Mod-01 Lec-18 Applications of Thermal analysis techniques

In the previous lecture, we looked at some
of the definitions of Magnetism, and how we measure magnetic response of a material and
some of the parameters that we usually link to a magnetic property in a material. And
we also looked at some of the basic definitions that categorize, magnetic materials into different
categories and how we can ascertain those magnetic properties.
In continuation to the previous lecture, I would like to dwell little bit more on the
classification of the magnetic property in a material and also I will try to attempt
showing, some of the group of compounds, which really stand out in today’s functional
applications. The cartoon that I have put in this in introductory slide shows that,
today the magnetic materials can be used in every
other applications, including toys to very sophisticated applications in avionic tubes
and in recording media. So, in today lecture, I
will try to take you through different classification of these magnetic materials and show
some example of how these materials can be used in applications. Just to recall the 2 group of compounds that,
we can easily categorize is based on the weak magnetic response and strong magnetic
response that materials display. So, weak magnetic response, diamagnetism and paramagnetism,
usually talks about influence of a external feet on the material, whereas in
strong magnetism, it is a intrinsic response of a
material in itself, when we say diamagnetism although, there is a very poor response
from the material point of view, to a net magnetic movement, but the diamagnetism per
say can be used for sophisticated applications, for example, magnetically levitating a
train, magnetic materials are used. And those materials has to be diamagnetic, for
magnetic levitation, therefore diamagnetism in essence is not a bad response, it is as
much used and exploited like a strong ferromagnetic material.
So, this is one group called weak magnetic response and strong magnetic response,
comes from ferromagnetism, antiferromagnetism or ferrimagnetism, we have already
seen ferrimagnetism is nothing but, a ferromagnetism. But, then there are some
antifferromagnetically aligned movements, which although is present, it is compensated
by a strong ferromagnetic response, therefore, ferrimagnetism is a special case of
ferromagnetism. And ferromagnetism in essence is mainly seen
evidently in metals and there are very few other compounds, which can be called as a
ferromagnetic material, whereas in ferrimagnetism, you can actually look at examples
other than metals, which show ferrimagnetic response and we will look at
some of the categories in the next few slides. Also to just group, this weak and strong magnetic
material, those which are weak, the diamagnetic materials or paramagnetic materials,
can be categorized from the others on 2 counts. One you talk about the susceptibility
value and you talk about the permeability value, both are a good measure of whether
the material is good or not in terms of its magnetic response.
Usually, if you look at the permeability for a diamagnetic material, it is going to be
less than 1 and susceptibility is definitely less
than 0, where as paramagnetic and antiferromagnetic ones have permeability greater
than 1 and susceptibility greater than 0. But, for neutral materials in ideal case permeability
has to be 1 and chi m has to be 0, in case of ferromagnetic and ferrimagnetic materials,
your permeability is far greater than 1 and definitely the susceptibility has to be
greater than 0. And what is the response between these 2 or what really makes them
candidly different is the hysteresis, that emerges out in the case of ferro or ferrimagnetic
materials, which is absent in the case of paramagnetic materials.
Therefore there is a clear divide between these 2 compounds and therefore, it is very
easy to understand, how a ferromagnetic material would respond against a
antiferromagnetic material. Now, the main thing that governs all this 3 group of
compounds, ferro, ferri and antiferromagnetic material is the presence of a magnetic
domain. Which is not a crystallite or it is not a structural domain, that we are talking
about it is the magnetic correlation, that is present within the material that makes
these materials very rich in it is chemistry.
Temperatures above a material dependent limit thermal vibrations leads to disalignment
of the magnetic moments, the materials becomes paramagnetic therefore, in ferro ferri or
antiferromagnetic material. We usually talk about a antiferromagnetic to paramagnetic
situation or antiferromagnetic sorry, ferromagnetic to paramagnetic situation. So, the
transitions are always to a totally disordered alignment of spins to a ordered or to a
antiferromagnetically ordered spins sticks. So, this is exactly, what we see, in ferro
and ferrimagnetism, usually for temperature greater temperature less than T C, you see
a very strong magnetic response and this is usually present in the case of metals. Whereas,
in the case of antiferromagnetism for temperature less than T N, that is Neel temperature
ferro and ferrimagnetism can occur in insulators. And this is more prevalent in
some of the selected metals and also in metal oxides.
