 Yeah, in the last three lectures we have been looking at the issue of magneto resistance and any material which loses its resistance in the presence or absence of magnetic field becomes very interesting. In the first example, we looked at rare earth manganites which actually has a genie inside the lattice, the magnetism controls the electrical conductivity and as a result there is a huge drop in resistance when you try to disturb the magnetic ordering there. And in the next example, we saw it need not be a ferromagnetic material in bulk, but if you can make stacked layers of ferromagnet an anti ferromagnet or ferromagnet and spacer layer then you would expect a colossal change in the resistance. So, we looked at few examples of this metallic multi layers, we are actually looking into the theme of the various aspects of magneto resistance, we call it GMR, we call it TMR, CMR. GMR and TMR are predominantly involving metallic ferromagnetic layers whereas CMR is confined more to oxides. So, we looked at the example of CMR, we looked at example of multi layers of GMR and we looked little bit into the mechanism of TMR which brings about a colossal change in resistance. Just to give you a recap of what is the central issue, the issue is to make use of the electron spin, electron has a down spin and electron has a up spin therefore, you can try to exploit the orientation of this electron spin up or spin down and you can try to moderate the electrical conductivity. In the bulk there are ferromagnetic materials, as you know paramagnetic material is best exemplified in this fashion, when magnetic field is removed the moments are oriented in random way whereas, when they are aligned when applied magnetic field is stronger and it can reorient all this in one direction. So, this is the situation of a paramagnet and then we also looked at a typical ferromagnet, typical ferromagnet gets ordered below T c, they are ordered like this and above T c they get reoriented into a paramagnetic phase. Now, the point is the presence of magnetic field produces only anisotropic magnitude resistance and the order of such change in resistance is very very small. As a result you cannot bring about a colossal change in resistance when you are looking at anisotropic magnitude resistance therefore, you need to use the same ferromagnetic material but you should align it in different way, you should stack it in different way then you can induce a colossal drop in magnitude resistance. So far we have seen the many phases of magnitude resistance, we call this as GMR, we call this as TMR and we call this as CMR. A GMR predominantly we are talking about metallic multilayers and organic inorganic multilayers, in TMR we looked at metal insulator, metal trial layers and the CMR predominantly rarer the manganese and ruthinates. Today I am going to spend little bit more time on this issue of organic inorganic multilayers and why it is advantages, why we need to go for such a combination of organic inorganic multilayers. Now, just to bring back to focus the two important mechanisms that governs, we said if there is a ferromagnet here which is aligned in this direction, ferromagnet here which is aligned in this direction and it is separated by a non-magnet of this dimension. Then you have two different resistance pathway, one is if the up spin electron is going from here to here then you have a resistance pathway in this form and if you have a down spin then you have a resistance pathway in this form. So, if your if it is an anti-parallel direction, your ferromagnets are anti-parallel aligned then you have a different pathway for resistance and this is the model that we propose. So, overall if you have a anti-parallel situation then you have larger resistance and if you apply a magnetic field you have a smaller resistance specially via this form which we call it a short circuit therefore, you can see a change in the magnet of resistance. This is mostly a spin dependence scattering which happens across the interface. The other one we also told about the mechanism of tunneling magnet of resistance. In the aligned case you actually have the spin up electrons going from here to here when they are aligned parallely and in that case the spin sub bands both are same. Whereas, when it is aligned in anti-ferromagnetic fashion the spin up band is located here whereas, in this case spin up band is located here therefore, there is a reluctance for this electron to go here and therefore, this resistance is going to be greater in the anti-parallel way. So, these are two important considerations for the resistance across multi layers. Another example that we can think of is instead of a insulator a non-magnetic insulator you can try to replace that with a ferromagnetic insulator. What would happen if there is a ferromagnetic insulator then electron when it goes from here to here if suppose it is up spin and then it has to retain its spin memory when it goes to this layer and in that case if there is a ferromagnetic ordering or if there is a ferromagnetic alignment in this insulating phase then the spin up can have this electron can have a spin memory which can be retained as it goes to this layer. So, this is also a useful concept instead of using a non-magnetic insulator you can go for a