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Report on Giant Magnetoresistance(GMR) & Spintronics


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Report on Giant Magnetoresistance(GMR) & Spintronics

  1. 1. GIANT MAGNETORESISTANCE A SEMINAR REPORT BY ECE Department 3rd year APRIL 22, 2014 A report submitted to the Respective Faculty in partial fulfillment of the Seminar on GIANT MAGNETORESISTANCE
  2. 2. 1 ABSTRACT Giant magnetoresistance (GMR) is a quantum mechanical magnetoresistance effect observed in thin film structures composed of alternating ferromagnetic and nonmagnetic layers. The effect manifests itself as a significant decrease (typically 10-80%) in electrical resistance in the presence of a magnetic field. In the absence of an external magnetic field, the direction of magnetization of adjacent ferromagnetic layers is antiparallel due to a weak anti-ferromagnetic coupling between layers. The result is high-resistance magnetic scattering as a result of electron spin. When an external magnetic field is applied, the magnetization of the adjacent ferromagnetic layers is parallel. The result is lower magnetic scattering, and lower resistance(3). The effect is exploited commercially by manufacturers of hard disk drives. The 2007 Nobel Prize in physics was awarded to Albert Fert and Peter Grünberg for the discovery of GMR.
  3. 3. 2 Contents 1 Introduction 4 2 Background A. Ferromagnetic Metals………………………………………………..... 7 B. Resistance …………………………..…………………………………… 9 C. Growth of Superlattices……………………………………………….. 10 D. Interlayer Coupling……………………………………………………… 12 3 Giant Magnetoresistance A. Introduction………………………………………………………………. 14 B. Mathematicl Model…………………………………………………….. 14 C. Carrier transport through a magnetic superlattice………………. 16 D. CIP & CPP Geometry………………………………………………….... 17 E. Experimental Findings…………………………………………………… 18 4 Spintronics A. What is Spintronics? ……………………………………………………….... 20 B. Logic of Spin…………………………………………………………………. 20 C. Half Metals…………………………………………………………………..… 21 5 Other improvements on GMR A. Tunneling Magnetoresistance …………………………………………….. 23 B. Colossal Magnetoresistance ………………………………………………. 24 6 Recent Developments & Future works A. Work so far………………………………………………………………………. 25 A.1. Magnetic switching and microwave generation by spin Transfer…………………………………………………………………… 25 A.2. Field Effect Spin Transistors……………………………………………. 26 7 Concluding Remarks 28 8 References 29
  4. 4. 3 1. Introduction The phenomenon called magnetoresistance (MR) is the change of resistance of a conductor when it is placed in an external magnetic field. For ferromagnets like iron, cobalt and nickel this property will also depend on the direction of the external field relative to the direction of the current through the magnet. Exactly 150 years ago W. Thomson (1) (Lord Kelvin) measured the behaviour of the resistance of iron and nickel in the presence of a magnetic field. He wrote “I found that iron, when subjected to a magnetic force, acquires an increase of resistance to the conduction of electricity along, and a diminution of resistance to the conduction of electricity across, the lines of magnetization”. This difference in resistance between the parallel and perpendicular case is called anisotropic magnetoresistance (AMR) (2). It is now known that this property originates from the electron spin-orbit coupling. In general magnetoresistance effects are very small, at most of the order of a few per cent. The MR effect has been of substantial importance technologically, especially in connection with read-out heads for magnetic disks and as sensors of magnetic fields. The most useful material has been an alloy between iron and nickel, Fe20Ni80 (permalloy). In general, however, there was hardly any improvement of the performance of magnetoresistive materials since the work of Kelvin. The general consensus in the 1980s was that it was not possible to significantly improve on the performance of magnetic sensors based on magnetoresistance. Therefore it was a great surprise when in 1988 two research groups independently discovered materials showing a very large magnetoresistance, now known as giant magnetoresistance (GMR). These materials are so called magnetic multilayers, where layers of ferromagnetic and non-magnetic metals are stacked on each other (figure 1). The widths of the individual layers are of nanometre size – i.e. only a few atomic layers thick. In the original experiments leading to the discovery of GMR one group, led by Peter Grünberg (3), used a trilayer system Fe/Cr/Fe, while the other group, led by Albert Fert (4), used multilayers of the form (Fe/Cr)n where n could be as high as 60.
  5. 5. 4 Figure 1. Schematic figure of magnetic multilayers. Nanometre thick layers of iron (green) are separated by nanometre thick spacer layers of a second metal (for example chromium or copper). The top figure illustrates the trilayer Fe/Cr/Fe used by Grünberg´s group (3), and the bottom the multilayer (Fe/Cr)n , with n as high as 60, used by Fert´s group (4). In figure 2 the measurements of Grünberg´s group are displayed (left) together with those of Fert´s group (right). The y-axis and x- axis represent the resistance change and external magnetic field, respectively. The experiments show a most significant negative magnetoresistance for the trilayer as well as the multilayers. The systems to the right, involving large stacks of layers, show a decrease of resistance by almost 50% when subjected to a magnetic field. Figure 2. After refs. (3) and (4). Left: Magnetoresistance measurements (3) (room temperature) for the trilayer system Fe/Cr/Fe. To the far right as well as to the far left the magnetizations of the two iron layers are both parallel to the external magnetic field. In the intermediate region the magnetizations of the two iron layers are antiparallel. The experiments also show a hysterisis behaviour (difference 1 and 4) typical for magnetization measurements. Right: Magnetoresistance measurements (4) (4.2K) for the multilayer system (Fe/Cr)n . To the far right (>HS , where HS is the saturation field) as well as to the far left (< – HS ) the magnetizations of all iron layers are parallel to the external magnetic field. In the low field region every second iron layer is magnetized antiparallel to the external magnetic field. 10 kG = 1 Tesla.
