Observation of magnetic vortex pairs at room temperature in a planar
α-Fe2O3/Co heterostructure
Nature Materials Nature Publishing Group
Authors:
FP Chmiel, NW Price, RD Johnson, AD Lamirand, J Schad, GVD Laan, DT Harris, J Irwin, MS Rzchowski, C-B Eom, PG Radaelli
Abstract:
Vortices are among the simplest topological structures, and occur whenever a
flow field `whirls' around a one-dimensional core. They are ubiquitous to many
branches of physics, from fluid dynamics to superconductivity and
superfluidity, and are even predicted by some unified theories of particle
interactions, where they might explain some of the largest-scale structures
seen in today's Universe. 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 only observed when the effects
of the dipole-dipole interaction is modified by confinement at the nanoscale,
or when the parameter associated with the vorticity does not couple directly
with strain. Here, we present the discovery of a novel form of vortices in
antiferromagnetic (AFM) hematite ($\alpha$-Fe$_2$O$_3$) epitaxial films, in
which the primary whirling parameter is the staggered magnetisation.
Remarkably, ferromagnetic (FM) topological objects with the same vorticity and
winding number of the $\alpha$-Fe$_2$O$_3$ 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 magnetisation), and indicate that the vortex/meron pairs
can be manipulated by the application of an in-plane magnetic field,
H$_{\parallel}$, giving rise to large-scale vortex-antivortex annihilation.
