Laser-induced demagnetization in van der Waals XY - and Ising-like antiferromagnets NiPS3 and FePS3

The critical behavior of laser-induced changes in magnetic ordering is studied experimentally in two-dimensional zigzag antiferromagnets XY-like ${\mathrm{NiPS}}₃$ and Ising-like ${\mathrm{FePS}}₃$. To examine laser-induced dynamics in flakes of these compounds, we employ time-resolved exchange linear dichroism effect sensitive to zigzag magnetic ordering and independent of the orientation of the antiferromagnetic vector. In both compounds laser excitation in the vicinity of the absorption edge induces partial quenching of the antiferromagnetic ordering manifested by exchange linear dichroism reduction. The amplitude of the effect varies with temperature as the derivative of the antiferromagnetic vector and exhibits a critical behavior with the exponents corresponding to XY and Ising models for ${\mathrm{NiPS}}₃$ and ${\mathrm{FePS}}₃$, respectively. Critical slowing down of the demagnetization in the vicinity of N\'eel temperature is found, however, only in ${\mathrm{FePS}}₃$. In contrast, the increase of the demagnetization time near the ordering temperature in ${\mathrm{NiPS}}₃$ is minor. We show that the difference in the demagnetization times correlates well with the spin specific heat in both compounds. Beyond the range of slowing down, the demagnetization times in ${\mathrm{NiPS}}₃$ and ${\mathrm{FePS}}₃$ are comparable, about 5--10 ps, and are longer than those reported earlier for ${\mathrm{CoPS}}₃$ and considerably shorter than for ${\mathrm{MnPS}}₃$. This points to the importance of the unquenched angular momentum of transition-metal ions in laser-induced demagnetization process.