The Effect of Cationic Disorder on Low Temperature Magnetic Properties of MnZn Ferrite Nanoparticles

MnZn ferrite nanoparticles (NPs) with Mn_xZn_1-xFe₂O₄ (x = 0 and 0.6) compositions have been synthesized by sol-gel process. As prepared (AP) particles were annealed at 1200 °C in air to compare the physical properties of NPs with their bulk counterparts. The crystallite sizes of AP samples were estimated to be 18 nm for x = 0 and 11 nm for x = 0.6. The cationic disorder was evaluated from the Rietveld analysis of X-ray diffraction (XRD) data, which decreases with increasing annealing temperature. The average particle size of the AP samples is observed to be below 20 nm and transform into single-phase spinel structure of ~500 nm long cylindrical rods of 60 nm-diameter for x = 0.6, when annealed at 1200 °C in air. The magnetic properties of AP NPs show ferrimagnetic behavior below the blocking temperature (T_B) and superparamagnetic behavior above the T_B. The observed magnetic characteristics corroborate well with the structural parameters obtained from the Rietveld analysis of the XRD data.

I. Introduction

Magnetic properties of Mn–Zn ferrites nanoparticles (NPs) have been investigated to enhance the performance of ferrites compared with their bulk counterparts for wide range of applications. Bulk MnZn spinel-ferrite compacts are commonly used in the medium range frequency applications, such as transformers, choke coils, inductors, and electromagnetic interference devices [1]. On the other hand, the NPs are envisaged as candidates for biomedical applications, such as drug delivery, magnetic hyperthermia treatments, and magnetic resonance imaging [2]–[4]. Soft magnetic ferrites belong to the cubic spinel structure and the general cation distribution formula is ( Fe Fe ( Co, Ni, Mn, and Zn), where the subscript is known as inversion parameter of spinel structure and it varies from 0 to 1. For normal spinel and for an inverse spinel . The bulk ZnFe₂O₄ is a normal spinel which is antiferromagnetically ordered below K [5], while ZnFe₂O₄, (MnZn)Fe₂O₄ NPs assume the inverse spinel structure [3], [6]. The distribution of cations over tetrahedral and octahedral sites will affect the magnetic properties of the system significantly. It is now established that the magnetic properties of the NPs depend on the method of synthesis, annealing temperature, particle size, cation distribution, and composition [3], [7]. In bulk ferrites, the competition between the two sublattice magnetization generates interesting – curves while the NPs undergo further changes as a function of temperature due to symmetry breaking effects. They give rise to superparamagnetic (SPM) behavior. It is observed that the blocking temperature ( increases with Mn concentration with nm particle size as SPM limit [3], [8]. However, on annealing at higher temperatures the increased for Zn-ferrite NPs synthesized by different methods [9], [10] while it decreased for free-standing and silica-coated Zn-ferrite NPs [11], [12]. These variations were attributed to size effects in the former case while degree of inversion rather than particle size for later.