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Alternatively, the magnetostrictive behavior observed in La0. Pioneering experiments with these materials [] revealed that many parameters needed to be tuned to take advantage of the properties of the individual constituents, just as the interactions between them had to be considered to ensure a maximized ME response. During this process, solid state reactions take place among the constituent oxides, for instance in the case of BaCO3 , Co3 O4 , Fe2 O3 , or TiO2 when they are heated below their melting point, particles adhere to each other.

The molar ratio of phases in sintered composites, the grain size of each phase, and the sintering temperature are easily controllable. Microcracks due to thermal expansion mismatch between constituents, impurities or undesired phases are also noticed to reduce the ME signal []. Certain constituents such as La0. Doping the Ni-ferrite with Co, Cu, and Mn leads to enhanced values for the magnetostriction, magnetomechanical coupling, and electric resistivity.

It should be noted that the dielectric transition temperature in these composites is close to the original dielectric Curie temperature of pure PZT []. Instead of sintering the constituents, researchers use laminated layers, and indeed enhanced ME responses are observed []. In spite of the obstacles encountered in experiments with composite materials, the obtained ME response exceeds the largest values observed in single-phase compounds [].

No obvious interfaces can be observed between these three layers reprinted with permission from [], copyright by the American Physical Society, URL: For the researchers at Philips, an excess of TiO2 1.

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Ultimately, the ME response depends on the relative orientation of the constituents []. Studies were also done for laminate designs with longitudinally magnetized and longitudinally polarized constituents []. At the same time, the ME coupling between the piezoelectric and magnetostrictive phases vanishes when the magnetostriction attains saturation []. The average spacing between the nanopillars was 20—30 nm, and the whole composite, nanopillars and matrix materials, was grown with a high degree of crystallographic orientation. In an alternative experiment, ferromagnetic NiFe2 O4 NFO nanoparticles were dispersed with their crystal orientations aligned in a ferroelectric Pb Zr0.

STO substrates using pulsed laser deposition. The advantage of having nanoparticles rather than nanopillars is that the geometry of the former reduces possible leakage current paths through the ferromagnetic phase. Even fewer theoretical approaches exist for nanostructured ME compounds []. Choosing appropriate ferroelectric and magnetic materials for an epitaxial nanocomposite requires consideration of lattice match and elastic properties, solid solubility, as well as chemical compatibility.

Growing CoFe2 O4 at elevated temperatures surrounded by ferroelectric BaTiO3 leads to self-assembly of the former into columnar nanostructures due to reduced solid solubility of the two phases Fig. These scans were repeated after every electrical poling or magnetizing process. Consequently, it was determined that 3. The bar is 30 nm. The magnetic anisotropy in the columnar CoFe2 O4 is perpendicular to the surface.

Furthermore, if these compounds are fabricated such that ferromagnetic and ferroelectric phases are made to coexist, then manipulating the ferromagnetic phase implies that setting or reading of a magnetic state can be obtained by means of a coexisting ferroelectric state, both within the same material []. Compounds with two or more phases are known as multiferroics [], since they combine two or more primary ferroic properties. Otherwise, the low resistance path created by the Terfenol-D particles will cause the polarization charges to leak via this path.

In such studies, a laminated composite of ring-type geometry containing circumferentially magnetized Terfenol-D and circumferentially polarized PZT was found to display a large ME voltage gain. The design of the transformer is based on the piezoelectric and piezomagnetic equations of state in a radial symmetric-mode vibration, and consists of a four-segment piezoelectric ring layer placed between two 62 3 Combined Phenomena in Novel Materials Fig.

Unwanted eddy-currents are minimized or eliminated when the conductive magnetostrictive layers are made very thin. A sketch of the transformer design is shown in Fig. As a result, the piezoelectric ring strains radially, producing an output voltage from each of the four segments of the ring. In a subgroup of hexagonal manganites e. However, it does not further reduce the symmetry of the crystal [].

