Study on the influence of annealing temperature and ferrite content on the structural and magnetic properties of x(NiFe₂O₄)/(100 − x)SiO₂ nanocomposites

Abstract

Magnetic nanocomposites of nickel ferrite nanoparticles uniformly dispersed in the silica matrix have been synthesized successfully by a sol–gel process using tetraethylorthosilicate (TEOS) and metallic nitrates as precursors. In addition, the influence of the annealing temperatures, varying from 400 to 900 °C, and NiFe₂O₄ contents, x(NiFe₂O₄)/(100 − x)SiO₂ (10 ≤ x ≤ 60 wt.%), on the structural and magnetic properties of the nanocomposite samples have been investigated. The studies carried out using XRD, FT-IR, TEM, STA (TG–DTG–DTA) and VSM techniques. The results indicated that the structural and magnetic properties of the samples showed great dependence on the variation of the particle size caused by the annealing temperature and NiFe₂O₄ content. The crystallization, saturation magnetization M_s and remenant magnetization M_r increased as the annealing temperature and NiFe₂O₄ content increase. But the variation of coercivity H_c was not in accordance with that of M_s and M_r, indicating that H_c is not determined only by the size of NiFe₂O₄ nanoparticles. TEM images showed spherical nanoparticles homogeneously dispersed in the silica network and were uniform in both morphology and particle size distribution with sizes of 10–15 nm. The results showed that the well-established silica network provided nucleation locations for NiFe₂O₄ nanoparticles to confinement the coarsening and aggregation of nanoparticles. The synthesized nanocomposites with adjustable particle sizes and controllable magnetic properties make the applicability of nickel ferrite even more versatile.

Introduction

Magnetic nanocomposites consisting of nanometric spinel ferrites embedded in an insulating matrix such as silica have attracted much attention in recent years due to their new magnetic properties [1] and their applicability in a variety of areas such as magnetic recording media, high-density information storage, ferrofluid technology, bioprocessing, magnetic drug delivery, catalysts, magnetic resonance imaging enhancement, gas sensors and magneto-optical devices [2], [3], [4], [5], [6], [7], [8], [9].

Among spinel ferrites, nickel ferrite is one of the most versatile and technologically important ferrite materials because of its magnetic properties, high electrochemical stability, catalytic behavior, abundance in nature, low conductivity and thus lower eddy current losses [10]. In addition, NiFe₂O₄ is the most suitable material for device applications in the upper microwave and lower millimeter wave ranges. Apart from its technological importance in the electronic and magnetic industries, NiFe₂O₄ has been used as a highly reproducible gas [9], [11] and humidity [12] sensor material.

Various synthetic routes have been reported in the literature for the preparation of nanoscale ferrites such as ceramic method [13], sol–gel [14], co-precipitation [15], solvent evaporation [16], hydrothermal [17], combustion [18], microemulsion [19] and citrate methods [20]. In the previous report, spinel cobalt ferrite nanoparticles have been synthesized by the polymeric precursor method [21]. While the nanoparticles obtained usually have a strong tendency to aggregate; this makes it very difficult to exploit their unique physical properties [22]. The preparation of magnetic nanocomposites through the dispersion of ferrite nanoparticles in a suitable matrix represents a route to obtain very fine nanoparticles by reducing particle agglomeration [23] leading to a narrow distribution of the dimensions. Also this technique allows one to stabilize the particles and study their formation reactions.

The interest in the preparation of magnetic nanocomposites has increased in the last years due to the properties presented by these materials, which are dependent on particle sizes, component ratio and distribution in the matrix. Magnetic nanocomposites showed considerable differences in the magnetic properties when compared with their equivalent pure and bulk materials. Different nanoparticles such as Fe [24], Ni [25], Fe₂O₃ [26], and NiZn-ferrite [27] dispersed in the silica matrix with applications in areas such as catalysis, sensors and electronic devices have been studied. Magnetic nanocomposites of nickel ferrite nanoparticles dispersed in the silica matrix have been studied [28], [29], [30], [31], [32], [33], [34], [35] revealing behavior different from that of bulk systems and as a model for the study of small particles.

Among various synthetic routes, the sol–gel process offers some advantages in making inorganic composite materials containing highly dispersed magnetic particles. The process facilitates a good and homogeneous dispersion of the particles into the inorganic matrix. The porous nature of the sol–gel derived amorphous silica matrix is an excellent host for supporting different types of guest nanoparticles which provides nucleation sites for magnetic nanoparticles and minimizes the aggregation imposing an upper limit to the size of the particles. This method makes possible the introduction of various concentrations of different components in a matrix with molecular homogeneity, which can be vitreous or crystalline, either porous or densified.

In this paper, the influence of annealing temperature and NiFe₂O₄ content on the structural and magnetic properties of x(NiFe₂O₄)/(100 − x)SiO₂ (x = 10, 20, 30, 40, 50, 60 wt.%) nanocomposites prepared by sol–gel method has been reported aiming at tuning the magnetic properties of NiFe₂O₄ nanoparticles dispersed in a silica matrix and greatly expanding the range of applications by adjusting the annealing temperature and NiFe₂O₄ content. Also, the alcogel precursors with different weight percents of components have been investigated. Therefore, special attention is given to the correlation between the structural and magnetic properties of NiFe₂O₄ nanoparticles embedded in a silica matrix, for different annealing temperatures and component ratios.