So, what really is a common understanding about metal oxides, transition metal oxides
is that, they are mostly antiferromagnetic, but
what you would see, in today’s application or
the new generation compounds, what is understood to be a antiferromagnetic compound
transience to become a very strong ferromagnet and metal. And that is the beauty of
chemistry, because the substitutional chemistry or materials chemistry totally puts the
landscape of transition metal oxides into a different perspective altogether, because
of the ease with which, we can engineer the materials.
So, if you know for sure for example, lanthanum cuprate L a 2 C o 4, which is a
antiferromagnetic oxide, you can actually make it metallic and you can make it
semiconducting by just substituting with strontium, at the lanthanum site. Similarly
another very important compound, which I will be discussing in module 4 and 5 is
lanthanum manganite, which is a perovskite. And this lanthanum manganite, actually is
a antiferromagnetic insulator, because the in the a b plane, the manganese oxygen sheets
are antiferromagnetically aligned along the c
axis whereas, they are ferromagnetically confined in the a b plane. But, when you try to
disturb this manganese oxygen manganese alignment by substituting with the strontium
for lanthanum, you can actually bring about a collinear ferromagnetism where. you try
to introduce a ferromagnetic exchange between
neighboring manganese via oxygen 2 p. And therefore, a radically antiferromagnetic
insulator is transformed into a ferromagnetic metal, which is very very astonishing phenomena,
in the case of oxides. So, when you look at metal oxides most of them are antiferromagnetic
oxides, but you can easily translate them into ferromagnetic oxides.
And there by effect the conductivity by mere chemistry and that is the specialty of materials
chemistry in these oxides. When you look at antiferromagnetic oxides
typically, the domains are like this each one
of this is domain in a antiferromagnetic oxide, which is antiferromagnetically coupled as
result, your chi m. In that case is going to be nearly 0 whereas, your permeability
is going to be approximately one and there is
full compensation of the magnetization by the
antiferromagnetic alignment. And in such a situation in compounds like
manganese oxide, iron oxide, cobalt oxide or
nickel oxide, if you allow the external field to go through these oxides, you would see
a situation like this where, there is no influence
of the flux on the system. So, this is typical of a antiferromagnetic oxide, because the
antiferromagnetic coupling is, so strong, that it cannot be removed by an external magnetic
field. So, if you have a domain structure like
this, the antiferromagnetic strength is of much higher order, that it cannot easily be
removed by an external force and that is exactly what you see here.
But, if there is a antiferromagnetic interactions coming due to some impurity effect or
defect effect, which is not intrinsic, but its extrinsic factor, then even with the little
of external field strength, it is possible to
remove those antiferromagnetic coupling. If possible, we will deal with this in the next
lecture, one of the most important or well studied antiferromagnetic oxide is manganese
oxide. And if you closely look at the manganese atom, these are the oxygen atoms
and these are your manganese items. Now, if you look at every alternate manganese
atom you would see that, they are antiparallely align, in any direction, they
are antiparallely aligned whereas, if you actually look at the 1 1 1 plane, then all
the manganese are ferromagnetically aligned. But, in essence this is a antiferromagnetic
coupling mainly, because the electron here, which is actually exchanging through the 2
p orbital of oxygen, to the neighboring oxygen, to the neighboring manganese site
is antiferromagnetically coupled. And this cannot be removed, because this is
a supper exchange coupling, which is dynamic via the 2 p orbitals of oxygen and
this cannot be removed at all. Therefore this is a very classic example of antiferromagnet,
in this case the spin orientation is due to the
3 d electrons of manganese 2 plus and 2 p electrons of oxygen 2, which is
antiferromagnetically aligned to the T 2 G orbitals of manganese 2 plus of the
neighboring atom. Partial overlapping of 3 d and 2 p orbitals result in antiparallel
alignment and this is actually dictated by the Hund’s coupling or Hund’s rule.
