Multiferroic behavior in silicate glass nanocomposite having a core-shell microstructure

Abstract

Nanosized iron core and barium titanate shell microstructure was generated within a silicate glass of composition 23.1 Na₂O, 23.1 BaO, 23.0 TiO₂, 7.6 B₂O₃, 5.8 Fe₂O₃, 17.4 SiO₂ by first reducing it at 893 K for 1/2 h and then subjecting it to heat treatment at 759 K for 4 h. Transmission electron microscopy showed the composite particles to have a mean diameter of 3.9 nm. The nanocomposite exhibited both ferroelectric and ferromagnetic behavior. The dielectric constant peak was not prominent because of a small thickness of the barium titanate phase. The magnetic hysteresis loop showed an asymmetric behavior giving rise to a small exchange bias field. This is believed to arise due to exchange interaction between the ferromagnetic iron core and the thin layer of Fe₃O₄ on the core surface with a spin glass-like behavior. The magnetization under zero-field cooled (ZFC) and field cooled (FC) conditions indicated superparamagnetic behavior at temperatures higher than 300 K. The optical absorption spectra exhibited a peak at around 325 nm. This was analyzed satisfactorily on the basis of a metal core-oxide shell nanoconfiguration. The extracted values of metal core conductivity showed a metal insulator transition for iron core diameters less than 2.4 nm. The present synthesis approach will lead to newer multiferroic nanocomposites and glasses with multifunctionalities.