Reduced surface effects in weakly interacting ZrO₂ coated MnFe₂O₄ nanoparticles

Surface effects in zirconium dioxide (ZrO₂) coated manganese ferrite (MnFe₂O₄) nanoparticles have been studied by using dc and ac magnetization. The average crystallite size of MnFe₂O₄ and ZrO₂ phase was about 9 and 4 nm, respectively as determined by Debye-Scherrer's formula. TEM images revealed that the nanoparticles are spherical in shape with less agglomeration. Selected area electron diffraction analysis shows two different crystalline species such as nanoparticle's core MnFe₂O₄ and coating ZrO₂, which was in agreement with the XRD analysis. Effective anisotropy constant (K_eff = 3 × 10⁰⁴ erg/cm³) as deduced from simulated ZFC/FC curves is close to bulk value (K_bulk = 2.5 × 10⁰⁴ erg/cm³) which is due to weak contribution of surface anisotropy. Saturation magnetization showed an increasing trend at low temperatures (more pronounced below 50 K) which is again due to reduced surface spins disorder. Temperature dependent coercivity revealed a sharp increase at 5 K, which is due to the surface spins freezing. The Arrhenius law fit to frequency shift of T_B in ac susceptibility revealed weak interparticle interactions. The nanoparticles showed slow spin relaxation in ZFC protocol which signify the presence of disorder in our nanoparticles, however the value of shape parameter β lies outside the spin-glass regime. In summary, non-magnetic ZrO₂ coating on these fine MnFe₂O₄ nanoparticles reduces their surface energy, magnetic interparticle interactions and surface effects, which are not sufficient to establish a spin-glass behaviour.