I Introduction
Zinc Ferrite nanoparticles have been getting attention due to their applications in magnetic resonance imaging as a contrast agent [1] and spintronics devices [2]. In spinel ferrites, magnetic properties originate from the AFM coupling between the octahedral and tetrahedral sublattices. In normal spinel (AB2O4), the tetrahedral sites are occupied by divalent (A) cations while trivalent (B) cations occupy octahedral sites. Contrasts to this, in inverse spinel structure, divalent cations occupy octahedral sites whereas trivalent cations are distributed equally among A and B-sites. General notation of the structure of spinel ferrites can be written as (Me2+1-XFe3+X)[Me2+XFe3+2-X]O4, whereas Me is divalent metal ion. The trivalent cations Fe3+ occupied tetrahedral sites is the degree of inversion . The resultant magnetization is the difference between the magnetizations of A- and B-sublattice, and therefore it is related to the distribution of the cations in the sublattices. In the bulk material, the majority number of divalent cations (Cu2+, Mn2+, Fe2+, Co2+, Ni2+, Mg2+, or Cd2+) prefer incorporation at the larger octahedral lattice sites. But in the case of Zn2+, it shows almost total preference to occupy the tetrahedral sites. Although Zn2+ is diamagnetic, when it is used in these mixed ferrites, the magnetic moment of the A-sublattice decreases, so the total magnetization of the crystal will be increased. Bulk ZnFe2O4 should have paramagnetic nature around room temperature, because it is the normal spinel with Zn incorporated mostly in the tetrahedral lattice sites, so the superexchange interaction between the two sublattices cannot be expected, and it should not come under ferrimagnetism. In the case of ZnFe2O4 nanoparticles, it shows ferrimagnetic behavior, and it is the matter of nanophase which influences the structure of crystal more than it can have partially inverse spinel structure [3]–[5]. Increase in the degree of inversion of ZnFe2O4 with decreasing particle size was found with neutron powder diffraction analysis [4]. Recently the EXAFS study of Zn-ferrite nanoparticles also has confirmed the increase in degree of inversion with decrease in particle size. Apart from the particle size, the preparation method and synthesis conditions or parameters can affect the value [6], [7].