Now, in that situation, if it is a antiferromagnetic
oxide, you would see a chi versus T plot to show something similar to this where,
you have a almost a linear dependency in this high temperature scale. But at Neel temperature,
the chi starts falling down. So, your susceptibility, drastically falls down at
the Neel temperature, typically this should have
actually increased in this fashion, but for a antiferromagnet, you would see this sort
of a cross over and this is a very important signal
for a antiferromagnetic oxide. And this is exactly what you see in this curve
here, that all this manganese are aligned antiferromagnetically and they are mediated
via this oxygen sites. And in such case the magnetic susceptibility is not governed by
curie law rather, it is governed by curie-weiss law, which is dictated by this expression
chi m. Therefore for antiferomagnetic oxides, your chi m will actually be related via curie-weiss
law. Now when you come to ferrimagnetism, as we
have already seen the partial compensation of the moments, actually results
in a net magnetization, in this form and as a result each domain actually contributes
to a net magnetization and this is possible only
in metal oxides or any metals, which are partially filled. And it also depends on the
crystal structure, which will see in some of the examples, what is that behavior of
this ferrimagnetic 1, the permeability will be
much stronger and your molar susceptibility will be greater than 0.
And if you look at the impact of the external flux line to these ferrimagnetic compounds,
you would see that there is a strong correlation or there is a interaction with the external
field strength. And as a result the domain will start getting influenced by the external
magnetic fields, a classic example of a ferrimagnet is A B 2 O 4 type of metal oxides,
which is called as Spinel metal oxides. Spinel compounds usually ferrites show a very
good response for ferrimagnetism and this is the unit cell for A B 2 O 4 type of
oxide where, you have the a cations sitting in
the tetrahedral sites and you have the b cation sitting in octahedral sites. In such case
you can actually generate a variety of spinel
compounds, this is a classic spinel, which is a
naturally occurring mineral and we can actually try to make ferrites of this formula.
Where you can have a site occupied by divalent metal oxides metal ions and b side by
trivalent metal ions, for ferrites b is always iron and you can generate magnesium ferrite,
manganese ferrite, cobalt nickel or zinc ferrite. The well known spinel is inverse spinel
that is iron oxide or magnetite where if you look at the occupation of the sites, you would
see that, the these 2 ions are actually in b site and your F e 3 plus S is actually in
a site. So, because you have a mixed valency of both
F e 2 plus and F e 3 plus in b site, this is
actually called as a inverse spinel, otherwise in a normal spinel phase of ferrite, you will
actually have always a F e 3 plus in b site and m 2 plus in a site. So, you would see,
this discrepancy in most of the ferrites the occupation
of the iron atoms sometimes will be between a and b site, so this categorizes
a group of compounds like ferrites where, ferrimagnetism is very much operative. And in such cases, you can actually try to
substitute with chromium, if it is chromium then we can still get a cubic ferrite, which
is magnetic, suppose we substitute aluminum in the place of iron then it becomes a non
magnetic ferrite. Similarly, a nonmagnetic ferrite, you will get if it is, if the a site
is substituted with zinc magnesium or barium whereas, you get a magnetic ferrite, if it
is manganese 2 plus nickel 2 plus cobalt 2 plus
and F e 2 plus. Some of the diamagnetic materials and paramagnetic
materials are listed here and this also tells, why we use those materials for
applications, for example copper gold silicon silver and zinc. These are very candid diamagnetic
materials and when we look at the A C susceptibility or susceptibility chi value,
you will always understand, that the susceptibility will be of the order of ten
power minus 5 whereas, in the case of ferromagnetic materials, this will be of the
order of minus 1 or it would be positive. So, the chi value should exactly tell you,
what sort of a material, you are talking about and most of these diamagnetic materials are
in this range and also paramagnetic materials like, chrome chromium, zirconium,
titanium, aluminum all these have a very low susceptibility value. And those, which