ferromagnetic insulator to maintain the spin memory as the electron goes from one electrode to the other electrode. So, in that case what is a situation this red m h loop m versus h loop what you see here corresponds to nickel ferrite n f o is nothing but n i f e 2 o 4 which is a spinal ferrite and this is a ferromagnetic and this is insulator typically it gives a magnetic hysteresis loop of this fashion and you also have the hysteresis loop of the un sintered ion layer which is having a rather different coercivity compared to the bottom layer which is actually grown at 250 degree c. So, you have essentially same thickness of ion but with different coercivity because in one case you grew the film at 250 therefore the coercivity changes and the other case you have ion which is deposited at room temperature therefore it has a different coercivity. So, you have two different coercive ferromagnetic electrodes and separated by a ferromagnetic insulator typically you would see the device showing the m versus h loop in this form. So, what is unique about this you have the staircase you have the staircase type of hysteresis loop which is typical for a device. So, if a device is performing then you have the staircase sort of feature and then the magneto resistance also shows a pronounced activity you can see although the order of percentage m r is rather low but you can clearly see this butterfly shaped butterfly wing shaped m r curve which clearly shows that this sort of magneto resistive feature can be accomplished with a ferromagnetic insulator. So, this is another example one can think of you can make this device in this fashion you first put ion electrode and then this is your NFO layer and then you put another stripe of Fe then you can measure the voltage that develops across this interface. So, this is one example where we can show that you can use variety of spacer layers not just non-magnetic layer you can use MGO we have already seen one example of MGO which is neither magnetic nor it is metallic but it is firm it is a anti ferromagnetic insulator and this is also showing a pronounced tunneling magneto resistance. Now GMR can also be seen in granular systems what is granular system for example, if you take cobalt silver these two are not immiscible in other words we can say they are immiscible they are immiscible alloys. So, if you actually deposit say silver layer and then you try to put cobalt layer it would not grow as a silver layer and then as a two dimensional cobalt layer what would happen because of the immiscibility all the cobalt will actually form clusters they will form clusters of cobalt atoms and it will be deposited on the silver matrix. Now the interaction between these clusters will determine what sort of magneto resistance that you can get and this is also called as granular system one drawback about the granular system is the field sensitivity is less we can also achieve that using iron and silver. Now you can make several such composites for example, one can run through nickel ferrite Ppy this is a example of how this GMR can be seen even in bulk because your NFO is nothing but ferromagnetic insulator you can try to provide the conducting pathway by coating it intimately with polypropylene or polypyrrole. So, in this case you take pyrrole and you try to polymerize it inside to with suspended nickel ferrite. So, as pyrrole is getting polymerized you will see that this nickel ferrite particles are coated intimately by pyrrole polypyrrole as a result you get a conducting matrix like this. So, the moments are actually aligned randomly and you have the polypyrrole matrix which is actually holding all this nickel ferrite clusters. Now this can give a some glue whether we can achieve magneto resistance in this sort of bulk composites because so far we have seen whether there is pronounced MR in metallic multi layers. Now we can also see whether there is any faint chance of harvesting a large percentage magneto resistance in bulk composites. If you actually take a look at the infrared spectra and the x ray diffraction spectra you would find it is very interesting that the pure NFO that is nickel ferrite gives a typical spinal pattern whereas the P P y which is a polymer gives you amorphous pattern. Now if you keep loading nickel ferrite in P P y to the order of 50 percent 70 percent or 90 percent in spite of loading that much of nickel ferrite even with little percentage of P P y you can see still the faint amorphous pattern dominates over the crystalline nickel ferrite what it suggests that nickel ferrite can be intimately coated by polypyrrole as a result you can try to measure the magneto resistance in bulk in such composites. The infrared spectra also gives you a clue about this particular band which is propping up with the increasing NFO because this is a characteristic nickel oxygen bond which can be seen in this NFO P P y matrix. So, one can actually make quite a bit of composite materials with the wide range of loading capacity and one can see whether this also gives pronounced gmr. If you actually take polypropylene itself which is a metallic polymer you would see that there is a positive magneto resistance and it is of the order of less than 1 percent although but it shows a more significant mr at 80 k. But