1.1 INTRODUCTION:
The naturally occurring ferrite is the ferrous ferrite (Fe3O4) as “Load stone”. In early days it was called as ferromagnetic material. L.Neel, tells us that these materials are “ferromagnetic” material due to uncompensated anti parallel spin arrangement. Due to interesting intrinsic properties magnetic materials are classified into Ferromagnetic materials, Ferrites. In last decades , in the field of the ferrites expensive development was done by many contributors and found to be technically, commercially useful magnetic material. These materials are at high frequencies, high electrical resistivity of magnetic temperature coefficient of resistance along with low magnetic loss.

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Basically, Ferrous ferrite (Fe3O4) consist double oxides FeO and Fe2O3 . The properties of these materials are alter by substituting divalent iron ion by divalent cation like Cd2+,Mg2+,Ni2+,Zn2+,Cu2+,Co2+ etc. from transition element. The spinel ferrites are represented as MFe2O4, where M is divalent cations. The magnetic properties are significantly improved by substituting trivalent iron ion by Al3+,Cr3+,Mn4+, Ti4+,Sn4+, Gd3+,Nd3+ etc. for particular applications because of their interesting magnetic and electrical properties with chemical and thermal stabilities[Gadkari A.B et.al. (2010)].
The applications of these materials in fields like electrical components, memory devices, microwave devices telecommunications, electronic industries, dada storage, data processing due to its high resistivity and low power losses. [Ernst Schloeman et al. (2000)]
Ferrites are of two type’s,one is hard ferrite and other is soft ferrite. The material which are easily magnetized and demagnetized, it is called soft magnetic material .That means, it can store or transfer magnetic energy in alternating or other changing wave forms. The ferromagnetic materials like iron, nickel, cobalt and some of the rare earths materials shows a unique magnetic behavior. All the magnetic moments of the individual ions or atoms are aligned parallel to some particular direction and the unpaired electron spins line up parallel with each other in the ferromagnetic material. The region in which magnetic dipoles align parallel to each other is called a domain. The structural properties of ferrite are very important. The structural properties of ferrites mainly depends on the manufacturing process of ferrite[Mangalaraja R.V. et al. (2003)].
In fundamental science, magnetic spinel ferrites are of great importance. They are useful especially for find out the fundamental relationship between magnetic properties and their crystalchemistry and structure [ Kalonji G.et.al.(1999)]. They find extensive applications in microwave devices, radar, digital recording, ferrofluids, catalysis and magnetic refrigeration systems [Horvath M.P. et.al. (2007)]. For the spinel ferrite the general formula is AB2O4and it is consisting of an almost perfect cubic closed packed oxygen arrangement, with the cations residing on tetrahedral and octahedralinterstices.
Nowadays, magnetic materials are used in various fields. The soft magnetic materials can be attracted to a permanent magnet and the hard magnetic materials become a permanent magnet. In case of soft magnetic materials, the large magnetic fields cannot be generated to the outside but in hard magnetic materials it generates magnetic fields. The magnetic materials like iron, cobalt and nickel ordinarily reveal prominent magnetic property. In industry, the ferromagnetic materials are widely used. In case of metal and alloy magnetic materials, due to their lower electrical resistivity the initial permeability and magnetic flux density is high and loss in eddy current is large at high frequencies. Nowadays, high-frequency characteristics are more useful, so this is occurred in ferrites which is multiplying the thin films. Due to higher electrical resistivity, the soft ferrites has excellent characteristics at high frequency. They are abundantly used for inductors or core materials of transformer. The hard ferrite is also used abundantly as permanent magnets for speakers and motors.
The soft ferrite is used into an alternating magnetic field. Ferrites are chemical compounds. They are composed of a ceramic material along with iron oxide as their main component. The magnetic property of the ferrite is due the structure and the arrangement of the ions in the sub lattice.
