Magnetic materials can be classified as diamagnetic, paramagnetic, ferromagnetic, antiferromagnetic, or ferrimagnetic based on their response to an external magnetic field. Ferromagnetic materials like iron exhibit a strong attraction to magnetic fields and retain magnetization after the external field is removed due to the alignment of magnetic dipoles within domains. Ferrimagnetic materials like ferrites have anti-parallel dipoles of different magnitudes, producing a net magnetic moment. Magnetic semiconductors that exhibit both ferromagnetism and semiconductor properties have applications in spintronics and quantum computing.

1. Magnetic Materials
Magnetic field:
The space around the magnet or the
current carrying conductor where the
magnetic effect is felt is called
magnetic field.
Magnetic dipole moment (m)
The product of magnetic pole strength and the distance between the two
poles.
Magnetic flux (φ)
Total number of magnetic lines of force passing through a surface is
known as magnetic flux. Unit – Weber (Wb)

2. Magnetic Flux Density (or) Magnetic Induction (B)
The No. of lines of forces passing through unit area of cross section (A) at that
point.
B = φ / A Wb/m2
Intensity of Magnetisation (I)
The term magnetisation means the process of converting a non - magnetic
material into a magnetic material.
Def: the magnetic moment per unit volume of the material.
I = M/V Wb/m2
Magnetic Field Intensity (or) Strength (H)
At any point in a magnetic field is the force experienced by a unit north pole
placed at that point.
Magnetic permeability (µ)
Magnetic flux density (B) is directly proportional to the magnetic field strength
(H)
B α H; B = µ H; µ = B/H
where µ is a constant of proportionality.

3. Relative permeability (µr)
The ratio between absolute permeability of a medium (µ) to permeability of a free
space (µ0) is known as relative permeability (µr).
µr = µ/ µo
Magnetic susceptibility (χ)
The ratio of intensity of magnetization (I) to intensity of magnetic field (H).
χ = I/H

4. Magnetic materials:
Classification of magnetic materials
Based on the response to applied external magnetic field, the
magnetic response are broadly classified as follows,
(i) Diamagnetic materials
(ii) Paramagnetic,
(iii) Ferromagnetic,
(iv) Anti ferromagnetic and
(v) Ferrimagnetic materials

5. Explanation
• In the absence of applied magnetic
field, each atom has zero magnetic
dipole moment.
• In the presence of a field, dipoles are
induced and aligned opposite to the
field direction.
Properties
• No permanent dipole moment
• Exhibit –ve magnetic susceptibility
• Relative permeability is slightly less than unity (µr <1).
• Magnetic susceptibility is independent of temperature
Diamagnetic materials
The atoms in diamagnetic materials are not possess permanent
magnetic moments. The materials which acquire weak magnetism
in the direction opposite to that of an applied external magnetic
field.
Ex. Au, Cu.

6. Explanation
• In the absence of applied magnetic field,
each atom has a net stable magnetic
moment due to orbital and spin
magnetic moments.
• In the presence of an applied magnetic
field, the magnetic dipoles align same
direction of applied magnetic field.
Properties
• Individual atoms have paramagnetic materials possess
permanent dipole moment.
• Exhibit +ve magnetic susceptibility
• Relative permeability is slightly grater than unity (µr > 1).
• Paramagnetic susceptibility strongly depends of temperature.
Paramagnetic materials
• The materials which acquire weak magnetism in the
same direction as that of an applied external magnetic
field.
Ex. Ferric oxide, Ferrous sulphate, nickel sulphate, etc

7. Ferromagnetic materials
Certain metals like Fe, Co, Ni and certain alloys exhibit high degree of
magnetisation. These materials show the spontaneous magnetization.
i.e., they have magnetisation even in the absence of an external
magnetic field.
Explanation
The adjacent magnetic dipoles are
equal magnitude align parallel to
each other.
Properties
• All the dipoles are aligned parallel to each other.
• They have permanent dipole moment and strongly attracted by the
magnetic field.
• They exhibit magnetism even in the absence of magnetic field.
• On heating, they lose their magnetism slowly.
• The magnetic susceptibility is very high
& it depends on temperature. It is given by,
C
(T Tc,Para.)
T
(T Tc,Ferro.)


 


m = mo

8. Domain theory of ferromagnetism
According to Weiss theory, molecular magnets in the ferromagnetic
materials are said to be aligned in such a way that, they exhibit a
magnetisation even in the absence of an external magnetic field
‘spontaneous magnetisation’.
According to Weiss hypothesis, a single crystal of ferromagnetic material
is divided into large number of small regions called domains.

9. Process of Domain Magnetisation
Fig. a (H=0). The direction of spontaneous magnetisation varies
from domain to domain and are oriented in such a way that the net
magnetisation of the specimen is zero.
H≠0
Fig. b. Translation of domain (weak field applied)
The movement of domain walls takes place in weak magnetic fields.
Fig. c. Rotation of domain (strong field applied)
The rotation of domain walls takes place in strong magnetic fields.

