Molecular intercalation in the van der Waals antiferromagnets FePS3 and NiPS3

We have performed electrochemical treatment of the van der Waals antiferromagnetic materials FePS3 and NiPS3 with the ionic liquid EMIM-BF4, achieving significant molecular intercalation. Mass analysis of the intercalated compounds, EMIMx-FePS3 and EMIMx-NiPS3, 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 31P nuclear magnetic resonance (NMR) studies of both systems. For EMIMx-FePS3, intercalation reduces the magnetic ordering temperature from TN = 120 to 78 K, and we find a spin gap in the antiferromagnetic phase that drops from 45 to 30 K. For EMIMx-NiPS3, 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.