Abstract
Polymorphism, commonly denoting diverse molecular or crystal structures, is crucial in the natural sciences. In van der Waals antiferromagnets, a new type of magnetic polymorphism arises, presenting multiple layer-selective magnetic structures with identical total magnetization. However, resolving and manipulating such magnetic polymorphs remain challenging. Here, phase-resolved magnetic second harmonic generation microscopy is used to elucidate magnetic polymorphism in 2D layered antiferromagnet CrSBr, demonstrating deterministic and layer-selective switching of magnetic polymorphs. Using a nonlinear magneto-optical technique, we unambiguously resolve the polymorphic spin-flip transitions in CrSBr bilayers and tetralayers through both the amplitude and phase of light. Remarkably, the deterministic routing of polymorphic spin-flip transitions originates from a ‘layer-sharing’ effect, where the transitions are governed by laterally extended layers acting as ‘control bits’. We envision that such controllable magnetic polymorphism could be ubiquitous for van der Waals layered antiferromagnets, enabling new designs and constructions of spintronic and opto-spintronic devices for probabilistic computation and neuromorphic engineering.
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Data availability
All data that support the findings of this study are available from the corresponding authors on reasonable request. Experimental source data for Figs. 2–6 are provided with this paper. Source data are provided with this paper.
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Acknowledgements
The work at Fudan University was supported by the National Key Research and Development Program of China (grant nos. 2022YFA1403302 and 2019YFA0308404), the National Natural Science Foundation of China (grant nos. 12034003, 11427902, 91950201 and 12004077), the Science and Technology Commission of Shanghai Municipality (grant nos. 20JC1415900, 21JC1402000, 23JC1400400 and 2019SHZDZX01), the Program of Shanghai Academic Research Leader (grant no. 20XD1400300), the Shanghai Municipal Education Commission (grant no. 2021KJKC-03-61) and the China National Postdoctoral Program for Innovative Talents (grant no. BX20200086). Z.L. acknowledges support from the National Natural Science Foundation of China (grant nos. 92365204 and 12274298) and the National Key R&D program of China (grant no. 2022YFA1604403).
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Shiwei W. conceived and supervised the project. Z.Sun and C.H. performed the experiments with assistance from Z.Sheng, Shuang W., Z.W. and B.L. Y.C., Q.M. and Z.L. provided the CrSBr single crystals. Z.Sun, C.H. and Shiwei W. analysed the data and wrote the paper with inputs from W.-T.L., Z.Y., Y.W. and J.S.
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Extended data
Extended Data Fig. 1 Illustration of phase-SHG measurements.
a, Schematic of the phase-resolved SHG setup. SCM, superconducting magnet; Obj, objective; HWP, half-wave plate; BS, beamsplitter; SBC, Soleil-Babinet compensator; SPF, short-pass filter; PMT, photomultiplier tube. b, Vector diagram of the orthogonal second-harmonic electric polarizations of CrSBr (\({\overrightarrow{{\boldsymbol{E}}}}_{CrSBr}^{2\omega }\)) and quartz (\({\overrightarrow{{\boldsymbol{E}}}}_{qartz}^{2\omega }\)), with the slow (\({\overrightarrow{SBC}}_{{\rm{slow}}}\)) and fast (\({\overrightarrow{SBC}}_{{\rm{fast}}}\)) axis of SBC aligned parallel to \({\overrightarrow{{\boldsymbol{E}}}}_{CrSBr}^{2\omega }\) and \({\overrightarrow{{\boldsymbol{E}}}}_{quartz}^{2\omega }\), respectively. The analyzer is set at an angle θ relative to the fast axis of SBC. c,d, Azimuthal SHG polarization patterns of 2 L CrSBr at ‘0 T (forward)’ (c) and ‘0 T (backward)’ (d). e, Azimuthal SHG polarization patterns of y-cut quartz. The angle ϕ = 0° denotes the alignment to the b-axis of CrSBr and the x-axis of quartz.
Extended Data Fig. 2 Phase-SHG of the 2L CrSBr with different external phase.
a,b, Phase-SHG images at ‘0 T (backward)’ (a) and ‘0 T (forward)’ (b). c, Phase-SHG intensity as a function of external phase shift. d, Phase-SHG hysteresis loop. The external phase shift in (a), (b) and (d) was set at 7π/6 (arrows in (c)). Scale bar: 5 µm.
Extended Data Fig. 3 SHG images of isolated 4L CrSBr.
SHG images at −0.2 T (forward) with stochastic domains during different field sweep cycles. Scale bar: 5 µm.
Extended Data Fig. 4 Magneto-PL hysteresis on the isolated 4L CrSBr.
PL hysteresis loops measured at the same position as in Fig. 3c.
Extended Data Fig. 5 Magneto-SHG images of the non-isolated 4L CrSBr with extended 2L.
The 4L region appears as a uniform ___domain extending over 20 μm throughout the field sweep. Scale bar: 5 µm.
Extended Data Fig. 6 Magneto-SHG loop and SHG excitation spectra on the non-isolated 4L CrSBr.
a, SHG hysteresis loop. b-e, SHG excitation spectra at the AFM (b), FM (c), ‘Type-I’ (d) and ‘Type-II’ (e) states, grouped by the number of FM and AFM interfaces listed in Extended Data Table 1. The error bars presented as mean values ± S.D. The excitation photon energy used for SHG loop in (a) is marked by the dashed line.
Extended Data Fig. 7 Alternative transition route for the non-isolated 4L CrSBr and extended 2L.
Compared to Fig. 5f, the magnetic structures of the bilayer with M = 0 and tetralayer with M = 0, ±2 are replaced by their spatial-inversion counterparts. The layer-sharing effect no longer appears in this magnetic evolution.
Extended Data Fig. 8 Magneto-SHG on different non-isolated 4L CrSBr.
a, Optical microscopic image of a non-isolated 4L CrSBr adjacent to a 2L on the left and a thin flake on the right. The 2L and 4L regions are delineated by white dashed lines. b, SHG image at −0.25 T (forward). The 4L region splits into two domains. c,d, SHG hysteresis loops measured at the green (c) and blue (d) dots in (b), with arrows pointing to the corresponding magnetic field. e, SHG image at −0.25 T (forward) obtained under a different thermal cycle. The magnetic domains and ___domain wall remain stable. f,g, The corresponding SHG hysteresis loops, which are repetitive. Scale bar: 5 μm.
Supplementary information
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Supplementary text, Fig. 1 and source data table.
Source data
Source Data Fig. 2
Experimental source data of Fig. 2.
Source Data Fig. 3
Experimental source data of Fig. 3.
Source Data Fig. 4
Experimental source data of Fig. 4.
Source Data Fig. 5
Experimental source data of Fig. 5.
Source Data Fig. 6
Experimental source data of Fig. 6.
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Sun, Z., Hong, C., Chen, Y. et al. Resolving and routing magnetic polymorphs in a 2D layered antiferromagnet. Nat. Mater. 24, 226–233 (2025). https://doi.org/10.1038/s41563-024-02074-w
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DOI: https://doi.org/10.1038/s41563-024-02074-w
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