  6. 6. 5 The effect is much smaller for the system to the left, not only because the system is merely a trilayer but also because the experiments led by Grünberg were made at room temperature, while the experiments reported by Fert and co-workers were per- formed at very low temperature (4.2K). Grünberg (3) also reported low temperature magnetoresistance measurements for a system with three iron layers separated by two chromium layers and found a resistance decrease of 10%. Not only did Fert and Grünberg measure strongly enhanced magnetoresistivities, but they also identified these observations as a new phenomenon, where the origin of the magnetoresistance was of a to-tally new type. The title of the original paper from Fert´s group already referred to the observed effect as “Giant Magnetoresistance”. Grünberg also realized at once the new possibilities for technical applications and patented the discovery. From this very moment the area of thin film magnetism research completely changed direction into magnetoelectronics. The discovery of giant magnetoresistance immediately opened the door to a wealth of new scientific and technological possibilities, including a tremendous influence on the technique of data storage and magnetic sensors. Thousands of scientists all around the world are today working on magnetoelectronic phenomena and their exploration. The story of the GMR effect is a very good demonstration of how a totally unexpected scientific discovery can give rise to completely new technologies and commercial products.
  7. 7. 6 2. Background A. Ferromagnetic metals Among the d transition metals (Sc…Cu, Y…Ag, Lu…Au, i.e. 3d, 4d, and 5d transition elements), the 3d metals iron, cobalt and nickel are well-known to be ferromagnets. Among the lanthanides (the 4f elements, La-Lu) gadolinium is also a ferromagnet. The origin of magnetism in these metals lies in the behaviour of the 3d and 4f electrons, respectively. In the following it is mainly the magnetism in the 3d elements that will be discussed. In the free atoms, the 3d and 4s atomic energy levels of the 3d transition elements are hosts for the valence electrons. In the metallic state these 3d and 4s levels are broadened into energy bands. Since the 4s orbitals are rather extended in space there will be a considerable overlap between 4s orbitals belonging to neighbouring atoms, and therefore the corresponding 4s band is spread out over a wide energy range (15–20 eV). In contrast to this, the 3d orbitals are much less extended in space. Therefore the energy width of the associated 3d energy band is comparatively narrow (4–7 eV). In practice one cannot make a clear distinction between the 3d and 4s orbitals since they will hybridize strongly with each other in the solid. Nevertheless for simplicity this two band picture will be used here and the 3d electrons will be considered as metallic – i.e. they are itinerant electrons and can carry current through the system, although they are still much less mobile than the 4s electrons. A useful concept in the theory of solids is the electron density of states (DOS), n(E), which represents the number of electrons in the system having energy within the interval (E, E+dE). According to the exclusion principle for fermions (in this case electrons), only one electron can occupy a particular state. However each state is degenerate with respect to spin and can therefore host both an electron with spin up and an electron with spin down. In the ground state all the lowest energy levels are filled by electrons and the highest occupied energy level is called the Fermi energy, EF. In figure 3 (left) the density of states is illustrated schematically for a non-magnetic 3d metal, sometimes referred to as a paramagnet, where there are equally many electrons with spin up as with spin down, i.e. there is no net magnetization. The so called spin polarization, P, [ P = (N↑ – N↓)/(N↑ + N↓), where N↑ ( N↓) = number of electrons with spin up (down)], is here equal to zero. For a ferromagnet N↑ is larger than N↓, so that there is a net spin polarization, P > 0. In order to com-pare the energy for the ferromagnetic state with the energy for the paramagnetic state one can start from the paramagnetic state and allow for a small imbalance in the number of spin up and spin down electrons. A transfer of spin down electrons from the spin down band into the spin up band leads to more exchange energy in the system, which means a lowering of the total energy (a gain) On the other hand such a process requires a transfer of electrons from spin down levels below the initial Fermi energy, into spin up levels situated just above the initial Fermi energy.
  8. 8. 7 Figure 3. To the left a schematic plot is shown for the energy band structure of a d transition metal. The density of states N(E) is shown separately for the spin up and down electrons and where a simplified separation has been made between the 4s and 3d band energies. For the non-magnetic state these are identical for the two spins. All energy levels below the Fermi energy are occupied states (orange and blue). The coloured area (orange + blue) corresponds to the total number of valence electrons in the metal. To the right the corresponding picture is illustrated for a ferromagnetic state, with a spin- polarization chosen to be in the up direction (N↑ > N↓; blue area > orange area). This polarization is indicated by the thick blue arrow at the bottom figure to the right. This will necessarily lead to a loss of band energy, “kinetic energy” and thus to an increase of the total energy (a loss). Thus there is a competition between two opposite effects. This can be formulated as the so called Stoner criterion (5) for magnetism, namely that when I N(EF) > 1, the system will be a ferromagnet. Here I is called the Stoner exchange parameter and N(EF) is the density of states at the Fermi energy. The Stoner parameter has a specific value for the individual element, while N(EF) depends much more on the particular spatial arrangements of the atoms relative to each other (like crystal structure). Furthermore, and most important, N(EF) tends to be high for systems with narrow energy bands as is the case for the heavier 3d transition elements (Fe, Co and Ni). This is the explanation for the ferromagnetism among the d transition metals. The situation for a ferromagnetic spin polarization is illustrated to the right in figure 3 (with a direction chosen to be upwards). The vertical displacement between the spin up and spin down densities of states exemplifies the exchange energy splitting between the spin up and spin down energy bands, which is relevant for the metals Fe, Co and Ni. In particular the density of states at the Fermi energy N(EF) can now be very different for the two spin bands. This also means that for a ferromagnet the character of the state at the Fermi energy is quite different for spin up and spin down electrons. This is an important observation in connection with the GMR effect. This picture of 3d energy bands (figure 3 to the right) for the ferromagnetic metals is often referred to as the itinerant model (6), also known as the Stoner-Wohlfarth model (5).
