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Antiferromagnetic half-skyrmions and bimerons at room temperature

Abstract

In the quest for post-CMOS (complementary metal–oxide–semiconductor) technologies, driven by the need for improved efficiency and performance, topologically protected ferromagnetic ‘whirls’ such as skyrmions1,2,3,4,5,6,7,8 and their anti-particles have shown great promise as solitonic information carriers in racetrack memory-in-logic or neuromorphic devices1,9,10,11. However, the presence of dipolar fields in ferromagnets, which restricts the formation of ultrasmall topological textures3,6,8,9,12, and the deleterious skyrmion Hall effect, when skyrmions are driven by spin torques9,10,12, have thus far inhibited their practical implementation. Antiferromagnetic analogues, which are predicted to demonstrate relativistic dynamics, fast deflection-free motion and size scaling, have recently become the subject of intense focus9,13,14,15,16,17,18,19, but they have yet to be experimentally demonstrated in natural antiferromagnetic systems. Here we realize a family of topological antiferromagnetic spin textures in α-Fe2O3—an Earth-abundant oxide insulator—capped with a platinum overlayer. By exploiting a first-order analogue of the Kibble–Zurek mechanism20,21, we stabilize exotic merons and antimerons (half-skyrmions)8 and their pairs (bimerons)16,22, which can be erased by magnetic fields and regenerated by temperature cycling. These structures have characteristic sizes of the order of 100 nanometres and can be chemically controlled via precise tuning of the exchange and anisotropy, with pathways through which further scaling may be achieved. Driven by current-based spin torques from the heavy-metal overlayer, some of these antiferromagnetic textures could emerge as prime candidates for low-energy antiferromagnetic spintronics at room temperature1,9,10,11,23.

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Fig. 1: Temperature evolution of AFM textures across TM in α-Fe2O3.
Fig. 2: Real-space topological textures from IP Néel vector maps.
Fig. 3: Temperature evolution of AFM feature sizes.
Fig. 4: Erasing and recreating (anti)merons.

Data availability

The data supporting the findings of this study are available within the article and its Supplementary Information files and can also be accessed at the repository site https://doi.org/10.5287/bodleian:GP57d0Z47.

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Acknowledgements

We thank S. Parameswaran for discussions, F. P. Chmiel for guidance with data reduction, and R. D. Johnson for assistance with experiments. We acknowledge the Diamond Light Source for time on Beam Line I06 under proposals MM23857 and S120317. The work done at the University of Oxford (H.J., J.-C.L., J.C., J.H. and P.G.R.) is funded by EPSRC grant no. EP/M2020517/1 (Quantum Materials Platform Grant). The work at the National University of Singapore (H.J., S.P., A.A. and T.V.) is supported by the National Research Foundation (NRF) under the Competitive Research Program (NRF2015NRF-CRP001-015) and we also acknowledge SSLS for time on Beam Line XDD, which is under NRF. A.A. thanks the Agency for Science, Technology and Research (A*STAR) under its Advanced Manufacturing and Engineering (AME) Individual Research Grant (IRG) (A1983c0034) for financial support. J.C. acknowledges a full scholarship from the Chinese Scholarship Council (CSC) and support from the National University of Defense Technology (NUDT). The work at University of Wisconsin–Madison (J.S. and C.B.E.) is supported by the Army Research Office through grant W911NF-17-1-0462. C.B.E. acknowledges a Vannevar Bush Faculty Fellowship, funded by ONR (N00014-20-1-2844).

Author information

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Authors

Contributions

H.J. performed material optimization, thin-film growth by PLD, and structural and magnetic characterization. H.J., J.-C.L., J.H., F.M. performed PEEM experiments. J.C., J.-C.L., H.J. performed data reduction and analysis. J.-C.L., S.P. performed overlayer growth and surface characterization. J.-C.L. assisted in magnetic characterization. J.S. prepared preliminary sputter-grown samples under the supervision of C.-B.E. A.A. and T.V. supervised the PLD film growth and characterization. H.J., under the guidance of P.G.R., prepared the theoretical model. P.G.R. and H.J. conceived the project and supervised the analysis. H.J. and P.G.R. prepared the first draft of the manuscript. All authors discussed and contributed to the manuscript.

Corresponding authors

Correspondence to Hariom Jani, T. Venkatesan or Paolo G. Radaelli.

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The authors declare no competing interests.

