Ferrimagnetism and soft magnetic materials

Ferrimagnetism (ferrites):

1. These magnetic materials in which there is permanent magnetization due to presence of magnetic moment of adjacent atoms or molecules in opposite direction but having unequal magnitude is known to have ferrimagnetisms.
2. Some ceramic materials exhibit ferrimagnetisms.
3. One example of material which exhibits ferrimagnetisms is Cubic ferrites. The chemical formula is \(M Fe_2 O_4\), where M is divalent metal ion \(M^{2+}\).
4. When \(M^{2+}\) is iron element then chemical si FeO, \(Fe_2O_3\)(ferrites).
5. Here \(Fe^{2+}\) ion exist in +2 and +3 valancy state in the ratio of 1:2 which corresponds to 4 and 5. Bohr’s magneton(\(\mu_B\)) respectively. For two types of ion. Here \(O^{-2}\) is ion magnetically neutral . Due to anti-parallel spin coupling interaction between two types of Fe ion, leads to Ferrimagnetisms.

Octahedral lattice site:

closed packing of spheres on a flat surface with second layer are added. The circle on the second layers re drawn small for clarity. The location of octahedral site is denoted by 3 spheres below it and 3 spheres and position of tetrahedral site is denoted by A.

The point on B is called octahedral site it is equidistance from 3 sphere below it and 3 spheres above it.

The points represented by A which is equidistance from 4 nearest neighbor is called tetrahedral site.

6. The two types of position that may be occupied by iron cation are tetrahedral site and octahedral site for tetrahedral site coordination number is 4 and for octahedral site is 6.
7. The spin magnetic moment of all \(Fe^{3+}\) ion in octahedral position are align parallel to each other. However, they are directed opposite to \(Fe^3+\) ions dispose in tetrahedral position. Thus spin moments of all \(Fe^3+\) ion cancel one another and makes no contribution to magnetization of solid. All \(Fe^{2+}\) ion have their magnetic moment align in the same direction. These ions are present in octahedral site where as \(Fe^{3+}\) ions are present in octahedral and tetrahedral sites.

Soft and hard magnetic materials:

Depending upon the strength of magnetic field required and total energy loss during complete cycle of magnetization and demagnetization. Ferromagnetic materials are classified into the two categories:

1. Soft magnetic materials
2. Hard magnetic materials

Soft magnetic materials

1. The area of hysteresis in B-H cycle gives total energy loss perunit volume of the material during magnetization and demagnetization. This loss in energy is generated as heat energy raising the temperature of the of the specimen.

In figure OR represents the magnetic flux density gain by the material which is called retentivity for a particular materials has importance to produce large magnetic field and this property is used to make electromagnet .

3. The opposite magnetic field to destroy the magnetization or magnet density gain by material is represented by OC and is called coercive force or coercivity. These material which have larger value of \(H_c\) are used as permanent magnetic materials or permanent magnets or permanent memory device.
4. For soft magnetic material the value of \(H_c\) is small and area of hysteresis loop is small. A soft magnetic material most has high initial permeability and low coercivity. A material having this properties reach its saturation magnetization with relatively low field.(i.e. magnetized and demagnetized, so soft magnetic material).

Soft magnetic materials are used to in device that are subjected to alternating magnetic fields in which energy losses must be low. For example: core of transformer, they are used in generators, motors dynamo and switching circuit. Examples of some soft magnetic materials are

99. Commercial iron (99.95 % Fe weight %)
100. Silicon iron alloy (97% Fe+3% Si)

• Perma alloy (55% of Fe +45% of Ni)

1. Superm alloy (79% Ni+ 15% alloy + 5% Mo+0.5% Mn)
2. Ferrox cube A (48% \(Mn Fe_2O_4, 52% ZnFeO_4\))
3. Ferrox X cube B(36% Ni\(Fe_2O_4, 64% ZnFeO_4\))

References:

Callister, W.D and D.G Rethwisch. Material Science and Engineering. 2nd. New Delhi: Wiley India, 2014.

Lindsay, S.M. Introduction of Nanoscience . New York : Oxford University Press, 2010.

Patton, W.J. Materials in industry . New Delhi : Prentice hall of India, 1975.

Poole, C.P. and F.J. Owens. Introduction To Nanotechnology. New Delhi: Wiley India , 2006.

Raghavan, V. Material Science and Engineering. 4th . New Delhi: Pretence-Hall of India, 2003.

Tiley, R.J.D. Understanding solids: The science of Materials. Engalnd : John wiley & Sons , 2004