  9. 9. 8 One important property of ferromagnets is that at high temperature their magnetism is lost. This hap - pens at a well-defined temperature, the so called Curie temperature, TC. For the present systems (Fe, Co and Ni) these critical temperatures are far above room temperature and can be neglected. B. Resistance An electrical current of electrons sent through a metallic system will always experience a resistance R. (Exceptions are the so called superconductors where below a certain temperature the current can flow without resistance). There are a number of reasons for this. In a crystal the atoms will always vibrate (phonons) around their equilibrium positions, thereby deviating from the perfect lattice positions, and the conduction electrons may be scattered by these deviations (electron – phonon interaction). Other important contributions to the resistance of a metal are scattering of electrons against impurities and defects. The only electrons that participate in the electrical conduction process are those at (or very close to) the Fermi level. For paramagnetic metals there is no difference between the spin up and spin down electrons, and they contribute equally to the resistance. Figure 4. To the left is the scattering event for spin dependent electrons due to impurities and defects. The spin is shown on the individual electrons by either up or down. The scattering event is shown by a star. To the right is the Mott’s equivalent circuit for the two current model. One part(Green) is due to the spin up electron and the other(Purple) is due to the spin down electron Already in 1936 Sir Nevil Mott (7) considered the electrical conductivity of d transition elements. He suggested that the conductivity was mainly determined by the 4s electrons which are easily mobile due to the wide energy range of the bands derived from the 4s-states. However in a scattering process the s electrons can scatter into the many d states which are available at the Fermi level. Therefore they experience a strong scattering giving rise to a considerable resistance. On the other hand for Cu, the element following Ni in the Periodic Table, all the 3d states are situated below the Fermi level and therefore not available for scattering processes.
  10. 10. 9 This explains the particularly high conductivity of Cu. In the 1960s and 1970s Fert together with Campbell studied in great detail the conductivity of 3d ferromagnetic materials (3, 8). They carried out extensive investigations of resistivity changes which occur when low concentrations of alloying elements, like Cr and other transition metals, are put as scattering centres into for example Fe and Ni. From these studies they could confirm that in a ferromagnet like iron there are two types of carriers, one made up from spin up electrons and one from spin down electrons. Since the density of states at the Fermi surface is quite different for the two spin states it follows that there is a significant difference in resistance for the spin up electrons and the spin down electrons. There could also be contributions to the resistance from scattering processes where the spins are flipped. This could for example be due to scattering against spin waves or from the spin orbit coupling. However these effects are small and will be neglected here. Thus the picture which is emerging is that the electrical current in a ferromagnet like iron, cobalt and nickel consists of spin up and spin down carriers, which experience rather different resistances. C. Growth of superlattices From the beginning of the 1970s the development in physics, chemistry and materials science had led to new experimental techniques allowing scientists to manufacture completely novel materials. Using what was called epitaxial growth one could start to produce artificial materials building one atomic layer after the next. Techniques that were introduced at this time involved for example sputtering, laser ablation, molecular beam epitaxy and chemical vapour deposition. Molecular beam epitaxy was already being used in the late 1960s to make thin semiconducting materials and at the end of the 1970s nanometre thick metallic layers could be produced. This was first applied to non-magnetic metals, but later also to metallic ferromagnets. At the same time, a number of characterization techniques had been largely improved, utilizing for example the magneto-optic Kerr effect (MOKE) and light scattering from spin waves. Using these methods it was possible to grow metallic multilayers involving for example iron and study their magnetic properties. In order to produce well-defined materials the choice of substrate on which to grow the material is of great importance. Commonly used materials are silicon, silicon dioxide, magnesium oxide and aluminium oxide. To obtain well-behaved metallic multilayers it is important that the lattice parameters for the different metallic layers match each other (figure 5) and it is also an advantage if the two metals forming the multilayer have the same crystal structure. This is the case for chromium and iron, where both metals adapt the bcc (body-centred cubic) crystal structure and where in addition they have very similar lattice spacing. This was important for the studies for which this Nobel Prize is awarded under-taken by the groups of Fert and Grünberg. In addition it was also extremely important that it was now possible to grow multilayers where the spatial separation between the magnetic layers is of the order of nanometres. In order to exhibit the GMR effect the mean free path length for the
  11. 11. 10 conduction electrons has to greatly exceed the interlayer separations so that the electrons can travel through magnetic layers and pick up the GMR effect. Without the new experimental growth techniques this requirement could not have been fulfilled and the GMR effect would have remained unknown. In this connection it should be mentioned that, in several publications prior to the work of Fert and Grünberg, there were reports of observations of substantial (of the order of a few per cent) magnetoresistance effects (9, 10, 11, 12). In none of them were the observations recognized as a new effect. Figure 5: Molecular Beam Epitaxy facility at Forschungszentrum Jülich - Peter Grünberg Institute in Julich at the University of Germany.
