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To understand the influence of surface interactions upon the magnetic properties of magnetic nanoparticles, the surface of manganese ferrite, MnFe[sub 2]O[sub 4], nanoparticles have been systematically modified with a series of para-substituted benzoic acid ligands (HOOC-C[sub 6]H[sub 4]-R; R = H, CH[sub 3], CI, NO[sub 2], OH) and substituted benzene ligands (Y-C[sub 6]H[sub 5], Y = COOH, SH, NH[sub 2], OH, SO[sub 3]H). The coercivity of magnetic nanoparticles decreases up to almost 50% upon the coordination of the ligands on the nanoparticle surface, whereas the saturation magnetization has increased. The percentage coemivity decrease of the modified nanoparticles with respect to the native nanoparticles strongly correlates with the crystal field splitting energy (CFSE) Δ evoked by the coordination ligands. The ligand inducing largest CFSE results in the strongest effect on the coercivity of magnetic nanoparticles. The change in magnetic properties of nanoparticles also correlates with the specific coordinating functional group bound onto the nanoparticle surface. The correlations suggest the decrease in spin-orbital couplings and surface anisotropy of magnetic nanoparticles due to the surface coordination. Such surface effects clearly show the dependence on the size of nanoparticles. [ABSTRACT FROM AUTHOR]