We have performed electrochemical treatment of the van der Waals antiferromagnetic materials ${\mathrm{FePS}}_3$ and ${\mathrm{NiPS}}_3$ with the ionic liquid $\mathrm{EMIM}\text{−}{\mathrm{BF}}_4$, achieving significant molecular intercalation. Mass analysis of the intercalated compounds, ${\mathrm{EMIM}}_x\text{−}{\mathrm{FePS}}_3$ and ${\mathrm{EMIM}}_x\text{−}{\mathrm{NiPS}}_3$, indicated respective intercalation levels, $x$, of approximately 27% and 37%, and x-ray diffraction measurements demonstrated a massive (over 50%) enhancement of the $c$-axis lattice parameters. To investigate the consequences of these changes for the magnetic properties, we performed magnetic susceptibility and ${}^{31}\mathrm{P}$ nuclear magnetic resonance (NMR) studies of both systems. For ${\mathrm{EMIM}}_x\text{−}{\mathrm{FePS}}_3$, intercalation reduces the magnetic ordering temperature from $T_N=120$ to 78 K, and we find a spin gap in the antiferromagnetic phase that drops from 45 to 30 K. For ${\mathrm{EMIM}}_x\text{−}{\mathrm{NiPS}}_3$, the ordering temperature is almost unaffected (changing from 148 to 145 K), but a change towards nearly isotropic spin fluctuations suggests an alteration of the magnetic Hamiltonian. Such relatively modest changes, given that the huge extension of the $c$ axes is expected to cause a very strong suppression any interlayer interactions, point to the conclusion that the magnetic properties of both parent compounds are determined almost exclusively by two-dimensional (2D), intralayer physics. The changes in transition temperatures and low-temperature spin dynamics in both compounds therefore indicate that intercalation also results in a significant modulation of the intralayer magnetic interactions, which we propose is due to charge doping and localization on the P sites. Our study offers chemical intercalation with ionic liquids as an effective method to control not only the interlayer but also the intralayer interactions in quasi-2D magnetic materials.

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• Antiferromagnetism
• Frustrated magnetism
• Magnetic interactions
• Magnetic phase transitions
• Honeycomb lattice
• Layered crystals
• van der Waals systems
• Nuclear magnetic resonance