1.2 SPINEL COMPOUNDS
The word spinel which is derived from Italian spinella, diminutive of spine, thorn (from its sharply pointed crystals). In the cubic system, Spinel crystallizes forming octahedral crystals. In spinel super group there are at least 30 oxide minerals included. The majority of spinel compounds belongs to the space group Fd3m. The formula for the principal member of the group has, AB2O4; out of which ‘A’ is a divalent metal ion such as magnesium, iron, nickel, manganese and zinc. The ‘B’ is trivalent metal ions such as aluminium, iron, chromium and/or manganese. Also, titanium Ti4+ and Pb2+ etc. may occupy this site. The solid solutioning which is common process in this group of minerals that means they may contain certain percentages of different ions in any particular specimen [Adams, D. M (1974)]. The oxygen ions are mostly larger than the metallic ions and the spinel structure can be formed by a cubic close packing of O2- ions, in most oxide structures in which the cations (e.g. Co2+, Fe3+) occupy certain interstices. So, the structure of a spinel compound and the highly symmetric structure of diamond is same. The position of the A ions and the positions of carbon atoms occupied in the diamond structure is identical. In this group this could discuss the relatively high hardness and high density. The arrangement in the structure of the other ions shows the symmetry just like diamond structure. This arrangement of the ions verifies the octahedral crystal structure which is the predominant crystal form and also the trademark of the spinels. Now a day, there are well over a hundred compounds are reported of the spinel structure. Most of them are oxides, sulphides, selenides and tellurides and some are halides. There are different cations may be introduced into the spinel structure and several charge combinations are possible, therefore, almost any combination that added and balances eight positive charges of anionic charges [Smyth,D.M.(2000)],for example Co2+Fe23+O4, Mg22+Ti4+O4, Li1+Al3+Ti4+O4, Li0.51+Al2.53+O4 and Na21+W6+O4, etc.
In oxide spinels, there are two types of cations which do not differ in size greatly because the spinel structure is stable only when the cations are rather medium sized and also the radii of the different ionic species in the same compound must be nearly equal. Therefore, similar cation combinations occur in sulphides, e.g. Zn2+Al23+S4 and Cu22+Sn4+S4. Hence, in halide spinels e.g. Li21+Ni3+F4 and Li1+Mn23+/ 4+F4 in which cations are limited to charges of plus 1 and plus 2, give an overall cation: anion ratio of 3 as 4.
Most spinels divided into three series determined by a B metal represent aluminate series with Al3+ (Hercynite, Gahnite, Galaxite); a magnetite series with Fe3+ (Magnetite,Magnesioferrite, Franklinite); the chromite series with Cr3+ (Chromite, Magnesiochromite). There is extensive cationic exchange (solid solution) within each series but very little between the series [King, R. J (2004)].
These spinels are classified on the basis of the distribution of cations in the two principal sites which are tetrahedral site (T-) and octahedral site (O-) [West, A. R. (1989)], into three types.
1.2.1 NORMAL SPINEL
In normal spinel A (BB) O4, all the divalent (A) cations placed on the tetrahedral (T-) sites and the trivalent (B) cations present on the octahedral (O-) sites. Which can be shown by the formula [A]tet [B2]oct O4. The examples of normal spinel are
MgO.Al2O3 = MgAl2O4 ——– (normal, parent mineral)
ZnO.Fe2O3 = ZnFe2O4 —— (normal)
FeO.Al2O3 = FeAl2O4 —— (normal)
CoO.Al2O3 = CoAl2O4 —— (normal)
MnO.Al2O3 = MnAl2O4 ——— (normal)
NiO.Al2O3 = NiAl2O4 ——- (normal)
1.2.2 INVERSE SPINEL
The inverse spinel, B (AB) O4, the divalent cations occupying on the O-sites and the trivalent cations which are equally divided among the T- and remaining other on O-sites. Which can be represented by formula, [B]tet [A, B]oct O4. The CoFe2O4 is conformably an inverse spinel with a formula;
CoxFe1-x (Co1-xFe1+x) O4(with x 0)
where x represents the cation distribution factor which discuss the fraction of tetrahedral sites occupied by Co2+ cations [Guire, M. R.D (1989)].
CoO.Fe2O3 = FeCoFeO4 ——— (inverse)
NiO.Fe2O3 = FeNiFeO4 ——- (inverse)
MgO.Fe2O3 = FeMgFeO4 —— (inverse)
1.2.3 RANDOM SPINEL
It has an intermediate cation distribution, represented as [B0.67 A0.33]tet [A0.67B1.33]octO4.
In this elementary unit cell of spinel structure consists eight tetrahedral and sixteen octahedral sites which are occupied by metal ions and the extreme cases, represent completely normal and inverse spinel, Therefore, the general cation distribution can be represented as
[ M(2)iq+M(1)1-ip+]AIV [M(1)p+i/2 M(2)(2-i)/2q+]2BvI O4
where M+(1)p+ and M(2)q+ are the minority and majority cations respectively. Hence, the first quantity in brackets shows the average occupancy of A-sites (coordination number of four (IV)) and the second quantity in brackets shows the average occupancy of B-sites (coordination number of six (VI)). The inversion parameter γ, shows the fraction of A-sites acquired by majority ions.