Paramagnetic
Materials:-
In some
materials the paramagnetism arise due to the presence of permanent magnetic
dipoles. When such magnetic materials are placed in a strong uniform magnetic
field, they get feebly magnetised in the direction of the field. The applied
magnetic field aligns their atomic dipoles in their own direction. In this way
the resultant magnetic field in the magnetised material increases and become
greater than the applied field. The tendency of a material to increase the
magnetic field by magnetisation in the materials is called paramagnetism. The
materials which exhibit this property are called paramagnetic materials. In the
absence of an applied field, the dipoles are randomly oriented and hence the
total magnetisation in any given direction is zero.
The relative permeabilities of paramagnetic materials
are slightly greater than unity 
and their susceptibilities are positive, of the order of 10-3 to 10-5. At low magnetic field
magnetisation I varies with
in a linear way
, but at very high fields
the deviation from proportionality occurs. Examples of paramagnetic materials
are aluminium, sodium, platinum, manganese, antimony, copper chloride, liquid
oxygen, solutions of salts of iron and nickel, ect.
and their susceptibilities are positive, of the order of 10-3 to 10-5. At low magnetic field
magnetisation I varies with in a linear way, but at very high fields
the deviation from proportionality occurs. Examples of paramagnetic materials
are aluminium, sodium, platinum, manganese, antimony, copper chloride, liquid
oxygen, solutions of salts of iron and nickel, ect.
The rise in temperature of a paramagnetic material
increases its thermal energy and hence the thermal agitation of its atom so
that the alignment of the dipoles becomes more difficult. At finite
temperature, the thermal agitation makes the dipole orientation random. Even
the strong field is unable to compete with the force due to thermal agitation
as a result of which magnetic dipoles slightly turn in the field direction,
that is why the paramagnetic substances are feebly magnetised in the field
direction.
The other characteristics of paramagnetic substances
are as follows:
1. When a bar of a paramagnetic material is suspended
freely in the region of intense magnetic field between the two strong magnetic
poles, the axis of the bar becomes parallel to the field and the poles
developed at the ends of the bar are opposite to the nearer magnetic poles
figure.
2. When a paramagnetic material is placed in a
non-uniform magnetic field, it tends to move from weaker to the stronger parts
of the field.
3. It is also noticed that the susceptibility of a paramagnetic material is
inversely proportional to the absolute temperature (that is, c = C/T, where C is the Curie constant). Its value
decreases with the rise in temperature and becomes negative at higher
temperature. At higher temperatures the paramagnetic material becomes
diamagnetic.
4. In the surrounding of move paramagnetic medium, a
paramagnetic material behaves like a diamagnetic.
5. When a bar of paramagnetic material is placed in the
region of intense magnetic field, the bar concentrates most of the lines of
force into it figure and thus they become more dense.
6. It is observed that a paramagnetic gas when allowed
to ascend between the pole pieces of a magnet, it spreads along the field.
Ferromagnetic
Materials:-
There are
certain substances the permanent atomic dipoles of which have a strong tendency
to align themselves in the direction of the field in spite of the randomizing
nature of thermal agitation of its atoms.This property of the material is
called ferromagnetism and the materials which exhibit this property are called
ferromagnetic materials. Even in the absence of any external field the
ferromagnetic materials show ferromagnetism that is why permanent magnets are
made from them. The phenomenon of ferromagnetism arises due to both the
interaction between the neighbouring atomic dipoles and the alignment of
permanent dipoles in atoms that result from unpaired electrons in the outer
shells. The ferromagnetic materials such as iron, cobalt, nickel, dysprosium,
gadolinium have a peculiar electronic structure such that their outermost shell
contains electrons even though the inner shell next to the outermost is still
unfilled. Due to this peculiarity in structure the spin magnetic moments of
ferromagnetic materials become drastically large, resulting large atomic dipole
moments. The arrangement of atomic dipoles in ferromagnetic materials is shown
in figure.
The relative permeabilities of ferromagnetic materials
are of the order of hundreds and thousands
. The susceptibillties of
ferromagnetic materials are independent of temperature and have very large and
positive values.
. The susceptibillties of
ferromagnetic materials are independent of temperature and have very large and
positive values.
Each ferromagnetic material
has a characteristic temperature, called curie temperature, below which they
exhibit the ferromagnetic behaviour of spontaneous magnetisation and above
which the ferromagnetic materials behaves as a paramagnetic. The curie
temperatures of iron, nickel, cobalt, dysprosium and gadolinium are 1043 K, 638
K, 1348 K, 105 K and 288 K respectively.
The other characteristics of
ferromagnetic substances are as follows :
1. The
proportionality between the intensity of magnetisation 
and magnetising field 
in a ferromagnetic material maintains for
smaller values of H (part OP of the curve shown in figure). For moderate values
of magnetising field H, I increases rapidly and acquire a constant value for
large H as shown in figure. It is also evident from figure that the
susceptibility remains constant for
smaller values of H, increases in the intermediate region and then decreases
for very large values of .
and magnetising field in a ferromagnetic material maintains for
smaller values of H (part OP of the curve shown in figure). For moderate values
of magnetising field H, I increases rapidly and acquire a constant value for
large H as shown in figure. It is also evident from figure that the
susceptibility remains constant for
smaller values of H, increases in the intermediate region and then decreases
for very large values of .
2. It is apparent from the magnetic induction 
versus curve shown in figure that the flux density B in a ferromagnetic
substance is not directly proportional to the magnetising force H, but B varies with H in a similar fashion as I varies with
H except that B does not
versus curve shown in figure that the flux density B in a ferromagnetic
substance is not directly proportional to the magnetising force H, but B varies with H in a similar fashion as I varies with
H except that B does not