On the other hand, a mere rotation of magnetization is not a symmetry operation of the parent phase, and therefore reduces the magnetic energy and increases the free energy of the crystal. Consequently, coupling between polarization and magnetization in a compound conserves the symmetry and makes it possible to obtain a large ME response []. It was also demonstrated that in some antiferromagnetic materials spin-rotation domains are generated that supplement the existing spin-reversal domains.

Attempts to combine magnetic and electric ordering in the same phase have proved unsuccessful so far, owing to the mutually exclusive nature of atomic-level mechanisms responsible for ferromagnetism or ferroelectricity. Yariv, Optical Electronics, 3rd ed. Holt, New York, SPIE , SPIE , 62 IEEE 82 4 , Optical Memory Tsukuba, Japan, , p.

SPIE , 15 Matter 18, Jackson, Classical Electrodynamics, 2nd ed. Wiley, New York, Ascher, in Magnetoelectric Interaction Phenomena in Crystals, ed. Schmid Gordon and Breach, London, B 2, Solids 27, Matter 18, L Selitsky, Ferroelectrics , 65 Rivera, Ferroelectrics , B 29, B 48, Gehring, Ferroelectrics , Lisnevskaya, Ferroelectrics , 63 B 50, B 60, 68 3 Combined Phenomena in Novel Materials B 70, B 63, Bichurin, Solid State Comm.

Hellwege Springer, New York, Matter 9, Petrov, Ferroelectrics , 33 J van Suchtelen, Philips Res. Lisnevskaya, Ferroelectrics , Uchino, H-E Kim, J. Electroceramics 7, 17 B 72, R B 66, B 25, B 67, Fekel, Solid State Commun. Ramesh, Science , A 74, S Soboleva, Ferroelectrics , Schmid, Ferroelectrics , Control 50, 10 B 53, R Shuvalov, in Magnetoelectric Interaction Phenomena in Crystals, ed.

Packaging 6, 68 Lett 90, C 10, L Tokura, Nature , 55 Additionally, the term spin valve has entered mainstream while being applied liberally and interchangeably with GMR, which is strictly speaking not correct.

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Since magnetic sensors enjoy an increasing popularity in ultra-high density recording applications, it is worth looking at what this is all about. What locks spins at an interface, which electrons pass and which do not are just a few questions not easily answered. The magnetoresistance of the system increases or decreases depending on the direction of magnetization in the second ferromagnetic layer.

If the magnetizations in the two ferromagnetic layers are parallel to each other the resistance decreases, while the converse is true for antiparallel layers. Depending on the shape of the magnetoresistance curve, the system is said to have positive magnetoresistance i. In the experiment of Baibich et al. The same type of ferromagnet existed on both sides of the Cr spacer, and yet GMR was observed.

Nevertheless, there is a limit for how thin these spacer layers can be before oscillatory interlayer exchange coupling sets in and reduces the GMR response [4]. Remarkably, in the Baibich et al. Having a paramagnetic layer with the thickness of a few lattice spacings between 4. Magnetic Impurities 73 two ferromagnetic layers can result in antiferromagnetic behavior of the overall system [6].

Because of its thinness, the Ru layer displayed a relatively high interlayer oscillatory exchange energy. By using these read heads, extremely small magnetic bits at an areal density of 2. This would not have been so undesirable if the insulation between the shields would not have occupied space, thereby limiting the recording density of the sensors.

When determining the capacity of these early hard disk drives not only the areal density needs to be taken into account, but also the size and number of disks or platters within the drive. Deskstar 16 GP contained mm-diameter disks, each with a storage capacity of more than 3. In GMR structures built with these materials, the magnetization vectors in the layers have to be ferromagnetically aligned at some point, so that the spin component with the preferred spin polarization can pass through the layers. Furthermore, impurities need to be present at the interfaces, as they are considered responsible for asymmetric spin scattering [11].

