Magnetic Properties of Zinc Ferrite Nanoparticles

In the present study, we report the synthesis of Zn-ferrite nanoparticles by a soft chemical coprecipitation method. The magnetic properties of Zn ferrite are sensitive to their average crystallite size ( d) which can be varied by altering the synthesis conditions. The d value was controlled by the pH of the solution which is related to the number of moles of OH^- ions (x ). The XRD pattern of as synthesized samples showed single phase spinel structure. The estimated d is in the range of 2 to 12 nm. The magnetization studies showed that the sample with d = 2 nm is paramagnetic whereas the samples with d >; 2 nm exhibited superparamagnetic behavior. The magnetization at 10 kG increases from 2.6 to 21.4 emu/g as d increases from 2 to 12 nm. The temperature dependent studies for the sample with d = 12 nm shows symmetric spectra with single resonance peak of Lorentzian shape. As the temperature decreases an additional peak at lower field side of the main signal appears. The g value and the linewidth (ΔH_PP) increase with decrease in temperature indicating the increase in antiferromagnetic (AFM) interactions between the ions in tetrahedral and the octahedral sites.

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].