**1. Introduction**

In the last decades, a wide range of nanomaterials were developed for applications in the field of magnetic recording and imaging, data and energy storage, refrigeration, electrical and communication devices, environmental depollution, catalysis, ceramics and pigments, sensors, medicine, etc. [1–8].

Spinel ferrite nanoparticles (NPs) with a general formula of MFe2O4 (M = divalent metal ion such as Mn, Cu, Co, Ni and Zn) have open a new and exciting research field because of their unique structural, magnetic, optical and electric properties. Among them, nanosized Co ferrite received a lot of attention due to its remarkable mechanical and chemical stability, wear resistance, dielectric character, electrical conductivity and excellent magnetic properties

such as high coercivity (*HC*) and moderate saturation magnetization (*MS*), high Curie temperature (*TC*) and large magnetocrystalline anisotropy [8, 9]. It also possesses some other unique characteristics such as good catalytic performance, small particle size, large surface area, narrow optical bandgap, non-toxicity and low production costs [10]. Depending on the composition, synthesis method and thermal treatment, Co ferrites have different structural, optical, electric, magnetic and biomedical properties [4, 11].

The doping with different elements adjusts the properties of ferrites by changing the structure, crystallinity and elements distribution among tetrahedral (A) and octahedral (B) sites [10]. The dopant amount, valency, size and site preferences define the structural, electrical and magnetic properties of doped Co ferrites [12]. The Co ferrite has an inverse spinel structure, but the doping with divalent metal ions could changes its structure into normal spinel [13]. The change from normal to inverse spinel depends also on the ratio between Co and the dopant ion. Co ferrite is a hard-magnetic material, but it may be softened by doping with non-magnetic ions [14].

The NPs embedding into or coating with insulating matrix such is silica could also considerably change the properties of the obtained nanocomposites (NCs), as the silica network can limit the particles growth, act as a buffer to protect the nanoparticles from mechanical stress and minimize the surface roughness and spin disorder [1]. Thus, by selecting the dopant, synthesis route and parameters, nanosized doped Co ferrites with tailored properties were produced for a broad range of applications [9, 12].

Various methods for the preparation of undoped and doped Co ferrite NPs have been reported such as: sol-gel, co-precipitation, polymerized complex, hydrothermal, thermal plasma methods, sol-gel, solvothermal, thermal decomposition, ultrasonic cavitation, mechanical alloying, ball milling, pulsed laser deposition, reverse micelle, micro-emulsion, microwave assisted synthesis, thermal decomposition, electrochemical and auto-combustion [6–8, 15, 16]. Although by using these methods, the required sizes and microstructures can be achieved, they are difficult to apply on large scale due to their complex and expensive procedures, long reaction times, high reaction temperatures, hazardous reagents and by-products and potential harm to the environment [16]. Among different synthesis procedures, the sol-gel method and post-annealing treatment is one of the simplest, feasible and most effective routes that produces high purity NCs at low temperatures and permits a good control over the particle size, morphology and chemical composition [1].

In this review, we summarize the recent, significant developments related to applications of Co ferrite NPs doped with divalent transitional metals in different fields based on their coloristic, magnetic, antimicrobial, biological, catalytic and dielectric properties.

## **2. Applications of coloristic properties**

The conventional coloring method of ceramics is based on the addition of pigments or dyes. Generally, the ceramic pigments are crystalline inorganic transition metal oxides powders with high chemical and thermal stability. They are soluble in glasses and glazes at high temperatures, have high tinting strength, high refractive index, low abrasive strength, and acid and alkali resistance. The color of pigments is determined by the presence of chromophore ions (usually transition metals) in an inert matrix (oxidic systems) or these ions may be part of their own matrix, as in the case of ferrites. The nano-pigments (nanoparticles dispersed in an organic vehicle) have a wide range of applications due to their high

**53**

*Progress, Challenges and Opportunities in Divalent Transition Metal-Doped Cobalt Ferrites…*

surface coverage, sharp spectral features, high scattering and uniform dispersion [17, 18]. The color performance of conventional ceramic pigments depends on the coloring efficiency and dissolution kinetics in the ceramic matrix, that are expected to be improved by small particle sizes. Magnetic inorganic pigments are also used in high-tech applications such as radar absorbing materials in military