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Peer review information Nature thanks Shinichiro Seki, Lucia Aballe, 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 Fig. 1 Characterization of epitaxial thin films.

a, HR-XRD 2θ − ω scan. The positions of the α-Fe2O3 film and α-Al2O3 substrate Bragg peaks are indicated. b, Rocking curve of the α-Fe2O3 film for the (0006) Bragg peak. c, XRR curve of the α-Fe2O3 film (black dots) and fit (red line) indicating a thickness of ~25.5 ± 0.6 nm. Doped films have similar HR-XRD, XRR scans and rocking curves. d, XAS spectra with LH- and LV-polarized X-rays, performed below the Morin transition. e, XAS spectra with LH-polarized X-rays, performed below and above the Morin transition. Spectra collected above TM result from averaging of many IP domains. f, Standard magnetic dichroism (XMLD,LH−LV) obtained by subtracting the LV curve from the LH curve when T < TM (OOP spins). g, Alternative form of magnetic dichroism (XMLD||avg−,LH), akin to other studies in the literature31,42. This was obtained by subtracting the LH curve collected when T < TM (OOP spins) from its counterpart when T > TM (IP spins). The absorption and dichroism results for our films are in good agreement with the α-Fe2O3 literature41,42. The energy positions (E1, E2) at which PEEM images give maximum AFM contrast are also indicated. See Supplementary Information section 1 for more details on the symmetry analysis and notations. a.u., arbitrary units.

Extended Data Fig. 2 Field-dependent magnetometry of α-Fe2O3 thin films.

a, b, MH loops collected on films without a metallic overlayer (a) and with a Pt overlayer (b), demonstrating that Pt does not alter the magnetic properties. The magnetic hysteresis loop is due to the presence of a very weak canted moment above TM, owing to bulk DMI, as is well known37,38.

Extended Data Fig. 3 Temperature evolution of AFM textures in α-Fe1.97Rh0.03O3 on warming.

a, Magnetometry data of α-Fe1.97Rh0.03O3 exhibiting the Morin transition at room temperature. Insets show the anisotropy reversal in the magnetic sublattices between OOP and IP orientations. bg, Evolution of the AFM textures as observed in LV-PEEM, at the α-Fe1.97Rh0.03O3–Pt interface, while warming across TM. Below TM (bd), the principal AFM features are OOP AFM domains (purple domains) separated by ADWs (yellow boundaries). Upon warming, ADWs widen and merge with IP domains nucleating nearby (de), until the IP domains themselves become the matrix (fg), above TM. As observed in pure α-Fe2O3 (Figs. 1, 2), the OOP bubbles (purple dots) get trapped in the IP matrix (yellow regions) when found at topologically non-trivial merger points of six-fold IP domains, resulting in the formation of merons and antimerons at room temperature. The grey region in (bg) corresponds to the defect on the sample surface. Scale bars are 1 μm.

Extended Data Fig. 4 Temperature evolution of AFM textures in α-Fe2O3 on cooling.

a–f, AFM textures observed in LV-PEEM at the α-Fe2O3–Pt interface upon cooling across TM. This temperature sequence was conducted before the warming run shown in Fig. 1. As expected, the evolution of the AFM textures is reversed, evolving from a predominantly IP state (yellow regions) with (anti)merons (purple dots) at high temperatures (ac), through the intermediate state (d) to an OOP state (purple domains) separated by ADWs (yellow boundaries) at low temperatures (ef). Comparing images at nearby temperatures during warming and cooling scans, for instance b with Fig. 1e, or d with Fig. 1c, demonstrates the hysteretic nature of the Morin transition. Grey regions in af correspond to the defect on the sample surface. Scale bars are 1 μm. Overall thermal evolution is shown in Supplementary Video 1.

Extended Data Fig. 5 XMCD-PEEM measurements.

a, b, Circular dichroism PEEM images at the α-Fe2O3–Pt interface in different regions at room temperature. The spatial positions for the two images are the same as in Fig. 2 and Extended Data Fig. 6, respectively. There is no notable ferromagnetic contrast in these images. Grey regions correspond to the defects on the sample surface. Scale bars are 1 μm.