are ferromagnetic the magnetic, spin moments are actually governed by the hund’s rule
and therefore, it is possible for us to calculate, how much of magnetic moment each of these
transition metal ions can contribute and this is measured in terms of bohr magneton. When we come to the range of compounds, we
usually have an idea that magnetic materials are meant only for some sort of
a magnetic force applications, like stickers or
magnets for isolating some magnetic materials. And in today’s application most of the
time, we encountered only permanent magnets, used in household applications, but what
we forget or what might slip out of attention is that the magnetic materials form a very
solid application core. And they are used in coils in transformers
in transducers and not only that in power electronics, but also it finds a very important
application in magnetic storage application. So, a range of applications are there, for
magnetic materials, I will basically make a
division between 2 types of materials before, I show some of the applications. One soft
magnetic materials and the other one permanent magnetic materials or hard magnetic
materials. Soft those, which show magnetic materials on application of a external
magnetic field, permanent or hard magnetic materials are those inherently have a
magnetic moment, you can kill the magnetic response only by applying a magnetic field. So, with this minimum distinction, let me
just take you through some of the functional applications, that the magnetic materials
hold, magnets function as transducers transforming energy from one form to the other,
without any permanent loss, of their own energy. And for example, permanent magnets
are used in a variety of applications, like they convert mechanical to mechanical
energy, in other words just used for either repulsion or for attraction.
So, this is one application, but you can also use the mechanical to electrical conversion
of energy, a as you see in generators and microphones, electrical to mechanical, in
motors, loudspeakers and in charged particle deflection. We can also translate this
mechanical energy into heat for example, in torque devices and in applications involving
eddy currents. And more so in the recent past, special applications have emerged out of
these materials, such as magneto resistance hall effect and magnetic resonance. I will
not deal with these applications precisely, because
I will be talking about this, when we discuss about electrical properties of materials,
so I will try to show some example, on the other aspects. This is a one of the cartoon that gives some
idea about, what sort of materials we characterize as soft magnetic materials, these
are actually, a range of compounds, which stands out compared to all the other known
soft magnetic material. So, I thought this would be, a good way to project some of the
representative soft magnetic materials, what has emerged recent, past is nano crystals,
which show soft magnetic response amorphous alloys are traditionally, soft magnetic.
And ferrites are mostly soft magnetic materials, but we also have bubbled memory
materials, which are hard ferrites, there is another group of compound called sendust,
then permalloy, which is used almost in every other applications. And then some of the
alloys also show soft magnetic material compared to other alloys, which are usually hard
in it is response. When we talk about soft magnetic materials,
we need to understand they have high permeability and very low coercivity, if coercive
force is very less and permeability is very high, then you can categorize that to
be a soft magnetic material, which is given by
mu, mu is a equal to B by H. So, B is your magnetic induction and for obtaining soft
magnetic materials, there are some clues one material with low magnetic anisotropy low magnetostriction and high saturation magnetization
has to be there. In the previous lecture, I told you that when
ferromagnetic or magnetic moments, they oscillate sometimes, they can dilate and bring
about magnetostriction response and those materials cannot be a good, soft magnetic
material therefore, it has to have a low magnetostriction. And how do we get this magnetic
soft magnetic materials, we can engineer it by carefully annealing with the
furnace or other annealing protocols where, we try to minimize on the defect, which can
help the domain wall to move easily, so that, we can get this response.
