Low Curie-transition temperature and superparamagnetic nature of Fe3O4 nanoparticles prepared by colloidal nanocrystal synthesis

Abstract

Colloidal size, narrow size-distributed magnetite (Fe3O4) nanospheres of 12 nm diameter were synthesized by colloidal nanocrystal synthesis protocols. X-ray diffraction and transmission electron microscopy studies reveal that the as-synthesized magnetite particles were single grain, spherical shaped and well crysallined in cubic spinel structure. Lattice vibrational studies confirms the existence of metal-oxide nanospheres and organic functional group (oleic acid) present on the particles surface. The nanospheres exhibits slightly enhanced energy band gap compared to counterpart bulk. The sample shows space-charge type polarization under low electric field frequencies (0.1–3 MHz) in the high temperature range (305–790 K), with Curie temperature at 713 K. Hence the dielectric constant (ε') reduces with enhance of electric field frequency. Dielectric loss (ε″) also reduces with enhance of frequency and the loss is 0.015 upto 650 K under 3 MHz. Hence it may be suitable for low loss device applications. AC electrical conductivity (σac) enhances with frequency and polaron hopping is slower than the site relaxation. Temperature dependent impedance spectra analysis reveals that grain contribution is predominant than grain boundary contribution with Debye-type relaxation. The nanospheres exhibits typical superparamagnetic behaviour with reduced saturation magnetization (Ms) due to disordered spins on the nanospheres' surface. Langevin function fit gives 10.5 nm magnetic domain diameter in the nanospheres.