Magnetism is one of the oldest and an important field in science and technology. By virtue of the different magnetic behaviour exhibited by different materials, they have been put into different applications. Certain materials has also been developed and designed artificially while others are tuned to suit different conditions, applications and needs. Over the years, magnetic materials have been part of the revolutionary development in the field of electronics as well.
On the basis of the response and behaviour of materials in the presence of an applied magnetic field, materials have been broadly classified into the following categories:
Different theories which are based on different assumptions have been developed to explain their distinct response to the applied magnetic field and the exhibition of persistent magnetisation by some materials even in the absence of an applied magnetic field.
Lodestone appears to be the first magnetic material to have been used and studied extensively. It belongs to the class of ferrimagnets. Its chemical formula is given by Fe3O4.
Ferrimagnets exhibits uncompensated antiferromagnetism. This is because the antiparallel magnetic moments in the constituent sub-lattices are not totally compensated. Thus they exhibit spontaneous magnetisation (though lower than ferromagnets) below a characteristic temperature called Neel temperature.
One of the properties of ferrites which make them an important class of materials is the exhibition of a notably strong magnetism and at the same time with a very high electrical resistivity (of the order of 10 10 times that of ferromagnetic iron.1 This property reduces the eddy current loss when they are used as cores of transformers at very high frequencies. The size can also be made so small that the transformer principle can be extended to digital circuits and computer memory arrays. The phenomenon of precession of that an unpaired electron can exhibit in response to an applied magnetic field, forms a basis for the application of ferrites in wave guiding. 2 Ferrofluids production is another application of ferrites with applications in industrial as well as medical fields.3
Fig-1: Structure of cobalt ferrite. 4
A unit cell of a ferrite crystal has 8 formula units. The oxygen atoms make up a f.c.c structural lattice. This arrangement gives rise to two types of site vacancies i.e., the tetrahedral (A) sites and the octahedral (B) sites for the divalent and the trivalent ions to occupy (Fig -1). In normal spinels, all the divalent metal ions occupy the A-sites and the trivalent metal ions occupy the B-sites. Whereas in inverse spinels, half of the divalent metal ions occupy the A-sites and the other half along with the divalent metal ions are distributed among the B-sites. Ferrites in general are very flexible in terms of tuning of their properties. Substitution of iron ions from Fe3O4 with either a transition metal or a rare earth metal changes its electrical as well as its magnetic properties.1
Cobalt ferrite is an example of inverse spinel ferrites. It forms a very stable structure with a very large fraction of volume which is left vacant thus makes it suitable for electron hopping which may be treated as cation migration. It is highly versatile and exhibit remarkable phenomena like moderate saturation magnetization, magnetic anisotropy and high coercivity.5
The electrical and magnetic properties of cobalt ferrite can be suitably tuned by substitutions and doping for cobalt or for iron with either transition or rare earth metal.
Among many researchers who have studied the effect of transition element substitution on electrical properties of cobalt ferrite, Y.D. Kolekar et al5 studied the effect of manganese substitution for iron in cobalt ferrite on its dielectric properties. Likewise A.C. Druc et al6 studied the change in electrical properties of magnesium ferrite due to cobalt substitution for magnesium.
R.C. Kambale et al7 have reported the effect of cobalt substitution for nickel in nickel ferrite on the its structure and electrical properties as well. They reported an increase in grain size with the increase in cobalt ion concentration in the material.
Many researchers have explored the effect of rare earth metal substitution for iron in ferrrites. Md. T. Rahman et al8 reported an increase in lattice parameter due to gadolinium substitution for iron in cobalt ferrite which is attributed to the large ionic radius of Gd3+ compared to those of Co2+ and Fe3+. They also reported the formation of secondary phase and a decrease in electrical conductivity with increase of gadolinium content. A. K. Pradhan et al9 on the other hand reported an increase in crystallite size with no notable change in lattice parameter because of molybdenum doping in cobalt-zinc ferrite. They also reported the domination of the contribution from grain boundary over that from the bulk.
Apart from the dependence of the properties of ferrites on the nature of the doping species and site occupation of the ions, the dependence of the properties on the methods of preparation and crystallite size have been studied as well. Mathew George et al10 have studied the difference in properties of cobalt ferrite with different crystallite size and reported the contribution of interfacial polarization in the different samples of cobalt ferrites. N. Ponpandian et al11 have studied the effect of grain size effect on the electrical properties of nickel ferrite which resulted in the change of the contribution of the grain boundary and the bulk of the grain relative to each other. This was reported in terms of the increase in the activation energy with grain size reduction.