A magnet produces a magnetic field that pulls on ferromagnetic materials like iron via attraction or repulsion. Magnetism originates from the orbital and spin motions of electrons, which produce magnetic moments. When a substance is placed in an external magnetic field, it exhibits different magnetic behaviors classified as diamagnetism, paramagnetism, ferromagnetism, or antiferromagnetism based on the sign and magnitude of susceptibility and dependence on field strength and temperature. Diamagnetic materials are repelled by a magnetic field while paramagnetic materials are attracted into a field. Ferromagnetism and antiferromagnetism arise from interactions between atomic spins in a crystal lattice.

2. Magnet:
A magnet is a material or object that produces a magnetic field. This
magnetic field is invisible but is responsible for the most notable
property of a magnet: a force that pulls on other ferromagnetic
materials, such as iron, and attracts or repels other magnets.

3. Origin of Magnetism:
According to classical model of an atom, the electron has two types of motion.
1. Orbital Motion: Produces magnetic field or magnetic moment which is called
orbital magnetic moment.
2. Spin Motion: Produces magnetic moment which is called spin magnetic moment or
simply spin moment of the electrons.
These two magnetic moments produce magnetic properties in substances. When a
substance which behaves as a magnet is placed between the poles of magnet, the
magnetic field produced within the substance interacts with externally applied
magnetic field different substances show different behaviour in the applied field.
The different magnetic behaviours are classified into
1. Dia magnetism
2. Para magnetism
3. Ferro magnetism
4. Anti-Ferro magnetism

4. Various magnetic behaviour differ in regard to
1. Sign and magnitude of x(Khi)
2. Field strength
3. Temperature
Definitions of some terms
1. Magnetic dipole: Magnetic dipole is a magnetic system in which the
north and south poles of a magnet equal and opposite in character are
separated by a short but definite distance.

5. 2.Pole Strength: A magnetic unit pole that repels or attracts a similar pole,
placed one cm away with a force of 1 dyne. However a single magnetic pole
cannot be isolated the force between two poles are taken into
consideration.
3. Intensity of magnetisation(I): This is also called magnetic moment per
unit volume. Intensity of magnetisation is defined as the pole strength
induced per unit area, thus if m is pole strength; Intensity of
magnetisation is distance between poles, A is area of pole.
I = M/A = m.I/A.I = magnetic moment/Volume
Hence I is magnetic moment per unit volume.
4. Magnetic Induction(B): When a substance is placed in a magnetic field
of strength H gauss, magnetic effect is induced in the substance and the
total magnetic induction or magnetic flux B induced in the sample is equal
to the sum of H and 4 π times of pole strength induced per unit area.
Thus B = H+4 π I

6. 5. Magnetic Permeability : (p) : “ P” represent the ration of the lines of
magnetic force per unit area in the sample to the lines of force per unit area
of magnetic field in vacuum. Thus P is given by
P= B/H
6. Magnetic susceptibility per unit volume: (χ)
It is a ratio of I/H and it gives the extent to which the substance is
susceptible to the external magnetic field or susceptible to magnetisation.
By placing B/H = P and I/H = X in egn.---(1)

7. 7. Gram Susceptibility (χg): In practico, the susceptibility is usually expressed per
unit mass rather than per unit volume.
Xg = Volume susceptibility/density = x/d
8. The molar susceptibility (Xm)
Xm = gram susceptibility X mol.wt.
Xm= Xg X M
Now let us discuss the details about various magnetisms.

8. Diamagnetism:
Diamagnetic materials are repelled by a magnetic field; an applied magnetic field
creates an induced magnetic field in them in the opposite direction, causing a repulsive
force.
The substances which when placed in a magnetic field decrease the intensity of
magnetic field are called diamagnetic substances and the property due to which they
show this behaviour is called diamagnetism. Diamagnetic substances are repelled by
the magnetic field and set themselves at right angle to the magnetic field. The
diamagnetism is caused by paired filled shell electrons.

9. Para magnetism:
The substances which when placed in a magnetic field, increase the intensity of the
magnetic field are called paramagnetic substances. The property due to which they
show this behaviour is called Para magnetism. The paramagnetic substances are
attracted into magnetic field. They set themselves parallel to the external magnetic
field. The para magnetism is caused due to the presence of unpaired electrons.

10. Ferromagnetism and Anti-Ferro magnetism:
The diamagnetic and paramagnetic properties characteristic of
individual atoms or complexes. The properties of ferromagnetism and Anti-
ferro magnetism depend on interactions of e- spins on many atoms and
arise from the co-operative behaviour of many unit cells in a crystal.
The magnitude of ferromagnetic susceptibility is very large. The magnitude of anti-
ferromagnetism is very small. These are temperature as well as field strength
dependent.
Spin only Magnetic Momentum:
The effective moment can be obtained by adding the magnetic moment due to
orbital motion and that due to the spin motion of the electron given by

11. Application of magnetic moments: From the knowledge magnetic
moments it is possible to find
1) No. Of unpaired electrons and oxidation state of the metal ion.
2) The bond type
3) Stereo chemistry of metal ion.
Ex: Anti Ferro Magnetics: Mno
Mno anti ferro magnetic below -151⸰
C.