Because the Fe layers are considerably thicker than the Cr layers, the Cr atoms in the layer of only three lattice constant thickness can be regarded as Cr impurities in bulk Fe. Similar concepts were introduced also in by Johnson and Silsbee for spin injection from a ferromagnet [12]. An electron beam ionizes the clusters, and a bias voltage is applied between the source and the substrate to propel the clusters onto the substrate while enhancing surface migration of adatoms. To preserve the clusters, an alternative method involves deposition of neutral clusters that are softly landed without ionization or acceleration [17].

This was because GMR is highly sensitive to nanoscale heterogeneity, and the magnetic state at the interface between the clusters and the host matrix. Easier magnetization switching, and hence enhanced GMR values have been reported [19] for magnetic layers that are magnetically textured displaying a preferred direction of magnetization. This is the case for layers containing nanowire arrays where the strong shape anisotropy of the latter has proved to be advantageous.

It was observed [19] that multilayered 4. A spin valve structure can encompass many layers, however the latter can be divided into three possible categories: Exchange coupling implies that the exchange interaction between magnetic moments is such that the magnetization within the pinned layer cannot change directions easily [23]. Thus, the hysteresis loop of the antiferromagnetic layer is asymmetrical, also said to be shifted horizontally [28]. With the invention of electroplating baths and improved fabrication processes for the heads at IBM, a plated inductive permalloy head [33] was employed in the early days of magnetic recording.

This electroplated permalloy was the only selected material for this purpose, as it could meet the strict requirements for both reading and writing processes. Its soft magnetic properties made it employable as both read and write element [35]. As the areal density of drives increased, Ni45 Fe55 started to gain ground. Although its soft magnetic properties were not as good as in permalloy, the increased magnetic moment due to the higher Fe content allowed writing in higher coercivity media.

However, trying to optimize the material to improve the writeability process was deteriorating readability, and vice versa. This is because inductive writing usually requires a better magnetic anisotropy, a smaller coercivity and a very low magnetostriction. The problem was solved with the introduction of the magnetoresistive read sensor [36] that was used in conjunction with the inductive write head [37], but with a physical separation between the writer and reader. Once the physical separation between the processes became possible permitting individual optimization of writing and reading, the soft magnetic property requirements of the inductive writer material became less stringent.

The head design was improved to handle the positive magnetostriction. Adding to the problematic fabrication conditions, the weak corrosion resistance made CoFeCu ternary alloys impracticable in magnetic recording. Furthermore, they did not display a visible advantage over Ni45 Fe55 in terms of head performance, therefore they were not implemented in magnetic recording. Preferably, a magnetic recording head should have a high moment, low coercivity, no magnetostriction, large electrical resistance, no internal stress and high corrosion resistance.

None of this is achievable in one single material, and compromises need to be made. However, this CoNiFe plating system was not consistently reproducible in the high moment region of the ternary diagram. Most samples underwent thermal annealing treatments without damage to their structure [40]. Hardening can be achieved by coupling that ferromagnetic layer to a thin antiferromagnetic layer via exchange biasing, as mentioned earlier.

If the layer is pinned, it has a shifted hysteresis loop, due to the exchange bias of the adjacent antiferromagnetic layer. In contrast, the hysteresis loop of the free layer is centered, displaying reduced coercivity. When the magnetizations of the pinned and free layer change direction from parallel to antiparallel, the magnetoresistance increases due to increased spindependent scattering. These changes in magnetoresistance are used to detect transitions between magnetic bits, allowing recovery of stored magnetic data. Basic principles of magnetic recording.

The read element scales down in size with the area of the magnetized regions. Their magnetic properties are highly dependent on their microstructure which is determined by the fabrication process, in this case, reactive sputtering from a CoFe2 target. Oxide pinning layers are preferred due to their increased corrosion resistance.

For the spin valve with the CoFe2 O4 pinning layer [54], changes in magnetoresistance of Additionally, the interactions between the free and pinned layer can render a too high free layer coercivity. Further improvements can be made by adding an insulating cobalt oxide CoO or Co3 O4 underlayer that pins without shunting current.