what we find here for optimum composition of 50 percent NFO which is NFO P P y composite we see mr is much more higher than even the 90 percent. So, there seems to be some influence of the NFO loading on polypyrrole and there seems to be some way that we can look for magneto resistance even in this bulk composites although the magnitude is less. We can also look at the other granular system that is cobalt silver again in this case you have the iron or cobalt silver which is dispersed in the silver matrix and we can achieve this sort of cobalt silver alloys by using sodium borohydride as the reducing agent and this is the overall reaction as to what happens sodium borohydride reacts with metal salts and it releases metal and boric acid and all these can be easily removed filtered therefore essentially you get a pure metal. If you are going to take 2 metals m n and m prime n plus then you can actually get alloy and this alloy can be nano in size as I told you if it is a granular mr the feature is something similar to this where you have the saturation value is rather high because the switching phenomena of this randomly oriented moments takes larger field as a result you have a very low saturation effect. Nevertheless we can see for a 60 percent cobalt doped silver alloy in other words 60 40 alloy we can say you see a very nice trend of a metallic behavior down to 4 kelvin and we have tried to measure the magnetic property or the TC for this alloy and this is reported to be above 350 kelvin because of the measuring constraints one cannot measure magneto resistance beyond 350. So for this reason we have tried to measure the mr at 350 and we see that near to TC this sort of granular alloy show a pronounced effect of up to 20 percent mr in this bulk cobalt silver composites. This is new because there are no reports where cobalt silver alloy is known to show such high values. It is possible from this work that one can try to prepare by either sputtering or other methods to prepare cobalt silver films and try to look at the TC close to sorry mr close to the TC. We can also try to do this with iron silver alloy composites where you see again a clear metallic behavior down to 4 kelvin and one can see that with increasing temperature the resistance increases and mr shows a very clear feature although the mr percentage is significantly low but it is showing higher proportion at 5 k compared to 300 k. So you have another composition iron silver 50 50 alloy which also shows similar trend where you see lesser mr percentage at room temperature compared to 5 kelvin and this can be engineered for applications if we can translate this composites into thin films. So I have shown you a variety of combinations of alloys which show mr behavior just to sum up at this stage what I have shown to you is that this stacking of multi layers is very important and one can try to engineer a variety of combinations as spacer layers and we can try to observe significant mr in cases of metallic multi layers and there is lot more work to be done for spacer layers which are not traditionally metallic. So we will go to another important issue of organic multi layers we can start from the origin where exactly this idea stems from this is known as a predominant feature in organic light emitting diodes. In organic light emitting diodes we have almost all the layers which are organic but for the anode and the cathode. So why this organics can be brought into spin tronics what is the need and what are the advantages over the metallic multi layers is the question. Now spin based electronics as you know we can try to read the information of each of this domains if we can look at the spin orientation. So depending on the spin orientation you can have a binary information storage that has non-volatility it can have high integration density and fast switching time and low power consumption. All these are advantages for spin based electronics which are obviously absent or incurs costly penalties when you think of the regular semi conducting industry. So for this reason we need to look at new possible avenues where we can try to look for wider applications involving organic spin tronics. Now why spins in organics there are two perturbing factors of spin orientation in bulk. Number one you have spin orbit interaction in this case interaction between electron spin and nuclear charge are becoming important and this is more pronounced for heavier atoms. The spin orbit interaction is more pronounced for heavier atoms and then you also have another competing interaction which is hyper fine interaction this interaction is between the electron spin and the nuclear spin. So you have two issues that you confront in the multi layer systems specially when you think of spin tronics one is electron spin interacting with nuclear charge another one electron spin competing with nuclear spin. As a result you have the spin tronic values or the magneto resistance values are considerably low specially when you involve heavier atoms because the spin orbit coupling contribution is of the order of z to the power 4, z is the nuclear charge as a result since this is dependent on z power 4 the contribution of spin orbit coupling is usually dominating and that is why we should try to see whether we can