Low-dimensional magnetism and spin evolutions probed by nonlinear magneto-optics in the vdW antiferromagnet CrSBr
ORAL · Invited
Abstract
Dimensionality plays a crucial role in shaping magnetic phase transitions and their associated spontaneous symmetry breakings. Yet, magnetism in low dimensions remains intrinsically challenging to investigate, limited by the small sample size and the low material volume. In this presentation, by leveraging rotational anisotropy second and third harmonic generations (RA SHG/THG) combined with rigorous group theory analysis, we investigate the effects of reduced dimensionality on magnetic order parameters and their fluctuations in the vdW antiferromagnet (AFM) CrSBr.
In the first part, we report the direct observation of an unusual separation between surface and bulk magnetic phase transitions in millimeter-thick CrSBr. Unlike “ordinary” phase transitions where surface and bulk develop long-range orders simultaneously, CrSBr is observed to exhibit surface ordering at a higher temperature (Ts=140 K) than the bulk (TN=132 K) – a rare and counterintuitive phenomenon given the enhanced thermal fluctuations expected at the surface. This spatial resolution of magnetic ordering is realized via combining surface-sensitive RA SHG and bulk-sensitive RA THG, by which both the order parameters and their fluctuations are captured.
In the second part, we investigate the development of magnetism and the evolution of the spin textures under external magnetic fields B in bilayer (2L) CrSBr. Magnetic-field-dependent RA SHG detects distinct spin evolutions under B along various crystallographic directions. Notably, under B‖c, the RA SHG patterns exhibit mirror symmetry breaking that cannot be explained by the conventional magnetic time-reversal-odd (c-type) SHG alone. Instead, this arises from the interference between c-type SHG and an unexpected time-reversal-even (i-type) SHG, which indicates an emergent electric polarization coexisting with magnetism, manifesting the linear magnetoelectric effect. Our work establishes a quantitative link between nonlinear optical tensor elements and physical quantities of matching symmetries, demonstrating the unique capability of nonlinear optics in selectively probing complex order parameters.
In the first part, we report the direct observation of an unusual separation between surface and bulk magnetic phase transitions in millimeter-thick CrSBr. Unlike “ordinary” phase transitions where surface and bulk develop long-range orders simultaneously, CrSBr is observed to exhibit surface ordering at a higher temperature (Ts=140 K) than the bulk (TN=132 K) – a rare and counterintuitive phenomenon given the enhanced thermal fluctuations expected at the surface. This spatial resolution of magnetic ordering is realized via combining surface-sensitive RA SHG and bulk-sensitive RA THG, by which both the order parameters and their fluctuations are captured.
In the second part, we investigate the development of magnetism and the evolution of the spin textures under external magnetic fields B in bilayer (2L) CrSBr. Magnetic-field-dependent RA SHG detects distinct spin evolutions under B along various crystallographic directions. Notably, under B‖c, the RA SHG patterns exhibit mirror symmetry breaking that cannot be explained by the conventional magnetic time-reversal-odd (c-type) SHG alone. Instead, this arises from the interference between c-type SHG and an unexpected time-reversal-even (i-type) SHG, which indicates an emergent electric polarization coexisting with magnetism, manifesting the linear magnetoelectric effect. Our work establishes a quantitative link between nonlinear optical tensor elements and physical quantities of matching symmetries, demonstrating the unique capability of nonlinear optics in selectively probing complex order parameters.
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Publication: Nature Communications 15, 6472 (2024)
arXiv 2509.13542 (2025)
Presenters
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Xiaoyu Guo
- University of Michigan
- Johns Hopkins University