Another way to get a soft magnetic responses to make a ring shape, so that you can
minimize on the shape anisotropy, because shape anisotropy can influence your magneto
crystalline anisotropy, as a result, it if you make a ring shaped one you can candidly
minimize on the magnetic shape anisotropy. So, let me just take you through, some
viewgraphs of each of this compound, just to show. What why, it is useful and how it is useful
permalloy as you would see, to since we handle the floppy disc, it is important for
us to understand, that permalloy is a material that, we use almost in everyday’s practical
application. As you see here, this is a 3 and
half inch floppy, which we no more use, we only use pen drives now, with much better
storage density, but permalloys are actually used in magnetic applications mostly as
shield. And permalloy is nothing but, a nickel iron
alloy where, 20 percent of iron and 70 to 80
percent nickel is used, permalloy is not the other way, usually we think that iron has
to be more. But, it is the other way about 20 percent
iron and up to 80 percent of nickel is there and what is important about, this it has largest
permeability as you can see here. And it is soft magnetic metal, as you can see from this
loop, it is a soft magnet and largest permeability. And what is the advantage, you
can generate strong magnetic fields with very weak electric currents by using a electromagnetic
core made of permalloy. So, you can see here, this is your permalloy
ring and using this with very small electric current, it is possible to generate or amplify
the magnetic field and not only that, we can try to block the magnetic flux from entering.
As a shield it can be used therefore, it can block the magnetic field, that is coming from
outside or it can be used as a gate, where you can confined the magnetic flux to be confined
to a particular place and that is why it is used in floppy’s in disc. So, permalloy is actually a very very important
alloy, soft magnetic alloy, used in magnetic storage, but conscious I am going
to avoid this, because I will be discussing this example later, in module 5, where I,
where permalloy is not only used as a magnetic shield, but is also being used for magnetic
read write applications. And these are some of
the cartoons, that you see here, permalloy is actually used as a magnetic shield in wide
range of applications, including as a sensor in fast trains. Permalloy processing is also very unique,
because its alloy, it is malleable, therefore you
can make it as a roll or as a sheet, in any form you can see this sort of big rolls of
permalloys can be made for functional applications and as you can see here. These are all
the permalloy applications, that you see in making disc and these are the magnetic
recording heads, that you would see in tape recorders. Today, we do not see tape
recorders or cassette players, but these are there head, that will actually read and help
you play, the songs or lyrics that you want to hear. And permalloy is essentially, the
head that reads the magnetic information. This is one application of permalloy and then
ferrites are also used systematically, in variety of applications, as I have already
touched upon spinel ferrites are known ferromagnets, some are ferrimagnets. And mainly,
because of their occupancy in the B site and the way they exchange between A and
B site will tell, whether it is going to be a
normal spinel or inverse spinel. You can make any sort of ferrite, if you know
how to play around with your A site combination, for example, you take manganese
zinc ferrite and nickel zinc ferrite, you can candidly see, how you can affect the other
properties, for example. Manganese ferrite have soft magnetic property, but they have
very low resistivity of the order of ohm centimeter, whereas, if you gambled with nickel
zinc ferrite, then you get resistivity of the order of ten power 3. And these 2 ferrites
are incidentally used very much in storage density applications, manganese zinc ferrite
and nickel zinc ferrite, both are by choice used for different applications, because one
gives you a almost metallic behavior and the other one behaves more like a insulator. And amorphous alloys are other group of compounds,
which show soft magnetic response and usually amorphous alloys are
fabricated by melt quenching or by vapor deposition. Because, you can deposit using
a physical vapor deposition method at room temperature then whatever is crystalline will
actually grow as amorphous compound. And then it is possible to characterize, the
magnetic behavior of a amorphous alloy and since our course is designed more to materials
chemistry, in the first module, I have increasingly stressed on the use of chemical
approaches to make amorphous alloys. I have discussed with you, based on sono chemistry,
how we can make such amorphous alloys in nano scale and how the properties
changes in one such example, I have mentioned. That what is usually conceived
to be a magnetic compound can become a nonmagnetic compound, what is considered to
be a nonmagnetic can transform into a magnetic compound. When you reduce the size
to nano scale and that is possible using, simple chemical routes for preparing amorphous
alloys. This is the materials, which can be engineered
as amorphous alloys are for example, a transition metal with a metalloid. So, you
can actually make borides phosphites carbides a silicides and we can also make alloys of
cobalt zirconium or niobium cobalt or we can make a rare earth manganites like gadolinium
cobalt or iron terbium compounds as alloys. These are usually used, in thin film
forms and when we try to do a casting or melt quenching, usually these are rendered in thin
film form. What is important about, this amorphous alloys
is that, they have a very local atomic arrangement and this is what we call it as,
short range order, because they are not totally disintegrated. They do have short range order,
but they do not have a long range order usually, this sort of amorphous material will
have x-ray pattern like this, where you have a hump at low angles, where as at higher angles,
you do not see any reflection. So, this low angle hump is a very clear indication,
that it is not completely glassy, but it has short range orders, so if you actually
look at the pair correlation functions, for such
amorphous alloys. You would find out that, there is a reflection or a response for the
transition metalloids or for alloys, which have, which gives an indication that, the
pair correlation functions indicate small polyhedrons
like this, of the order of say 4 to 12. They constitute a short range order as a result,
you can get a very different magnetic phenomena compared to crystalline materials. Nano crystalline alloys, they are comprise
primarily of crystalline grains, having at least
1 dimension, one interesting thing about this nanocrystalline alloys is that, they are
usually considered to be a single domain size. In other words they are lesser than a single
domain magnitude and most of this single domain particles are exchange couple
materials, which can provide soft magnetism and the way we processes soft magnetic
materials, specially the nano crystalline alloys is by sudden quenching.