Aside from its electric insulating properties, this underlayer provides a template for better crystalline growth, maintaining a higher coercivity and thermal stability. Also, free layer properties are preserved, comparable to PtMn based sensors 4. That, and the reduced critical thickness of the CoPt pinning layer renders the latter a viable alternative to IrMn or PtMn antiferromagnets for small gap sensors in high density magnetic recording. The CoPt layers are deposited at room temperature by dc magnetron sputtering in an argon pressure of 2 mTorr using two separate Co and Pt targets.

Composition changes are obtained by varying the sputtering power of the Pt source. For pinning applications in spin valves it is required for the magnetic moment of the CoPt pinning layer to balance the magnetic moment of the reference layer, CoFe. Its resistance can be similar or greater than the total resistance of the actual spin valve structure. In related investigations by Maat et al. High coupling constants had been previously measured in Ir-coupled layers [58].

The strong coupling was retained even when patterned into nanopillars of 50— nm. While the sense current is passed not in plane, but perpendicularly through the device, the spacer layer is replaced by a very thin insulating tunnel barrier. The magnetic moments of the two contacts rotate together, while remaining parallel to each other [60].

Prior to GMR read heads, the magnetic recording industry employed anisotropic magnetoresistive sensors made of two thick, soft magnetic layers shielding a sensing layer in the middle [63]. The anisotropic magnetoresistive heads achieved a low performance of merely a few percent variation in magnetoresistance at room temperature. This was due to the fact that anisotropic magnetoresistance is attributed 4. In particular, ferromagnetic alloys containing permalloy display a random distribution of magnetic atoms causing spin-dependent bulk scattering [66, 67].

However, the latter has a relatively small impact [68] on GMR, as compared to spin-dependent interface scattering.

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The EMR is modulated by the Lorentz force that acts on the electrons that form a current between two adjacent conduction channels formed by a high mobility semiconductor bar and a low-resistance metallic shunt, both nonmagnetic. In these, the current travels between the two magnetic shields Fig. The spacing between the two shields determines the downtrack resolution of the recorded bits. The overall width of the sensor is responsible for the crosstrack resolution, while the sensor height needs to scale with the track width for good magnetic stability [71].

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Increases in GMR can be achieved by partially oxidizing layers within the structure [71,72]. The CPP sensor resistance is characterized by the product: This low product makes CPP GMR sensors more compatible with high density recording heads than MTJ devices that display a too high resistance to be employed in high data rate disk drives [73]. However, parasitic resistance from underlayers, antiferromagnets, cap layers, and other inactive layers reduce the measured GMR. The Ru layer facilitated the antiferromagnetic coupling between the pinned layer and the reference layer, creating the actual spin valve, and minimizing the magnetostatic coupling to the free layer.

The total stack height of the sensor was kept just 4. Multilayer stack for single and dual spin valves. The single spin valves are obtained by terminating the structure with a cap after the free layer FL reprinted from [74] copyright with permission from the American Institute of Physics under 50 nm [70]. Additionally, the shape of the magnetoresistance curve changes with spacer layer thickness [76], as well as due to the fact that the resistance of these materials is strongly temperature dependent. Only antiferromagnetic coupling leads to noteworthy GMR [77].

It was observed that in layered structures of permalloy and Cu, the intermixing of the two at the interface reduces GMR [81]. Less scattering and therefore less dissipation of energy occurs when the magnetization vectors of the layers are aligned, allowing the favored spin component to carry electric current with lower resistivity, shunting out the more strongly scattered component. The high density of states and low d electron velocity make it easier for the highly mobile valence sp electrons to make the transition. In contrast to ferromagnetic Fe, in nonmagnetic Cu the d bands are fully occupied and not exchange split.

The valence sp bands have low density of states and high velocity electrons, 4. When the conduction electrons leave the nonmagnetic spacer layer and enter the ferromagnetic layer, they are scattered dependent on spin. Clearly, the higher MR is obtained for larger spin polarizations. It should be noted in passing that, in GMR spin valves and in MTJ electron transport occurs near the Fermi level, however it is possible for electron transport to occur at much higher energies. These are the so-called hot electrons, and they are utilized in spin valve transistors made of a spin valve base with two semiconductor substrates constituting the emitter and collector.

