 In the previous lecture, we have looked at some of the broad classifications on magnetic materials and we also looked at a clear divide between two sets of magnetic materials. One is soft and other one is hard magnetic materials and we looked at some example of soft magnetic materials like permaloid, alnico, and samarium cobalt and also some ferrites which show soft magnetic material properties and we also looked at a broad classification on permanent magnets and what are the criteria for a permanent magnet and broad classification and we also looked at some of the alloys which are used mostly alloys and borides show this permanent magnet property and also we looked at some of the ceramic materials which show this permanent magnet property. In today's lecture, I am going to deal further on some of the examples of magnetic materials specifically related to magnetic recording media and lastly I am going to touch on some magnetic phenomena which always comes out when we specially deal with ferromagnetic materials. When we think of paramagnetic diamagnetic materials, we do not see so much of a manifestation as much as we deal with ferromagnetic. So in ferromagnetic materials, there are certain issues that we need to have in mind specially when we deal at low temperature. Therefore those issues which are specifically important for ferromagnetic materials, I would like to discuss in the later part of the lecture. So let us quickly go through some of the examples that we saw in the previous lecture on magnetic materials. We looked at the properties of permanent magnets, the classification based on metal alloys or inter metallic compounds specially those with borides and then a ceramic ferrite which is called barium hexaferrite. Now the numbers that we usually look for in permanent magnet is those of coercivity and the energy product. Energy product is defined as B H max and this is expressed in kilojoules or in mega orston and the more the energy product better is the behavior as a permanent magnet and as you would see here the inter metallics usually have a larger proportion of energy product compared to alloys and we also have a substantial degree of contribution from ferrite which is a ceramic compound. Ceramic compounds although they show this property but the fabrication is a intricate issue because it is often brittle therefore you cannot sinter it. But this particular compound barium hexaferrite you can achieve up to 95 percent theoretical density therefore it is possible to use this as a permanent magnet. Not only it is used as a permanent magnet is reported but barium hexaferrite is also used in thin film recording media we will see that shortly. So when we look at the permanent magnet applications we need to bear in mind these two parameters and the more the energy product better its application we will quickly go back to all the compounds that we have seen and the range of applications that these magnetic materials hold. These are a wide variety of alloys that we can see and alloy processing itself is a challenge and therefore there are lot of chemistry routes material chemistry route that is adopted to prepare magnetic materials specially those of amorphous alloys. Because amorphous alloys is very difficult to prepare by conventional metallurgical means because when you make alloy using a conventional melting technique you usually crystallize that alloy or they are crystalline in nature but amorphous alloys have to be suddenly quench therefore the bottom up approach is usually favored for making this material one of the most important route is sonochemistry which we have seen in one of the lectures in module one. And then we need to understand in perspective that ceramic compounds mainly those which are iron based compounds find use in applications as you would see here gamma Fe 2 O 3 and CRO 2 both are simple oxides but they actually hold the magnetic recording media and even now this hold the market although we have several other candidates there but when we think of tape material still gamma Fe 2 O 3 and CRO 2 hold potential and this is a multi billion dollar industry which can never be compromised or substituted by any other material therefore we will study little bit about this two oxides in this lecture and then we have seen yesterday how by changing the zinc ferrite either substituting with manganese or nickel you can change the resistivity by orders and this is also a soft material which is used in magnetic recording today I will also mention little bit about YIG which is a garnet and hexafarite which is used in several other applications like microwave technology permanent magnets and magnetic recording media. If you look at this samarium cobalt which is a very popular alloy you see this is a very tricky or a very complex structure although they crystallize in hexagonal symmetry this is your samarium cobalt compound and this is samarium 2 cobalt 17 and these are very highly ordered alloys and if you look at this samarium cobalt 5 it is exactly the base material for the samarium cobalt 17 except for some of the samarium atoms are replaced by this sort of double shaped copper cobalt cobalt dimer. So this is a very intricate alloy that one has to engineer to prepare it carefully so is the case for neodymium iron boride where you can see the structure is quite complex one with the tetragonal unit cell. Now preparing these compounds is a material challenge and that is where materials chemistry is also a valued component most of these compounds are usually prepared by powder metallurgy and they are melted and when they are melted they are ground into micrometer sized particles and once you get