  12. 12. 11 D. Interlayer coupling It has been known for a long time that disturbances like defects and impurities in metallic systems become screened by the surrounding conduction electrons. The disturbance gives rise to decaying oscillations of the electron density as a function of the distance from the disruption (so called Friedel oscillations). Similarly, a magnetic impurity atom in a metallic surrounding gives rise to an induced spin polarization of the electron density. With increasing dis-tance from the magnetic impurity there will be an oscillation in the sign of the polarization and the disturbance will also decay in magnitude with distance. As a consequence, the magnetic moment of a second impurity placed relatively close to the first one, will become aligned parallel or antiparallel to the magnetic moment of the first moment depending on the sign of the induced polarization at that particular distance. This coupling (exchange coupling) between magnetic moments (schematically shown in figure 6) was well-known for the rare-earth metals where each atom possesses a magnetic moment originating from the very tightly bound (and localized) 4f electronic configuration positioned deep inside the atom. In fact the magnetism of the heavier lanthanide metals originate from this interaction. As already mentioned gadolinium is a ferromag- net where the magnetic moments originate from the localized 4f electrons on each atom having a 4f7 configuration. That is, all the 4f magnetic mom- ents point in the same direction and surrounding these moments there are three conduction elec- trons per atom which mediate the interaction between the 4f magnetic moments. In 1986 Majkrzak et al. (13) published work on a super- lattice of Gd/Y/Gd where they reported an anti- parallel mag-netic moment alignment between the Gd layers for the case of 10 monolayers of Y. This could be understood from the way that a ferromagnetic Gd layer induces an oscillatory spin polarization of the normally non- magnetic Y metal and that the second Gd layer happens to be at a distance where an antiferro-magnetic alignment is preferred. Practically simultaneously Grünberg et al. (14) discovered an antiferromagnetic coupling between the iron layers for the Fe/Cr/Fe tri-layer. This can be explained in a similar way to the Gd/Y/Gd case. It should be remarked, however, that in both cases, due to the geometry, there are important contributions to the interlayer exchange coupling from quantum interference of the Figure 6. Schematic illustration of the behaviour of the exchange coupling as a function of distance.
  13. 13. 12 electron waves reflected at the magnetic layers (15). In the present context it is however sufficient to conclude that the important role of the electrons of the non- magnetic layer(s) is that they provide the coupling mechanism between the magnetic layers. The next step was to investigate the dependence of the coupling on the thickness of the intermediate non-magnetic layers. Several groups identified a change of sign with increasing thickness (13, 15, 16, 17,18). A thorough study of the dependence of the oscillatory behaviour on the thickness of the non-magnetic layer, its dependence on the material of the non-magnetic layer as well as on the dependence of the material of the magnetic layer itself was made by Parkin (1). Here he actually utilized the GMR effect as a tool to study this dependence. In the preparation of the multilayers Parkin used a magnetron sputter deposition technique. With this method it was possible to prepare a large number of samples under comparable conditions. This extensive work was important for the further development of the GMR effect into a working device (20, 21,22).
  14. 14. 13 3. Giant Magnetoresistance A.Introduction In 1988, the Giant Magnetoresistance (GMR) effect was discovered independently by two groups led by Albert Fert and Peter Grünberg. In recognition of their achievement, Albert Fert and Peter Grünberg and were awarded with the Nobel Prize in 2007. Both groups found that the electric conductance of a layer system consisting of ferromag- netic and non-magnetic layers would depend on the relative orientation of the magnetization in neighbouring ferromagnetic layers. The resistance was small when the magnetization was parallel and vice versa. B. Mathematical Model The resistance of a GMR device can be understood from the following somewhat simplified picture. In figure 7 a plot of the magnetic configuration for the FM/NM/FM (ferromagnetic/non-magnetic/ferromagnetic) multilayer is made together with the corresponding electron density of state for the two ferromagnetic sides (FM). In the absence of a magnetic field (at the top) the two FM layers are separated from each other in such a way that they have opposite magnetization directions. In the presence of a magnetic field the magnetizations of the two FM layers will be parallel (at the bottom). An electrical current is now sent through the system for both configurations. As already mentioned above the current through the FM layer is composed of two types – one spin up current and one spin down current – and the resistance for these two currents will differ. When an electron leaves the first iron layer and enters the non-magnetic metal there will be additional scattering processes giving rise to extra resistance. Since the spin up and spin down particles have different density of states at the Fermi level (or rather, they originate from energy levels having different character), the resistance not only within the FM layers, but also that originating from the FM/NM interface will be different for the two spins. Inside the NM layer the up and down spins will experience the same resistance, but generally this is low compared to those in the FM layers and FM/NM interfaces and can here be neglected.