Nevertheless, the energy of the hot electrons is limited to about 0. On the other hand, in the magnetic tunnel transistor the energy of the hot electrons can be adjusted by varying the bias voltage between the emitter and base across the tunnel barrier. This allows the magnetic tunnel transistor to operate over a wide energy range while obtaining high collector currents through large voltage bias [88]. The transmission of hot electrons injected into the empty states above the Fermi level is possible because of the longer attenuation lengths of majority spin hot electrons, as there is a spin dependence of the number of states available for inelastic scattering by electron—hole pair excitation [89].

Due to the nonequilibrium nature of tunneling magnetic phenomena, when the spin-polarized hot electrons are injected into the states above the Fermi level of one ferromagnetic electrode, holes are simultaneously injected into the states below the Fermi level of the counterelectrode, also by a tunneling process. In a p-type magnetic tunnel transistor, after spin-dependent transmission through the ferromagnetic base, holes are collected in the valence band of the p-type semiconductor, provided their energy and momentum allow them to overcome the Schottky barrier [90].

The question arises whether charge and spin of electrons can both be utilized at the same time to broaden device applications. Nonmagnetic materials, such as GaAs or similar III—V semiconductors, are currently employed in devices that use charge. Apparently, they can be made magnetic if high concentrations of magnetic elements are introduced [92], although this is sometimes being disputed at magnetic semiconductor conferences.

Semiconductors are particularly preferred for these types of experiments, as they have the ability to be doped with impurities and thereby change their properties [93]. If these impurities are magnetic, molecular beam epitaxial growth is employed under low temperature equilibrium conditions to enhance their solubility [94]. The result is a diluted magnetic semiconductor, for instance Ga0.

The advantage of diluted magnetic semiconductor materials is that ferromagnetism and semiconducting characteristics coexist [96]. In Ga,Mn As, the ferromagnetic double exchange between nearby pairs of Mn ions is stronger than the antiferromagnetic superexchange. On the other hand, p-type Ga,Mn N is supposed to have a Curie temperature above room temperature [], which would add more capabilities to GaN-based structures, already in use in photonics and high power electronics. According to theoretical [] and experimental [, ] investigations, the n-type Ga,Mn N does not display ferromagnetism above 1 K.

For example, Fe3 O4 with its high Curie temperature [] K , [] CrO2 , Heusler alloys, and manganites are considered to be half-metallic ferromagnets. A variety of mechanisms have been proposed as an explanation, such as electron—phonon coupling [], double exchange [], electron—magnon interactions [], or phase- and charge-segregation []. Introducing Sr at Ca-sites sharpens the transition from metal to insulator [], whereas hole doping with [] Pb leads to a completely stable ferromagnetic character.

If additionally, a compound such as La0. Adding metal ions improves physical properties, especially around room temperature []. Sintering and partial melting seem to open up new percolating conduction channels between grains, helping the ordering of Mn spins and restoring the ferromagnetic state at elevated temperatures []. The large aspect ratios of nanowire length vs. Therefore, the CPP geometry is still preferred where improved designs can lead to low resistance-area products for the spin valves, combined with high GMR values [, ], making it feasible to fabricate low-resistance, nanometer-sized devices for high data rate magnetic recording applications.

Not only can the density of bits of information on the disk be increased, but also reading the information is now faster. Other interesting applications of spin valves, such as the bipolar spin switch [], the micromagnetometer [], or the micro-SQUID [], have also emerged in the previous decade, and still more are yet to come. Prepatterning is done by photolithography and ion-beam milling. The movement of the bridge is controlled by an applied voltage on the gate. References 93 References 1. Nguyen van Dau, F. B 39, 3. Parkin, in Ultrathin Magnetic Structures, ed.

