FERROMAGNETIC MATERIALS

Those substances, which when placed in magnetic field are strongly magnetised
in the direction of the magnetising field.

e.g.: - Nickel, Iron, cobalt, rare earth metals

Domain Theory
 The atom of a Ferro-magnetic material also possesses non-zero magnetic
moment as in case of a paramagnetic substance.
 However due to a quantum mechanical effect, called exchange
interaction (a process by which exchange energy of unpaired
electrons is minimised) an unpaired electron in one atom interacts
strongly with the unpaired electron in the neighbouring atom in such
a way that they spontaneously align themselves in a common
direction over a small volume of the material.
 These small volumes of uniform magnetisation are called domains.
Although domains are extremely small in size (approx. 10^-18 in volume)
, yet each domain contains a large number of atoms (approx. 10 ^11 atomic
magnetic dipole moments)
Arrangement of Domains

1) In the absence of external magnetic field, the direction of magnetic moments

in different domains are oriented randomly in different directions.
2) Since probability of orienting in different directions is same we get a net
zero magnetic moment due to domains in the absence of external magnetic
field.
3) The magnetic moments arrange in two ways depending on the applied
external field
a) When a weak or moderate magnetic field is applied to a pure and
homogeneous ferromagnetic material the domains in which the
magnetic moments are parallel to the external field expand
continually at the expense of remaining domains and ultimately the
magnetic moments align themselves along the direction of the
magnetic field.
b) If the external magnetic field is strong , thren the magnetic
moments in all domains become parallel to applied magnetic field.

Properties of Ferromagnetic materials

a) A ferromagnetic material is strongly attracted by a magnet.

b) When a rod of ferromagnetic substance is suspended in magnetic field, it

quickly aligns itself along the direction of the magnetic field.

c) The ferromagnetic materials move from weaker part of magnetic field

applied to stronger part of the field. Ferromagnetism is not present in
liquids and gases. If a finely powered ferromagnetic solid in a watch glass
is placed on the pole pieces of a magnet, the material rises in the
middle. When the pole pieces are moved apart, the material depresses
in the middle.

d) When a ferromagnetic material is placed in a magnetic field, the magnetic

field lines becomes highly dense in the ferromagnetic substance.
 e) For a ferromagnetic materials the following parameters are very high
positive value.

(a) Intensity of magnetisation (M - Magnetic moment per unit

volume) :This is because on being in a magnetic field, a
ferromagnetic substance gets strongly magnetised in the
direction of the field.

(b) The magnetic susceptibility Xm : Large positive value, this

follows from the relation that.

𝑀
Xm =
𝐻
(c) The relative permeability (µr ) :Large positive value
from the relation
µr = 1+ Xm
(d) The magnetic flux density (B) :In the ferromagnetic
substance is much larger than that in vacuum (µ0H).

B = µ0(H + M)

Magnetic susceptibility and Temperature

 The susceptibility of ferromagnetic materials decreases with increase in

temperature in complicated manner.
 At a particular temperature called Curie point or Curie temperature, a
ferromagnetic material starts behaving as a paramagnetic substance.
 At temperatures below the curie's point, the magnetic moments are
partially aligned within the magnetic domains in ferromagnetic materials.
 As the temperature is increased from below the curie's point, thermal
agitation starts destroying this alignment, until the net magnetisation
becomes zero at and above the Curie Point.
 Above the Curie point, the material is purely paramagnetic in nature.
 The effect is primarily used in magneto-optical storage media, where it is
used for erasing and writing of new data.
 Other uses include temperature control in soldering irons and in general,
where a temperature- controlled magnetisation is desirable.
  The loss of ferromagnetic behaviour of Fe at a temperature above its
Curie point can be used to explain that the earth core cannot be a
permanent magnet.
 For Fe Curie point is about 1043K this temperature is reached at about
20km depth in the earth. At the outer boundary of the core temperature is
about 20000C.
 This is well above the Curie point of Fe. So although the core is largely
Fe that Fe is no longer ferromagnetic. Hence earth's core cannot be a
permanent electromagnet.

Material Fe Co Ni Gd MnB MnAs MnBi CrBr3 GdCl3

Curie Point 1043 1388 627 293 578 318 760 37 2.2

Hysteresis

The phenomenon of 'lagging behind' of induced magnetic field behind the

magnetising field applied is called hysteresis.
 The loop is generated by measuring the magnetic flux of a ferromagnetic
material while the magnetizing force is changed. A ferromagnetic
material that has never been previously magnetized or has been
thoroughly demagnetized will follow the dashed line as H is increased.

 As the line demonstrates, the greater the amount of current applied (H+),
the stronger the magnetic field in the component (B+).

