Polarizing an antiferromagnet by optical engineering of the crystal field

Abstract

Strain engineering is widely used to manipulate the electronic and magnetic properties of complex materials. For example, the piezomagnetic effect provides an attractive route to control magnetism with strain. In this effect, the staggered spin structure of an antiferromagnet is decompensated by breaking the crystal field symmetry, which induces a ferrimagnetic polarization. Piezomagnetism is especially appealing because, unlike magnetostriction, it couples strain and magnetization at linear order, and allows for bi-directional control suitable for memory and spintronics applications. However, its use in functional devices has so far been hindered by the slow speed and large uniaxial strains required. Here we show that the essential features of piezomagnetism can be reproduced with optical phonons alone, which can be driven by light to large amplitudes without changing the volume and hence beyond the elastic limits of the material. We exploit nonlinear, three-phonon mixing to induce the desired crystal field distortions in the antiferromagnet CoF2. Through this effect, we generate a ferrimagnetic moment of 0.2 μB per unit cell, nearly three orders of magnitude larger than achieved with mechanical strain.

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Fig. 1: Piezomagnetic effect in CoF2.
Fig. 2: Breaking symmetry with phonons.
Fig. 3: Driving degenerate infrared phonons in CoF2.
Fig. 4: Time-resolved magneto-optical measurements.
Fig. 5: Characterization of the pump-induced state.
Fig. 6: Pump-induced magnetization dynamics.

Data availability

Source data are available for Figs. 46. All other data that support the plots within this paper and other findings of this study are available from the corresponding author on reasonable request.

Change history

  • 30 June 2020

    An amendment to this paper has been published and can be accessed via a link at the top of the paper.

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Acknowledgements

We thank J. Chen for help preparing the samples and assistance with the optical experiment. This work received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP7/2007–2013)/ERC (grant agreement no. 319286 (QMAC)) and the Cluster of Excellence ‘CUI: Advanced Imaging of Matter’ of the Deutsche Forschungsgemeinschaft (DFG), EXC 2056, project ID 390715994. Work done at the University of Oxford was funded by EPSRC grant no. EP/M020517/1, entitled Oxford Quantum Materials Platform Grant. A.S.D. was supported by a fellowship from the Alexander von Humboldt Foundation.

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Contributions

P.G.R. and A.C. planned the project together with A.S.D. and M. Först. A.S.D. designed and performed the experiments with help from B.L., T.F.N. and M. Först. A.S.D. analysed the experimental data. M. Fechner carried out the first-principles calculations. M. Fechner, A.S.D. and P.G.R. developed the phenomenological model. D.P. prepared the samples. A.S.D. and A.C. wrote the manuscript with feedback from all co-authors.

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Correspondence to Ankit S. Disa or Andrea Cavalleri.

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Supplementary information

Supplementary Information

Supplementary Information, Figs. 1–6, Tables 1–3 and refs. 1–25.

Source data

Source Data Fig. 4

Data for measured Faraday rotation and circular dichroism plotted in Fig. 4.

Source Data Fig. 5

Data for measured temperature, field and frequency dependences plotted in Fig. 5.

Source Data Fig. 6

Data for induced magnetization from experiment and theory plotted in Fig. 6.

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Disa, A.S., Fechner, M., Nova, T.F. et al. Polarizing an antiferromagnet by optical engineering of the crystal field. Nat. Phys. (2020). https://doi.org/10.1038/s41567-020-0936-3

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