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A combination of hard phase CoFe2O4 and soft phase MnFe2O4 as the bimagnetic nanocrystals in a core-shell architecture has been synthesized, and their magnetic properties have been systematically studied. Both HRTEM andi EDS results confirmed the formation of bimagnetic core-shell structured nanocrystals. On the basis of the systematic and comparative studies of the magnetic properties of a mechanical mixture of pure CoFe2O44 and MnFe2O4 nano- crystals, chemically mixed Co1-xMn-xFe nanocrystals, and bimagnetic core-shell CoFe2O4@MnFe2O4 and MnFe2O4@CoFe2O4 nanocrystals, the bimagnetic core-shell nanocrystals show very unique magnetic properties, such as the blocking temperature and coercivity. Our results show that the coercivity correlates with the volume fraction of the soft phase as the theoretical hard-soft phase model has suggested. Furthermore, switching the hard phase CoFe2O4 from the core to the shell shows great changes in the coercivity of the nanocrystals. The bimagnetic core- shell nanocrystals evidently demonstrate the rational design capability to separately control the blocking temperature and the coercivity in magnetic nanocrystals by varying the materials, their combination, and the volume ratio between the core and the shell and by switching hard or soft phase materials between the core and shell. Such controls via a bimagnetic core-shell architecture are highly desirable for magnetic nanocrystals in various applications. [ABSTRACT FROM AUTHOR]

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]