 At point "a" almost all of the magnetic domains are aligned and an
additional increase in the magnetizing force will produce very little
increase in magnetic flux. The material has reached the point of
magnetic saturation. +

 When H is reduced to zero, the curve will move from point "a" to point
"b." At this point, it can be seen that some magnetic flux remains in the
material even though the magnetizing force is zero. This is referred to as
the point of retentivity on the graph and indicates the remanence or
level of residual magnetism in the material.
(Some of the magnetic domains remain aligned but some have lost their
alignment.)

 As the magnetizing force is reversed, the curve moves to point "c",

where the flux has been reduced to zero. This is called the point of
coercivity on the curve. (The reversed magnetizing force has flipped
enough of the domains so that the net flux within the material is zero.)
The force required to remove the residual magnetism from the material is
called the coercive force or coercivity of the material.

 As the magnetizing force is increased in the negative direction, the

material will again become magnetically saturated but in the opposite
direction (point "d"). Reducing H to zero brings the curve to point "e." It
will have a level of residual magnetism equal to that achieved in the
other direction. Increasing H back in the positive direction will return B
to zero.

 Notice that the curve did not return to the origin of the graph
because some force is required to remove the residual magnetism.
The curve will take a different path from point "f" back to the saturation
point where it with complete the loop.
From the hysteresis loop, a number of primary magnetic properties of a material
can be determined.

 Retentivity - A measure of the residual flux density corresponding to the

saturation induction of a magnetic material. In other words, it is a
material's ability to retain a certain amount of residual magnetic field
when the magnetizing force is removed after achieving saturation. (The
value of B at point b on the hysteresis curve.)

 Residual Magnetism or Residual Flux - The magnetic flux density that

remains in a material when the magnetizing force is zero. Note that
residual magnetism and retentivity are the same when the material has
been magnetized to the saturation point. However, the level of residual
magnetism may be lower than the retentivity value when the magnetizing
force did not reach the saturation level.

 Coercive Force - The amount of reverse magnetic field which must be

applied to a magnetic material to make the magnetic flux return to zero.
(The value of H at point c on the hysteresis curve.)

 Permeability - A property of a material that describes the ease with

which a magnetic flux is established in the component.

 Reluctance - Is the opposition that a ferromagnetic material shows to the

establishment of a magnetic field. Reluctance is analogous to the
resistance in an electrical circuit.

Permeability
As previously mentioned, permeability is a material property that describes the
ease with which a magnetic flux is established in a component. It is the ratio of
the flux density to the magnetizing force and is represented by the following
equation:


It is clear that this equation describes the slope
of the curve at any point on the hysteresis
loop. The permeability value given in papers
and reference materials is usually the
maximum permeability or the maximum
relative permeability. The maximum
permeability is the point where the slope of the B/H curve for the unmagnetised
material is the greatest. This point is often taken as the point where a straight
line from the origin is tangent to the B/H curve.

The relative permeability is arrived at by taking the ratio of the material's

permeability to the permeability in free space (air).

(relative) = (material) /air

where: air = 1.256 x 10-6 H/m
The shape of the hysteresis loop tells a great deal about the material being
magnetized. The hysteresis curves of two different materials are shown in the
graph.

Relative to other materials, a material

with a wider hysteresis loop has:

 Lower Permeability
 Higher Retentivity
 Higher Coercivity
 Higher Reluctance
 Higher Residual Magnetism

Relative to other materials, a material

with the narrower hysteresis loop has:

 Higher Permeability
 Lower Retentivity
 Lower Coercivity
 Lower Reluctance
 Lower Residual Magnetism.

In magnetic particle testing, the level of residual magnetism is important.

Residual magnetic fields are affected by the permeability, which can be related
to the carbon content and alloying of the material. A component with high
carbon content will have low permeability and will retain more magnetic flux
than a material with low carbon content.
ENERGY LOSS DUE TO HYSTERESIS

 If hysteresis loop is drawn by plotting a graph between magnetic

induction(B) and intensity of magnetisation (M), then area of the
hysteresis loop is numerically equal to the work done per unit volume
(or energy absorbed per unit volume) in taking the magnetic specimen
over a complete cycle of magnetisation.

 From the hysteresis loops for soft iron and hard iron we can figure out the
following result.

 The area of hysteresis loop for soft iron is much smaller than that for hard
iron. In other words , soft iron is much more strongly magnetised (or
more susceptible to magnetism) than steel.

Explain the below selection criteria.

1) Permanent Magnets - High retentivity and High coercivity

2) Electromagnets - High retentivity and Low coercivity

3) Transformers - Least possible area of hysteresis