**3. Applications of magnetic properties**

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 to bulk materials [22].

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 communication [25].

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].

*Advanced Functional Materials*

ions [14].

applications [9, 12].

dielectric properties.

**2. Applications of coloristic properties**

netic and biomedical properties [4, 11].

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, mag-

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

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

Various methods for the preparation of undoped and doped Co ferrite NPs have

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

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

**52**

The magnetic properties of Co1−x−ySrxZnyFe2O4 (x = 0.0, 0.01, 0.05, 0.3 and y = 0.0, 0.05, 0.1, 0.4, 0.5, 0.7) NPs synthetized by spontaneous gel autocombustion (Pechini) technique were strongly influenced by the presence of both dopant ions, resulting in a superparamagnetic behavior [23]. The decrease of *MS* values with increasing dopant ions content and decreasing particle size is due to the surface anisotropy of nanoferrites, while the decrease of *HC* values is the result of some structural defects, such as dislocations, grain boundaries and anisotropy. The obtained results recommended the Zn-Sr co-doped Co ferrite as excellent candidate for various applications such as information storage devices, contrast agents in magnetic resonance imaging and gas sensors [23].

The addition of surfactants assures the control of the crystal nucleation and growth, due to their capability to act as a protective coating for NPs, reduces coalescence and enhances the crystallite size, porosity and specific surface. All these parameters further allow the control of the magnetic properties. In this regard, Co0.5Zn0.5Fe2O4 NPs prepared by co-precipitation method with ethanol as a surfactant show good *MS* and large *HC* [13]. When Co2+ ion with higher magnetic moment replaced Ni2+ ion with lower magnetic moment at B-sites, the *HC* and *MR* of CoxNi1−xFe2O4, (x = 0.0–0.4) [26] and NixCo1−xFe2O4 (x = 0, 0.25, 0.5, 0.75, 1.0) [27] increased, while *MS* changed randomly. This increase is the result of cations distribution at the octahedral (B) and tetrahedral (A) sites in lattice structure, in spin canting and spin disorder [26, 27]. The Ni1−xCoxFe2O4 (x = 0.0, 0.15, 0.3, 0.45, 0.6, 0.75, 0.9, 1.0) synthesized by Pechini's sol-gel method showed an increase of *MS*, *HC* and *TC* by Co2+ doping. Also, the number of magnetic domains increases and domain wall movement is facilitated by increasing particle size [28].

The magnetic properties of Co ferrite are also modified by incorporating Mn2+ ions. In case of MnxCo1−xFe2O4 (x = 0.2, 0.4, 0.6, 0.8) synthesized by sol-gel precipitation method, the *MS* increases (up to x = 0.4) and then decreases (up to x = 0.8) with increasing Mn2+ content, due to the surface disorders resulted from the distortion of the magnetic moments at the surface and to the antiferromagnetic nature of the Mn2+ ions. The *K* decreased with increasing Mn2+ content, indicating the interaction between grains [29]. The *MS* and magnetic moment increase with increasing Co2+ content in Mn1−xCoxFe2O4 (x = 0.2, 0.4, 0.6, 0.8) obtained by auto-combustion the ferrite structure [7].