  15. 15. 14 Figure 7. Schematic illustration of the electronic structure of a trilayer system with two ferromagnetic layers (light green) on both sides separated by nonmagnetic material (grey). The top figure is for the case without external magnetic field (H=0), i.e. when the two magnetic layers have opposite magnetizations (indicated by the thick blue and orange arrows at the top of the topmost figure). The bottom figure is for the case when an external magnetic field (H ≠ 0) has forced the two magnetizations to be parallel (two thick blue arrows at the bottom of the lower figure.).The magnitude of the four magnetizations is the same. When the electrons enter the second iron layer they will again experience spin dependent scattering at the NM/FM interface. Finally the spin up and spin down electrons go through the second iron layer with the same resistance as in the first iron layer, which still of course differs for the two spins. For simplicity the resistance for the spin up (down) electrons through the FM layer and the scattering at the interface to the NM layer will be called R↑ (R↓). Thus when the two layers have parallel spin polarizations (magnetizations), i.e. in the presence of an external magnetic field (H), the resistance for the spin up channel is 2 R↑ and for the spin down channel it is 2 R↓. Standard addition of resistances for a parallel current configuration gives the following total resistance, RH, in the presence of an external magnetic field; RH = 2R↑R↓/(R↑ + R↓). In the case of no external magnetic field, (H=0), the configuration between the two magnetic layers is antiparallel (top part of figure 8). In this case the first scatterings in the left part of the multilayer system are exactly the same as before for the lower part of the figure. However, when a spin up electron enters into the second FM layer it finds itself in a totally upside-down situation where the conditions are now exactly the same as they were for the spin down electron in the initial FM layer. Thus the spin up particle will now experience a total resistance of R↑ + R↓. The spin down particle will be affected in the same (but opposite) way and its resistance will be R↓ + R↑. The total resistance will accordingly be 𝑅0 = 𝑅↑+𝑅↓ 2
  16. 16. 15 Figure 8. The same physical system as in figure 6. The magnetic layers are now represented by resistances R↑ and R↓. This shows very clearly that the total resistance for the two cases are different, i.e. there is a magnetoresistance effect. In case R↑ >> R↓ it is practically only the lowest of the four possibilities which will permit a current. In the lower picture with parallel magnetizations the resistance for the spin up (spin down) electrons will be R↑ (R↓) in both magnetic layers. In the upper picture with antiparallel magnetizations the spin up (spin down) electrons will have a resistance R↑ (R↓) in the first magnetic layer to the left. In the second magnetic layer the resistance for the spin up (spin down) electron will be R↓ (R↑), since the magnetization environment has here become totally opposite compared to the first magnetic layer. Thus the difference in resistance between the two cases (magnetic field or not) becomes: R = RH – R0 = - (𝑅↑−𝑅↓)2 2((𝑅↑+𝑅↓) Thus the larger the difference between R↑ and R↓ the larger the negative magnetoresistance. This expression clearly shows that the magnetoresistance effect arises from the difference between the resistance behaviour of the spin up and down electrons. C. Carrier transport through a magnetic superlattice Magnetic ordering differs in superlattices with ferromagnetic and anti- ferromagnetic interaction between the layers. In the former case, the magnetization directions are the same in different ferromagnetic layers in the absence of applied magnetic field, whereas in the latter case, opposite directions alternate in the multilayer. Electrons traveling through the ferromagnetic superlattice interact with it much weaker when their spin directions are opposite to the magnetization of the lattice than when they are parallel to it. Such anisotropy is not observed for the antiferromagnetic superlattice, as a result, it scatters electrons stronger than the ferromagnetic superlattice and exhibits a higher electrical resistance.(31)
  17. 17. 16 Figure 9: Carrier transport through a structured FM/NM/FM layer. To the left we can see that the current consisting of spin up and spin down electron faces low resistivity when the spins are parallel due to external applied magnetic field. To the right the current is facing more resistance due to the anti-parallel alignment of the ferromagnetic layer and gets scattered away. Applications of the GMR effect require dynamic switching between the parallel and antiparallel magnetization of the layers in a superlattice. In first approximation, the energy density of the interaction between two ferromagnetic layers separated by a non-magnetic layer is proportional to the scalar product of their magnetizations: 𝜔 = −𝒥(𝑀1. 𝑀2) The coefficient J is an oscillatory function of the thickness of the non-magnetic layer ds; therefore J can change its magnitude and sign. If the ds value corresponds to the antiparallel state then an external field can switch the superlattice from the antiparallel state (high resistance) to the parallel state (low resistance). D. CIP and CPP geometries Electric current can be passed through magnetic superlattices in two ways. In the current in plane (CIP) geometry, the current flows along the layers, and the electrodes are located on one side of the structure. In the current perpendicular to plane (CPP) configuration, the current is passed perpendicular to the layers, and the electrodes are located on different sides of the superlattice.(31) The CPP geometry results in more than twice higher GMR, but is more difficult to realize in practice than the CIP configuration
  18. 18. 17 Figure 10: To the left is shown the current in plane (CIP) geometry where the contact electrode is connected parallel to Structure To the right is shown the current perpendicular to the plane (CPP) where the contact electrode is connected perpendicular to Structure. CPP results in better performance than CIP Image courtesy Seagate E. Experimental findings The trilayer system of Grünberg et al. consisted of a Fe-Cr-Fe sandwich(3) that was grown epitaxially on a GaAs substrate. The 12 nm thick iron film grow epitaxially. The thickness of the chromium layer amounted to 1nm. With that particular thickness, the magnetization in the two iron layer assume an anti-parallel orientation due to interlayer coupling(52). In a strong magnetic field parallel to the direction in the film plane the magnetizations in the iron film turn parallel. The electrical resistance measured parallel to the layer is 1.5 smaller in that case. The reason for smaller resistance is that conducting electrons traversing the chromium layer from one iron layer into the other carry their spin orientation along. If the magnetization of both iron films point in the same direction, a spin-up electron originating in one iron layer easily finds matching states concerning its k-vector and Figure 11: Magnetoresistance in a three layer sandwich from Grunberg et al. In zero field, the magnetization in the two iron layers are held antiparallel via interlayer coupling through the antiferromagnetic chromium. On application of external magnetic field the magnetization are forced to become parallel hence electrical resistance decreases.