So, quenching or by a preferred heat treatment of a amorphous alloy precursor, it is
possible to get nano crystalline alloys for example, let us take the case of one
composition. Before we look at the other examples, iron copper niobium silicon boride,
actually has nano crystalline alloy of the composition alpha F e 3 S i where, additives
like copper is added mainly to initiate nucleation, because iron and copper are
immiscible. Therefore, it will not form a alloy, therefore
you will get a massive nucleation, which is promoted, when you put some copper into it
and you can put some niobium in order to restrict the grain growth. So, to get a amorphous
phase, you put this as a additive, but actually your compound is iron silicon. So,
when you have such a composition, it is possible to realize nano crystalline alloys,
one of the reasons why we look for a nano crystals, when we look for soft magnetic properties
is that. The spins in a cluster are exchange coupled
ferro magnetically and the spins between the clusters are also exchange coupled in the
range of coupling length called L and that is
what we see here, in this cartoon. So, these 2 are clusters, but the clusters have a
correlation within itself and between the crystals and both of these nano crystals will
have some amount of a grain boundary influence. And if this exchange can overcome the grain
boundary influence and the exchange length, then they can be strongly coupled
to the extent that, they show a net magnetization. And this is viewgraph, which
tells how the exchange correlation or exchange length can influence the net magnetization,
the effective magnetic anisotropy, which is actually contributed mostly by the
magneto crystalline anisotropy is given by this expression where, your magneto crystalline
anisotropy is actually governed by the correlation length.
If it is short then the crystalline anisotropy is going to be stronger whereas, if the
correlation length is going to be larger, than your magneto crystalline anisotropy is
going to be weak. And therefore, the reason for
soft magnetism is coming from the grain size, which is very small, which does not influence
the domain wall movement and the cluster is made of a soft magnetic material, with
very small magneto striction or almost 0 magneto striction. And the reason for soft
magnetism also is due to the exchange coupling between, clusters, which is larger
than the magneto magnetic anisotropy, because you are minimizing on the magneto
crystalline anisotropy, if your correlation length is going to be lesser. This is another compound, which is used widely,
it is called sendust, this is basically discovered by those researchers in sendai
and this is actually a material, which is peculiar, because it is very very brittle
therefore, it cannot be used in any other form,
other than as a dust or as a powder. So, it is called as sendust, the reason sendust is
interesting is that, you have a very very less magneto striction by the way, the
composition for the sendust, varies in terms of its aluminium content and it can vary
from 5 to 15 or so. But, usually silicon 10 percent will affect
the magnetic property of your B C C iron. So,
doping iron and aluminum, can give you a special composition and you can see herem
the magneto strictionm for a sendust is somewhere here. And it has a very high
permeability for this particular composition, which is almost amounted to 20000
therefore, this is one compound. Which is actually used for reducing core loss
and what you do here is that, we this can be
actually coated as a lamination in transformers, therefore, it can bring down the joule
heating in copper coils. This material is for power and distribution transformers and
the heating affect can be minimized, when you
try to spray or laminated with this sendust. And this particular cartoon gives you an idea,
how little amount of silicon can rapidly alter some of the properties, for example
saturation magneto striction or resistivity, you
can see how the resistivity varies, just with very little amount of silicon, then the curie
temperature also varies down, with the doping of silicon up to 5 to 10 percent. Now, we will take a quick look at some of
the materials, which stands out as permanent magnets, just want to highlight that, the
main divide between a hard magnet and a soft magnet comes, basically by looking at the
hysteresis loop. As you know the hysteresis loop, tells us about a unmagnetised state
or virgin state of a magnetic material on application of a magnetic field, it get saturated.