10. Types of Energy involved in the process of domain growth
Exchange energy
The interaction energy which makes the adjacent dipoles to align
themselves is known as exchange energy (fig.a).
Magneto-static energy
The energy can be reduced by dividing the single domain into two
domains (fig. b).
The process of subdivision may be carried further, untill the reduction of
magnetic energy is less than the increases in energy to form another
domain and its boundary (domain wall or block wall_ Fig. c,d) .
They have zero magneto-static energies due to the introduction of
triangular domains at the top and bottom of the crystal (fig.e).

11. Crystal anisotropy energy
The difference in magnetic energy to
produce saturation in an easy [1 0 0]
direction and hard [1 1 1] direction is called
crystal anisotropic energy.
Magneto-strictive energy
When the domains are magnetised in different directions, they will either
expand or shrink. Therefore, there exists a deformation (i.e) change in
dimension of the material, when it is magnetised, it is found that it suffers
a change in dimensions. This phenomenon is known as magneto-striction
energy.

12. Hysteresis - M versus H Behavior (Fe, Iron)
Hysteresis:
When a ferromagnetic material is made to undergo a cycle of
magnetisation, the intensity of magnetisation (I) and the magnetic flux
density (B) lags behind the applied magnetic field (H) and the process is
known as Hysteresis.
Retentivity or Residual or
Remanence magnetism:
It is the residual intensity of magnetisation
retained by the specimen even when the
external magnetic field is cutoff.
(OB from loop).
Coercivity or Coercive force:
It is the strength of reverse magnetic
field required to completely remove
the residual magnetisation or demagnetise
the material. (OC from loop).

13. Ferri-magnetism (or) Ferrites
It is a special case of magnetic material and it is composed of two
sets of different transition metal ions having different values of
magnetic moment with anti-parallel alignment (even absence of
external field).
These materials have anti-parallel magnetic moments of different
magnitudes, giving rise to fairly large magnetic moment in the
presence of external magnetic field.
Properties
• The susceptibility (χ) is +ve and low.
• > Neel temperature (Tc) χ decreases.
• Low coercivity.
C
(T Tc )
T


 


14. Structure of Ferrites
Ferrites are the magnetic compounds consisting of two different
kinds of atoms. Generally ferrites expressed as
There are two types of ferrite structure.
Normal spinel and Inverse spinel.
Normal spinel
In normal spinel structure all (2+) divalent metal ions occupy A-sites
(4 - O ions in a tetrahedral sites) and (3+) trivalent ions ions occupy
B-sites (6 – O ions in an octahedral sites) the structural formula of
such ferrites is
2 3 2 2 2 2 2 2
2 4
2 3 2 2 3 2
2 4 2 4
X Fe O X Mg , Zn , Fe , Mn etc .
Eg. Ni Fe O , Fe Fe O et
[
c
]
       
     

2 3 2
2 4
Mg Fe O
  

16. Inverse spinel
In this spinel structure, the Fe3+ ions (trivalent) occupies all the A-
sites (tetrahedral site) and half of the B-sites (octahedral sites) also.
Thus the left out B sites will be occupied by the divalent (Fe2+). The
structural formula of these ferrites is 3 2 3
4
Fe [Fe Fe ]O
  

17. Mixed spinel
In this spinel structure, the cation of divalent metal ions and trivalent
metal ions occupy both A & B sites. The structural formula of this
ferrites is
2 3 2 3
4
1- 2-
A B [ A B ]O , isthedegreeof inversion.
    
   


18. Magnetic semiconductors
• Definition: Magnetic semiconductors are semiconductor
materials that exhibit both ferromagnetism (or a similar
response) and useful semiconductor properties.
• Example: Mn doped GaAS system, (high Tc up to 200 K.)
Importance of Magnetic semiconductors
• If it is applied in devices, these materials could provide a new
type of control of conduction.
• But, traditional electronics are based on control of charge
carriers (n or p type).
• Practical magnetic semiconductors would also allow control of
quantum spin state (up or down).
• This would theoretically provide near-total spin polarization,
which is an important property for spintronics.

19. Dilute magnetic semiconductor (DMS)
• These are based on traditional semiconductors, but they are
doped with transition metals instead of, or in addition to
electronically active elements.
• They are of interest because of their unique spintronics properties
with possible technological applications.

20. DMS

21. Examples:
• Mn doped InAs and GaAs
• Mn doped ZnO
• N-type Co doped ZnO
• P-type Co doped ZnO
• Co doped TiO
• Fe doped TiO
Applications of magnetic semiconductors
• They are used to make quantum computing architecture using
spin polarized electron.
• They are used in magneto optic applications.
• They are used to fabricate spin transistors and spin polarized
Light Emitting Diodes (LEDs).
• They are used to exhibit favorable dilute magnetism.