the preferred dimension then these are actually oriented magnetically by an external magnetic field and then they are density densified by further sintering. So this is a very intricate procedure to get these permanent magnets it is not a simple grinding and heating but it involves a careful orientation of these grains along a particular axis mainly the magnetic field is applied along the z axis so that these are elongated particles along a preferred axis and as you would see from the magnetic behavior they have in all these magnets they have a preferred easy axis of magnetization meaning it cannot be randomly magnetized only in a preferred direction you can magnetize it therefore preparation of these materials become very very important and challenging. This is a ceramic ferrite or a ceramic magnet which is called a barium hexafarite. Barium hexafarite is named such a way because it is a solid solution of barium oxide and six formula units of Fe2O3 you can take this and grind it to get a single phase like this but normally to prepare a single phase material of barium hexafarite is very very difficult therefore soft chemistry roots have been adopted very much to engineer this compound. What makes this particular compound interesting is that it has a very high intrinsic coercivity of the order of 160 to 240 kilo amps per meter so because of this very high coercivity this is a very preferred compound for permanent magnetic materials. However when we try to look at the morphology if you look at the morphology of this barium hexafarite these are hexagonal platelets and not necessarily the desired shape for recording media but what happens is the easy axis of magnetization actually remains along the perpendicular direction to the plane of the surface as a result they usually exhibit a perpendicular anisotropy. Particles may be platelet type but the orientation of the magnetization will be perpendicular to the platelet morphology as a result this is usually used in perpendicular recording media not only that one hassle is there in using this for recording media as a memory storage material but the intrinsic coercivity is a problem because such a high magnitude is not decided for magnetic recording so if you want to use this for permanent magnet you look for high coercivity when you want to use the same material for recording you need to tailor down the coercivity how do you do that usually we substitute the iron atoms with little bit of cobalt or titanium immediately the coercivity comes down so with that you can still use this for a recording purpose or for making a thin film recording media and the point that we need to bear in mind is because of the perpendicular anisotropy this is one of the most preferred material which is not exhibited in any of the other soft ferrites although they are used for course in several electrical applications this is the only compound which is used for magnetic recording because of its perpendicular anisotropy so that way this stands off as a most preferred material compared to some of the alloys we will come little bit more into discussion on magnetic storage when we think about magnetic storage and applications of these magnetic materials we need to understand in perspective there are two things one is the material that is used for storing the memory magnetic memory and the other one is the material that is used for reading the magnetic information both are important and to read the material that is stored you need a magnetic material and to store you need a magnetic material so in this the materials chemistry again plays a very important role and as you see one of the important component in magnetic storage is the read head there are several ways that you can do that one is in one configuration the same material can write in another configuration the material can read or it can do only one job but today the new generation computers or storage devices usually have a head which is capable of writing as well as reading so these are popularly known as read and write heads so read write heads are also specially designed based on new generation magnetic materials so this is to do with the read head which is currently used in most of our recording applications and the other one is the material that is used for storage if you look at the storage media there are two things that are prevalent now we are in the age where we are using pen drives and few years back we were using CD and we were using floppy disk the floppy disk actually has a tape material compared to the CD so that was the transformation between a particulate magnetic media and thin film magnetic media so that was a clear divide between this floppy disk and the CD ROMs once the CDs came then the next question was how much I can condense this CDs into a smaller devices that is how the iPod came and the pen drives came so when we think about magnetic materials in all these applications you have two sets of two generation of material which are used but nevertheless when we think about the entertainment industry it is all actually stored in tapes in high quality tapes so the material that is used for recording media is usually alpha or sorry gamma Fe2O3 which is a low temperature form of iron oxide this has a very particular specific manifestation or it has a very preferred orientation where it can prove effective for magnetic media and if you are thinking about a thin film magnetic recording media then we look for alloys and the most preferred one is cobalt platinum chromium alloy or cobalt chromium tantalum alloy so what you see as a hard disk is nothing but it is a material that is made out of alloy what you see as a tape