  19. 19. 18 energy in the second to continue its path without being scattered. In case of anti- parallel orientation, the spin-up states in the second layer are shifted by the exchange energy. In general there will be no matching states for electronic the second iron layer. The electron is hence scattered to loose its preferential direction, which increases the electric resistance.
  20. 20. 19 4. Spintronics A. What is Spintronics? To enhance the multifunctionality of devices (for example, carrying out processing and data storage on the same chip), investigators have been eager to exploit another property of the electron—a characteristic known as spin. Spin is a purely quantum phenomenon roughly akin to the spinning of a child’s top or the directional behavior of a compass needle. The top could spin in the clockwise or counterclockwise direction; electrons have spin of a sort in which their compass needles can point either “up” or “down” in relation to a magnetic field. Spin therefore lends itself elegantly to a new kind of binary logic of ones and zeros. The movement of spin, like the flow of charge, can also carry information among devices. One advantage of spin over charge is that spin can be easily manipulated by externally applied magnetic fields, a property already in use in magnetic storage technology. Another more subtle (but potentially significant) property of spin is its long coherence, or relaxation, time—once created it tends to stay that way for a long time, unlike charge states, which are easily destroyed by scattering or collision with defects, impurities or other charges. B. Logic of spin Spin relaxation (how spins are created and disappear) and spin transport (how spins move in metals and semiconductors) are fundamentally important not only as basic physics questions but also because of their demonstrated value as phenomena in electronic technology. Researchers and developers of spintronic devices currently take two different approaches. In the first, they seek to perfect the existing GMR-based technology either by developing new materials with larger populations of oriented spins (called spin polarization) or by making improvements in existing devices to provide better spin filtering. The second effort, which is more radical, focuses on finding novel ways both to generate and to utilize spin-polarized currents—that is, to actively control spin dynamics. The intent is to thoroughly investigate spin transport in semiconductors and search for ways in which semiconductors can function as spin polarizers and spin valves. This is crucial because, unlike semiconductor transistors, existing metal-based devices do not amplify signals (although they are successful switches or valves). If spintronic devices could be made from semiconductors, however, then in principle they would provide amplification and serve, in general, as multi-functional devices. Perhaps even more importantly, semiconductor-based devices could much more easily be integrated with traditional semiconductor technology. Although semiconductors. In addition to the near-term studies of various spin transistors and spin transport properties of semiconductors, a long-term and ambitious subfield of spintronics is the application of electron and nuclear spins to quantum information processing and quantum computation. The late Richard Feynman and others have pointed out that quantum mechanics may provide great advantages over classical physics in computation. However, the real boom started after
  21. 21. 20 Peter Shor of Bell Labs devised a quantum algorithm that would factor very large numbers into primes, an immensely difficult task for conventional computers and the basis for modern encryption. It turns out that spin devices may be well suited to such tasks, since spin is an intrinsically quantum property. Figure 12: Spins can arrange themselves in a variety of ways that are important for spintronic devices. They can be completely random, with their spins pointing in every possible direction and located throughout a material in no particular order (upper left). Or these randomly located spins can all point in the same direction, called spin alignment (upper right). In solid state materials, the spins might be located in an orderly fashion on a crystal lattice (lower left) forming a nonmagnetic material. Or the spins may be on a lattice and be aligned as in a magnetic material (lower right). C. Half-metals Since magnetoresistance deals with electrical conductivity it is obvious that it is the behaviour of the electrons at the Femi surface (defined by the Fermi energy) which is of primary interest. The more spin-polarized the density of states (DOS) at the Fermi energy, i.e., the more N↑ (EF) deviates from N↓ (EF), the more pronounced one expects the efficiency of the magnetoelectronic effects to be. In this re-spect a very interesting class of materials consists of what are called half-metals, a concept introduced by de Groot and co-workers (23). Such a property was then predicted theoretically for CrO2 by Schwarz in 1986 (24). The name half-metal originates from the particular feature that the spin down band is metallic while the spin up band is an insulator. This is shown schematically in figure 13, and it is clear that there is a 100% spin polarization at the Fermi level. The theoretical prediction for CrO2 was later confirmed by experiment (25,26).
  22. 22. 21 Figure 13. Schematic illustration of the density of states for non- magnetic (left) and ferromagnetic CrO2 (right). As can be seen immediately for the ferromagnetic case (right), the spin down electrons (orange) are placed in a metallic band, while the spin up electrons (blue) show an electronic structure that is typical for an insula-tor. The net spin polarization is shown by a thick blue arrow. In figure 14 it is shown the two DOS (spin up and spin down) for the ferromagnetic state. For a trilayer of two ferromagnetic half-metals with one non- magnetic metallic layer between them, it becomes very easy to appreciate the mechanism behind the GMR effect. When the magnetizations of the two half-metals are parallel there will be a current made up exclusively of spin down electrons. However, for an antiparallel magnetization, the spin down channel will be totally blocked for conduction of electricity. Hence a magnetic field which can switch between these two configurations will give rise to a large change in resistance, i.e. will show a strong magnetoresistance behaviour. An enhanced magnetoresistance for the half-metal CrO2 was confirmed experimentally by Hwang and Cheong (27). Figure 14. Illustration of the magnetoresistance effect for a half- metal. Two ferromagnetic half-metallic layers (light green) separated by a non- magnetic metal (grey). In the absence of an external magnetic field (H = 0) the two ferromagnets have antiparallel spin polarizations (blue and orange thick arrows at the top). In the presence of an external field (H ≠ 0) the two magnets have parallel spin polarization (two thick blue arrows at the bottom of the figure). As can be immediately understood, there will be no current for the upper case. For the lower case there will only be a spin down current.
