Coercivity and exchange bias in size reduced iron obtained through chemical reduction

Abstract

Size dependent coercivity of Fe particles synthesized through polyol process in a vast average size range from 210 nm to 19 nm has been reported. The large particles exhibit multidomain behaviour whereas the smaller particles show single domain nature and exchange bias. Saturation magnetization upto 211 emu/g, close to bulk Fe, has been obtained for particles with the largest average size. The major contribution to magnetization reversal is influenced by the magnetocrystalline anisotropy of Fe particles. The room temperature coercivity of multidomain particles with an average size of 210 nm is less than 100 Oe while that of 50 nm single domain particles reaches a maximum of 328 Oe. The TEM analysis of 50 nm and 19 nm particles reveal core shell morphology, with an iron oxide shell of thickness 3–4 nm. The increase in exchange anisotropy contribution due to size reduction has been studied from field cooled hysteresis loop measurements at 15 K. The interfacial exchange coupling between the core and shell gives rise to an exchange bias field of 16 Oe and 38 Oe for the 50 nm and 19 nm particles, respectively. Exchange bias blocking temperature of 50 K is observed for particles of average size 19 nm. The increase in exchange bias field for the particles of average size 19 nm has been correlated to interfacial effects using the Malozemhoff’s random field model with a roughness factor of 2.51.