Magneto-optical properties of Mn³⁺ substituted Fe₃O₄ nanoparticles

Abstract

Mn_xFe_3−xO₄ (0.0≤x≤1.0) nanoparticles were synthesized by the polyol synthesis method and the effect of Mn³⁺ substitution on structural, magnetic and optical properties of Fe₃O₄ was studied. X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), UV–visible spectroscopy and vibrating sample magnetometer (VSM) were used to study the physical properties. The crystallite (from XRD) and particle sizes (from TEM and SEM) are in close agreement with each other. Lattice parameter increases with increasing Mn³⁺ concentration, due to the respective larger ionic radius of Mn³⁺ ion compared with the Fe³⁺ ion. The magnetic hysteresis (M–H) curves revealed superparamagnetic characteristics of the products. The extrapolated specific saturation magnetization (σ_s) values decreased from maximum value of 47.3 emu/g to the minimum value of 25.6 emu/g by increasing Mn composition. The particle size dependent Langevin function was applied to determine the magnetic particle dimensions (D_mag) around 15 nm. The observed magnetic moments of NPs are in range of (1.06–1.96) µ_B and significantly less than 4 µ_B of bulk Fe₃O₄. Magnetic anisotropy was offered as uniaxial and calculated effective anisotropy constants (K_eff) are between 34.47×10⁴ Erg/g and 21.83×10⁴ Erg/g. The size-dependent saturation magnetization suggests the existence of a magnetically inactive layer as 1.638 nm on FeMn_xFe_2−xO₄ NPs. The UV–vis diffuse reflectance spectroscopy (DRS) and Kubelka−Munk theory were applied to determine the optical properties. The estimated optical band gap (E_g) values dropped almost linearly from 2.05 eV to 1.17 eV with increasing Mn composition.

Introduction

From the few decades, there has been a great attention on the preparation and characterization of superparamagnetic metal oxide nanoparticles of spinel ferrite. A strange behavior of nanoparticles has been discussed briefly in a previous research work [1]. As compared with bulk ferrites, the nanoparticles have special optical and magnetic properties due to their large volume occupied by the atoms at the grain boundary area, surface anisotropy, spin canting, dislocations and superpargamagnetism [2].

For the synthesis of spinel magnetic oxide nanoparticles, several methods have been developed, including hydrothermal, solid state, microemulsion, sol–gel, microwave, and coprecipitation methods. Among these synthesis methods, polyol (thermal decomposition) method of metal complexes in the presence of ligand (as capping agent) appears currently to be the most promising to ensure the control of the NPs׳ size, shape, and composition. The polyol process, which is known for providing monodisperse fine metal particles, afforded us the opportunity to synthesize ferromagnetic metal particles smaller than 2 µm and to investigate their dynamic properties [3], [4], [5], [6], [7], [8]. Then the polyol process appears to be a suitable method to provide quasi-spherical monodisperse metal particles over a wide range of particle size with accurate control of this size. And the polyol process also appear to be very convenient materials for studying the influence of the particle size upon the microwave permeability [9]. In addition, nanoparticles synthesized by this method are in situ coated by an organic layer of ligands, which ensure their good colloidal stability in organic solvents.

Cation distribution of metal ions effect the magnetic properties of the host compound [10]. Magnetite (Fe₃O₄) are usually isomorphically substituted by divalent (Co, Ni, Zn, Cu, Mn, etc.), trivalent (Al, V, Cr, etc.) or tetravalent (Ti) cations. Although this substitution maintains the spinel structure [11], it changes the catalytic, electromagnetic and thermoelectric properties of the magnetite [12], [13], [14], [15], [16]. As a result of that the substitution of trivalent metal into spinel ferrites may produce the promising materials for catalytic and enhanced memory storage applications [17]. MnFe₂O₄ belong to a group of soft ferrite materials characterized by high magnetic permeability and low losses. Manganese ferrite nanoparticles with spinel structure are important for technological applications owing to their soft magnetic properties. They are widely used for microwave, inductance, magnetic recording media, electronic devices, magnetic storage devices, magnetic contrast and agent applications due to its high electrical resistance and high magnetic permeability [18], [19], [20], [21], [22], [23]. As a potential member of contrast agents in Magnetic Resonance Imaging (MRI), the superparamagnetic manganese ferrite (MnFe₂O₄) NPs have been found to have a very large relaxivity and high magnetization owing to their large magnetic spin magnitude [24], [25]. There are several methods employed to synthesize manganese ferrite such as solid-state reaction, high-temperature solution phase reaction of metal acetylacetonates, coprecipitation method and self-ignited high temperature synthesis [23], [26], [27], [28], [29], [30]. Even polyol techniques provide many advantages such as control over the particle size and shape, and mass production, which is main factor for technological applications.

In this study, Mn_xFe_3−xO₄ (x=0.0, 0.2, 0.4, 0.6, 0.8 and 1.0) NPs were synthesized by polyol route and the effect of Mn³⁺ substitution on structural, optical and magnetic properties of Fe₃O₄ was studied for the first time.