material is nothing but one which is made either of gamma Fe2O3 or another popular one is CRO2 which is also a good tape material so a particulate type and thin film type are the two generation storage materials now this is a cassette which I fondly like to remember because if you have gone got exposed to any of the tape recorder which is no more available now in market tape recorders usually had a option to say whether it is CRO2 tape because for CRO2 tape the way the read head will read or scan the tape will have a different sensitivity compared to gamma Fe2O3 based tape materials so this is a cassette which is a CRO2 cassette it is called CRO2 90 minute virgin cassette as you can see this is a very expensive one sold those days but we have lot of advances in this storage media so a typical cassette tape has a material which is made of chromium dioxide chromium dioxide originally known as a chromium dioxide tape it is good high frequency sensitivity actually helps in minimizing on the tape noise because when you play the tapes you usually have a noise the background coming and that is because of the sensitivity and greatly that has been improvised by application of CRO2 and the company which really marketed or made billions of dollars out of this CRO2 tape is the TDK company and even now the video tapes that are available those are made out of a black compound if you have time and just take a look at the tape that is running in those video tapes you would see it is a dark one whereas the audio tapes that were sold will be brown in color so the brown ones are usually gamma Fe2O3 and the black ones are usually those made of CRO2 and the main contrast between these two is gamma Fe2O3 is easily made compared to CRO2 therefore CRO2 is used for very specific applications because to make CRO2 in a preferred geometry is very important so what is what are the numbers that we need to remember regarding particle particulate magnetic recording media the most important thing is the shape anisotropy of small elongated magnetic particles which will actually give you the required coercivity that is needed for superior recording performance so we need to know that when we talk about particle particulate magnetic recording the most governing principle there is the shape anisotropy and CRO2 therefore can be successfully made using hydrothermal process I have already dealt in one of my lectures in module 1 where hydrothermal conditions are the only preferred way to get CRO2 in a needle or acicular shape so actually this is the way you have the aspect ratio of these particles and that is exactly needed for the magnetic recording in a preferred direction and mainly because you need to smear this CRO2 particles on a tape material and they need to have a particular axis of orientation it cannot be random therefore for particulate medium which is usually a tape material which has a polymer base you prefer needle shaped CRO2 particles so this is one of the criteria for tape applications the other one is ion oxide that is used and ion oxide again has acicular or needle shaped geometry and acicular ion oxide particles are magnetically stable since the shape induce uniaxial magnetic anisotropy is actually dominating the magnet of crystalline anisotropy therefore this is of a particular advantage over other materials this is a review graph to tell you what exactly I was talking about the shape anisotropy and this is the desired form of gamma Fe2O3 if you want to use it for tape because there are several chemical roots by which gamma Fe2O3 can be made but those usually end up with this sort of irregular shaped or spherical shaped particles but those are not preferred for the tape applications so this is made specially out of a particular chemical process so if you really need to win over in terms of applications we need to control the morphology and whenever we think about controlling the size we usually resort to a particular chemical process and hydrothermal condition usually comes as a aid mainly because you use high pressure to force the morphology. The next point that I want to drive home is the use of this magnetic materials for thin film media so thin film magnetic recording media usually is used for CDs and also for making the read hits read hits are never tape materials those are those are some iron cores which is coated with a particular magnetic material so in addition to the composite particulate magnetic recording media we have this magnetic thin film media which actually has taken over the hard disk applications so the criteria for thin film recording media is again similar to the tape material that we mentioned it should have magnetic hardness in terms of high coercivity, high remanence magnetization and high coercivity squareness and also the energy product also we need to have a low noise this would ensure good application for thin film media the candidates that are usually used are mostly metals or alloys the most preferred one is the cobalt based alloys and several processes have been developed not just the physical vapour deposition but if you want to make this in a larger scale the most convenient and inexpensive route is sputtering where you do not really look for a very high quality of epitaxy but just a particular orientation that is possible using sputtering because you can really make a large area deposition for this and mainly this has to be on a real real to real basis so this is achieved using vacuum technology so just want to make some sum up on these two range of materials for magnetic recording the numbers that we are looking for is the coercive field and this coercive field is actually controlled by