Impact of rare earth europium (RE-Eu³⁺) ions substitution on microstructural, optical and magnetic properties of CoFe_2−xEu_xO₄ nanosystems

Abstract

In this study, pure cobalt ferrite (CoFe₂O₄) nanoparticles and europium doped CoFe₂O₄ (CoFe_2−xEu_xO₄; x = 0.1, 0.2, 0.3) nanoparticles were synthesized by the precipitation and hydrothermal approach. The impact of replacing trivalent iron (Fe³⁺) ions by trivalent rare earth europium (RE-Eu³⁺) ions on the microstructure, optical and magnetic properties of the produced CoFe₂O₄ nanoparticles was studied. X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectra exposed the consistency of a single cubic phase with the evidence of Eu₂O₃ phases for x ≥ 0.2. FTIR transmittance spectra showed that, the all investigated samples have three characteristic metal-oxygen bond vibrations corresponding to octahedral B-site (υ₁ and υ₂) and tetrahedral A-site (υ₃) around 415 cm⁻¹, 470 cm⁻¹ and 600 cm⁻¹ respectively. XRD and energy dispersive X-ray spectroscopy studies affirmed the integration of RE-Eu³⁺ ions within CoFe₂O₄ host lattice and decrease of average crystals size from 13.7 nm to 4.7 nm. Transmission electron microscopy (TEM) analysis showed the crucial role played by RE-Eu³⁺ added to CoFe₂O₄ in reducing the particle size below 5 nm in agreement with XRD analysis. High resolution-TEM (HR-TEM) analysis showed that the as-synthesized spinel ferrite, i.e., CoFe_2−xEu_xO₄, nanoparticles are single-crystalline with no visible defects. In addition, the HR-TEM results showed that pure and doped CoFe₂O₄ have well-resolved lattice fringes and their interplanar spacings matches that obtained by XRD analysis. Magnetic properties investigated by the vibrating sample magnetometer technique illustrated transformation of magnetic state from ferromagnetic to superparamagnetic at 300 K resulting in introducing RE-Eu³⁺ in CoFe₂O₄ lattice. At low temperature (~5 K) the magnetic order was ferromagnetic for both pure and doped CoFe₂O₄ samples. Substitution of Fe³⁺ ions in CoFe₂O₄ nanoparticles with RE-Eu³⁺ ions optimizes the sample nanocrystals size, cation distribution and magnetic properties for many applications.

Introduction

One of the fascinations, yet challenges, of the magnetic nanosized materials, is how to optimize their structural properties for a new magnetic, electronic, optical and catalytic configuration [[1], [2], [3], [4], [5], [6]]. One of the magnetic nanomaterials which attract many across the research community is cobalt spinel ferrite (CoFe₂O₄). Cobalt ferrites are at the focus of the research scientists due to its uniqueness of structural, optical and magnetic characteristics such as high magnetocrystalline anisotropy, strong spin-orbit coupling, controllable optical energy gap, moderate saturation magnetization at 300 K, high Curie temperature and its high chemical and thermal stability [7,8]. Such properties make CoFe₂O₄ a good candidate for many important technological applications such as spintronics, gas sensing, magnetic-resonance imaging, memory devices, diagnostic imaging, refrigeration, microwave absorbers and ferrofluid technology [9]. Also, CoFe₂O₄ is found in Lithium-Ion batteries, solar cells, electrocatalytic oxidation and medical drug targeting [10].

For the development of new applications of cobalt nano-ferrites, it is important to adapt the magneto-optic properties of these nanomaterials. The essential approaches, by which, one can modify these properties for a specific application are; first, the optimization of the fundamental fabrication processes. Second, the modification of the constituents of the cobalt ferrite nanomaterials such that the properties of the newly added impurities are partially transported to the properties of the original cobalt ferrite compounds. In this context, researchers have been broadly investigating the relation between the methods of preparation and the physical properties of cobalt ferrite nanostructures to develop new techniques for controlling the particle size and shape of those spinel compounds [11,12].

In previous studies, researchers found that the introduction of the trivalent (RE) cations into the spinel ferrite host lattice can induce RE³⁺–Fe³⁺ interactions [[13], [14], [15]]. When RE-ions diffuse into the ‘B-lattice’ sites of CoFe₂O₄ lattice, a displacement of proportionate number for Fe³⁺ from ‘B-lattice’ to ‘A-lattice’ positions occur. This modifies the magnetic properties such that it results in an improved application for RE-doped cobalt ferrites in comparison with non-RE-doped cobalt ferrite systems [16]. As an example of such improved behavior is the usage of cobalt ferrite nanostructures in many magneto-optical devices such as recording media, light modulators, and deflector. These devices are non-efficient and less practical due to the high Curie temperature which affects the heat-assisted magnetic recording. Doping cobalt nano-ferrites with RE-ions is found to lower their Curie temperature and improves their magneto-optical response [17]. In a different biomedical application, in the absence of an external magnetic field, post a medical procedure, the superparamagnetic behavior of the nanoparticles prevents their aggregation and improves the colloidal stability. This facilitates the elimination of these nanoparticles through the body’s natural filters such as the liver or the immune system [18]. In other words, transforming the magnetic behavior of cobalt nano-ferrites from ferromagnetic state to superparamagnetic state by doping with RE-ions such as Eu³⁺ helps the spinel ferrite nanoparticles, coated with biocompatible materials, attachment on the cell surface. Thus, making these nanomaterials key role players in various biomedical applications such as cell manipulation, tumor therapy, magnetic hyperthermia, targeted drug delivery, DNA/RNA isolation and tissue repair [19,20].

Cobalt nano-ferrite doped with RE-ions are fabricated by several techniques used to obtain materials in the form of nanostructure such as the standard solid-state, microwave-assisted, sonochemistry-based, sol–gel, hydrothermal, solvothermal, and precipitation methods [[21], [22], [23]]. Precipitation and hydrothermal methods are favorable to prepare these materials for many reasons. For instance, preparation of nano-ferro materials by precipitation does not involve the use of organic solvents and allows maximum control of the particle size and the stoichiometric composition. However, this method requires high sintering temperature for the resulting precipitates to achieve a suitable particle size. The usage of the hydrothermal method prevents clustering, improves the colloidal stability of nanoparticles, and requires minimal preparation energy. Taking into account all preceding considerations, we used a mixture synthesis technique that features the benefits of the two previous methods. This devised method guarantees smooth and homogeneous material, narrower nanoparticle size distribution and improved production rates at low reaction temperatures.

Despite many reports investigated the effect of RE³⁺ ions substituting the Fe³⁺ ions on the CoRE_xFe_2-xO₄ nano-ferrite systems [[24], [25], [26]], yet the synthesis of RE-Eu³⁺ doped CoFe₂O₄ nanoparticles in high yield and controlled physicochemical properties still not fully understood. In this work, we report on the precipitation and hydrothermal joint-approach for the preparation of CoEu_xFe_2-xO₄ nanoparticles and the characteristically improvements due to the change of its microstructure and magneto-optical characteristics.