Real-space imaging of non-collinear antiferromagnetic order with a single-spin magnetometer

Abstract

Although ferromagnets have many applications, their large magnetization and the resulting energy cost for switching magnetic moments bring into question their suitability for reliable low-power spintronic devices. Non-collinear antiferromagnetic systems do not suffer from this problem, and often have extra functionalities: non-collinear spin order1 may break space-inversion symmetry2,3 and thus allow electric-field control of magnetism4,5, or may produce emergent spin–orbit effects6 that enable efficient spin–charge interconversion7. To harness these traits for next-generation spintronics, the nanoscale control and imaging capabilities that are now routine for ferromagnets must be developed for antiferromagnetic systems. Here, using a non-invasive, scanning single-spin magnetometer based on a nitrogen–vacancy defect in diamond8,9,10, we demonstrate real-space visualization of non-collinear antiferromagnetic order in a magnetic thin film at room temperature. We image the spin cycloid of a multiferroic bismuth ferrite (BiFeO3) thin film and extract a period of about 70 nanometres, consistent with values determined by macroscopic diffraction11,12. In addition, we take advantage of the magnetoelectric coupling present in BiFeO3 to manipulate the cycloid propagation direction by an electric field. Besides highlighting the potential of nitrogen–vacancy magnetometry for imaging complex antiferromagnetic orders at the nanoscale, these results demonstrate how BiFeO3 can be used in the design of reconfigurable nanoscale spin textures.

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Figure 1: Ferroelectric and magnetic order in BiFeO3.
Figure 2: Mapping the magnetic texture of BFO with NV magnetometry.
Figure 3: Electrical control of the spin cycloid.
Figure 4: Quantitative analysis of the spin cycloid magnetic texture.

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Acknowledgements

We thank J. P. Tetienne and T. Hingant for experimental assistance in the early stages of the project. We are grateful to J. M. D. Coey for discussions. This research was supported by the European Research Council (ERC-StG-2014, IMAGINE), the European Union Seventh Framework Program (FP7/2007-2013) under the project DIADEMS, and by the French Agence Nationale de la Recherche (ANR) through project FERROMON and PIAF. This work is supported by a public grant overseen by the French National Research Agency (ANR) as part of the ‘Investissements d’Avenir’ program (Labex NanoSaclay, reference: ANR-10-LABX-0035).

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Contributions

I.G., W.A., L.J.M. and S.C. performed the NV magnetometry experiments; I.G., W.A., L.J.M. and V.J. analysed the data and performed magnetic modelling with assistance from M.V.; K.G. and C.C. fabricated the BFO sample; V.G. and S.F. performed the structural analysis and the piezoresponse force microscopy experiments; P.A. and P.M. engineered diamond tips hosting single NV defects; I.G., W.A., V.G., S.F., M.B. and V.J. wrote the manuscript. All authors contributed to the interpretation of the data and commented on the manuscript.

Corresponding author

Correspondence to V. Jacques.

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Reviewer Information Nature thanks J. Mundy and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Figure 1 Structural properties of the magnetic thin film.

a, The surface topography of the 6 μm × 6 μm 32-nm-thick BiFeO3 (BFO) thin film grown on a DyScO3 substrate, showing single-unit-cell atomic steps. b, X-ray diffraction ω–2θ pattern of the same film displays only (00l) peaks for BFO and DyScO3 (in monoclinic notation). D (in red colour) and B (in blue colour) subscripts stand for DyScO3 and BiFeO3, respectively. c, Zoom along the (001) peak of DyScO3, showing clear Laue fringes.

Extended Data Figure 2 Reciprocal space mappings (RSMs) of the 32-nm-thick BFO film grown on SrRuO3/DyScO3.

Shown are RSMs around a, (002)D, b, (004)D, c, , d, (013)D, e, , f, (113)D, g, and h, (103)D planes of DyScO3. All the planes are indexed in monoclinic notation and the subscripts D and B correspond to DyScO3 and BFO, respectively. Two different domains can be identified for monoclinic BFO (green and blue). Qx,y and Qz indicate the in-plane and out-of-plane reciprocal space units, respectively.

Extended Data Figure 3 Determination of polarization variants in BFO thin films.

a, Local out-of-plane PFM hysteresis loop with bias voltage. d33 is the out-of-plane piezoelectric coefficient. b, Homogeneous out-of-plane PFM phase corresponding to polarization variants pointing downward in a 6 μm × 6 μm area. c, In-plane PFM phase and d, amplitude for the cantilever parallel to [100]c. e, Sketch of the PFM cantilever and the four possible in-plane variants of polarization in BFO. f, Sketch of the [110]c direction of the cantilever, with the corresponding in-plane PFM amplitude (g) and phase (h). i, Sketch of the direction of the cantilever with the corresponding in-plane PFM amplitude (j) and phase (k). All the images in c to k have been acquired in the same 3 μm × 3 μm area.

Extended Data Figure 4 Measurement of the probe-to-sample distance.

a, The scanning-NV magnetometer (‘diamond tip’) is used to measure the magnetic field (grey arrows) produced at the edges of an uniformly magnetized ferromagnetic wire (blue arrows). b, Typical Zeeman-shift profile measured by scanning the NV defect across the edges of a 500-nm-wide wire of Pt/Co(0.6nm)/AlOx with perpendicular magnetic anisotropy. The circles are experimental data and the red solid line is data fitting from which distance d is extracted24,40. We note that only the absolute value of the magnetic field is measured in this experiment.

Extended Data Figure 5 Schematic of the geometry used for the stray field calculation.

The thickness, t, of the film is divided into N monolayers of thickness a. The blue plane represents the observation plane at a distance d from the BFO film. (x, y, z) and (x′, y′, z′) represent, respectively, the coordinates of the observation point and the magnetic moment with respect to the laboratory frame. The red dashed lines indicate the remaining monolayers in the film that are not illustrated.

Extended Data Figure 6 Data fitting and uncertainty analysis.

a, Magnetic field distribution reproduced from Fig. 4a of the main text. b, The black symbols are the experimental data and the coloured solid curve is the result of a two-dimensional fit using equation (17) with d = 49 nm, meff = 0.07μB, λ = 70 nm, a = 0.396 nm, t = 32 nm and (θ, ϕ) = (128°, 80°). The linecut shown in Fig. 4c of the main text corresponds to the white dashed line in a. c, Summary of the relative uncertainties on the fitting parameter mDM for the six parameters pi = {λ, meff, t, d, θ, ϕ} (see Methods for details).

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Gross, I., Akhtar, W., Garcia, V. et al. Real-space imaging of non-collinear antiferromagnetic order with a single-spin magnetometer. Nature 549, 252–256 (2017). https://doi.org/10.1038/nature23656

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