Dysprosium doping induced shape and magnetic anisotropy of Fe_3−xDy_xO₄ (x=0.01–0.1) nanoparticles

Highlights

• Report on formation of nanorods in magnetite prompted by Dy doping.

• Observation of anisotropic magnetic behaviour emanating from the shape anisotropy.

• Evidence of Dy³⁺ ions occupying octahedral site in place of Fe³⁺ ions.

• Nanorods envisaged to be useful as catalysts and in biomedical applications.

Abstract

The effect of dysprosium doping on evolution of structural and magnetic properties of magnetite (Fe₃O₄) nanoparticles is reported. A standard route of co-precipitation was used for the synthesis of undoped and doped magnetite nanoparticles Fe_3−xDy_xO₄ (x=0.0–0.1). Transmission electron microscopy (TEM) shows formation of round shaped particles with diameter in the range of 8–14 nm for undoped sample. On doping beyond x=0.01, the formation of rod like structures is initiated along with the round shaped particles. The number of rods is found to increase with increasing doping concentration. Magnetic characterization using Vibrating Sample Magnetometer (VSM) revealed doping dependent magnetic properties which can be correlated with the crystallite size as determined from X-ray diffraction (XRD). Enhancement in the saturation magnetization in the initial stages of doping can be explained on the basis of incorporation of Dy³⁺ ions in the inverse spinel structure at the octahedral site in place of Fe³⁺ ions. Subsequent decrease in saturation magnetization observed beyond x=0.03 could be attributed to precipitation of excess Dy in form of dysprosium ferrite phase.

Introduction

Magnetite (Fe₃O₄) nanoparticles are very widely used ferromagnetic material due to their unique properties like high magnetization, high resistivity, non-linear optical behavior and biocompatibility [1], [2], [3], [4]. Efforts have been done to further improve on these properties by using techniques like doping. Improved magnetization properties with doping have been reported in cases of Zn, Ni, La, Dy, Eu, and Tb doped Fe₃O₄ [5], [6], [7], [8]. Fe₃O₄ possesses inverse spinel structure with eight Fe³⁺ and eight Fe²⁺ ions in octahedral sites [site-A] and eight Fe³⁺ ions in tetrahedral site [site-B] [9]. Due to symmetrically placed Fe³⁺ ions at the two sites with opposite spins, their spin interaction nullifies each other's effect and Fe²⁺ ions govern the magnetic behavior. However, substitution of other element ion in place of Fe³⁺/Fe²⁺ ion can result into modified magnetic behavior. It is proposed that dysprosium could prove to be a good candidate for such substitution due to its high magnetic moment (10.6 μ_B). There have been earlier reports on modification of magnetic and structural properties caused by dysprosium doping in Co–Zn ferrite, Ni ferrite and cobalt ferrite [10], [11], [12]. In this work we investigate the effect of Dy doping on the properties of magnetite.

Here, the co-precipitation method has been used for synthesis of Fe_3−xDy_xO₄ particles with x varying from 0 to 0.1. The synthesized nanoparticles were characterized by using techniques like XRD, TEM, XPS and VSM. TEM images reveal round shaped particles of ~8–14 nm diameter in case of undoped Fe₃O₄ nanoparticles whereas rod like structures with aspect ratio in the range of 6–12 are visible for different Dy doping concentrations. The magnetic characterization of the oriented thin layers in parallel and perpendicular direction of applied magnetic field show anisotropic behavior emanating from the shape anisotropy. The saturation magnetization value shows gradual increase in the initial stages of doping with maximum value reaching for x=0.03. XRD indicates formation of dysprosium ferrite phase beyond x=0.03. To the best of our knowledge this is the first report on formation of nanorods in magnetite prompted by rare earth doping, where no surfactant or template was used. Such rod shaped particles can be used as catalysts and for biomedical application because the effect of functional materials attached to them will be over the large area as compared to spherical particles.