Reduced surface effects in weakly interacting ZrO2 coated MnFe2O4 nanoparticles

Abstract

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 (Keff = 3 × 1004 erg/cm3) as deduced from simulated ZFC/FC curves is close to bulk value (Kbulk = 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 TB 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.