two parameters one is the magnet of crystalline anisotropy where your k l or k u dominates k l or k u is nothing but the magnet of crystalline anisotropy coefficients which are specific for either a cubic or a uniaxial ferromagnet and then your shape anisotropy also comes into picture so shape anisotropy and magnet of crystalline anisotropy they determine the value that you can generate out of it so the values that you generate for shape anisotropy is roughly of the order of 800 kilo amps per meter so these are the estimates specifically mentioned for small magnetic particles the fascinating material which not only shows the room temperature metallic property because this is the only oxide which is both ferromagnetic as well as it is metallic at room temperature so this is a very important oxide in magnetic recording not only that in the recent past we have observed another unique feature of this c r o 2 particles especially when you talk about small particles not as a thin film if you look at the small particle you can see the resistivity of this metal seems to be rising at low temperature when they rise at low temperature that means the exchange coupling between the particles are affected because of the random orientation of this ferromagnetic domains so as result if you try to look at the influence of magnetic field on c r o 2 you can see here there is a substantial increase in the magnet of resistance ratio in other words c r o 2 can be used not only for magnetic tape material but it can also be used for read head application mainly because it is responding to a field and it varies the resistance how it varies the resistance at 0 field you see the resistance is at its maximum and at higher field the resistance almost drops down and this is the case with the with change in temperature as you would see here nearly 44 percent of negative magnet of resistance ratio is achieved at 4.2 Kelvin for this c r o 2 particles and that is mainly happening because your transfer integral of one spin to the other spin is actually governed by cos theta i j which is the angle that is made between two spins so when you apply higher field and at low temperature you can actually bring about a collinear exchange between two c r o 2 particles so as a result you see as large drop in resistance even at very fairly low field so this is one of very important breakthrough in the recent past on c r o 2 technology where c r o 2 can not just be used for as a tape material but it can also be used for magnetic sensor applications but the challenge in c r o 2 is that it has to be prepared under extremely difficult conditions because if you take a c r o 3 which is a lab reagent and then you heat it all you would get is c r 2 o 3 which is chromium tri oxide and this is a green compound which is thermodynamically stable so your c r o 3 6 hexavalent compound comes to a trivalent compound and it is very difficult to promote this c r 3 plus here to c r 4 plus that is why it has to be stabilized under high pressure condition so one of the most important way by which we can stabilize c r o 2 is by hydrothermal process where you apply very high pressure to a starting compound so that you can stabilize this phase so this is a very narrow range where you can stabilize c r o 2 and the production of c r o 2 has actually crippled the massive use of this in the recording medium and this is actually crystallizing in a t o 2 rutile phase and you can see this is a typical x-ray pattern that you should get but what you would see is a bit of c r 2 o 3 coming as impurity which is not a desired so to get 99.999 percent pure c r o 2 is a challenge which is the challenge for the materials chemist now the other important application in magnetic memory is bubble memory what is this bubble memory there are some compounds which actually in the presence of external field will not show a systematic orientation of domains rather the domains will coalesce into a bubble and this bubble can actually move and in this bubble you can try to store the memory and this is a cartoon which tells what is this bubble memory about it forms a bubble where the read write chamber is placed and you can sense this magnetic information as a 0 1 bit and this is confined more in a circular motion not in a linear fashion that is why it is called bubble memory but this bubble memory actually has one critical disadvantage because it takes more time to read the information as a result what was predicted to be a breakthrough in the early 70s later faded out because lot of new compounds started coming which was a pleasant replacement for this bubble memory devices some of the notable materials which are used as bubble memory devices are perma-loy the first discovery was actually on perma-loy and later ferrites were used so just run through some of the definition for what this bubble memory is about it is a type of non-volatile computer memory that means it is non-volatile because even after you remove the magnetic field the magnetic information will still be there whereas in non in volatile memory devices with the moment you remove the magnetic field the memory information is lost whereas in this magnetic bubble memory devices it is a non-volatile computer memory and this actually uses a thin film magnetic material and this thin film magnetic material can be magnetized into small bits or small bubbles and therefore you can actually hold the memory in each bubble and you can treat each of this bubble as a bit so that is why you call this as bubble and not domains each of this stores one bit of data so bubble memory started out in 1970s but it failed because of the commercial disadvantage it