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

The magnetic properties of nanofilms prepared by Langmuir-Blodgett nanoarchitectonics with CoFe_2-xY_xO₄ (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, M_S(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 M_S(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 CoFe_2–0.8Y_0.2O₄ were obtained, indicating that the contribution of freezing spins to M_S(T) is negligible above approximately 5 T_f.

Coercivities are higher for the NPs with yttrium substitution due to their smaller size and M_S 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 (K_eff) of the nanofilm prepared with CoFe_1.8Y_0.2O₄ NPs (4.2 × 10⁵ J/m³) is smaller than K_eff for the nanofilm made of CoFe₂O₄ NPs (5.6 × 10⁵ J/m³). Nonetheless, K_eff values for both nanofilms are relatively large, making these nanoarchitectures promising for different applications.