completely ignore this contribution so that you can maximize on the GMR or the magneto resistance ratio. There comes the issue of organic molecules if I need to subdue this both this effects then I should look for organic molecules which are lighter atoms and they are better alternatives because you can minimize both on spin orbit coupling and hyper fine interaction to a larger level as a result there is a search for new compounds. Now where does organic come into picture in electronics the classic example is that of organic LED because in the year 1987 it was Wanslik and Tang who actually reported organic electro luminescent devices which appeared in applied physics letters and a typical organic LED configuration is like this. You have the transparent ITO which is compounded with p dot p s s which is a whole doping layer and then you also have aluminum as cathode with a small barrier lithium fluoride. Now if this is sandwiching a organic semiconductor then light comes out of the ITO layer which brings about a new generation of display devices these are all the old devices which are presently coming into market you can bring down the screen size because you can go for large area and also you can minimize on the deposition intricacies. So organic semiconductors brings about a new generation of devices which involves spin too. Now spins in organics is construed in this way what is the mechanism of this organic LED as you see here electrons come from the anode and then holes go from the cathode and they do combine here when they combine first they are held by a columbically bound electron hole pair and this value 1 3 specially refers to 1 spin singlet and 3 spin triplets both in electron hole pairs and they together combine as excitons. So as a electron hole pair the proportion is 1 spin singlet and 3 spin triplets now they together combine to form a exciton with a singlet proportion and a triplet nature. So you can actually have the singlet exciton and the triplet exciton of which the excitonic spin singlets are the ones which radiate the fluorescent light. In other words if you look at the spin statistics you have 25 percent or one fourth of the possibility is the singlet exciton and three fourth of the possibility is the triplet exciton and because of spin selection rule only the singlet excitons are allowed to radiate and therefore they account for the light that you see in a organic LED. In other words of the excitons that are produced due to electron hole combination you have only 25 percent which is responsible for the light emission and 75 percent is spin forbidden. There are ways to harvest this if you can annihilate this triplets then you can convert this into singlet pair and thereby you can increase the efficiency of this singlet excitons which radiate fluorescent light. So this is what I said the singlet which is responsible for the fluorescence and the triplet excitons which are responsible for the phosphorescence. This is the theoretical limit and the experimental observations are slightly higher than 25 percent in various cases. Now these are some of the organic molecules which are used in the current OLED devices and mostly these are all polymers PPV, PFO, MEH, PPV and P3H and so on. Now how can we translate this spin LED into a spin magnet or resistive device? If you carefully look at this configuration this is your OLED device and if you want to convert it into a spin valve all you need to do is replace this anode cathode and anode by ferromagnetic electrodes. So if you replace ITO with a ferromagnetic electrode if you replace aluminum with a ferromagnetic electrode then you are essentially making a spin valve device which involves a organic layer. This ferromagnetic electrode can be a metal inorganic metal such as say iron or it can be cobalt. So essentially you are making a iron organic iron electrode device where you can look for the spin valve operation and this is typically the way we can cartonize the spin valve structure that involves a organic where you have a ferromagnet 1 and ferromagnet 2. This is the organic layer and as you see that when the ferromagnets are aligned then there is more of electron mobility as a result you have a current high situation when the spin valve is open. Now if they are anti-parallel aligned then you have current low and therefore even with this sort of a configuration ferromagnet organic layer ferromagnet device you can essentially bring about a spin valve response using organic. And this is the way that a spin valve will work if you have this sort of a configuration then you will see a sharp rise in the resistance as a result you can imitate this to be like a inorganic spin valve. So in organic spin valve what are the examples? Park actually reported this first work where he used LSMO and cobalt as the ferromagnetic electrodes. One of the reason why LSMO which is lanthanum strontium manganese oxide which is used is because this is known to be a 100 percent spin polarized half metal and therefore we can use this as a bottom electrode and the interface also can by and large can be moderated if you are going to put organic layer. P3HT is nothing but a thiophene moiety with substitutions and this is a good hold transport layer as a result we can try to have this in between