Engineering photomagnetism in collinear van der Waals antiferromagnets

Achieving efficient ultrafast optical control of antiferromagnetic spin dynamics is a central goal for next-generation high-speed THz spintronic and magnonic devices. Resonant optical pumping of crystal-field-split d-d orbital multiplets in magnetic TM ions directly modulates exchange and spin-orbit interactions, inducing large-amplitude coherent spin precession. However, such effects are limited to a handful of systems and there is no general strategy to enhance d-d photomagnetism in antiferromagnets. Here, we demonstrate the engineering of photomagnetism via TM-ion doping in collinear van der Waals antiferromagnets. In Mn$_{1-x}$Ni$_x$PS$_3$, small amounts of Ni$^{2+}$ activate a strong photomagnetic response while largely preserving the Néel ground state. Even 10% Ni boosts the response by more than an order of magnitude compared to pure MnPS$3$, with resonant pumping of Ni$^{2+}$ d-d transitions driving large-amplitude coherent spin precession and providing helicity-dependent phase control. Tuning the pump energy across the full Mn${1-x}$Ni$_x$PS$_3$ composition range shows that Ni excitations remain effective across competing Néel and zig-zag antiferromagnetic states while supporting tunable-frequency coherent spin precession. These results establish TM-ion doping as a versatile strategy to harness orbital multiplet excitations for ultrafast, low-dissipation spin control in van der Waals antiferromagnets.

💡 Research Summary

The paper addresses a central challenge for future terahertz (THz) spintronic and magnonic technologies: achieving efficient, ultrafast optical control of antiferromagnetic (AFM) spin dynamics. The authors focus on resonant excitation of crystal‑field‑split d‑d orbital multiplets in transition‑metal (TM) ions, a mechanism that can directly modulate exchange interactions and spin‑orbit coupling, thereby launching large‑amplitude coherent spin precession. While this photomagnetic effect has been demonstrated in a few bulk antiferromagnets, a systematic strategy to amplify it and make it broadly applicable has been lacking.

To develop such a strategy, the authors study the van‑der‑Waals (VdW) antiferromagnets MnPS₃ and NiPS₃, which share the same monoclinic crystal structure but host different TM ions (Mn²⁺, 3d⁵ and Ni²⁺, 3d⁸) and consequently different magnetic ground states: MnPS₃ exhibits a Néel‑type out‑of‑plane AFM order, whereas NiPS₃ displays an in‑plane zig‑zag AFM order. By forming solid solutions Mn₁₋ₓNiₓPS₃ with varying Ni concentration (x), the authors can continuously tune the magnetic ground state while keeping the lattice essentially unchanged.

The magnetic phases of the alloys are mapped using two symmetry‑selective magneto‑optical probes: magnetic second‑harmonic generation (mSHG), which is allowed only when spatial inversion symmetry is broken (Néel order), and magnetic linear birefringence (mLB), which appears for the zig‑zag order that preserves inversion symmetry but creates optical anisotropy. The phase diagram shows that Mn‑rich compositions (x < 0.5) retain Néel order, Ni‑rich compositions (x > 0.5) retain zig‑zag order, and near x ≈ 0.5 long‑range order is suppressed, hinting at a possible spin‑glass regime.

For the photomagnetic experiments, the authors employ pump‑probe spectroscopy. A broadband pump (0.8–2.4 eV) is tuned across the distinct d‑d multiplet resonances of Mn²⁺ and Ni²⁺ while a weak probe at 1.2 eV (well below the bandgap) monitors the transient change in mSHG (ΔmSHG). The pump fluence is fixed at 6 mJ cm⁻². In Mn‑rich samples (x = 0.35), the Fourier spectra of ΔmSHG reveal two coherent magnon modes at ~45 GHz and ~25 GHz. Their amplitudes peak when the pump photon energy matches the Ni²⁺ multiplet transitions at 0.925, 0.98, 1.498, and 1.71 eV (3A₁g, 3E_g, 1A₁g, 3T₁g). Remarkably, despite Mn being the dominant magnetic ion, pumping the Mn²⁺ 4T₁g transition (~1.9 eV) produces essentially no coherent spin response.

When the Ni concentration is reduced to 10 % (x = 0.1), the Néel order remains robust, and the same two magnon branches appear at higher frequencies (92 GHz and 39 GHz), reflecting the weakening of exchange due to Ni substitution. Even in this Mn‑dominant regime, the Ni²⁺ multiplet excitations dominate the photomagnetic response, delivering a spin‑precession amplitude more than an order of magnitude larger than that generated by Mn²⁺ excitations.

The authors rationalize these observations with quasiparticle self‑consistent GW (QS‑GW) calculations. The calculations show that the Ni²⁺ 3A₁g excitation is highly localized, has minimal hybridization with the surrounding S ligands, and retains substantial orbital angular momentum after excitation. This combination yields a strong coupling to the spin sector, efficiently modulating the exchange constant and anisotropy on femtosecond timescales. In contrast, Mn²⁺ excitations, although spin‑flip in nature, possess weak oscillator strength and delocalized wavefunctions that strongly hybridize with ligands, resulting in poor spin‑orbit coupling efficiency.

Three key design principles emerge: (i) the oscillator strength of the transition is less important than the locality and orbital angular momentum of the excited state; (ii) excitations that lie well below the fundamental bandgap and avoid strong ligand hybridization are most effective at transiently reshaping magnetic interactions; (iii) the lowest‑energy Ni²⁺ 3A₁g state, despite its weak absorption cross‑section, is the most potent driver of coherent spin dynamics.

Finally, the authors demonstrate helicity‑dependent phase control: circularly polarized pump pulses resonant with the Ni²⁺ 3A₁g transition imprint a deterministic phase on the magnon oscillations, a capability absent in pure MnPS₃. This shows that the photomagnetic effect can be used not only for amplitude modulation but also for coherent phase engineering of spin waves.

In summary, the work establishes TM‑ion doping as a versatile, materials‑by‑design approach to amplify d‑d photomagnetism in collinear antiferromagnets. By judiciously introducing a minority of Ni²⁺ ions into MnPS₃, the authors achieve >10× enhancement of coherent spin precession while preserving the original Néel ground state. The findings provide a clear roadmap for exploiting orbital multiplet excitations to achieve ultrafast, low‑dissipation spin control in van‑der‑Waals magnets and suggest that similar doping strategies could be applied to a broad class of magnetic materials for next‑generation THz spintronic applications.