 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 categorizes 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 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's lecture, I will try to take you through different classification of this magnetic materials and show some example of how these materials can be used in applications. Just to recall, the two 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 external field on the material. Whereas in strong magnetism, it is an 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 moment, but the diamagnetism per se can be used for sophisticated applications. For example, in levitating a train, 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, anti ferromagnetism or ferrimagnetism. We have already seen ferrimagnetism is nothing but ferromagnetism, but then there are some anti ferromagnetically aligned moments 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 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 materials, those which are weak the diamagnetic materials or paramagnetic materials can be categorized from the others on two 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 whereas paramagnetic and anti ferromagnetic 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 for greater than 1 and definitely the susceptibility has to be greater than 0 and what is the response between these two 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 two compounds and therefore it is very easy to understand how a ferromagnetic material would respond against a anti ferromagnetic material. Now the main thing that governs all this three group of compounds ferro, ferri and anti ferromagnetic 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 this materials very rich in its chemistry. At temperatures above a material dependent limit thermal vibrations lead to disalignment of the magnetic moments the material becomes paramagnetic. Therefore, in ferro, ferri or anti ferromagnetic material we usually talk about a anti ferromagnetic to paramagnetic situation or anti ferromagnetic sorry ferromagnetic to paramagnetic situation. So, the transitions are always to a totally disordered alignment of spins to a ordered or to a anti ferromagnetically ordered spin state. So, this is exactly what we see in ferro and ferri magnetism usually for temperature greater than 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 anti ferromagnetism for temperature less than T n that is the nil temperature ferro and ferri magnetism 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 transitional metal oxides is that they are mostly anti ferromagnetic but what you would see in today's application or in the new generation compounds what is understood to be a anti ferromagnetic compound transcends 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 anti ferromagnetic oxide you can actually make it metallic and you can make it semi conducting 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 magnet which is a perovskite and this lanthanum magnet actually is a anti ferromagnetic insulator because the in the A B plane the manganese oxygen sheets are anti ferromagnetically 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 anti ferromagnetic insulator is transformed into a ferromagnetic metal which is a very very astonishing phenomena in the case of oxides. So, when you look at metal oxides most of them are anti ferromagnetic oxides but you can easily translate them into ferromagnetic oxides and thereby affect the conductivity by mere chemistry and that is the specialty of materials chemistry in this oxides. When you look at anti ferromagnetic oxides typically the domains are like this each one of this is a domain in a anti ferromagnetic oxide which is anti ferromagnetically coupled as a result your chi m in that case is going to be nearly 0 whereas your permeability is going to be approximately 1 and there is full compensation of the magnetization by the anti ferromagnetic 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 this oxides you would see a situation like this where there is no influence of the flux on the system. So, this is typical of anti ferromagnetic oxide because the anti ferromagnetic coupling is so strong that it cannot be removed by an external magnetic field. So, if you have a domain structure like this the anti ferromagnetic 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 anti ferromagnetic interactions coming due to some impurity effect or defect effect which is not intrinsic, but it is a extrinsic factor then even with a little of external field strength it is possible to remove those anti ferromagnetic coupling if possible we will deal with this in the next lecture. One of the most important or well studied anti ferromagnetic oxide is manganese oxide and if you closely look at the manganese atom these are the oxygen atoms and these are your manganese atoms. Now, if you look at every alternate manganese atom you would see that they are anti parallely aligned in any direction they are anti parallely aligned whereas if you actually look at the 1 1 1 plane then all the manganese are ferromagnetically aligned. But in essence this is an anti ferromagnetic 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 anti ferromagnetically coupled and this cannot be removed because this is a super 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 anti ferromagnet 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 anti ferromagnetically 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 anti parallel alignment and this is actually dictated by the Hoon's coupling or Hoon's rule. Now, in that situation if it is a anti ferromagnetic 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 higher temperature scale but at nil temperature the chi starts falling down. So, your susceptibility drastically falls down at the nil temperature typically this should have actually increased in this fashion but for a anti ferromagnet you would see this sort of a crossover and this is a very important signal for a anti ferromagnetic oxide and this is exactly what you see in this curve here that all these manganese are aligned anti ferromagnetically 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-Weis law which is dictated by this expression chi m. Therefore, for anti ferromagnetic oxides your chi m will actually be related via Curie-Weis law. Now, when you come to ferri magnetism 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 we will see in some of the examples. What is the behavior of this ferrimagnetic one 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 this 