two ferromagnetic electrodes and the cobalt can be used as a top electrode which is reported by Majumdar and co workers and they have reported this in 2006. So this is a typical configuration of a organic spin valve where you are essentially using ferromagnetic electrodes and your middle layer is your organic but what are the problems here? Problems in making this is to do with the interface because to grow a organic and a inorganic interface it is very very difficult because this organic layer should be good enough to wet the inorganic layer or the inorganic layer has to be atomically flat so that you can make a very thin two dimensional layer of your organic which is the challenge otherwise many such structures could have been realized by now. So far the limitation is you cannot grow a good interface here because of the roughness that is coming from the inorganic layer and because of the growth mode which can vary for the organic layer. Nevertheless for the device that we saw which involves cobalt P3HT LSMO layer you can see here the magneto resistance at 300 k and magneto resistance at 5 k it is sufficiently remarkable responses there. In this case you can see at 5 k a very large response and in the other case you see a faint response of about 3 percent at 300 k which is not a bad number considering the metallic multi layers at room temperature definitely there is a good response for this device. Spin injection and spin transport and spin detection seemingly are proved in this organic electronic device these are small but distinct signature at room temperature therefore spin polarized transport through organic materials is therefore possible. So what is this organic magneto resistance which can be called as OMAR organic multi layers first even before a typical organic spin wall was done a typical OLED device was taken and magnet was kept in closer proximity and this is the response that you would see a huge change in the resistance with the driving voltage is realized when you have PFO as the organic layer and therefore a typical OLED device can also give you a large magneto resistance it need not necessarily involve a ferromagnet organic ferromagnet trial layers even a typical organic LED can give you large room temperature response because of the spin statistics that are involved in such phenomena. Using this as a clue towards printed magnetic senses based on organic diodes Majumdar group have come out with another structure which involves silver aluminum and P3HT and put it between the magnets and then you can see a clear MR behavior that can be that can be seen this gives us the challenge to go for printed electronics which is another good development in this field using organic layers. So one can go for different models and how do we understand the organic magneto resistance there are several issues that are being addressed one is the bipolaronic issue then the charge pair issues and also the spin-spin interactions all these are discussed in different examples which are quoted in the recent past. I would like to leave with the one more example on organic insulator we can take two ferromagnetic layers and we can put a spacer and this spacer can be a non-magnetic insulator which is a organic layer and what would happen in such situation we can try to make this sort of organic layers using pulse electron deposition which is a rugged facility and this is a typical pulse electron deposition chamber which has facility for a six target carousel and you can use this pulse electron beam to ablate the material from the target and typically during a ablation protocol you would see the plasma that is coming out because you can you can essentially use this pulse beam for any type of material not only metallic but you can use organic material you can use insulating material to ablate this sort of compounds and this is the overall setup while in use. So we can see some examples as to how we can look for magneto resistance using organic layers in the next slide. So this is a very good facility for making organic trial layers mainly because the insulating organic material can easily be ablated this is very different compared to pulse laser deposition pulse laser deposition is a technique where laser plume laser light falls on the target something like this and then it ablates the material but if the material is insulating and it has a wide band gap then it is difficult for the laser light to be absorbed by the material and ablation will be remarkably low. So for this reason you can replace that with the electron beam then you can try to ablate any material either metallic or insulating material. So you can make any device applications and this is typically for the compound PTFE which is commercially known as Teflon. Teflon is a good insulator and we can use Teflon as a organic layer between two ferromagnetic electrodes and typically if you have the PTFE deposited using PED you can see the thickness profile and this thickness profile is evaluated from a profile of meter and you can actually go up to 21 22 nanometers 22 nanometer thick PTFE layer can be deposited and this is how you can look at your thickness profile one can also go down up to 5 nanometer comfortably with a continuous deposition of this film in two dimensional way. So after