And then when you remove or reverse the field it loses, it is saturation and this
is the remanence that you get out of it. And then,
you can completely remove the magnetization or demagnetize, when you reverse it.
Therefore you are response, in the second quadrant becomes very very important, for
your permanent magnetic material and as you would see here, a permanent magnetic
material, when you try to reverse the field. In the second quadrant here, whatever is
happening here will tell, you the nature of your magnetic material or the strength of
your magnetic material. So, compare to a soft ferrite,
which is here a hard ferrite or a hard magnet will actually have a very high hysteresis
and large coercivity and therefore, this can be used as a permanent magnet. So, how does this affect or what is the parameter,
that we use to ascertain for a permanent magnet, we can just look at it briefly,
a permanent or a hard magnetic material is 1, which is having a remanence, even at
0 magnetic field or in the absence of magnetic field. And you need a very large field to
demagnetize the material, soft magnetic material on the other hand requires application. So, then demagnetization curve or the second
quadrant of the magnetization or magnetic induction is the most preferred property that,
dictates a permanent magnet suitability as a
magnetic device. So, when you try to magnetize a compound and try to reverse the
magnetization, the second quadrant behavior is very important, to measure the strength
of a permanent magnet. The notable parameters that, we use for judging
is coercivity or you talk about magnetic saturation magnetization saturation or magnetic
induction saturation B saturation and popularly, this B H max is what is used to
ascertain the strength of a permanent magnet which is called energy product. If the energy
product is very high, which is A factor of B
cross H, then more the energy factor, more stronger the field strength as a result, it
can be used as a magnetic material. This is the plot for a neodymium iron boride,
which is a permanent magnet and as you can see here, the width of the coercive field
is very very high and this is typical of a hard
magnet. And the behavior of your, B versus H and M versus H, usually is different and
how they respond in this second quadrant is what is important for the permanent magnet. And this is how, we see a response for samarium
cobalt compound, this is your M versus H and this is your B versus H plot for a samarium
cobalt. And you can see that, the second quadrant
is very clearly altered, when you try to increase the temperature and for example,
the B r, B r is nothing but, your remanence induction, in this case or M R, that is your
remanence magnetization, both changes distinctly. As you increase the temperature
and you can see for B remanence, it drops down by at least 2000 Oersted and B H max
is your energy products strength and significantly, it changes with temperature,
that means, it loses it is magnetization with increase in temperature.
Therefore, you can decide, which sort of permanent magnet you can use for, what
application. So, at high temperatures, if they are going to drastically drop the energy
product, then you it put limits on its application. So, you can use this permanent magnets
in, a variety of environment. And this is another viewgraph just to tell
you, where all these permanent magnets are used, this is the neodymium iron boride and
this is aluminum, cobalt, nickel called alnico, this is another permanent magnet.
And these are ceramic magnets, usually made of ferrites and this is your samarium cobalt
magnet and this one is new generation magnets, which is nothing, but a composite
which are called flexible magnets. Usually flexible magnets contain a polymer
support where, these permanent magnets can be intersperse and rendered into tapes or
any other flexible shapes. So, by and large when
we think of permanent magnets, you have borides or alnico, which is a alloy or a rare
earth alloy or ceramics, usually these are oxides and classifieds as ferrites, which
are used for applications. Alnico it is one of the magnet, which is used
as a permanent magnetic material and in these materials elongated magnetic particles
are precipitated throughout the matrix, during the manufacturing processes. And therefore,
in the case of alnico, it is what is important is the shape anisotropy and this
is usually spin cast into a particular form. Therefore the shape anisotropy is more important
as far as alnico is concerned, the samarium cobaltite are preferred over borides,
because of the processability, but both are essentially very good materials for alloys.