II Springer, Berlin, p. F 6, 6. B 44, 8. Tsang, US Patent 5,,, B 16 19 , B 37, B 44 18 , A, B 58 18 , B , Romankiw, Advances in Electrochemical Science and Engineering, ed. Romankiw, US Patent 3,,, 23 Sept. PV, 39 J-W. PV, E. PV, C. PV, P. PV, H. PV, S. IEEE 91 5 , S.

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B 70, K. Padmore, Science , M. B 47, A , Lodder, Science , Ohno, Science , 96 4 Magnetoresistance and Spin Valves B 59, B 57 4 , R B 55, R Yoshino, Physica E 10, Dagotto, Science , B 61, Dou, Solid State Commun. Cheong, Nature , B 53, B 43 1 , B 51, B 31, Mailly, in Superconducting Devices and Their Applications, ed. Lubbig Springer, Berlin, , p. An elusive property of electrons known as spin may just be the next entity for data encoding.

This connection between electron transport and spin plays a major role in information transport through the device. Since spin is usually associated with quantum mechanics, any devices relying on it are termed quantum devices. In the present chapter, a new area of electronics is discussed, an area that has seen various degrees of experimental success in proving that information can indeed be encoded, transported and stored using both electron charge and spin. When dealing with spin it is expected that tiny length scales within the device come into play, and that phenomena not generally encountered in bulk materials come to light.

Furthermore, for information to be reliably encoded and transported from one part of the device to another using the spin associated with electrons, certain conditions need to be met. Aside from examining some of these key conditions, this chapter also highlights what has been done to eliminate challenges encountered in spin injection, transport and detection, as well as a few developments where the majority of research seems to be concentrated nowadays.

Thus, information can be encoded via electron spin, and transported from one part of the device to another using electron current [5], if certain conditions are 5 Some Basic Spintronics Concepts met for the phase coherence to be maintained. The coding can be changed by remagnetizing the metal ferromagnet containing the information. Also, the transport of information only takes place provided the device is built on a short length scale for the quantum mechanical phase coherence of the electronic wave function to be preserved [6].

The quantum mechanical phase coherence is measured by the transverse relaxation time T2 [7]. Control of spin-polarized electrical conduction while maintaining phase coherence is likely to have a great impact on quantum information technology [9]. Spintronics is based on three factors [11]: Nanotechnology can nowadays construct devices on these scales [13]. Majority Spin Carriers The injection of spin from a ferromagnet into an otherwise nonmagnetic material is one of the prerequisites for any spintronic device [14].

But how do we get to these spins? Minority spin carriers constitute a good supply of spins when the current passes from a ferromagnet to a nonmagnetic material.

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Nevertheless for low electron densities, electron—electron interactions can cause a 2D homogeneous system to become unstable to spontaneous spin polarization as a result of exchange [20]. Reports of an enhanced g-factor and anomalous spin susceptibility have indicated that spontaneous spin polarization does exist when the disorder is low [21]. Consequently, this perpendicular spin is transferred as a torque exerted on the moment due to conservation laws 5 Some Basic Spintronics Concepts that have to take place.

Furthermore, when a phenomenon termed spin transmission resonance [26] takes place, the magnetic moment in the ferromagnet remains unaltered, and no spin transfer occurs. Thus, there is more than one way to control the injection of spin. The currentperpendicular-to-plane CPP geometry Fig. On the other hand, the current-in-plane CIP geometry Fig. Current-in-plane CIP ; bottom: For actual spintronics devices, electrical spin injection is preferred [34], where a magnetic electrode is connected to a specimen.

A current from the electrode drives spin-polarized electrons to the sample, creating a nonequilibrium spin accumulation in the latter [35]. To restore equilibrium, a process of spin relaxation occurs, giving rise to an exponential spin accumulation decay from the interface. If any spin accumulation is to be achieved, the spin relaxation needs to be counterbalanced by a certain spin accumulation rate [39].

