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Observation of magnetic vortex pairs at room temperature in a planar α-Fe2O3/Co heterostructure

Abstract

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 nanoscale1,2,3, or when the parameter associated with the vorticity does not couple directly with strain4. Here, we observe an unprecedented form of vortices in antiferromagnetic haematite (α-Fe2O3) 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-Fe2O3 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.

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Fig. 1: Vector map of the α-Fe2O3 antiferromagnetic domain configuration from X-ray photoemission microscopy.
Fig. 2: Magnetic domain structure of α-Fe2O3/Co film.
Fig. 3: Topological defects in α-Fe2O3 and Co.
Fig. 4: Vortex annihilation with an ex-situ applied field.

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Acknowledgements

We acknowledge Diamond Light Source for time on Beam Line I06 under Proposals SI16338 and SI15088. We thank S. Parameswaran for discussions and T. Hesjedal and S. Zhang for assistance with initial film growth. The work done at the University of Oxford (F.P.C., N.W.P., R.D.J. and P.G.R.) is funded by EPSRC grant no. EP/M020517/1, entitled Oxford Quantum Materials Platform grant. The work at University of Wisconsin-Madison (J.S., J.I., M.S.R. and C.-B.E.) is supported by the Army Research Office through grant nos W911NF-13-1-0486 and W911NF-17-1-0462. R.D.J. acknowledges support from a Royal Society University Research Fellowship.

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F.P.C., N.W.P., R.D.J. and A.D.L. performed the experiment. F.P.C. and A.D.L. performed the data reduction. F.P.C and N.W.P. performed the data analysis. J.S grew the films. D.T.H. made the α-Fe2O3 sputtering target. J.S and F.P.C. characterized the epitaxial relation of the films. J.I. performed the MOKE measurement. G.v.L. performed calculations of the XMLD signal. N.W.P. performed the micromagnetic simulations. P.G.R conceived and designed the experiment and supervised the analysis together with R.D.J, while C.-B.E. supervised the film growth. M.S.R. supervised the MOKE measurement. P.G.R. and F.P.C. prepared the first draft of the manuscript. All authors discussed and contributed to the manuscript.

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Correspondence to C.-B. Eom or P. G. Radaelli.

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Supplementary Notes, Supplementary Figures 1–7, Supplementary References 1–3

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Chmiel, F.P., Waterfield Price, N., Johnson, R.D. et al. Observation of magnetic vortex pairs at room temperature in a planar α-Fe2O3/Co heterostructure. Nature Mater 17, 581–585 (2018). https://doi.org/10.1038/s41563-018-0101-x

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