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Magnetic and structural Behavior of Fe-CoO Nanocomposites Mechanically Milled

The Fe60(CoO)40 nanostructured alloys have been prepared from pure iron and cobalt oxide powders by mechanical alloying technique within a high energy planetary ball-mill. Morphology, microstructural and magnetic properties of this powder were investigated by a Scanning Electron Microscope (SEM), X-ray diffraction (XRD) and Vibrating sample magnetometer (VSM). The effect of time of milling on magnetic behaviour of Fe(CoO) nanostructured composite has been investigated. Apparition of new phase polycrystalline sample having a size in the range of 12 and 26 nm, it is confirmed by X-ray diffraction testing. The enhanced magnetic properties and structural behaviour of the nanoparticle are due by the diminution of size of crystallite. After 40 hours of milling, the appearance of spinel structure of CoFe2O4. The reduction in particle size leads to a significant increase in magnetic hardening, the coercive field at room temperature increases from 6 Oe to 208 Oe


Introduction
A magnetic powder material is among the first areas of research in recent time for material researchers in accordance to its emerging potential applications in various technological fields and as well as new structural and magnetic properties at the essential level [1]. The CoO is used extensively in the ceramics industry and transition metal oxides, the CoO have higher magneto-crystalline anisotropy [2][3]. In other way, iron nanostructured particles have attracted significant attention for the reason of their attractive magnetic properties and applications in high density magnetic recording media [4]. The main applications of cobalt iron oxide nanoparticles are as a nanoscale magnetic material with applications in molecular imaging and drug delivery [5][6]. In this paper, we are studying the magnetic properties in the Fe/CoO nanostructured alloy. The effect of crystallite size in the hysteresis loops has been observed by vibrating sample magnetometer.

Experimental procedure
The particle size and purity of elemental powders are 80 µm, 99.5% for Fe and 100 µm, 99.9% for CoO. The mixture of Fe, CoO powders was grinded in planetary ball mill (PM400) under controlled atmosphere used a hard chromium grinding balls. The initial powder mixtures are contains of 60% wt of Fe and 40% wt of (CoO). The mass ratio of powder to balls was approximately 1:30.The time of milling was varied from 5 to 40 hours. The mechanical alloying method was used to elaborate Fe60 (CoO)40 powder alloys and to decrease the particles sizes after severe plastic deformation. Morphology, particle size and particle distribution of powders alloy formed after mechanical milling was characterized by scanning electron microscopy (Gemini SEM 300) attached with Energy Dispersive X-ray unit (EDX). Crystallite size, lattice strain and lattice parameters were determined by X-ray diffraction analysis (XPERT PRO) using Co K radiation. Magnetic measurement properties were determined by Vibrating Sample Magnetometer (VSM).

Powder morphology
Morphology of the Fe60(CoO)40 powder mixture milled for different times was shown in figure 1. During the milling time, Fe60(CoO)40 powder having uniform particle size distribution, flattened and irregular shapes, this morphology is caused by severe plastic deformation, welding process and fracture process [12][13]. Cold welding and fracture process lead to considerable changes in the morphology of the powder particles during mechanical alloying. The mean particle size of Fe60(CoO)40 powder alloy after 40 hours of milling (figure 1.e) is smaller than 5, 10,15 and 25 hours of milling, indicating that the milling effect speed up the reduction of particle size.      The EDS analysis presented in figure 2e shows the presence of Fe and CoO in Fe60(CoO)40 powder milled to 40h with mass percentage 57.63 % Wt and 42.37%Wt respectively. The repartition of these elements (Fe, Co and O) presented in figure 2d shows a good homogeneity in the powder alloy. Figure 3 shows the progression of the crystallites size and lattice strain, kinetics of formation, evolution of lattice parameters determined by X-ray diffraction analysis of Fe60 (CoO)40 powder alloy during mechanical milling time to 5 until 40hours.  (1)

Structural characterization of Fe60 (CoO)40
(2)  Figure 4 shows the variation of average crystallite size and lattice strain of Fe60(CoO)40 powder alloy during mechanical milling time, the reduction of the size of crystallite is due of the increase of energy stocked inside particles during plastic deformation. The increase of lattice strain and the decrease of the size of crystallite in function of mechanical milling time are caused by distortion effect in the crystallite lattice with a high density of dislocations.  Figure 5 shows the Change of the lattice parameter of Fe60(CoO)40 powder alloy during of mechanical milling time, the increase in the lattice parameter during milling time has been noticed. This variation is easily explained in the first step by the dislocation introduced during mechanical alloying. The decrease in the next step can be explained by a dynamic equilibrium between the mechanical alloying effects and the restoration of particle powder.   Figure 6 show the hysteresis loops for Fe60(CoO)40 powder alloy milled during mechanical alloying. The hysteresis loops determined seem as soft magnetic materials with the magnetic properties varied versus mechanical milling time. The variation of coercive force (Hc), magnetization saturation (Ms), and residual magnetization (Mr) during mechanical alloying is shown in figure 6.

Conclusion
The Fe60(CoO)40 alloy powders were successfully elaborated by mechanical milling technique using PM400. The magnetic behavior, morphological of powder and structural parameters of Fe60(CoO)40 powder alloy could be determined. The structural parameters such as the crystallite size modify the magnetic behavior of Fe60(CoO)40 powder alloy.
The mappings of the powders alloy are distributed in a homogeneous way and kept the same proportion with the initial chemical composition. The coercivity of the Fe60(CoO)40 alloy highly depends on the structural parameters, the saturation magnetization is highly dependent on the chemical composition of powder alloy.