ensuring that it is possible for us to make a device structure of this configuration iron PTFE iron tri layer and how do you go about it first you put a iron stripe like this and then we can put the organic layer and then you can put one more metallic layer on the top and you can measure across the electrodes and then you can also see the AFM images of the iron layer which is deposited as a bottom electrode this is actually deposited at 200 degree C and then you can put a 6 nanometer thick PTFE film the surface looks like this and then the top layer which is actually deposited at room temperature shows a much larger grain size compared to smaller grain size for iron electrode. Now how does the device respond if you individually take the top and the bottom electrodes iron electrodes you can see the coercivity is different because when you deposit the iron at 200 degree C or so then the coercivity shrinks whereas the top layer shows higher coercivity which is deposited at room temperature and this is the typical signature for the device when device is there then you would see this two step hysteresis loop which is showing the clear device operation now you can vary the thickness we can go down to 2 nanometer, 4 nanometer, 3 nanometer and 6 nanometer and as you increase the PTFE thickness you can see that this step is more resolved compared to smaller one thicknesses now the question is when you can achieve such small thickness of this PTFE layers whether the device can clearly show magneto resistance is the question you would see that in the next slide when you have 3 nanometer and 4 nanometer although you have this two step hysteresis loop which is characteristic of a device but when you look at the magneto resistance they are essentially showing negative magneto resistance negative MR which means the top iron layer is actually getting coupled with the bottom iron layer only then you will see a negative magneto resistance but if they are clearly separated by PTFE layer then you should actually see positive magneto resistance which is nothing but the response for tunneling magneto resistance device so what does that mean even though you have a two step magnetic hysteresis there is shorting between shorting between the bottom and the top iron electrodes it is short circuiting as a result we can say that this layer is not flat or it is not fully covered there are pin holes in this layer which is actually bringing about a short circuit between the top and the bottom electrode so what you do if you go further to 6 nanometer if you go to 6 nanometer then you see this response is much clearer and if you look at the magneto resistance compared to the previous one in previous one it is a inverted response whereas when you go to higher thickness you see that the response is now positive so what does that mean at a critical thickness of say 6 nanometer you are able to clearly demonstrate a tunneling magneto resistance behavior as a result you see a positive MR although the value is very less still it is appreciable to show that organic spintronic can be demonstrated now what is the clue for whether this device is working if you look at the resistance value for such a device you can see in the presence and absence of field the resistance is varying and the value of resistance gives you a clue as to whether such a device is working suppose the top electrode and the bottom electrode are short circuiting then the value of this resistance will be less than 5 ohms or this may be in milli ohms because it is essentially coming from ion electrode the fact that you are seeing a very high resistance it means the organic layer is able to decouple the resist the bottom electrode from the top electrode as a result you see a pronounced TMR value so this is another example that we can look more positively into making organic spin wall or organic TMR junctions which can give you pronounced GMR behavior there are lot more things one has to do especially in understanding how the inorganic organic surfaces work just to conclude what we have seen so far spin and spintronics will pay way for future electronics number one and organics in spintronics is a fundamentally logical and plausible goal provided you have a ways and means to deposit this organic films in a very sequential way and spin polarized transport in organic is now proved even at room temperature if you can carefully look for a suitable combinations then you can achieve maximum spin polarized transport via organic layers and we can also say organic magneto resistance phenomenon opens possibilities for new application of this existing technology. So, we have actually in essence seen different phases of the MR phases of MR one we saw about a GMR which is in stacked multi layers and then we saw some examples of tunneling magneto resistance in trial layers we have seen some example of granular MR in bulk composites and we have also seen organic GMR where we are bringing the hyperfine interactions and the spin orbit coupling interactions into focus and we have tried to see whether organic spintronics can become a vital tool to address to the issue of magnetic storage. So, with this I will stop and we will continue with other examples of electrical conductivity in inorganic materials in the next lecture.