Curie temperature of NiFe2O4 ferrite nanocrystals grown by solid-state reaction

Ferrites are compounds characterized by specific electric and magnetic properties. The anomalous changes of these properties are recently observed in nanocrystalline ferrites, hav-ing ultrafine grain sizes [1-2]. In the present work, we focus on the formation of ferrimagnetic nickel ferrite (NiFe2O4) by thermally activated solid-state reactions between equimolar mix-tures of antiferromagnetic NiO and Fe2O3 powders. The growth of the these nanocrystals was obtained under isothermal conditions (Tiso) for some selected annealing times (tann). Some dif-ferent samples have been obtained by varying tann with Tiso ranging from 550°C up to 1000°C. The kinetic of the reaction has been investigated using the Magnetic Field Gradient Thermogravimetric technique and the results have been interpreted by applying the Tam-mann’s equation and the Huttig’s model [3]. The formation of three distinct stages for the nickel ferrite have been identified by measuring the corresponding activation energies of 2.0•102 kJ/mol, 3.1•102 kJ/mol and 1.7•102 kJ/mol, respectively. The morphology of nickel ferrites has been examined by Scanning Electron Microscopy (SEM). The SEM images sug-gest that nickel ferrite samples have a rod-like morphology having a diameter of few hundred nanometers, while their lengths are dependent on the conditions of used thermal treatment. Magnetization curves of the studied samples have been measured at room temperature using a Quantum Design SQUID magnetometer. The dependence of coercivity and residual magneti-zation on Tiso and tann indicates that nickel ferrites behave like multi-domain fine particles. We have observed that the Curie temperature of the samples is directly proportional to Tiso, but it is independent on tann. According to the one-electron molecular-orbital Dionne’s model [4], the magnetic exchange energy depends on the modifications of the spinel NiFe2O4 structure. Indeed, Dionne concluded that the Curie temperature is not reduced greatly when the divalent Ni2+ ions move from tetrahedral to octahedral sites occurring in presence of the spinel changes from normal to inverse. In order to check if Tiso may influence the distribution of the Ni2+ metal ions over the tetrahedral and octahedral sites, and consequently the Curie temperature of the nanocrystalline spinel ferrites, some Fourier Transform Infra-Red (FTIR) absorption spectra have been meas-ured. The FTIR absorbance spectra have been recorded for some nickel ferrite samples ob-tained by different thermal treatments. The measured spectra show between 600 and 400 cm-1 two vibrations attributed to tetrahedral lattice sites of NiFe2O4 [5]. The absorbances of these vibrations, found at 418 and 565 cm-1, have been used to determine the content of NiFe2O4 formed by thermal treatment of the starting oxide mixture. Further FTIR analyses are in pro-gress to correlate the spinel NiFe2O4 structure with the corresponding ferrimagnetic proper-ties. [1] Sanjukta Ghosh, et al. Phys. Lett. A 325 (2004) 301. [2] S. Chakraverty, et al. Phys. Rev. B 71 (2005) 024115. [3] R.C. Turnbull, J. Appl. Phys. 32 (1961) 380S. [4] Gerald F. Dionne, J. Appl. Phys. 99 (2006) 08M913. [5] P.A. Miles et al., Rev. Mod. Phys. 29 (1957) 279