Magnetic Material

Magnetic Materials

 Materials attracted by magnet are magnetic materials
 Magnetic materials become “magnetized” when they are placed in a magnetic field.
 Steel and Iron are magnetic materials.
 The first accounts of magnetism date back to the ancient Greeks who also gave magnetism its name.
History of Magnet
• The name magnetism is derived from Magnesia, a Greek town and province in Asia Minor-Part of Persia-now called Turkey, the etymological origin of the word “magnet” meaning “the stone from Magnesia.
• This stone consisted of magnetite and it was known that a piece of iron would become magnetized when rubbed with it.
• It is mainly 20th century physicists (Gauss,Maxwell, and Faraday) who must take the credit for giving a proper description of magnetic materials and for laying the foundations of modem technology
Magnetic field or Magnetic Field strength H:
The magnetic field strength H at any point in a magnetic field is the force experienced by a unit north pole placed at that point. Its unit is
(Am-1) Or −1
= Am B H
Magnetic Induction-B
• It is defined as the number of lines of magnetic force passing perpendicularly through unit area. Unit of magnetic induction is Wb/m2 or Tesla. Intensity of Magnetization (I or M) 
–It is defined as the magnetic moment per unit volume. Unit of magnetic intensity is ampere per meter (Am-1).
It is denoted by either I or M.
–Since the magnetization is induced by the magnetic field, we can assume that I or M , the intensity of magnetization is proportional to H.
Magnetic Dipole Moment( µ )
• It is defined as the product of pole strength and dipole length
Mathematically ,
µm=m×2l
m= magnetic pole strength, 2l is the
length of the magnet. Unit of µm is
Am2
Magnetic Permeability (µ)
• It is defined as the ratio between the magnetic induction B in the sample to the applied magnetic field intensity H. µ, the permeability (absolute permeability) of a material is a measure of the degree of how much/easily, the filed lines penetrate or permeate through the material.
Relative Permeability
• The relative permeability of a material is the ratio of the absolute permeability of the material to the permeability of free space. 
Relation between relative permeability and magnetic susceptibility,
• Where, B= magnetic induction, M= intensity of
magnetization and H= magnetizing filed/Magnetic field strength.
• i.e. When a material is kept in a magnetic field, two types of induction arise: one due to the magnetizing filed, H and the other as a consequence of the magnetization, M of the material itself.
Origin of Magnetism
The magnetic properties arise due to electrons undergoing two types of motions in all the atoms, giving rise to magnetic dipole moments.
These magnetic dipole moments in turn are responsible for the magnetic properties of materials.
ORIGIN OF MAGNETISM IN MATERIALS
Nuclear spin
Orbital motion of electrons Origin of Magnetism Spin of electrons
A moving electric charge, macroscopically or “microscopically” is responsible for Magnetism
Weak effect
Unpaired electrons required for net Magnetic Moment
Magnetic Moment resultant from the spin of a single unpaired electron
→ Bohr Magneton = 9.273 x 10−24 A/m2 
ATOMIC MAGNETS
Electrons have 2-types of motion in an Atom; Orbital motionalong orbit and Spin motion about its axis.
Magnetic moment associated with (a)orbital & (b) spin of an electron
Classification of Magnetic Materials
• Magnetic Materials can be classified into five(5), different Categories.
• Diamagnetic
• Paramagnetic
• Ferromagnetic
• Antiferromagnetic &
• Ferrimagnetic materials.
Diamagnetic Materials
• Diamagnetic materials are those which experience a repelling force when brought
near the pole of a strong magnet.
• In non uniform magnetic field, they are repelled away from stronger parts of the field.
Ex: copper, bismuth, lead, water, mercury, zinc.
• They have low relative permeability (μr < 1), which neither changes with the strength of the applied field, nor with the temperature.
• Diamagnetic materials develop magnetization in a direction opposite to that of the magnetizing field because of which they have negative susceptibility of the order of 10-5 which neither changes with temperature nor with the applied field strength
• They repel the magnetic lines of force,
• The magnetization becomes zero on removal of the external magnetizing filed.
• They do not have permanent dipole moment,
• The magnetic susceptibility for this group of magnetic materials is independent of temperature and external field,
• They have negative magnetic susceptibility,
• Magnetic effects are very weak for these materials.
Paramagnetic Materials
• These materials experience a feeble attractive force when brought near the pole of a magnet.
• They are attracted towards the stronger parts of an inhomogeneous magnetic field
• Ex: platinum, aluminium, manganese chloride & salts of iron & nickel.
