Engineering photomagnetism in collinear van der Waals antiferromagnets

Abstract

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.

Figures (5)

• Figure 1: Tuning the antiferromagnetic ground state and photomagnetic properties of Mn$_{1-x}$Ni$_x$PS$_3$.(a) The transition-metal (TM) ions form a hexagonal lattice and are surrounded by trigonally-distorted S$_6$ octahedral cages. The Tanabe-Sugano diagram of Mn$^{2+}$ and Ni$^{2+}$ ions in the octahedral ($O_h$) crystal field with a trigonal ($D_{3d}$) distortion is shown in the center, along with spin-flip excited states. MnPS$_3$ (left) favours a Néel (N)-type antiferromagnetic (AFM) order with the Néel vector out-of-plane. In contrast, the larger spin-orbit coupling leads to an in-plane anisotropy for NiPS$_3$ and zig-zag (ZZ) AFM order (right). In the pump-probe experiment, the laser fundamental (1.2 eV) is detuned from the multiplet resonances and is used as a probe. The pump pulse, with an incident fluence of 6 mJ/cm$^2$, is tuned from 0.8 to 2.4 eV across multiple multiplet resonances. (b) The ground state magnetic phase diagram of Mn$_{1-x}$Ni$_x$PS$_3$ mapped by symmetry-selective magneto-optical probes. The spatial inversion symmetry breaking in the N-type AFM order gives magnetic second-harmonic generation (mSHG, green) and the ZZ-type order gives magnetic linear birefringence (mLB, red); these probes are used to determine the onset of the AFM order at the Néel temperature ($T_\text{N}$). The Mn (Ni)-rich compounds have an N (ZZ)-type AFM ground state reflective of the pure compound, while long-range magnetic order is not observed for $x = 0.5$. (c) Optical absorbance of pure NiPS$_3$ and MnPS$_3$ compounds measured at 10 K, showing the below-bandgap multiplet resonances.
• Figure 1: Ab-initio calculation of Ni$^{2+}$ multiplets. Excitonic wavefunctions are computed in the framework of QS$G\hat{W}$Cunningham2023 theory for the mixed compounds Mn$_{1-x}$Ni$_{x}$PS$_3$, with $x = 0.25$ (details in Supplementary Sec. 6). (a) The in-plane distribution of Mn and Ni ions for the $x = 0.125$ and $x = 0.25$ alloys. (b) The electronic band structure of Mn$_{1-x}$Ni$_{x}$PS$_3$, for $x = 0.25$. The colors of the markers indicate the weight of S-$p$, Mn-$d$, Ni-$d$ orbitals, respectively. The Brillouin zone and high-symmetry points are shown in the inset, with the $k$-path denoted in blue. (c,d,g) The wavefunction of the excitonic-hole for the $^3A_{1g}$, $^3E_{g}$ and $^3T_{1g}$ excitations. The Ni and Mn ions are shown in purple and turquoise, respectively. S and P are shown in gray and brown. The $^3A_{1g}$ and $^3E_g$ excitations are localized to the Ni-ion, while the $^3T_{1g}$ state is delocalized over multiple hexagonal plaquette. (e,f,h) Electronic band structure plotted along $k$-path in the inset of b for the $^3A_{1g}$, $^3E_{g}$ and $^3T_{1g}$ excitations. Marker size indicates the weight of the excitonic hole $c_{n, \mathbf{k}}$, raised to the power of 4 for visualization purposes. Projection show that the $^3A_{1g}$ ($^3E_{g}$) excitation is projected onto the $m_l = |2|\,(0)$ Ni-$d$ states, while the $^3T_{1g}$ excitation is delocalized over the S-$p$ valence bands.
• Figure 2: Launching coherent spin dynamics via resonant multiplet excitation in Mn$_{1-x}$Ni$_{x}$PS$_3$.(a, c) The pump-induced change of the magnetic second harmonic signal ($\Delta$mSHG) as a function of pump-probe delay measured for $x = 0.35$ and $x = 0.1$ at $T = 10$ K, respectively. The trace at 0.94 (0.95) eV is highlighted. (b, d) The frequency spectrum of the coherent dynamics as a function of pump photon energy in the $x = 0.35$ and $x = 0.1$ samples. Colors indicate $\Delta$mSHG intensity in linear scale. In panel b, clear resonances are observed at $E_\text{Ni} = [0.925, 0.97, 1.498, 1.708]$ eV, corresponding to multiplet resonances in Ni$^{2+}$ in the trigonally distorted octahedra. In panel d, a clear resonance is observed at 0.95 eV, and smaller resonances at 1.55, and 1.98 eV, corresponding to multiplet excitations in Ni$^{2+}$ (purple) and Mn$^{2+}$ (turquoise). The intensity between the gray dashed lines is integrated and shown for the $x = 0.35$ (left) and $x = 0.1$ (right) samples and fit with four Gaussian peaks.
• Figure 3: Spin-orbit efficiency of Ni$^{2+}$ excitations in Mn$_{0.75}$Ni$_{0.25}$PS$_3$.(a) The orbital-to-spin coupling strength $\chi_{so}$ of the $^3A_{1g}$, $^3E_{g}$ and $^3T_{1g}$ multiplets, which is calculated as the intensity of the spin dynamics (the amplitude of the corresponding Gaussian peaks in Fig. \ref{['Fig: fig2']}b) divided by the absorption peaks (the values of absorption at corresponding photon energies in NiPS$_3$ presented in Fig. \ref{['Fig: fig1']}c) and normalized to 1. (b-d) The wavefunction of the excitonic-hole for the (b) $^3A_{1g}$, (c) $^3E_{g}$ and (d) $^3T_{1g}$ multiplets. The Ni and Mn ions are shown in purple and turquoise, respectively. S and P are shown in gray and brown. The $^3A_{1g}$ and $^3E_g$ excitations are localized to the Ni-ion, while the $^3T_{1g}$ state is delocalized over multiple hexagonal plaquette. $m_l = |2|, (0)$ labels the Ni $d$ states onto which the $^3A_{1g}$ ($^3E_g$) excitation is projected.
• Figure 4: Phase control of spin dynamics in Mn$_{1-x}$Ni$_x$PS$_3$.(a) Helicity dependence of the magnon excitation for the $x = 0.1$ (top) and $x = 0.35$ (bottom) samples at 10 K. (b) Linear polarization dependence of the magnon excitation for the $x = 0.1$, $x = 0.2$ samples, respectively, at 10 K. The orange curve in the bottom panel is vertically scaled by a factor of 5 for visualization purposes. (c) Frequency of the magnons driven by the Ni $^3A_{1g}$ excitation (0.95 eV) as a function of Ni fraction, measured with magnetic linear birefringence. Black (magenta) colors indicate the absence (presence) of phase-flip. The star markers for $x = 0$ and $x=1$ are values reported in literature afanasiev2021matthiesen2023.