Surface contributions to the alternating current and direct current magnetic properties of oleic acid coated CoFe2O4 nanoparticles

In the present work, we have studied the surface layer effects on the dc and ac magnetic measurements of oleic-acid coated CoFe2O4 nanoparticles. The blocking temperature for the nanoparticles is found to be 278 K with 100 Oe field and above this temperature, the nanoparticles show superparamagnetic behavior. A spin-glass correction term to the Bloch law (to account for a magnetically disordered surface layer) was found to accurately describe the temperature dependence of the saturation magnetization. By studying the frequency dependence of the ac magnetization, the coated nanoparticles were shown to have moderate particle interaction strength with anisotropy energy and anisotropy constant of 160 meV and 2.23 × 10⁵ erg/cm³, respectively. A feature in the ac magnetization at 53 K was confirmed to be the result of surface spin-glass freezing on the individual particles. Because of decreased interaction strength due to the oleic acid coating, the presence of a magnetically disordered surface layer due to superexchange frustration (possibly as a result of decreased oxygen ion coordination) was observed.