Non-linear magnetotropic susceptibility in FePS3

Magnetotropic susceptibility is the thermodynamic coefficient that maps the curvature of free energy with respect to an applied magnetic field orientation, providing a means to quantify the magnetic anisotropy of a crystal. In this context, non-linear magnetic torque behavior has been reported in FePS3, motivating the investigation of similar non-linear characteristics in its magnetotropic susceptibility. In this work, we derive the non-linear magnetotropic susceptibility expressions for FePS3 in both ac*-and bc*-planes using complementary approaches: by taking the first derivative of torque and through the formal calculation of the magnetotropic susceptibility. Higher-order terms in the magnetization are included, and the final equations are obtained by applying symmetry constraints imposed by the C2h point group of the material. We analyze the behavior of the resulting non-linear expressions and identify the contributions of each parameter. Our theoretical results show good agreement with preliminary, unpublished experimental data, offering meaningful guidance for ongoing and future experimental work.