For applications where temperature is stable and it is just above room temperatures,
samarium alloys are used whereas, borides usually used for, high temperature
applications. And we have a range of applications coming from ceramic ferrites, the only
permanent magnets, which are used from the oxides are mostly ferrites. And specifically, one particular compound
its a hard ferrite, which is barium hexa ferrite, how to chose permanent magnet materials, we
have a range of numbers, that will help you understand, what sort of materials, that
you can use. We look at the maximum energy product value, we look at the coercivity
and we also look at the machinability and the working temperature. All these 4 parameters
decide, what sort of compound that, you want to chose for example, flexible materials,
the limitation is you cannot go more than 100, but you can actually render it into any
form. Hard ferrites, you can play around up to 300,
but processability machinability is very poor, there by that way, but if you look at
the alloys and the borides alloys, you can use it
in high temperature applications, whereas machinability is fairly decent and not very
easy whereas, for borides, it is possible, for you to machine it, because it is a rugged
material. So, these are the some of the parameters,
that will tell you, which one to use and alnico is
one of the most popular material, I cannot run through all the datas, but we a preferred
one is a composition, which will have maximum energy product up to 7.5 and your
coercive force is up to nearly 1000 Oersted. So, there are various compositions of alnico,
which will determine, what sort of energy product that, we are looking for the
compositions are aluminum nickel cobalt with adhesions of copper and titanium. As far as the ceramic magnetic materials are
concerned, we have barium hexaferrite or strontium hexaferrite, which does the job
and as you would see here, maximum that, you can achieve, in the energy project product
is of the order of 3.5 megagauss Oersted and coercive forces up to 3000 can be achieved
in this barium hexaferrite. Apart from barium hexaferrite, you also have
strontium hexaferrite, which can do the job, as a permanent magnet these magnets offer,
the best value when comparing the cost, because alloys are pretty much costlier, therefore
ferrites are usually preferred. And one important thing about these ceramic
oxides is that, you can actually make very dense compact to the extent that, the porosity
can be minimized less than 5 percent, that is the essential beauty of this barium hexaferrite
and they are also highly resistive. So, this can be used for specific applications,
because you can sinter it, to near to theoretical density. And rare earth manganites, there are 3 particular
compositions, which I want to touch upon number 1 is the cobaltite, usually it
is samarium cobalt, which is very useful material. As
you would see here, in this particular viewgraph, the energy product keeps
on increasing from 16 to 50 and you can tailor based on, the composition that, you are
using. And the coercive forces can touch up to 10000
Oersted, if you can chose the right rare earth material.
So, the first one is usually samarium cobalt based compounds and the maximum that,
you can achieve is 9000 Oersted and your energy product is up to 22 mega gauss
Oersted. Whereas, when you go to, the next batch of compounds, these
are usually a transition metal and samarium base ones, instead
of cobalt samarium, if you go for samarium iron copper cobalt, then you can
increase on the energy product or we can go for another batch of compounds, which are
usually borides. These are neodymium iron boride based compounds and
these are samarium iron base compounds, as would you see
here, as you go down this list, the energy product can improve and mostly it is the borides, which take edge over samarium cobalt.
And these are a range of iron chromium cobalt magnets that
are available . And the next important application of this
magnetic materials is in the area of magnetic storage, which I will continue in the next
lecture and also I will try to discuss what are all
the magnetic phenomenas, that are involved when we try to look at
these magnetic materials. So, in the next lecture, I will
give some examples of a materials, that are used
for magnetic storage and I will
also discuss with you about some of the magnetic phenomena, that happens, which we can suitably
minimize or enhance, if we get to know what is the application, that we are looking for.

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