The remanence ratio in CoFe(2)O(4) nanoparticles with approximate single-domain sizes

Approximately single-domain-sized 9-, 13-, and 16-nm CoFe(2)O(4) 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 CoFe(2)O(4) nanoparticles and to the pinning effect of CoFe(2)O(4) on CoFe(2) for the slightly reduced nanoparticles. [Figure: see text]