Molecular intercalation in the van der Waals antiferromagnets FePS3 and NiPS3

We have performed electrochemical treatment of the van der Waals antiferromagnetic materials FePS_3 and NiPS_3 with the ionic liquid EMIM-BF_4, achieving significant molecular intercalation. Mass analysis of the intercalated compounds, EMIM_x-FePS_3 and EMIM_x-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 ^31P nuclear magnetic resonance (NMR) studies of both systems. For EMIM_x-FePS_3, intercalation reduces the magnetic ordering temperature from T_N = 120 K to 78 K, and we find a spin gap in the antiferromagnetic phase that drops from 45 K to 30 K. For EMIM_x-NiPS_3, the ordering temperature is almost unaffected (changing from 148 K 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 unequivocally to the conclusion that the magnetic properties of both parent compounds are determined solely 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.