Abstract
Approximately single-domain-sized 9-, 13-, and 16-nm CoFe2O4 nanoparticles are synthesized using the thermal decomposition of a metal-organic salt. By means of dilution and reduction, the concentration, moment, and anisotropy of nanoparticles are changed and their influence on the magnetic properties is investigated. The relation of M r/M s ∝ 1/lgH dip is observed, where M r/M s is the remanence ratio and H dip is the maximum dipolar field. Especially, such relation is more accurate for the nanoparticle systems with higher concentration and higher moment, i.e., larger H dip. The deviation from M r/M s ∝ 1/lgH dip appearing at low temperatures can be attributed to the effects of surface spins for the single-phase CoFe2O4 nanoparticles and to the pinning effect of CoFe2O4 on CoFe2 for the slightly reduced nanoparticles. Graphical Approximately single-domain-sized 9-, 13-, and 16-nm CoFe2O4 nanoparticles were synthesized and then the concentration, moment, and anisotropy of these NPs were changed. The correlation of M r/M s ∝ 1/lgH dip was observed, independent of the size, concentration, moment, and anisotropy, and especially, such correlation is more accurate for the nanoparticle systems with higher concentration or moment, i.e., stronger dipolar interaction, which has not been reported before as far as we know.
Highlights
Nanoscale magnetic materials often exhibit novel properties, differing from those of their bulk polycrystalline counterparts [1,2,3,4,5], as a result of several effects including the finite size effect, surface effect, and interparticle interaction [6,7,8]
One typical phenomenon of the size effect is that the coercivity (Hc) reaches the maximum as the particle size (D) decreases to a single-domain critical dimension Dc, and reduces monotonically to zero when D is further decreased to a certain size below Dc [9]
Surface spin is another factor to affect the magnetic properties for the nano-sized magnetic materials
Summary
Nanoscale magnetic materials often exhibit novel properties, differing from those of their bulk polycrystalline counterparts [1,2,3,4,5], as a result of several effects including the finite size effect, surface effect, and interparticle interaction [6,7,8]. The NPs exhibit the superparamagnetic behavior with the theoretical remanence (Mr) to saturation (Ms) magnetization ratio (Mr/Ms) being zero Surface spin is another factor to affect the magnetic properties for the nano-sized magnetic materials. The total magnetization of a nanoparticle composes of the surface and core spins [10, 11], which is
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