moments in the AFM layer, which can be partially rotated by the exchange coupling with the FM layer. Devices showing EB perpendicular to the layered surface are especially promising for low power consumption and ultrafast circuits, as well as for high-performance memories 4,7 .Ultrathin CoO films number among the most interesting AFM layers for spintronic devices. At room temperature (RT), bulk CoO paramagnetic phase crystallizes in the rocksalt structure where pure Co and O planes alternate along the [111] axis ( fig.1). It has a Néel temperature (T N ) of 293 K and a magnetic moment of 3.98 µ B 8,9 . The magnetic moment lies far above the 3 µ B value, revealing a large orbital contribution. The strong interaction between spin and orbital magnetic moments through the spin-orbit coupling drives the magnetic anisotropy energy 10 . Below T N , the AFM ordering develops concomitantly with a monoclinic distorted phase 9 . The AFM structure is described as a stacking of FM hexagonal sheets of high spin Co 2+ ions coupled antiferromagnetically along the [111] direction. The spin structure is found collinear, ...