Electron theory of ferromagnetism (Part - 25) - Matter & Magnetism, Physics, Class 12 Video Lecture

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electron theory of ferromagnetism so the
first thing that we will discuss while
discussing Ferro magnetism is the domain
claim like I told you that a special the
special thing which takes place in case
of ferromagnetic substances is domain
formation so what does this domain
formation basically in case of a
ferromagnetic substance also each atom
has a permanent dipole moment but in
this case so how do those domain
coordination takes place as I told that
each atom here will have a permanent
dipole moment now several atoms together
will group and form a domain so a domain
many atoms together many happens
together will form a domain right
several atoms so one domain they contain
approximately some 10 to the power 10
atoms right there will be so many atoms
in one domain now inside each domain a
special interaction exists between the
atoms as a result of which all the
dipole moments are in one direction so
that means in this case each atom has
permanent dipole moment right now atoms
form domain now inside each domain
inside domain there is a special
interaction inside domain there is a
special interaction between the atoms
such that
all dipole moments are in one particular
direction right so what will happen as a
result each domain will behave like a
magnet each domain will behave like a
magnet net because the domain as a whole
has all the dipole moments in one
direction that means the domain has a as
a whole has a net dipole moment
so therefore the domain will behave like
a magnet but there are several domains
inside the substance it till now we were
just talking about electrons inside
atoms and atoms inside the substance but
now we are saying that electrons are
there inside atom and the atoms together
are forming domains and many domains
form the substance now the several
domains they are all randomly oriented
so what happens the next thing is random
orientation of domains then now since
the domains are randomly oriented
therefore the substance as a whole is
not magnetic so the substance is not
magnet so even though each domain
behaves like a magnet because they have
a net magnetic moment but the domains
are randomly oriented therefore the
substance as a whole does not behave
like a magnet for example here you can
see in this diagram which I have drawn
the green lines represent the net
magnetic moment of each domain so each
triangle represent a domain so if you
look at this particular domain if you
look at this particular domain what do
you see all the magnetic moments and
aligned along this direction and this
has a net magnetic
towards right so that means this
particular domain behaves like a magnet
but there are four such domains and they
are all randomly oriented and as a
result of which the net magnetic moment
of this entire substance is zero so the
next substance as a whole will not
behave like a magnet but what will
happen when an external electric field
is applied so in the absence of electric
field domain formation by atoms random
orientation of domains therefore the net
magnetic moment in any direction is
equal to zero so the substance does not
behave like a magnet so now what will
happen in presence of magnetic field
when a magnetic field is applied the
substance which was not a magnet as a
whole will get strongly magnetized but
how when they apply an external field
two things happen the first thing is the
domains which are in the direction of
magnetic field they will increase in
size for example if we apply an external
magnetic field in this direction then
out of the four domains which domain was
in this direction this domain right so
this right MINIX magnetic field was not
applied till that time all the domains
where equal in size right so now if I
apply an external magnetic field in this
direction this particular domain will
increase in size that is what happened
here if you if you see the topmost
domain this domain increased in size
because it is in that direction of the
applied magnetic field right so what
will happen since this domain increased
in size so then now the net magnetic
moment of this substance will be nonzero
and the net magnetic moment will be in
this direction so that is one way by
which the net magnetic moment of the
substance increases and the other way is
domains rotate till they all align in
the direction of external magnetic field
let us suppose I apply the external
magnetic field along this direction so
what is happening this was already in
this direction the other three domains
also start
till they all align in the same
direction so now since all are aligned
in the same direction therefore the net
magnetic moment of the substance as a
whole is also nonzero so we see that by
applying an external magnetic field the
substance get strongly magnetized
because now all the domains will be
aligned along the direction of magnetic
field so in this case in presence of
magnetic field domain boundaries
displacement so domain boundaries
displacement is what we referred here
that means increase in size of the
domains which are aligned along the
direction of the magnetic field that is
one has domain boundaries displacement
domain rotation is what we have
described here that is that all the
domains will keep rotating till they all
align themselves along the direction of
the magnetic field now in this case also
therefore the net magnetic moment in any
direction is non zero now and therefore
the substance gets strongly magnetized
right so here also another thing to be
noted is what happens when a
ferromagnetic substances heated when the
ferromagnetic substance is heated we
talked about the special interaction
right we talked we told that inside the
domain there is a special interaction
because of which all the dipole moments
inside the domain is in one direction
now when you heat a ferromagnetic
substance that special interaction
disappears as a result they dope that
speciality which takes place inside the
domain that is lost because the
difference between Ferro magnetism and
para magnetism arises only because of
the domain formation and the domain
formation is crucial because of that
special interaction between the atoms
inside a domain because of which all the
magnetic moments inside a domain are in
one direction so when you heat it that
special interaction is lost as a result
that the special role of domain
formation is also lost and the
substance behaves like a paramagnetic
substance so this is an important point
to be noted here when heated it turns
paramagnetic why because when heated the
special interaction inside the domain
disappears as a result it becomes or it
behaves like any other ferromagnetic
substance so I hope that I am able to
explain you the three classification
diamagnetic paramagnetic and
ferromagnetic and also the electron
theory behind each of them right so now
it is time that we talk about few other
important topics thank you please visit
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