Uncompensated surface states in antiferromagnetic FeF2 nanoparticles induced by mechanical milling

Abstract 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 (TN) 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 TN indicating a strong exchange coupling between compensated core and uncompensated shell.