• They develop magnetization proportional to & in the same direction of the applied magnetic field
• They have a positive susceptibility, but small in magnitude of the order of 10-3
• The susceptibility is positive and it is dependent on temperature,
• The susceptibility χ decreases with increase of temperature obeying the relation χ = C / T, where C is the Curie constant & T is the temperature of the substance in absolute scale. 
• When subjected to the influence of a magnetic field, the paramagnetic materials cause a slight convergence of flux lines.
• This indicates a weak attractive force exerted by a material.
• They have relative permeability μr > 1,
which is independent of both temperature & applied field strength.
Ferromagnetic materials
• Ferromagnetic materials are those materials which experience a very strong attractive force when brought near the pole of a magnet.
• These materials apart from getting magnetized parallel to the direction of the applied field, will continue to retain the magnetic property even after the magnetizing field is removed.
• Ex: Iron, nickel, cobalt & their alloys
• Ferromagnetic materials develop magnetization in the same direction as that of the applied magnetic field, but not proportional to it.
• The susceptibility χ is very large & depends not only on the magnetizing force but also on the previous magnetic history (i.e., hysteresis) of the specimen.\
• All ferromagnetic materials lose their ferromagnetism at critical temperature called ferromagnetic Curie temperature .
• Above , the material behave like an ordinary paramagnetic material & the variation of χ obeys the relation , Called Curie’s law. Here T is the temperature of the substance in absolute scale.
• When ferromagnetic material is subjected to the influence of magnetic field, there will be crowding of flux lines within its body. This indicates a powerful attractive force exerted by the materials.
• This indicates a powerful attractive force exerted by the materials.
• Their relative permeability is very high 
i.e., μr > > 1 & even so high that μr = 106.
Ferromagnetic materials strongly attract the magnetic lines of force. Even in the absence of a magnetic field , ferromagnetics exhibits magnetization, because of the spontaneous magnetization,
They have permenanet magnetic dipole moment,
The magnetic dipoles are arranged parallel to each other,
Domain Theory of Ferromagnetism
• In 1907,Weiss proposed the concept of magnetic domains.
• By the domain concepts, properties of ferromagnetic materials and their hysteresis effects have been explained satisfactorily.
• Magnetic domain: the small region within which all spin magnetic moments are aligned in a single direction is known as magnetic domain.
• A magnetic domain is fully magnetized and has definite boundaries.
• A Ferromagnetic material consists of large number of domains.
• Each domain acts as a single magnetic dipole and is oriented in random directions.
• Each domain is separated from other domains by a wall known as domain wall also called Bloch.
Domain Theory of Ferromagnetism
Magnetic Hysteresis
• Magnetic hysteresis is the lag of the flux density behind the applied field
• When a ferromagnetic material is subjected to varying magnetic fields, the magnetic induction changes.
• When plotted, the changes in the magnetic induction against the applied magnetic field results in a hysteresis loop
• This can be explained with a magnetic hysteresis curve, or B-H curve
Magnetic materials can also be classified as soft and hard magnets depending upon the ease with which they get magnetized and able to retain magnetic fields.
Magnetic materials can also be classified as soft and hard magnets
• This classification depends upon the ease with which they get magnetized and able to retain magnetic fields.
• Soft magnetic materials can be easily magnetized and demagnetized.
• Hard magnetic materials can not be easily magnetized and demagnetized.
Soft Magnetic materials
• Soft magnetic materials have high permeability,
• low coercive force, and
• low magnetic losses,
• can be magnetized and demagnetized easily Soft Magnetic materials-exhibit SLIM hysteresis loop  
Soft Magnetic material
• The hysteresis loop for a soft magnetic material is characterized by a steeply ascending curve
• It has low remanent flux density, and low coercive field as shown in figure
• Since the coercive field strength is very small, the area within the hysteresis loop
is also small. 
Applications of Soft Magnetic materials
• They are used, in microwave isolators and storage components
• In electrical components such as motors, transformer cores and sensors
• In high frequency rotating materials and alternators
• Iron-Silicon alloys are used in electrical equipments. 
Hard Magnetic materials
• Hard magnetic materials are also called permanent magnetic materials
• They retain their induced magnetic field almost permanently.
• They are characterized by high remanence, Br, a large coercive field, Hc and naturally a large area of the hysteresis loop Hard Magnetic materials exhibit BULKY hysteresis loop Hard 
• In contrast to soft magnetic materials, hard magnetic materials have a high
resistance to demagnetization, and low permeability
• A larger hysteresis loop area obviously means large magnetic energy losses
Applications of Hard Magnetic materials
• They are used in D.C motors and measuring devices.
• They are used in microphone instead of conventional magnet
• They are used in compass needles
• They are used to produce materials having permanent magnetism.