Magnetic nanofilms prepared by Langmuir-Blodgett nanoarchitectonics using Co and Co-Y ferrite nanoparticles

Abstract

The magnetic properties of nanofilms prepared by Langmuir-Blodgett nanoarchitectonics with CoFe2-xYxO4 (x=0; 0.2) nanoparticles (NPs), are studied. Magnetization measurements M(H) of the nanofilms and the NPs were performed, with maximum applied magnetic fields of ± 5 T at different temperatures from 5 K to 300 K, as well as zero-field-cooling, field-cooling curves (ZFC-FC). In the nanofilms, saturation magnetization as a function of temperature, Ms(T), and ZFC-FC curves exhibit a sharp increase below 50 K, which is attributed to the existence of a disordered spin shell in the NPs forming the nanofilms, where superficial effects are more relevant than in compacted NPs. The Ms(T) curves were fitted with the modified Bloch’s law plus an additional term corresponding to surface spins. Freezing temperatures of (6 ± 1) K for the nanofilm prepared with CoFe2O4 and (14 ± 3) K for the one made with CoFe2-0.8Y0.2O4 were obtained, indicating that the contribution of freezing spins to Ms(T) is negligible above approximately 5 Tf. Coercivities are higher for the NPs with yttrium substitution due to their smaller size and Ms reduction as a consequence of yttrium inclusion in the spinel lattice. Both NPs and nanofilms follow the modified Kneller’s law, with the exponent greater than 0.5, indicating the presence of magnetic dipolar interactions. At room temperature, the effective magnetic anisotropy (Keff) of the nanofilm prepared with CoFe1.8Y0.2O4 NPs (4.2 x 105 J/m3) is smaller than Keff for the nanofilm made of CoFe2O4 NPs (5.6 x 105 J/m3). Nonetheless, Keff values for both nanofilms are relatively large, making these nanoarchitectures promising for different applications.