  23. 23. 22 6. Other improvements on GMR A. Tunnelling magnetoresistance Another variation of multilayers in the present context is to grow layered materials with an alternation between metallic and insulating layers. Here the insulating material should be only a few atomic layers thick so that there is a significant probability that electrons can quantum mechanically tunnel through the insulating barrier (figure 15). In this manner a current can be sent through the multilayer. The first publication on such a system was made by Julliere (28). This work was done for a trilayer junction with the following structure Fe/amorphous Ge/Co. The experiments were done at low temperature and an effect of about 14% was reported. Figure 15. Illustration of tunneling magnetoresistance (TMR). Two ferromagnetic layers separated by an insulating layer (i = electron current). The next work of this type was carried out by Maekawa and Gäfvert (29). They investigated junctions of the type Ni/ NiO /FM, where FM stands for Fe, Co or Ni. The magnetoresistance they found was of the order of a few per cent, again at low temperatures. These two reports remained essentially unnoticed for a long time. In fact it was only after the discovery by Fert and Grünberg that attention focussed on these types of systems again. The breakthrough came in 1995 when two groups reported significant progress. Thus Moodera and his group (30,31) measured tunneling layers on CoFe / Al2O3 /Co (or NiFe) and found resistance changes of 24% at 4.2 K and 12% at room temperature. Similarly, Miyazaki and Tesuka (32) used a Fe /Al2O3/ Fe junction and found resistance changes of 30% and 18% at 4.2 K and room temperature, respectively. Today it is rather common to find changes of the order of 50% at room
  24. 24. 23 temperature. This is indeed higher than the resistance changes found in “standard” GMR materials. Recently barriers of Fe/MgO/Fe have been shown to give rise to TMR- values that sometimes exceeded 200% (33,34,35). Due to the better performance of the magnetic tunnel junctions they are expected to become the material of choice when it comes to technical applications. Their use in connection with non-volatile magnetic random access memories (MRAM) is of particular interest and MRAM systems based on TMR are already on the market. One expects that TMR based technologies will become dominant over the GMR sensors. However, the discovery of the GMR effect paved the way for the TMR technology. B. Colossal magnetoresistance The discovery of the GMR effect for magnetic multilayers gave rise to an increased interest in finding related effects among bulk materials (36). Thus von Helmolt and his group (37) found even larger magnetoresistance effects than for GMR in certain manganese perovskites. These materials are some-times referred to as mixed valence systems. Jin and co-workers (38) also found these effects, where the resistance change in an applied magnetic field could be several magnitudes higher than for GMR. Hence the observed effect became known as colossal magnetoresistance (CMR). These extraordinary systems exhibit a very rich variety of exceptional properties, where electron correlations play a very central role. However it is unlikely that they will become of technological interest, mainly because the required magnetic fields are very high.
  25. 25. 24 7. More recent developments & Future work A.Work so Far Here I just mention a few of the vast number of different research areas which represent more recent trends regarding spin materials and their applications. One such area is for example magnetic semiconductors, where the Ohno’s group demonstrated the potential of such materials (39,40) using the semiconductor (Ga, Mn)As. Another area concerns spin injection. Here the early work by Johnson on metallic systems should be mentioned (41,42). Injection of spin from a metallic ferromagnet into a semiconductor was successfully accomplished by Zhu et al. (43) and Hanbicki et al. (44), using Fe and GaAs. Injection of spins from a magnetic semiconductor to a non-magnetic semiconductor was shown by Ohno et al. (45) and by Fiederling et al. (46). The question of how far spin-polarized electrons can travel in a material while maintaining their spin polarization is of great importance and promising work has been reported by Awshalom and his colleagues (47,48,49). Very intense work is now being directed towards magnetic switching induced by spin-currents. This interest started from two theoretical papers where it was shown that a spin-current through a magnetic multilayer can lead to a magnetization reversal (50,51). This prediction was soon verified experimentally (52,53,54). The realization of current induced domain wall motions (55,56,57) forms the basis for the idea of a magnetic “Race-Track Memory” (58). A.1 Magnetic Switching And Microwave Generation By Spin Transfer The study of the spin transfer phenomena is one of the most promising new directions in spintronics today and also an important research topic in our CNRS/Thales laboratory. In spin transfer experiments, one manipulates the magnetic moment of a ferromagnetic body without applying any magnetic field but only by transfer of spin angular momentum from a spin-polarized current. The concept, which has been introduced by John Slonczewski and appears also in papers of Berger, is illustrated in Fig. 16. As described in the caption of the figure, the transfer of a transverse spin current to the “free” magnetic layer F2 can be described by a torque acting on its magnetic moment. This torque can induce an irreversible switching of this magnetic moment or, in a second regime, generally in the presence of an applied field, it generates precessions of the moment in the microwave frequency range. The first evidence that spin transfer can work was indicated by experiments of spin injection through point contacts by Tsoi et al.(23) but a clear understanding came later from measurements performed on pillar-shaped metallic trilayers (Fig. 11a). In Fig.11b-c, I present examples of the experimental results in the low field regime of irreversible switching, for a metallic pillar and for a tunnel junctions with electrodes of the dilute ferromagnetic