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 field. A classic example of a ferrimagnet is AB204 type of metal oxides which is called as spinal metal oxides. Spinal compounds usually ferrites show a very good response for ferrimagnetism and this is the unit cell for AB204 type of an oxide where you have the A cations sitting in the tetrahedral sites and you have the B cations sitting in octahedral sites. In such case you can actually generate a variety of spinal compounds this is a classic spinal 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 oxide and metal ions and B site by trivalent metal ions for ferrites B is always ion and you can generate magnesium ferrite, manganese ferrite, cobalt, nickel or zinc ferrite. The well known spinal is inverse spinal that is ion oxide or magnetite where if you look at the occupation of these sites you would see that these two ions are actually in B site and your Fe3 plus is actually in A site. So because you have a mixed valency of both Fe2 plus and Fe3 plus in B site this is actually called as a inverse spinal otherwise in a normal spinal phase of ferrite you will actually have always Fe3 plus in B site and M2 plus in A site. So you would see this discrepancy in most of the ferrites the occupation of the ion 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 ion then it becomes a non-magnetic ferrite similarly a non-magnetic 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 Fe2 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 AC susceptibility or susceptibility chi value you will always understand that the susceptibility will be of the order of 10 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 set of a material you are talking about and most of this diamagnetic materials are in this range and also paramagnetic materials like chromium, zirconium, titanium, aluminum all this have a very low susceptibility value and those which are ferromagnetic the magnet spin moments are actually governed by the Hund's rule and therefore it is possible for us to calculate how much of magnetic movement each of this transition metal ions can contribute and this is measured in terms of Bohr magnet arm. 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 encounter only permanent magnets used in household applications but what we forget or what might slip out of our 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 applications. So a range of applications are there for magnetic materials I will basically make a division between two 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 an external magnetic field, permanent or hard magnetic materials are those inherently have a magnetic movement 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 use for either repulsion or for attraction. So this is one application but you can also use the mechanical to electrical conversion of energy 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 this materials such as magneto resistance hall effect and magnetic resonance. I will not deal with this 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 one of the cartoon that gives us 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 materials. 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. Magnetic materials but we also have bubble memory materials which are hard ferrites there is another group of compound called send dust then permaloid 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 its 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 is 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 magnetic restriction 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 soft magnetic materials we can engineer it by carefully annealing with a 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 response is to make a ring shape so that you can minimize on the shape anisotropy because shape anisotropy can influence your magnet of crystalline anisotropy as a result if you make a ring shaped one you can candidly minimize on the magnetic shape anisotropy. So let me just take you through some view graphs of each of this compound just to show what why it is useful and how it is useful. Permaloid as you would see since we handle the floppy disk it is important for us to understand that permaloid is a material that we use almost in everyday's practical application as you see here this is a three and a half inch floppy which we no more use we only use 10 drives now with much better storage density but permaloids are actually used in magnetic applications mostly as shield and permaloid is nothing but a nickel iron alloy where 20 percent of iron and 70 to 80 percent of nickel is used. Permaloid 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 permaloid. So you can see here this is your permaloid 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 confine the magnetic flux to be confine to a particular place and that is why it is used in floppy's in disk. So permaloid 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 five where I where permaloid is not only used as a magnetic shield but it is also being used for magnetic read write applications and these are some of the cartoons that you see here permaloid is actually used as a magnetic shield in wide range of applications including as a sensor in fast trains. Permaloid processing is also very unique because it is 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 permaloids can be made for functional applications and as you can see here these are all the permaloid applications that you see in making a magnetic recording disk 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 their head that will actually read and help you play the songs or lyrics that you want to hear and permaloid is essentially the head that reads the magnetic information. So this is one application of permaloid and then ferrites are also used systematically in variety of applications as I have already touched upon spinal 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 spinal or inverse spinal. 