CoFe₂O₄ magnetic ceramic derived from gel and densified by spark plasma sintering

Highlights

• Cobalt ferrite (CoFe₂O₄) has been processed by sol–gel and spark plasma sintering technique (SPS).

• CoFe₂O₄ ceramic shows low electrical conductivity (σ_AC∼0.0022 at 10⁴ Hz and room temperature).

• CoFe₂O₄ ceramic shows complex dielectric relaxation phenomena.

• CoFe₂O₄ ceramic shows values of Ms and Hc for microwave or radio frequency devices.

Abstract

Cobalt ferrite (CoFe₂O₄) has been successfully synthesized by sol–gel technique. Pellets were prepared by spark plasma sintering technique (SPS) from CoFe₂O₄ sol–gel derived powder. The rapid sintering of CoFe₂O₄ pellet by SPS at 950 °C, leads to a dense ceramic (97%ρ_theoretic) with average crystallite size of 71 nm. As revealed by X-ray diffraction and Mössbauer measurements, the obtained powder and SPS pellets are spinel ferrite with a cubic symmetry. Complex dielectric investigation performed on CoFe₂O₄ ceramic reveals multiple relaxation mechanisms while the magnetic measurements indicated a saturation magnetization value of ∼83 A m²/kg at room temperature, which make them useful for applications in microwave domain.

Introduction

CoFe₂O₄ spinel ferrite is of high interest because of its many applications in advanced technologies e.g., lithium ion battery, high-density data storage, magnetic fluids, magnetic drug delivery, medical diagnostics, microwave devices, magneto-optics devices, sensors, high frequency applications and catalysis [1], [2], [3], [4]. The cobalt ferrite (CoFe₂O₄) is a hard magnetic material with high coercivity (80 kA/m–250 kA/m, depending on grain size etc.), moderate magnetization (80 A m²/kg) and high Curie temperature (520 °C) [5], [6]. Furthermore, it shows high cubic magnetocrystalline anisotropy, high magnetostrictive coefficient, great physical and chemical stability [7]. Therefore it has emerged as an important technological material being one of the most promising candidates for high-density recording media such as audio and videotape. Despite of its properties, much attention is given in the last years to the substituted cobalt ferrite with Gd, Mn, Zn, Cr, Al, etc. Moreover, recently, many synthesis of double phase multiferroic composites like CoFe₂O₄–Pb(Zr,Ti)O₃ [8] and CoFe₂O₄–BaTiO₃ [9], [10] start to emerge. However, the detailed reports on cobalt ferrite pellets have not drawn enough interest so far. Only, several papers report on the structural and magnetic properties of CoFe₂O₄ sintered ceramic [11], [12], [13], [14], [15]. Furthermore, there is high interest in literature regarding the electrical conduction and dielectric behaviour of these ferrites [15], [16], [17], [18], [19], CoFe₂O₄ ferrites being potential candidates for booth magnetic and electric applications. The dielectric investigation can provide important information on the behaviour of localized electric charge carriers, giving rise to a better understanding of the mechanism of dielectric polarization. Specifically, dielectric behaviour is one of the most important properties of ferrites which depend on the preparation conditions, e.g. sintering time, and temperature and heating and cooling rate [15], [16], [17], [18], [19]. Hence, further investigation of the CoFe₂O₄ ferrite ceramic structure evolution, dielectric, and magnetic properties is required.

The magnetic properties of the of CoFe₂O₄ particles depends on their size, shape, and purity which are very sensitive to the preparation method. These particles should be nanosized, pure phase and single domain in order to improve their functional properties. For this reason, several papers report on the conditions and methods used for the preparation of cobalt ferrite nanopowders, such as: chemical co-precipitation [2], hydrothermal method, microwave synthesis, sol–gel method [20], [21], [22], complexometric method [23], polyol method [24] and others. The sol–gel route is one of the most efficient methods to prepare ultra-fine particles with a high purity and homogeneous composition [23], [24], [25]. The consolidation of nanopowders to produce dense bodies while maintaining their fine crystallite size is usually difficult by conventional methods such as hot pressing, etc. Furthermore, in order to get the high magnetic performance as that of powders, avoiding detrimental effects, a highly densified pellet is necessary. Therefore, motivated by these factors, we report in the present paper the preparation of CoFe₂O₄ nanopowder and bulk by sol–gel and by SPS sintering, respectively. The aim is to obtain fine-grained sintered cobalt ferrite ceramics, with preserved or improved magnetic characteristics. The dependence of the dielectric properties and AC conductivity on frequency, at room temperature, has been investigated in details. Furthermore, ferromagnetic properties of the ceramic are analysed by recording magnetization (M) vs. magnetic field (H) loops at different temperatures.