Co ferrite is a black pigment widely used in the ceramic industry due to its excellent properties such as chemical and thermal stability [17]. There are only few studies reporting the use of divalent transition metal doped Co ferrites NPs as pigments. Sol-gel synthesis followed by post annealing pathway was used to obtain Zn doped Co ferrites (Co0.3Zn0.7Fe2O4 and Co0.7Zn0.3Fe2O4) embedded in SiO2 matrix in order to be used as dark gray to black color ceramic pigments [19]. The coloring properties of the Zn0.6Co0.4Fe2O4 NPs were tested by embedding in opaque and transparent tile glazes, and their application on ceramic tile. For pigments, the cartesian coordinates confirmed the dark gray color, that becomes almost black in bulk, while by disper-

The magnetic properties of nanomaterials, associated with the spin of electrons, make them suitable for various applications in biotechnology, telecommunications and electronic industries. The magnetic properties of cubic spinel ferrites depend upon their metallic composition, particle size and cationic distribution between tetrahedral (A) and octahedral (B) sites [7, 21]. In case of magnetic NPs, the presence of large number of atoms at the surface due to high-surface-to-volume ratio and finite size effects result in several interesting and superior properties compared

Co ferrite is well-known magnetic nanomaterial with high *HC* and *MS*. The *MS*, *HC*, *TC* and anisotropy constant (*K*) of Co ferrite decrease by doping with nonmagnetic ions decrease the hard-magnetic behavior and change the ferromagnetic to superparamagnetic behavior, leading to various applications [14, 23, 24].

Zn doped Co ferrites are soft magnetic materials with good chemical stability and high *HC* [25]. The *HC*, remanence magnetization (*MR*) and squareness ratio (*MR/MS*) decrease by doping, as a result of the anisotropic nature of spinel Co-Zn ferrites and the non-magnetic moment of Zn2+ ions. The *MS* values increase with the increasing content of dopant ions and their preference for tetrahedral (A) site. The dopant ions displace Fe3+ from tetrahedral (A) to octahedral (B) sites, resulting in weak magnetic interactions and low Neel temperature. High content of Fe3+ and Co2+ magnetic ions at the octahedral (B) sites leads to enhancement of B–B exchange interaction and weakening of A–B interaction. Nanosized magnetic zinc-cobalt ferrites with different Co to Zn ratio were obtained by co-precipitation [24], autocombustion [13] and sol-gel [14] methods. The high *MS* values make Zn-Co ferrites potential candidate for high-frequency inductors, information technology and

In case of Co1−xCdxFe2O4 (x = 0, 0.1) obtained by auto-combustion, the codoping with diamagnetic (Zn2+ and Cd2+) ions brings interesting change in the magnetic properties of Co ferrite. The Zn-Cd co-doped Co ferrites (ZnxCd0.375-x Co0.625Fe2O4, x = 0.0, 0.075, 0.125, 0.25) synthesized via chemical co-precipitation route are recommended as soft magnets. The variation of *MR*, *MS* and *HC* is due to the different chemical composition, crystal structure, particles size and arrangements at the lattice sites. The *HC* also decreases by increasing Zn content, due to the

lower magneto-crystalline anisotropy of Zn compared to Co and Cd [21].

sion in glazes the dark pigment present a bright gray color [20].

**3. Applications of magnetic properties**

*DOI: http://dx.doi.org/10.5772/intechopen.93298*

applications [2, 3].

to bulk materials [22].

communication [25].

*Progress, Challenges and Opportunities in Divalent Transition Metal-Doped Cobalt Ferrites… DOI: http://dx.doi.org/10.5772/intechopen.93298*

surface coverage, sharp spectral features, high scattering and uniform dispersion [17, 18]. The color performance of conventional ceramic pigments depends on the coloring efficiency and dissolution kinetics in the ceramic matrix, that are expected to be improved by small particle sizes. Magnetic inorganic pigments are also used in high-tech applications such as radar absorbing materials in military applications [2, 3].

Co ferrite is a black pigment widely used in the ceramic industry due to its excellent properties such as chemical and thermal stability [17]. There are only few studies reporting the use of divalent transition metal doped Co ferrites NPs as pigments. Sol-gel synthesis followed by post annealing pathway was used to obtain Zn doped Co ferrites (Co0.3Zn0.7Fe2O4 and Co0.7Zn0.3Fe2O4) embedded in SiO2 matrix in order to be used as dark gray to black color ceramic pigments [19]. The coloring properties of the Zn0.6Co0.4Fe2O4 NPs were tested by embedding in opaque and transparent tile glazes, and their application on ceramic tile. For pigments, the cartesian coordinates confirmed the dark gray color, that becomes almost black in bulk, while by dispersion in glazes the dark pigment present a bright gray color [20].