was actually Paul Charles who actually worked with perma-loy who observed that the domains can actually orient perpendicular to the plane of the film and that gives an idea about a perpendicular anisotropy so magnetic domains are in orthogonal directions with the film and a group of compound which showed this property is from the ceramic ferrites in other words oxides and they are called as ortho ferrites. Ortho ferrites are usually RE Fe O 3 where your RE is nothing but rare earth so we can talk about lanthanum Fe O 3 or we can say neodymium Fe O 3 these are called ortho ferrites and one of the reason why they are called ortho ferrites is because they show magnetization which is orthogonal to the thin film surface and this is very peculiar because not all the magnetic compounds can show such a orientation and if you actually try to channelize it by running wires on this thin film and in x y direction what you can do is you can generate a array of this bubbles and this bubbles can be used to store datas it was later by IBM personnel Bobbeck who could use magnetize small spots perpendicular to the surface and could move the magnetic spots more in whatever fashion you can design and some of the ways that you can do that is seen in the next few slides the difference between the ortho ferrite and the normal magnetic material is this when you place the magnetic material between poles usually the opposite poles attract the respective ones so in this case the shaded colors tell tells you how a typical magnet would respond but ortho ferrite will actually respond this way it will not align opposite but rather it will exhibit a dipole so it will it will remain perpendicular to the pole and therefore you can get a domain structure like this a bubble memory device or a material will actually show your domain in this form and these shaded areas or colored regions are the bubbles perpendicular to the plane of the film so now if you try to place it between a poles of a magnet then you can see all these bubbles whatever you are seeing this coalesce into small bubbles and then if you try to use some small magnets then you can easily move this bubbles throughout so what you can do you can try to put some circular wires and if you try to homogeneously apply a field then if these are the shapes of your ferrites then you can actually rotate this bubbles in any direction that you want so if you want this to go in a linear direction then this is the configuration that you need to keep so you can generate each of this red once what you see here is nothing but a bubble that you can generate and you can push it along a particular direction if you want V shaped motion then you can design something like this so this is one of the very distinct feature of a bubble memory device which is used for storage application and to know whether these are really homogeneous you can get micrographs of these ortho ferrites and you can see here this MFM mapping can clearly give you the domains which are rich in ferromagnetic component and the regions which are rich in anti ferromagnetic component so this can be clearly studied and the phase homogeneity can be modified. Another compound which is of interest is garnet garnet is also used in storage media this is believed to have no preferential direction of magnetization since the garnet is cubic so it is not supposed to show any isotropic behavior so this was again used by Bobak to explore the application for a bubble memory applications. One of the important feature of this garnet is that it is used more in magneto optical devices reason they can this YIG or etium ion garnet is actually capable of rotating the plane of light so this can be used in magneto optical applications specially for faraday effect where you can use this more as a polarizer so if you use this light you can plane polarize it when you pass it through a polarizer made of YIG and in that case you can actually get a plane polarize light as a output therefore you can study the ferromagnetic response of any magnetic material so this is one of a very important application of garnet that is used in today's device for example notable application is in the field of smoke smoke is nothing but surface magneto optical care effect this is a optical way of studying the care rotation so if a magnetic material if a material is magnetic then it will actually rotate the plane polarize light in different directions so you will get a hysteresis like this similar to what you get in a VSM in a vibrating sample magnetometer or using a squid if you see a hysteresis the same hysteresis can be generated using a plane polarize light and this is specifically used for recording media these are some of the view graphs of YIG shows how the ferromagnetic domains are aligned this is mapping under magnetic imaging this is a MFM image of the same thing and this is a unusual structure which is not as simple as the spinal structure or the orthophoretic structure so this has its own characteristics of crystal and lastly I would like to conclude on the classification of this magnetic materials with those which are present generation materials used for read heads this is a cartoon that you would see in an IBM website where the current read head mechanism is very clearly mentioned and what is being used nowadays is a permaloid based read head material that is nothing but a multi layer device so if you look at the read head structure this is where your read headed which is a very small spot and if you try to magnify that to see this is a multi layer which is kept over inductive coils and the tip of this read head is nothing but a multi layer which is of this geometry