Magnetic materials

3.3 CLASSIFICATION OF MAGNETIC MATERIALS ON THE BASIS OF MAGNETIC MOMENT
Magnetic materials are classified according to the presence or absence of the permanent magnetic dipoles. Generally, every two electrons in an energy state of an atom will form a pair with opposite spins. Thus the resultant spin magnetic moment is zero. Hence they don’t have permanent magnetic moments and they are called as diamagnetic materials.
Example: Gold, germanium, silicon, etc.
But in some magnetic materials like iron, cobalt, etc., there exists unpaired electrons. The spin magnetic moment of these unpaired electrons interact with the adjacent atoms unpaired electron spin magnetic moment in a parallel manner resulting in enormous permanent spin magnetic moment. These materials are classified into para, ferro and ferromagnetic materials with respect to the electron spins.

3.3.1 DIAMAGNETIC MATERIAL
Ina diamagnetic material, the electron orbits are more or less random, and mostly all the magnetic moments are cancelled. Similarly all the spin moments are almost paired i.e., they have even number of electrons and has equal number of electrons spinning in two opposite directions. Hence the net magnetic moment in the diamagnetic material is zero. Therefore most of the materials do not have magnetism in the absence of magnetic field.
Effect of external field
When an external magnetic field is applied, the electrons reorient in such a way that they align perpendicular to the field direction and their magnetic moments opposes the external magnetic field. This will reduce the magnetic induction present in the specimen.
Properties

1. They repel the magnetic lines of force
2. Susceptibility is negative and it is independent of temperature and applied magnetic field strength
3. Permeability is less than 1
4. There is no permanent dipole moment, so they are called weak magnets
5. When temperature is less than critical temperature diamagnetic become normal material

Examples: Gold, Germanium, Silicon, etc.,
3.3.2 PARAMAGNETIC MATERIALS
In the case of paramagnetic materials, the spins in two opposite directions will not be equal. There exist some unpaired electrons which gives rise to spin magnetic moment. Hence the resultant magnetic will not be equal to zero.

However in the absence of external field the magnetic moments are oriented randomly. Due to its random orientation some magnetic moments get cancelled and the materials possess very less magnetization in it.
Effect of external field
When an external field is applied, the magnetic moments of individual molecules reorient itself along the direction of the magnetic field and the material is magnetized.
Properties

1. The magnetic lines of force pass through the material
2. Magnetic susceptibility is positive and it is given by
3. Permeability is greater than one
4. They possess permanent dipole moment
5. When the temperature is less than Curie temperature, paramagnetic materials becomes diamagnetic material.

Examples: CuSO4, MnSO4, Platinum etc.
3.3.3 FERROMAGNETISM
In a ferromagnetic material the numbers of unpaired electrons are more. Most of these spin magnetic moments point in one direction as shown in figure.

Hence even in the absence of external field, the magnetic moments align themselves parallel to each other and give rise to magnetic field.

Effect of magnetic field
To these materials even if a small external magnetic field is applied, the magnetic moments which are already aligned parallel, reorient itself along the direction of the magnetic field and they become very strong magnets.
Properties

1. Since some magnetization is already existing in these materials, all the magnetic lines of force passes through it
2. They have permanent dipole moment. So they act as strong magnets.
3. They exhibit magnetization even in the absence of external field. This property is called Spontaneous magnetization.
4. Its susceptibility is positive and high and it is given by
5. When the temperature is greater than Curie temperature, ferromagnetic material becomes paramagnetic material.
6. Permeability is very much greater than 1

Examples: Ni, Co, Fe, etc.,