In recent years, metal cobalt (Co) and cobalt oxide (Co3O4) have attracted a great deal of attention thanks to their special properties. Based on these special properties, they have a wide range of applications including sensors, the components of information storage, catalyzers, etc.  . Cobalt oxide has been known as a semi-conduc- tor compound of p type with a Spinel structure. Therefore, they are used as gas sensor and absorbent of solar energy. Furthermore, they are employed as an effective catalyze in environmental clearing -. Metal cobalt is important owing to its various crystal structures (fcc, hcp, ε), the type of the structure of cobalt affects its magnetic and electronic properties . The important point in fabrication of pure metals is their stability against oxidation. This is because as the size of particles declines, their resistance to oxidation diminishes as well. Recently, walked into particles have been prepared by some methods including thermal degradation  , vapor condensation  , decreased salt of cobalt  , and deposition method .
In the thermal gradation method, carbonyl cobalt Co2(CO)8 is often used as a precursor, though cobalt carbonyl is very toxic and expensive. Recently, Shao et al conducted thermal degradation using cobalt acetate with cobalt nanoparticles of 8 - 200 nm . Similarly, Lee et al have prepared cobalt nanoparticles through thermolysis of Co2+-Oleat2 . In order to prevent clotting of the particles, organic surfactants are used . Triphenyl phosphine (TPP) is widely used for stabilizing gold nanoparticles and other metals . The phenyl groups in TPP develop more spatial disturbance than chain alkyl groups .
2. Method of Fabrication
In materials science, the sol-gel process is a method for producing solid materials from small molecules (Figure 1). The method is used for the fabrication of metal oxides, especially the oxide of cobalt.
The process involves conversion of monomers into a colloidal solution (sol) that acts as the precursor for an integrated network (or gel) of either discrete particles or network polymers.In this method, metal nanoparticles and the metal oxide have been prepared through thermal degradation of the metal-surfactant complex in a hot surfactant solution. This method is schematically shown in Figure 2, demonstrating the fabrication of cobalt nanoparticles.
Figure 1.The sol-gel method for fabrication of nanoparticles
Figure 2.The procedure of fabrication of Co3O4 nanoparticles
3. Magnetic Properties
Firstly, the magnetic properties and hysteresis loops of magnetic compounds are generally discussed, followed by investigation of the properties of cobalt and cobalt oxide nanoparticles.
Magnetic compounds have been among interesting issues for human beings for over 4000 years. In large societies, magnetic compounds including computer disks, credit cards, speakers, coolers, automatic doors, and many other devices can be observed on a daily basis .
There are various magnetic compounds including diamagnetic, paramagnetic, and ferromagnetic. Further, ferromagnetic itself is categorized into two smaller groups called anti-ferromagnetic and ferrimagnetic. When magnetic moments are positioned randomly in relation to each other, the pure magnetic moments of the Crystal is zero. This state is called paramagnetic. However, the ferromagnetic crystal has a pure magnetic moment. The ferrimagnetic crystal also has a pure magnetic moments, but magnetic areas are positioned at opposite directions to each other. One of the areas is larger than the other and thus eventually a net magnetic moment will be present. However, in the antiferromagnetic crystal, the magnetic areas are absolutely of the same size and in opposite direction. Two important properties of magnetic compounds are Curie temperature and magnetic hysteresis. Exchanged coupling and thus the Heisenberg’s exchange energy are directly related to Curie temperature (Tc) of ferro and ferrimagnetic compounds. At temperatures lower than Tc, the magnetic moment is the same as the specific crystallographic direction of the zero axis of these compounds. It is preferred that this axis be called easy axis of magnetic crystal. This axis is developed in response to pairing this electron spin and the angular momentum of electron orbital. Due to presence of the easy axis, by applying an external magnetic field, the formation of the crystal of the compounds is controlled. When the magnetic field with the crystal’s easy axis and its direction towards the external magnetic field overcome the barrier energy between the magnetic areas, it changes their direction towards the magnetic field. This barrier energy is known as Magnetocrystalline Anisotropy, the atomic region of the magnetic hysteresis behavior of magnetic compounds. EA is one of the most important principles of the magnetic properties of compounds, determining the stability of compounds for special uses .
Ferromagnetic compounds are of interest based on their applications. Their properties should be identified quantitatively using the hysteresis loop of magnetic compounds. A hysteresis loop can be measured by placing a sample in a magnetometer and the compound’s response (M, σ) to the exerted magnetic field (H). Several quantities can be obtained from the hysteresis loop .
Magnetic saturation (Ms) or special magnetic saturation (σs) are the cases that show the extent of magnetization when all bipolars have been ordered in the direction of the exerted magnetic field.
The remaining magnetic (Mr) is the magnetization of the sample in a magnetic field of zero. The inhibition force (Hc) is a force of the magnetic field required for changing the remaining magnetization. The change in the field's bias (HE) indicates the extent of displacement off the center of hysteresis loop.
Figure 3 demonstrates the magnetization curve of a ferromagnetic compound. The total changes of magnetization of the sample M has been shown in terms of the intensity of the exerted DC field (H). Initially, when the
Figure 3. Hysteresis curve of a ferromagnetic.
exerted field increases, so does M, until it reaches the saturation point Ms. When the exerted field decreases from the saturation point, M does not reach its initial value, rather it stops at a point higher than the reduced field. This condition is called “residual”. It happens when the areas that have been aligned with the increase in the field do not return to their original orientation with the reduction of the field. When the exerted magnetic field H reaches zero, the magnet has still magnetization called residual magnetization (Mr). As can be seen in Figure 3, in order to remove the residual magnetization, the Hc field should be exerted in the opposite direction of the initial field. This field is called driving field, which causes the areas to rotate and return to their original positions. The properties of the magnetization curve of a ferromagnetic compound is a strong reliance for applying magnetic compounds.