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 gamble with the nickel zinc ferrite then you get resistivity of the order of 10 power 3 and these two 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 almost a 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 a amorphous compound and then it is possible to characterize the magnetic behavior of amorphous alloy. And since our courses design 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 sonochemistry 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 non-magnetic compound what is considered to be a non-magnetic 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 or for example a transition metal with a metalloid. So, you can actually make borites, phosphites, carbides, silicides and we can also make alloys of cobalt zirconium or neobium cobalt or we can make rare earth manganades 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 a x-ray pattern like this where you have a hump at low angles whereas 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 gives an indication that the pair correlation functions indicate small polyhedra 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 comprised primarily of crystalline grains having at least one dimension. One interesting thing about this nano crystalline 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 coupled materials which can provide soft magnetism and the way we process soft magnetic materials especially 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 a nano crystalline alloy of the composition alpha Fe 3 Si where additives like copper is added mainly to initiate nucleation because iron and copper are immiscible therefore it will not form 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 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 nano crystals when we look for soft magnetic properties is that the spins in a cluster are exchange coupled ferromagnetically 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 two 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 a view graph 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 then your magneto crystalline anisotropy is going to be weak. Therefore, the reason for shaft 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 restriction or almost 0 magneto restriction 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 send dust 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 send dust the reason send dust is interesting is that you have a very very less magneto restriction. By the way the composition for the send dust varies in terms of its aluminum content and it can vary from 5 to 15 or so, but usually silicon 10 percent will affect the magnetic property of your BCC ion. So, doping ion and aluminum can give you a special composition and you can see here the magneto restriction for a send dust is somewhere here and it has a very high permeability for this particular composition which is almost amounted to 20,000. 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 effect can be minimized when you try to spray or laminate it with this send dust and this particular cartoon gives you an idea how little amount of silicon can rapidly alter some of the properties. For example, saturation magneto restriction 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 un magnetized state or a virgin state of a magnetic material on application of a magnetic field it gets saturated and then when you remove or reverse the field it loses its 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, your 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, compared 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 one 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 ion boride which is a permanent magnet and as you can see here the width of the coercive field is 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 Orsted and B H max is your energy product strength and significantly changes with temperature that means it loses its 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 it puts limit on its application. So you can use this permanent magnets in a variety of environment and this is another view graph just to tell you where all these permanent magnets are used this is the neodymium ion 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 this permanent magnets can be interspersed and rendered into tapes or any other flexible ships. So by and large when we think of the permanent magnets you have borides or alnico which is alloy or a rareth alloy or ceramics usually these are oxides and classified as ferrites which are used for applications. Alnico 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 process 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 process ability 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 are 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 is a hard ferrite which is barium hexa ferrite. How to choose permanent magnet materials we have a range of numbers that will help you understand what sort of material 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 four parameters decide what sort of compound that you want to choose. 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 process ability machinability is very poor 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 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 data's but a preferred one is a composition which will have a maximum energy product up to 7.5 and your coerci force is up to nearly 1000 Orsted. 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 additions of copper and titanium. As far as the ceramic magnetic materials are concerned we have barium hexafariate or strontium hexafariate 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 mega gas Orsted and coerci forces up to 3000 can be achieved in this barium hexafariate. Apart from barium hexafariate you also have strontium hexafariate 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 ferrits are usually preferred. And one important thing about this 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 hexafariate 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 magnets there are 3 particular compositions which I want to touch upon number 1 is the cobaltite usually it is samarium cobalt which is a very useful material. As you would see here in this particular view graph 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 10,000 orsted if you can choose the right rare earth material. So, the first one is usually samarium cobalt based compounds and the maximum that you can achieve is 9,000 orsted and your energy product is up to 22 mega gauss orsted. Whereas when you go to the next batch of compounds these are usually a transition metal and a samarium based 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 borates. These are neodymium iron borate based compounds and these are samarium iron based compounds as you would see here as you go down this list the energy product can improve and mostly it is the borates which take edge over samarium cobalt and these are 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 phenomena's that are involved when we try to look at this magnetic materials. So in the next lecture I will give some examples of 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.