  26. 26. 25 semiconductor Ga1-xMnxAs. For metallic pillars or tunnel junctions with electrodes made of a ferromagnetic transition metal like Co or Fe, the current density needed for switching is around 106-107 Amp/cm2, which is still slightly too high for applications, and an important challenge is the reduction of this current density. The switching time has been measured in other groups and can be as short as 100 ps, which is very attractive for the switching of MRAM The spin transfer phenomena will have certainly important applications. Switching by spin transfer will be used in the next generation of MRAM and will bring great advantages in terms of precise addressing and low energy consumption. The generation of oscillations in the microwave frequency range will lead to the design of Spin Transfer Oscillators (STOs). One of the main interests of the STOs is their agility that is the possibility of changing rapidly their frequency by tuning a DC current. They can also have a high quality factor. Figure 16. Illustration of the spin transfer concept introduced by John Slonczewski45 in 1996. A spin- polarized current is prepared by a first magnetic layer F with an obliquely oriented spin-polarization with respect to the magnetization axis of a second layer F2. When this current goes through F2, the exchange interaction aligns its spin-polarization along the magnetization axis. As the exchange interaction is spin conserving, the transverse spin polarization lost by the current has been transferred to the total spin of F2, which can also be described by a spin-transfer torque acting on F2.This can lead to a magnetic switching of the F2 layer or, depending on the experimental conditions, to magnetic oscillations in the microwave frequency range A.2 Field effect Spin Transistor The first scheme for a spintronic device based on the metal-oxide- semiconductor technology familiar to microelectronics designers was the field effect spin transistor proposed in 1989 by Supriyo Datta and Biswajit Das of Purdue University. In a conventional field effect transistor, electric charge is introduced via a source electrode and collected at a drain electrode. A third electrode, the gate, generates an electric field that changes the size of the channel through which the source-drain current can flow, akin to stepping on a garden hose. This results in a very small electric field being able to control large currents. In the Datta-Das device (60), a structure made from indium-aluminum-arsenide and indium-gallium-arsenide provides a
  27. 27. 26 channel for two-dimensional electron transport between two ferromagnetic electrodes. One electrode acts as an emitter, the other a collector (similar, in effect, to the source and drain, respectively, in a field effect transistor). The emitter emits electrons with their spins oriented along the direction of the electrode’s magnetization, while the collector (with the same electrode magnetization) acts as a spin filter and accepts electrons with the same spin only. In the absence of any changes to the spins during transport, every emitted electron enters the collector. In this device, the gate electrode produces a field that forces the electron spins to precess, just like the precession of a spinning top under the force of gravity. The electron current is modulated by the degree of precession in electron spin introduced by the gate field: An electron passes through the collector if its spin is parallel, and does not if it is antiparallel, to the magnetization. The Datta-Das effect should be most visible for narrow band-gap semiconductors such as InGaAs, which have relatively large spin- orbit interactions (that is, a magnetic field introduced by the gate current has a relatively large effect on electron spin). Another interesting concept is the all-metal spin transistor developed by Mark Johnson at the Naval Research Laboratory. Its trilayer structure consists of a nonmagnetic metallic layer sandwiched between two ferromagnets. The all-metal transistor has the same design philosophy as do giant magnetoresistive devices: The current flowing through the structure is modified by the relative orientation of the magnetic layers, which in turn can be controlled by an applied magnetic field. In this scheme, a battery is connected to the control circuit (emitterbase), while the direction of the current in the working circuit (base-collector) is effectively switched by changing the magnetization of the collector. The current is drained from the base in order to allow for the working current to flow under the “reverse” base-collector bias (antiparallel magnetizations). Neither current nor voltage is amplified, but the device acts as a switch or spin valve to sense changes in an external magnetic field. Figure 17: In the spin transistor invented by Mark Johnson, two ferromagnetic electrodes - the emitter and collector—sandwich a nonmagnetic layer—the base. When the ferromagnets are aligned, current flows from emitter to collector (left). But when the ferromagnets have different directions of magnetization, the current flows out of the base to emitter and collector (right). Although it can act as a spin valve, this structure shares the disadvantage of allmetal spintronic devices in that it cannot be an amplifier.
  28. 28. 27 8. Concluding remarks The discovery by Albert Fert and Peter Grünberg of giant magnetoresistance (GMR) was very rapidly recognized by the scientific community. Research in magnetism became fashionable with a rich variety of new scientific and technological possibilities. GMR is a good example of how an unexpected fundamental scientific discovery can quickly give rise to new technologies and commercial products. The discovery of GMR opened the door to a new field of science, magnetoelectronics (or spintronics), where two fundamental properties of the electron, namely its charge and its spin, are manipulated simultaneously. Emerging nanotechnology was an original prerequisite for the discovery of GMR, now magnetoelectronics is in its turn a driving force for new applications of nanotechnology. In this field, demanding and exciting scientific and technological challenges become intertwined, strongly reinforcing progress. The field of spintronics already has promising future for future electronics with the most important being the Quantum computing. Although much remains to be understood about the behaviour of electron spins in materials for technological applications, but much has been accomplished. A number of novel spin-based microelectronic devices have been proposed, and the giant magnetoresistive sandwich structure is a proven commercial success, being a part of every computer coming off the production line. In addition, spintronic-based non-volatile memory elements may very well become available in the near future. But before we can move forward into broad application of spin-based multifunctional and novel technologies, we face the fundamental challenges of creating and measuring spin, understanding better the transport of spin at interfaces, particularly at ferromagnetic/semiconductor interfaces, and clarifying the types of errors in spin-based computational systems. Tackling these will require that we develop new experimental tools and broaden considerably our theoretical understanding of quantum spin, learning in the process how to actively control and manipulate spins in ultra-small structures.
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