Optically Inferring Magnetic Interactions in 2D van der Waals Antiferromagnets

Jana et al., Advanced Science, 2025 National University of Singapore · LNCMI-EMFL · Université Grenoble Alpes · Université de Toulouse · National Laboratory of the Rockies · Charles University, Prague · King’s College London, London · Warsaw University of Technology · Polish Academy of Sciences

This paper, published in Advanced Science in 2025, focuses on two representative van der Waals magnetic antiferromagnets, MnP_S and NiPS₃. It addresses the question, how do magnetic order and optical transitions couple in van der Waals antiferromagnets, and what can optical measurements reveal about the underlying magnetic interactions?

Understanding how macroscopic spin arrangements influence optical transitions is vital for creating protocols to probe and manipulate magnetic states using light. However, the interplay between magnetic ordering and sub-bandgap optical transitions makes it challenging to pinpoint the mechanisms driving spin-entangled optical transitions and to determine the fundamental single-particle bandgap. This work meets the challenge by combining advanced electronic structure calculations with high-field magneto-optical spectroscopy.

MnP_S and NiPS₃ are structurally similar, but the former possesses an out-of-plane easy spin axis and the latter an in-plane easy spin axis. This work shows that the electronic structure and attendant effective magnetic parameters are quite different in the two systems.

Theoretical Methodology & Electronic Structure

To provide a parameter-free, unbiased description of the electronic states, the authors employed two complementary, advanced theoretical methods:

1. Many body perturbation theory (MBPT) in the form of Quasi-particle Self-consistent GW Theory ($\text{QS}G\hat{W}$/$\text{QS}GW$) : Imposes self-consistency over both self-energy and charge density to accurately compute both one-particle properties (fundamental bandgap) and two-particle properties (spin-allowed excitonic transitions).

2. Exact Diagonalization Dynamical Mean Field Theory (ED-DMFT), using $\text{QS}GW$ as the starting point: Captures nonperturbative, locally exact atomic multiplet transitions. Crucially, DMFT accounts for spin-flip transitions that are missing from standard MBPT/GW frameworks.

The single-particle bulk bandgap computed by ($\text{QS}G\hat{W}$/$\text{QS}GW$) is 3.9 eV for MnPS₃ and 2.2 eV for NiPS₃. No reliable experiment is available, but $\text{QS}G\hat{W}$ – the extension of $\text{QS}GW$ to include ladder diagrams in the polarizabiity – has been shown to yield accurate bandgaps in a wide range of bulk systems, and 2D van der Waals magnets as well, e.g. in CrSBr. Both materials have charge-transfer character, with sulfur p-states dominating the valence band maximum.

Origin of the Narrow Optical Resonance (“X-feature”)

Theoretical calculations successfully distinguish between broad, extended excitonic states and highly localized Frenkel states. The ultra-narrow, sub-bandgap optical resonance—labeled as transition X—is proven to be an on-site d-d spin-flip transition strictly confined to the transition-metal atom (Mn or Ni). It shares a direct physical analogy with the spin-forbidden transitions described by traditional Tanabe-Sugano diagrams.

The computed spin-flip energies (2.64 eV for Mn, 1.47 eV for Ni) match experiment well.

Optical response and magnetic parameters from magneto-optics

Using low-temperature 5  photoluminescence (PL) and PL Excitation (PLE) spectroscopy, the authors analyzed the materials’ emission and absorption behaviors:

MnPS₃: Shows broadband transitions in its ligand-field spectra. Because changing the electronic spin configuration disrupts interatomic bond strengths, the optical transitions strongly couple to lattice phonons, broadening the absorption bands and leading to a heavily Stokes-shifted 1.2 eV emission.

NiPS₃ Exhibits a delocalized, near-band-edge spin-allowed exciton near the fundamental bandgap (with broad emission at $1.4\text{ eV}$), but its optical response is dominated by an exceptionally sharp, ultra-narrow spin-flip X-resonance at 1.47 eV.

By tracking how the X-feature evolves in external magnetic fields (up to 30 T) applied both parallel and perpendicular to the easy spin axis, key magnetic interaction parameters are extracted from varous models:

The drastic difference in exchange constants and saturation limits emphasizes the underlying structural anisotropy of vdW antiferromagnets: magnetic interactions are far weaker when the easy spin axis aligns with weak interplanar vdW forces MnPS₃ compared to when it aligns with strong, covalent, in-plane atomic bonds NiPS₃.

Broader Significance

This work establishes magneto-optical spectroscopy as a general all-optical probe of antiferromagnetic order in 2D materials, establishing that fundamental exchange and anisotropy values can be extracted directly from microscale flakes. It establishes how to characterize microscale thin flakes – inaccessible to neutron scattering – with all-optical technoques and paves the way to ultrafast manipulation of 2D antiferromagnetic domains.