Structure of Ferrites

STRUCTURE OF FERRITES

Ferrites are the magnetic compounds consisting of two or more different kinds of atoms. Generally ferrites are expressed as X²⁺ Fe₂³⁺ O₄, where X²⁺ stands for suitable divalent metal ion such as Mg²⁺, Zn²⁺, Fe²⁺, Mn²⁺, Ni²⁺ etc.

Examples

(i) If X²⁺ is replaced by Ni²⁺, then the ferrite (Ni²⁺ Fe₂³⁺O₄) is formed, thus named as nickel ferrite,

(ii) If X²⁺ is replaced by Fe²⁺ then the ferrite (Fe²⁺Fe₂³⁺O₄) is formed, thus named as ferrous ferrite.

Ferrites formed usually have a face centered cubic structure of oxygen ions closely packed together with the divalent and trivalent metal ions in the interstitial sites. This structure is called spinal structure. There are two types of ferrite structures

(i) Regular spinal

(ii) Inverse spinal

Regular spinal: Eg. Mg²⁺Fe³⁺O₄

In this type each metal atom (divalent) is surrounded by four O^{2 ‒} ions in a tetragonal fashion. For example, if the metal atom is Mg²⁺, then the structure is as shown in Fig. 1.14 and it is called 'A' site. Totally in an unit cell, there will be 8 tetrahedral (8A) sites.

Each Fe³⁺ (trivalent) is surrounded by six O^2‒ ions and forms an octahedral fashion as shown in Fig. 1.15. Totally, there will be 16 such octahedral sites in the unit cell. This is indicated by 'B' site.

Thus in a regular spinal, each divalent metal ion (Mg²⁺) exists in a tetrahedral form (A site) and each trivalent metal ion (Fe³⁺) exists in a octahedral form (B site). Hence the sites A and B combine together to form a regular spinal ferrite structures as shown in Fig. 1.16.

Inverse spinal: Eg., Fe³⁺ [Fe²⁺ Fe³⁺]O₄

In this, the Fe³⁺ ions (trivalent) occupies all the A sites (tetrahedral) and half of the B sites (Octahedral) also. Thus the left out B sites will be occupied by the divalent (Fe²⁺). The inverse spinal structure is as shown in Fig. 1.17.

Types of interaction present in the ferrites

The spin arrangement between the A site and B site is in an antiparallel manner and it was explained by Neel. According to him, in ferrites, the spin arrangement is antiparallel and there exists some interaction between the A and B sites which is represented as AB interaction.

Apart from this, there are two more interactions (i.e.,) AA and BB interaction which is negative and considerably weaker than AB interaction.

The tendency of AB interaction is to align all A spins parallel to each other and antiparallel to all B spins, but the tendency of AA and BB interaction is to spoil the parallel arrangement of A and B spins respectively.

Since AB is very strong as compared with AA and BB, the effect of AB interaction dominates and give rise to antiparallel spin arrangement.