Properties of Magnetic Substances

Magnetic Materials

Types of magnetic materials

On the basis of mutual interactions or behavior of various materials in an external magnetic field, the materials are divided in three main categories.

(i) Diamagnetic materials: Diamagnetism is the intrinsic property of every material and it is generated due to mutual interaction between the applied magnetic field and orbital motion of electrons.

(ii) Paramagnetic materials: In these substances the inner orbits of atoms are incomplete. The electron spins are uncoupled, consequently on applying a magnetic field the magnetic moment generated due to spin motion align in the direction of magnetic field and induces magnetic moment in its direction due to which the material gets feebly magnetized. In these materials the electron number is odd.

(iii) Ferromagnetic materials: In some materials, the permanent atomic magnetic moments have strong tendency to align themselves even without any external field.

These materials are called ferromagnetic materials.

In every unmagnetised ferromagnetic material, the atoms form domains inside the material. The atoms in any domain have magnetic moments in the same direction giving a net large magnetic moment to the domain. Different domains, however, have different directions of magnetic moment and hence the materials remain unmagnetised. On applying an external magnetic field, these domains rotate and align in the direction of magnetic field.

COMPARATIVE STUDY OF MAGNETIC MATERIALS

Property	Diamagnetic substances	Paramagnetic substances	Ferromagnetic substances
Cause of magnetism	Orbital motion of electrons	Spin motion of electrons	Formation of domains
Explanation of magnetism	On the basis of orbital motion of electrons	On the basis of spin and orbital motion of electrons	On the basis of domains formed
Behaviour In a non-uniform magnetic field	These are repelled in an external magnetic field i.e. have a tendency to move from high to low field region.	These are feebly attracted in an external magnetic field i.e., have a tendency to move from low to high field region	These are strongly attracted in an external magnetic field i.e. they easily move from low to high field region
State of magnetisation	These are weekly magnetised in a direction opposite to that of applied magnetic field	These get weekly magnetised in the direction of applied magnetic field	These get strongly magnetised in the direction of applied magnetic field
When the material in the form of liquid is filled in the U-tube and placed between pole pieces.	Liquid level in that limb gets depressed	Liquid level in that limb rises up	Liquid level in that limb rises up very much
On placing the gaseous materials between pole pieces	The gas expands at right angles to the magnetic field.	The gas expands in the direction of magnetic field.	The gas rapidly expands in the direction of magnetic field
The value of magnetic induction B	B < B0	B > B0	B >> B0

Magnetic susceptibility χ	Low and negative |χ| ≈ 1	Low but positive χ ≈ 1	Positive and high χ ≈ 102
Dependence of χ on temperature	Does not depend on temperature (except Bi at low temperature)	Inversely proportional to temperature χ ∝ 1/T  or χ= C/T.This is called Curie law, where C = Curie constant	χ ∝ 1/T-Tc  or χ = C/T-Tc.This is called Curie Weiss law. Tc = Curie temperature
Dependence of χ on H	Does not depend independent	Does not depend independent	Does not depend independent
Relative permeability (μr)	μr < 1	μr > 1	μr >> 1 μr = 102
Intensity of magnetisation (I)	I is in a direction opposite to that of H and its value is very low	I is in the direction of H but value is low	I is in the direction of H and value is very high.
I-H curves
Magnetic moment (M)	The value of M is very low (χ 0 and is in a direction opposite to H.)	The value of M is very low and is in the direction of H	The value of M is very high and is in the direction of H
Transition of materials (at Curie temperature)	These do not change.	On cooling, these get converted to ferromagnetic materials at Curie temperature	These get converted into paramagnetic materials above Curie temperature
The property of magnetism	Diamagnetism is found in those materials the atoms of which have even number electrons	Paramagnetism is found in those materials the atoms of which have majority of electron spins in the same direction	Ferro-magnetism is found in those materials which when placed in an external magnetic field are strongly magnetised
Examples	Cu, Ag, Au, Zn, Bi, Sb, NaCl, H2O air and diamond etc.	Al, Mn, Pt, Na, CuCl2, O2and crown glass	Fe, Co, Ni, Cd, Fe3O4 etc.
Nature of effect	Distortion effect	Orientation effect	Hysteresis effect

where  B₀  is the magnetic induction in vacuum