Originally, the magnetic properties of compounds can be understood and controlled through magnetic pairings. Such pairings have a close relationship with the type of the chemical compound and the magnetic structure of compounds, though no accurate relationship is known between the magnetic properties of compounds, their chemical composition, and their crystal structures at an atomic level. There are various factors contributing to proper understanding of the changes of magnetic areas at atomic level by external exerted fields as well as the magnetic properties and magnetic pairings at atomic level. The structure of magnetic nanoparticles includes unique magnetic areas. Multi-area structures are not desirable in terms of energy due to their small size. Without the presence of the walls of areas, the magnetic pairing of atomic balance is directly associated with the magnetic properties of nanoparticles. Certainly, understanding and controlling the magnetic properties of nanoparticles will clarify the mechanism of magnetic properties of compounds together with its design and control. Magnetic nanoparticles are promising thanks to reduction of magnetic areas and thus development of super-paramagnetic properties. The super-paramagnetic properties of nanoparticles are directly influenced by magnetic Anisotropy of nanoparticles. As is evident in Figure 3, due to the symmetry of the precursor, there are sites with equal energy balances on the surface of the metal kernel. TPP is a surfactant with a high boiling point, developing a great spatial inhibition of controlling the magnetic properties of magnetic nanoparticles, despite the three phenyl groups. In addition to TPP in the mixture, presence of oleyl amine and the complexity of the precursor significantly diminish the size of particles. Oleyl amine is known as a ligand that has a suitable bond with the surface of metal nanoparticles. The combination of the effects of TPP and oleyl amine greatly contributes to development of individual nanoparticles.
4. Results and Discussion
4.1. Magnetic Results
As shown in Figure 4 the curve exhibits an antiferromagnetic behavior. The ferromagnetic behavior of the nanoparticles can be explained as follows: bulk Co3O4 has a normal spinel structure with antiferromagnetic exchange between ions which occupy the tetrahedral and octahedral sites .
Figure 4. CompoundCo3O4 nanoparticles .
The hysteresis curve for Co3O4 nanoparticles prepared at different temperatures were carried out at room temperature. As shown in Figure 5 the hysteresis curve for the Co3O4 nanoparticles prepared at 400 k exhibits a weak ferromagnetic behavior with a saturation magnetization of 0.125 emu∙g−1 at the maximum field of 9 kOe applied while the hysteresis curve for the Co3O4 samples prepared at 450 K and 500 K in the Figure 6 and Figure 7 display higher ferromagnetic properties with saturation magnetization values of 0.23 and 0.31 emu∙g−1 at the applied field of 9 kOe, respectively. To confirm that the ferromagnetic behavior originates from the nanoparticles, this measurement was also conducted on a bulk sample.
The change from an antiferromagnetic state for bulk Co3O4 to a weakly ferromagnetic state for the Co3O4 nanoparticles can be ascribed to the uncompensated surface spins and finite size effects. It is well known.
Figure 5. The hysteresis curve of Co3O4 nanoparticles at 400 K.
Figure 6. The hysteresis curve of Co3O4 nanoparticles at 450 K.
Figure 7. The hysteresis curve of Co3O4 nanoparticles at 500 K.
4.2. XRD Results
Figure 8 presents the X-ray Diffraction (XRD) pattern on the prepared samples. The peaks in Figure 8 are related to 111, 200, 220, 200, and 311 planes of hcp cobalt. Due to the very small size, cobalt nanoparticles are easily oxidized when exposed to air. The XRD pattern reveals no peaks related to the cobalt oxide phase.
This is due to the protection of the surface of cobaltnanoparticles against oxidation by oleyl amine and TPP. The developed peaks are wide, attributable to the reduction in the size of particles. This shows that all cobalt particles are in nano size. It is possible to obtain the mean size of the particles by Debye-Scherrer Equation. The mean size of these samples is 20 nm. Figure 9 demonstrates the XRD pattern of cobalt nanoparticles following exposure to air. In this figure, no peak associated with metal cobalt can be seen. The image is related to the cubic phase of Co3O4 with a Spinel structure. The network parameter is equal to a = 8.085 Å. No other pure peak is seen in this pattern, suggesting that the Co3O4 is completely pure. Separation of peaks is very good, implying that the crystal structure is single phased in the cubic crystal structure.
4.3. TEM Results
Figure 10 illustrates the TEM image of cobalt nanoparticles. The majority of Co3O4 nanoparticles are irregular (Figure 10). The mean size of Co3O4 nanoparticles is 10 nm.In HRTEM image (Figure 11), the distance be-
Figure 8. The XRD pattern of Co.
Figure 9. The XRD pattern of Co3O4 nanoparticles.
Figure 10. TEM image of Co3O4 nanoparticles.
Figure 11. HRTEM image of Co3O4 nanoparticles.
tween the two adjacent planes (d) is 4.6 A. The TEM images showed that the product nanoparticles consisted of dispersive quasi-spherical particles with a narrow size distribution ranged from 5 to 15 nm and an average size around 10 nm.
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