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Imaging the Néel vector switching in the monolayer antiferromagnet MnPSe3 with strain-controlled Ising order

Nature Nanotechnology volume 16, pages 782–787 (2021)Cite this article

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Abstract

Antiferromagnets are interesting materials for spintronics because of their faster dynamics and robustness against perturbations from magnetic fields. Control of the antiferromagnetic order constitutes an important step towards applications, but has been limited to bulk materials so far. Here, using spatially resolved second-harmonic generation, we show direct evidence of long-range antiferromagnetic order and Ising-type Néel vector switching in monolayer MnPSe3 with large XY anisotropy. In additional to thermally induced switching, uniaxial strain can rotate the Néel vector, aligning it to a general in-plane direction irrespective of the crystal axes. A change of the universality class of the phase transition in the XY model under uniaxial strain causes this emergence of strain-controlled Ising order in the XY magnet MnPSe3. Our discovery is a further ingredient for compact antiferromagnetic spintronic devices in the two-dimensional limit.

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Fig. 1: Characterization of bulk MnPSe3 samples.
Fig. 2: Ising-type Néel vector switching in thick MnPSe3 samples.
Fig. 3: SHG and Néel vector switching of atomically thin MnPSe3 samples exfoliated on SiO2/Si in a glove box.
Fig. 4: Strain-tunable Néel vector in MnPSe3.
Fig. 5: Renormalization group flow of the Landau theory.

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All data needed to evaluate the conclusions in the paper are present in the paper and the Supplementary Information. Additional data related to this paper could be requested from the authors.

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Acknowledgements

We thank S. W. Cheong and O. Tchernyshyov for helpful discussions. The project design, data collection and analysis, and preparation of the manuscript are supported by L.W.’s startup package at the University of Pennsylvania. The development of the SHG photon counter is supported by the ARO YIP award under grant W911NF1910342 to L.W. The measurement by atomic force microscopy is supported by the ARO MURI under grant W911NF2020166 to L.W. The acquisition of the oscillator laser for the SHG experiment is supported by the National Science Foundation through Penn MRSEC (DMR-1720530). E.J.M. acknowledges support from National Science Foundation EAGER 1838456. C.L.K is supported by a Simons Investigator grant from the Simons Foundation. D.G.M acknowledges support from the Gordon and Betty Moore Foundation’s EPiQS Initiative, grant GBMF9069. H.W. and X.Q. acknowledge support from National Science Foundation DMR-1753054 and the Texas A&M University President’s Excellence Fund X-Grants Program. B.X. and C.B. are supported by the Schweizerische Nationalfonds by grant no. 200020-172611. The density functional theory calculations were conducted with the advanced computing resources provided by Texas A&M High Performance Research Computing.

Author information

Authors and Affiliations

  1. Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, PA, USA

    Zhuoliang Ni, E. J. Mele, C. L. Kane & Liang Wu

  2. Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN, USA

    A. V. Haglund & D. G. Mandrus

  3. Department of Materials Science and Engineering, Texas A&M University, College Station, TX, USA

    H. Wang & X. Qian

  4. Department of Physics and Fribourg Center for Nanomaterials, University of Fribourg, Fribourg, Switzerland

    B. Xu & C. Bernhard

  5. Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA

    D. G. Mandrus

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  1. Zhuoliang Ni
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Contributions

L.W. conceived the project and coordinated the experiments and theoretical work. L.W. designed the SHG imaging set-up and built it with Z.N.; Z.N. performed the experiments and analysed the data under the supervision of L.W.; L.W., Z.N., E.J.M. and C.L.K. discussed and interpreted the data. E.J.M. performed the spin model calculation. C.L.K. performed the Landau theory calculation. A.V.H. and D.G.M. grew the crystals and performed the magnetization measurements. H.W. and X.Q. performed the first-principles calculation. B.X. and C.B. performed the optical conductivity measurement. L.W. and Z.N. wrote the manuscript with the input of all authors. All authors edited the manuscript.

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Correspondence to Liang Wu.

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Extended data

Extended Data Fig. 1 Thickness characterization of atomically thin MnPSe3 samples.

a, Optical image of a multilayer MnPSe3 exfoliated on the SiO2/Si substrate. Scale bar: 10 μm. b, Atomic force microscopy image of the same sample in a. The step between monolayer and bilayer is around 0.78 nm. Scale bar: 10 μm. c, Optical contrast of samples with different layer numbers. Green circles are data extracted from sample shown in a and orange circles are data from other samples. The black line is a linear fit.

Extended Data Fig. 2 SHG data for monolayer S2 exfoliated on SiO2/Si in a glove box.

a, Optical image of the monolayer S2. There are some small bilayer/trilayer islands inside the monolayer sample. Scale bar: 10 μm. The dashed region is the area for SHG mapping. b, SHG intensity mapping of the monolayer S2 at 5 K. c, Temperature dependence of SHG intensity measured at the green point marked in a for one thermal cycle. d-e, Polarization-dependent SHG patterns measured at the green point in (d) crossed and (e) parallel configuration after two different thermal cycles and at 5 K. Data from both domains are shown. The dots are experimental data and the solid lines are the best fit. The patterns are different from the monolayer S1, indicating the angles between the Néel vector and the crystalline axis are different in the two samples.

Extended Data Fig. 3 SHG data for monolayer S3 exfoliated on SiO2/Si in a glove box.

a, Optical image of the monolayer S3. Scale bar: 10 μm. The dashed box denotes the region of SHG mapping. b, SHG intensity mapping of the monolayer S3. Scale bar: 10 μm. c-d, Polar patterns at the two points denoted by purple and green dots are measured after thermal cycles. c, Crossed patterns of two domains of the purple point measured at 5 K after two different thermal cycles. d, Crossed patterns of two domains of the green point measured at 5 K after two different thermal cycles. Different crossed polar patterns also with different orientations at the purple and the green points indicate that their Néel vectors have different directions. e, Temperature dependence of SHG intensity at the purple point for one thermal cycle. f, Temperature dependence of SHG intensity at the green point for one thermal cycle.