Extended Data Fig. 6 Reproduction of AFM topological textures and the role of pinning.

a, We constructed a large-area image of the AFM textures at the α-Fe2O3–Pt interface after in situ rewarming across the Morin transition. This confirms the reproduction and random regeneration of the AFM OOP cores and thereby (anti)merons across the sample, upon performing a Kibble–Zurek transition. The image was prepared by combining four square images (each with sides ~10 μm) about the central defect indicated in grey. Scale bar is 3 μm. bd, LV-PEEM images taken at the same spatial position as in Fig. 1 after three different cooling and rewarming cycles across TM. The red and green ellipses encircle OOP cores in c and d that are close to the original positions from their respective previous thermal cycles, that is, b and c, respectively. The black ellipses encircle OOP cores that remain permanently pinned at the original positions over multiple cycles (bd). Scale bars in bd are 1 μm. In the overall distribution, we notice that about 85% of the textures are located at different positions in the three images, confirming that they evolve randomly, whereas the remaining ~15% are permanently pinned at the same positions (in black ellipses). Moreover, there is some cycle-to-cycle spatial reproduction for ~40% of the textures (in red or green ellipses). It should be noted that pinning could also manifest as change of local magnetic parameters (for example, anisotropy, exchange)50,51,52,53 resulting in possible deviation of experimental (anti)meron core sizes in comparison to our theoretical calculations in Fig. 3.

Extended Data Fig. 7 Schematic representations of AFM antiphase domain walls (ADWs).

a, b, AFM ADWs contain spins rotating from OOP (left) → IP (centre) → OOP (right). Two layers with opposite sublattice magnetizations are depicted with Néel-type (a) and Bloch-type (b) characters. Their respective widths, WN and WB, are defined in Supplementary Information section 2.

Extended Data Fig. 8 Schematic representations of AFM Néel (anti)merons and bimerons.

Two layers with opposite sublattice magnetizations are depicted. Blue and red colours indicate the OOP spin directions near the cores (blue, down; red, up). Topological charges and winding numbers are indicated as (Qw). The effect of the time-reversal operation is the same as swapping the sublattices, leading to a sign change of Q but not of w (Methods). ab, Variants of AFM Néel antimerons with winding number w = −1 and topological charges Q = ±1/2. a and b are related by reversal of core polarization. cd, Variants of AFM Néel merons with winding number w = +1 and topological charges Q = ±1/2. ef, Magnified views of the corresponding AFM textures seen in Fig. 2c, with double-headed arrows indicating the sense of rotation of spins away from the core. The regions shown correspond to an antimeron (e) and a meron (f), and clearly demonstrate that merons and antimerons are distinguishable. By contrast, a and b and their time-reversed counterparts are indistinguishable by our XMLD-PEEM, and so are c and d and their time-reversed counterparts. g, AFM Néel bimeron with topological numbers Q = +1, w = 0 obtained by merging b with d. The additive property of topological numbers is evident, because, for example, a Néel bimeron with Q = −1 can be constructed by combining a with c. h, Merging a with d results in a topologically trivial meron pair (TTMP).

Extended Data Fig. 9 Schematic representations of AFM Bloch (anti)merons and bimerons.

Colours and legend are as in Extended Data Fig. 8. ab, Variants of AFM Bloch antimerons with winding number w = −1 and topological charges Q = ±1/2, which are interrelated as in Extended Data Fig. 8. cd, Variants of AFM Bloch merons with winding number w = +1 and topological charges Q = ±1/2. ef, Magnified views of the AFM textures in Fig. 2c, with double-headed arrows indicating the sense of rotation of spins away from the core. The regions shown correspond to an antimeron (e) and a meron (f). Again, XMLD-PEEM enables us to distinguish merons from antimerons, but topological variants that only differ by the sign of Q are indistinguishable. g, AFM Bloch bimeron with topological numbers Q = +1, w = 0 obtained by merging b with d. Once again, the additive property of topological numbers applies. h, Merging a with d results in a TTMP. It should be noted that the overall AFM backgrounds in the bimerons are uniform.

Supplementary information

Supplementary Information

This file contains Supplementary Sections 1-4 and Supplementary Figures S1-S3.

Video 1

Temperature evolution of AFM textures in α-Fe2O3 as seen in LV-PEEM images, during the cooling and warming sequences across the Morin transition. All images were collected in the same location and orientation.

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Jani, H., Lin, JC., Chen, J. et al. Antiferromagnetic half-skyrmions and bimerons at room temperature. Nature 590, 74–79 (2021). https://doi.org/10.1038/s41586-021-03219-6

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Further reading

  • Reversible hydrogen control of antiferromagnetic anisotropy in α-Fe2O3

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    • , Elke Arenholz
    • , Paolo G. Radaelli
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