Surface effects in uncoated and amorphous SiO2 coated cobalt ferrite nanoparticles

Abstract

Surface effects in uncoated and amorphous silica (SiO2) coated cobalt ferrite nanoparticles have been studied by using dc magnetization. Both uncoated and SiO2 coated nanoparticle samples were synthesized by using sol–gel method. SiO2 coated nanoparticles exhibit smaller average crystallite size as compared to uncoated nanoparticles. Saturation magnetization (Ms) revealed decreasing trend at low temperatures for uncoated nanoparticles as compared to coated nanoparticles which may be due to magnetically dead nanoparticle's surface layer for uncoated nanoparticles. A step-like behavior near remanent field was observed for both the samples but more pronounced for SiO2 coated nanoparticles. This fast magnetic relaxation near remanent field in coated nanoparticles is due to lesser dipolar (interparticle) interactions among nanoparticles. Bloch's law shows a good fit for coated nanoparticles. SiO2 coated nanoparticles showed larger values of coercivity at low temperatures due to their enhanced surface anisotropy as compared to uncoated nanoparticles. Kneller's law for temperature dependent coercivity shows a good fit for SiO2 coated nanoparticles and is attributed to lesser dipolar interactions among coated nanoparticles as compared to uncoated nanoparticles. The exchange bias phenomenon was also observed for both the samples, which signify the presence of core–shell interactions. However, SiO2 coated nanoparticles exhibit larger values of exchange bias at low temperatures due to their strong surface anisotropy as compared to uncoated nanoparticles. The presence of exchange bias in uncoated nanoparticles indicates that their surface layer is not totally magnetically dead. In summary, (i) the SiO2 coated nanoparticles have lesser dipolar interactions due to SiO2 coating and (ii) surface layer in uncoated nanoparticles is not magnetically dead but may have different preferred ordering at low temperatures as compared to SiO2 coated nanoparticles.