Uncompensated surface states in antiferromagnetic FeF2 nanoparticles induced by mechanical milling

Here we report an efficient way to prepare antiferromagnetic nanoparticles comprising core-shell structure. Surface modification affecting structural and magnetic properties of FeF₂ nanoparticles are also extensively studied. The samples were prepared by the high energy planetary ball-milling method with varying milling hour. Expected structural changes are observed and studied using X-ray diffraction and transmission electron microscopic measurements which show the nanoparticles decrease in size with an inflation in lattice micro-strain. Hyperfine analysis manifests two paramagnetic doublets revealing the existence of multiple charge states of Fe ions. The enhancement in percentage area with milling duration of the doublet linked with Fe³⁺ ions enunciates the distribution of these ions near or at the surface. Magnetization studies show a vertical shift of M-H curve and an increase in coercive field below Neel temperature (T_N) implying the possible effect of exchange bias between the FeF₃ uncompensated surface spins and the compensated FeF₂ core. Temperature dependent ZFC-FC magnetization curves exhibit a large bifurcation below observed T_N indicating a strong exchange coupling between compensated core and uncompensated shell.