and this is made of several structures and the popularly this is called as spin valve structure because with these it is possible for you to rotate the magnetic moment of the top layer so if your layer has to read your magnetic memory or magnetic information in 0 1 bit then this top layer magnetic moment has to rotate very freely and that is why it is called spin valve it can easily be flipped so the spin valve is the current generation 1 and that is one of the reason why we are able to store lot of memory even in a small area so the field sensitivity of this spin valve is very very important and the compound that really shows such a read head capability or the spin valve device is nothing but bi layers of iron chromium or nickel iron and copper sort of multi layers the view graph that we see here is nothing but a response of resistance normalized resistance over field and what you see is a multi layer device which is made of iron chromium repeat of the order of 30 such bi layers or 36 or 60 such bi layers you can make where after every 3 nanometer iron layer you see a 0.9 nanometer thin chromium layer so this is nothing but a magnetic non-magnetic bi layers so if you stack such magnetic non-magnetic bi layers what you are effectively bringing into perspective is a ferromagnetic and a anti-ferromagnetic coupling between two magnetic layers so if you have this sort of a anti-ferromagnetic ferromagnetic coupling then in the presence of the field and in the absence of the field you see a change in the response and that is what we call it as magneto resistance so let us take the case of a simple bi layer like this at 0 field it has a very high resistance and at say 10 kilo gauss you can see that the resistance has shifted but if you control the size of this interlayer from 1.8 nanometer if you go down to 0.9 nanometer you can see this fall is quite steep so thinner the non-magnetic layer higher the magneto resistance response so what essentially it means is in the absence of field and in the presence of field I can control the resistance so that is why it is called as a magnetic sensor and this is precisely a sort of layer that is used in the current read head applications so this is actually taking over most of the attention compared to the traditional magnetic materials that are used in the read head applications so what we have seen so far is the range of magnetic materials from ferromagnetic to ferromagnetic we have seen some of the examples of diamagnetic materials and we have largely looked at the different classification of compounds between hard and soft magnets and some of the candidates for magnetic storage media. In the next few minutes I am going to discuss with you about some of the problems one would encounter when we handle ferromagnetic compounds although ferromagnetic compounds are a luxury to use in several applications the way the magnetic response happens at a wide range of temperature say room temperature and low temperature actually brings about interesting physics and chemistry so I am going to show some of the magnetic phenomena which are embedded in system which we need to understand in perspective some of these examples I will be discussing in detail in the next module but I will show only the examples to highlight the point of magnetic phenomena for example one issue that often we confront in ferromagnetic material is spin glass it is not only seen in oxides but also in other metallic compounds to show what the spin glass behavior is I take this popular example of a phase diagram of lanthanum calcium magnet if you take lanthanum magnet and dope it with calcium then as a function of calcium you can see the ferromagnetic response varies in other words the curie temperature keeps on varying so there is a domain where you can see this is the domain where you get a very strong ferromagnetic response but as you keep on substituting calcium before and after here and here the ferromagnetic response vanishes completely and you get into a sort of related phenomena and that is exactly one of the phenomena is spin glass which you can actually see in this domain spin glass and what we also observe in the same ferromagnetic material is a charge ordering these are all embedded along with the system and we can understand how to eliminate or what the origin of these mechanisms are now what makes this lanthanum magnet more interesting is that if you look at the band structure of this lanthanum magnet these are the 2 p bands from oxygen and this is the 3d band from manganese and you would see that this is the situation when the material is above Tc temperature is above the curie temperature this is the band structure but once you go to temperature below Tc when the system or the material is showing ferromagnetic nature you can see that at the Fermi level the up spin electrons or the d bands dominate over the other sub band that is the low spin sub band so as a result this has high degree of spin polarization and this makes it very important for magnetic application spin polarization but what happens this half metallicity or 100 percent spin polarization of one particular carrier of electron is actually distorted by several other factors which happens in the lattice now this is the cartoon which tells what is important about this lanthanum magnet and what is the magnetic phenomena usually we confront with what is happening here is you have a manganese 3 plus and you have a manganese 4 plus so this manganese 3 plus has a eg electron which actually goes via oxygen 2p to manganese 4 plus and then this becomes a manganese 3 and this becomes manganese 4 then the same electron can go reverse back which is