Extended Data Fig. 4 More data of the monolayer S1 shown in the main text.

a, Optical image of the sample S1. Scale bar: 10 μm. The dashed region is the area for SHG mapping. b, SHG intensity mapping at ϕ = 120∘ (the peak of domain 1) in the crossed pattern. c, SHG intensity mapping at ϕ = 160∘ (the peak of domain 2) in the crossed pattern. Maps in b,c are after the same thermal cycle. The domain walls are highlighted by the green dashed lines. d-i, Crossed patterns of six different points marked by different colors in b after the same thermal cycle. P1-P5 are the same points in Fig. 3f in the main text. P6 is very close to sample corner and displays smaller SHG intensity with a slightly different orientation, which indicates the Néel vector at P6 has a slightly different direction. All the SHG measurements are performed at 5 K.

Extended Data Fig. 5 SHG data of atomically thin samples exfoliated on SiO2/Si in air.

a, Temperature dependence of square root of the SHG intensity in samples with different thickness. All of the samples in this figure are exfoliated in air. b, SHG intensity at 5 K as a function of layer numbers in a log-log plot. Data are shown in yellow dots. The solid line is a fit for I ∝ N2. c, Néel temperature as a function of layer numbers. Note that there is a large reduction of both SHG signal and Néel temperature of monolayer compared to those exfoliated in the glove box due to the aging effect. See more data on the aging effect on a bilayer sample in Supplementary Figure 10. d, SHG intensity mapping of a monolayer MnPSe3 exfoliated in air at 5 K. Scale bar: 3 μm. Inset: Optical image of the monolayer sample. f, Crossed polar pattern measured at the center of the monolayer sample shown in d at 5 K.

Extended Data Fig. 6 Néel vector switching of a bilayer sample exfoliated on SiO2/Si in air.

a, Optical image of the bilayer sample. Scale bar: 10 μm. b, SHG intensity mapping of the bilayer sample at 5 K. Note that the SHG intensity of this sample is quite uniform, which indicates that the Néel vector direction is nearly aligned. c, SHG intensity of 6 consecutive cooling runs across TN. The curves collapse into two, indicating the existence of two domains. d, Crossed and e, parallel polar patterns measured after each cooling at 5 K. The measured data are shown in dots. The blue and red shaded regions are guides for the eye, corresponding to the two different AFM domains.

Extended Data Fig. 7 Strain tunability of Néel vector of a trilayer sample on a PDMS substrate.

The strained sample is prepared using the same method as in the main text. The measured thickness is around 3.2 nm, which is a typical thickness for a trilayer sample. The measured Néel temperature is also consistent with a typical trilayer sample. a, Optical image of the sample exfoliated on the PDMS before adding strains. The trilayer sample is marked by the black box. Scale bar: 10 μm. b-c, Optical images of the sample when vertical and horizontal strain are added (marked by the red arrows), respectively. The sample and PDMS are mounted on the metal platform and the contrast of the sample is low. A 5% strain is added on the PDMS by a micro-manipulator. The directions of the cracks of the ~ 60-nm thick flake on the top also indicate the strain directions. d, SHG intensity mapping of the dashed area in a under different strain directions (marked by the red arrows) and ϕ in the crossed pattern. The dark intensity maps in d indicate that the Néel vectors are mainly along the strain direction, and rotated by around 90∘ when the strain directoin is swithced from vertical to horizontal. e, Crossed pattern at the center of the trilayer sample with vertical strain direction. f, Crossed pattern at the center of the trilayer sample with horizontal strain direction. All of the above SHG measurements are operated at 5 K.

Extended Data Fig. 8 Strain dependence on the Néel temperature of an 8-L sample on a PDMS substrate.

The MnPSe3 samples in the strain-tuning experiment (Fig. 4 in the main text, extended Fig. 7 and supplementary Fig. 13) are exfoliated on a ~ 30 μm PDMS, which is too thick to have the best thermal conductance. One needs to calibrate the sample temperature by measuring a thick ( > 50 nm) flake on the same PDMS each time. To measure the sample temperature without calibration, we use thinner home-made ( ~ 5 μm) PDMS to increase the thermal conductance and encapsulate the sample after exfoliation and attach the ~ 5 μm thick PDMS to the metal platform after applying strain (shown in a). b, Optical image of an as-exfoliated sample ( ~ 8 L) on the PDMS substrate before being put onto the metal platform. Scale bar: 10 μm. c, Optical image of the same sample under a horizontal strain. A 5% strain on PDMS is added by a micromanipulator and then the PDMS is attached to the metal platform. Scale bar: 10 μm. d, SHG intensity mappings of the as-exfoliated and strained sample measured at ϕ = 0∘ and ϕ = 90∘ in the crossed polar pattern. The as-exfoliated sample shows that the Néel vectors are not oriented horizontally while the strained sample favors a horizontal Néel vector orientation. The data is collected at 5 K. e-f, Temperature dependence of SHG intensity of the (e) as-exfoliated and the (f) strained sample. The difference of the Néel temperature is within 1 K.

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Ni, Z., Haglund, A.V., Wang, H. et al. Imaging the Néel vector switching in the monolayer antiferromagnet MnPSe3 with strain-controlled Ising order. Nat. Nanotechnol. 16, 782–787 (2021). https://doi.org/10.1038/s41565-021-00885-5

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