called as double exchange ferromagnetism and when this double exchange ferromagnetism happens if you look at the AB plane which is nothing but manganese oxygen manganese oxygen manganese so this is your AB plane the ferromagnetism is actually ordered in this direction and this is a strong ferromagnet it is a correlated for ferromagnetism but when you go to low temperatures what happens some of these ferromagnetic clusters can actually disalign and they can get frozen into a cluster therefore this can freeze as it is as a disordered state but each of this can be grouped as a ferromagnetic domain so this is not completely it has lost ferromagnetism but there is a competing interaction to the strongly ferromagnetic character that you would see at the TC so at room temperature when it is a ferromagnet this is the situation when you go to low temperatures you have competing interactions which we call it as spin glass or as cluster glass or as cluster glass what happens this is a ferromagnetic domain this is a ferromagnet but they are misaligned as a result you won't get a strong ferromagnetic loop rather in the absence of a magnetic field you would see that it is almost going towards zero so how do we see that in a typical magnetic response this is a m versus t data which shows there is a strong ferromagnetic transition at low temperature there is no problem it is strongly ferromagnet and as you bring it to the curie temperature it becomes a paramagnetic situation so this is a simple m versus t response of a typical ferromagnetic compound but what happens when you try to remove the field and cool the substance what is happening here is the same compound which is showing a very sharp transition here when you do a ZFC that is zero field cooling it immediately shows a response like this so what is this response this is called as blocking temperature Tb and this is called as Tf which is called a freezing temperature why because at this place the ferromagnetic domains are totally frozen into a spin glass it is more like a cluster glass and how do I know that you if you carefully look at the relative or the complementary response of this resistance versus temperature you can see corresponding to this transition you can see a very clear metal insulated transition but as you go down the temperature with the fields that are freezing you can see the resistance is sharply going so it is showing metallicity in this regime which is corresponding to this ferromagnetic ordering when it is getting cluster glass situation or when it is frozen as a spin glass then you see that the domains are not coupling together or there is no exchange coupling between two domains as a result you can see the resistance is going so when you have a complementary response between resistance and magnetization and when you see this set of typical behavior then you can classify this as cluster glass now what is the difference between a cluster glass and a anti ferromagnet in a anti ferromagnet you would exactly see the whole thing the moment sharply going to 0 which means even with very little field it is it would not be possible for me to reverse the anti ferromagnetic coupling so if you apply an external field say 1000 R state or 500 R state it is not possible to remove this anti ferromagnetic coupling whereas in spin glass or cluster glass even with very little field for example it is mentioned here as 100 R state with 100 R state if I apply somewhere if I sit here at this temperature and apply 100 R state then immediately this behavior comes back so this is the signature of a cluster glass or a spin glass but why we are studying this why this is important because this can certainly bring about a candid response or influence on the electrical behavior so if you want your material to be a metallic material down to 100 K or 50 K and if you want a magnetic response then there should not be any spin glass behavior in that case we need to modify the composition so that we can get a strong ferromagnetic ordering for example if you take another compound or another substituted compound of ruthenium lanthanum magnate you can see between this and this there is a very small opening it is not as this one what it means is the spin glass behavior in this particular composition is very very less than this one therefore the competing anti ferromagnetic interactions are minimal in this case compared to this composition so this much of information we can try to understand for want of time I will not be able to discuss other issues but I just want to sum up on this cluster glass here is another compound but this compound is actually varied with different applied external field and the ZFC and FC curves are recorded here as you would see here if I do the ZFC FC curve at 2 tesla you hardly see any change between these two which means the cluster glass influence is actually suppressed by external magnetic field whereas if you lessen the external field to just 300 r std then the opening becomes bigger and if you go still further if you just apply very meager magnetic field then you can see a very clear spin glass or cluster glass behavior coming so what it means is this anti ferromagnetic interactions can be largely minimize when you study this material under high magnetic field with this I should be able to end because I do not have much time to cover other examples so I would just sum up with one particular view graph that we have seen some of the issues related to magnetic phenomena in ferromagnets especially the spin glass and I have also touched upon in this lecture on special materials which are specific to bubble memory devices and also storage recording media.