Nevertheless, a drop in resistance is observed even in a CIP sample when the magnetizations are aligned in the two ferromagnetic layers. Therefore, while the magnetizations in the ferromagnets are aligned one spin type is heavily scattered 5 Some Basic Spintronics Concepts in both layers, leaving the other spin type relatively unscattered [29]. On the other hand, if the layers have antiparallel magnetization, neither spin type has high mobility due to heavy scattering in one ferromagnet or the other.

This group of ferromagnets contains Heusler alloys [45], ferromagnetic oxides [46, 47], and some diluted magnetic semiconductors DMS [14]. Heusler alloys are cubic materials with four interpenetrating fcc sublattices. Group IV magnetic semiconductors are of particular interest due to their potential compatibility with Si-based processing technologies.

When unwanted phase separations occur, the materials obtained are disordered and inhomogeneous. Usually, doping with several elements is preferred over single ones, as the dopants alter the local kinetic and strain conditions [52]. While Co in Ge contracts the lattice, doping with Mn expands it [53]. At the same time, strain stabilizes growth in Co-rich samples due to the small lattice mismatch. There may exist an optimum doping concentration for which the lattices may no longer be mismatched. There is also a possibility that both electrons and holes are spin polarized, thereby opening new avenues for heterojunction applications [53].

Half-metallic ferromagnetic oxides consist of compounds that have itinerant electrons such as CrO2 and Sr2 FeMoO6 , or compounds with localized electrons as is the case for Fe3 O4. The discovery of unexpected properties in ferromagnetic oxides has sparked a general interest in these materials all over the world [53]. This in turn leads to Ti being fully 5. Nevertheless, this material has great thermal stability as it does not change its magnetic properties when annealed in vacuum at K, as opposed to pulsed-laserdeposition-grown Co: Compositional variations are easily implemented into high-throughput synthesis methods followed by a rapid characterization of structural, electrical, and magnetic properties.

For instance, it was applied to the investigation of new magnetic oxide semiconductors such as ZnO and TiO2 doped with 3d transition metals. Generally, anatase TiO2 is unstable and very sensitive to the choice of substrate relative to the rutile mineral phase that is easier to grow. In this case, oxygen vacancies were noted to lead to n-type conductivity, while Co mole fractions of up to 0. These materials display a coexistence of spontaneous ferromagnetic or antiferromagnetic and ferroelectric order, where the coupling between magnetization and polarization gives rise to a ME response [58].

The DMS are nonmagnetic semiconductors doped with a few percent of magnetic impurities. Depending on the percentage of the dopant and the complex chemistry involved, these materials can exhibit ferromagnetism at and above room temperature. Furthermore, it is assumed that the metals are similar, in the sense that the spin polarizations are of the same sign.

This is because the second ferromagnet, also a half-metal has density of states for only one spin type. In case the magnetizations are antiparallel, the second ferromagnet will not accept the other type of spins. If the two ferromagnets in the above example are of opposite polarization, the majority spins in one ferromagnet are not a majority in the other ferromagnet. Therefore, a parallel alignment of magnetizations will lead to an increase in resistance, while the converse is true for antiparallel orientation. The overall GMR response of the system is in this case negative.

Several design and fabrication techniques [63] have been employed [64] to obtain dissimilar switching of the magnetizations in the two ferromagnetic layers [65,66], a practice common to spin valve [14] heads in magnetic recording. Spin valves are essentially based on how the spins scatter depending on the alignment of magnetizations, and the layer coercivities. Some aspects are also discussed in Chaps. Even if the two materials are closely lattice matched, the interface is still nonideal [67]. A tunnel contact between a ferromagnet and a semiconductor is spin selective due to the spin polarization of the density of states.

This may be achievable if another layer of some material is introduced between the ferromagnet and the spacer. The role of this intermediate layer would be to take over the spin polarization and deliver it mostly unchanged to the spacer layer. The intermediate layer may be a magnetic or nonmagnetic tunneling barrier, or a Schottky barrier whose depletion zone is the actual intermediate layer. In case of a nonmagnetic tunneling barrier, the voltage drop across the barrier would be very large and totally control the injected current and its polarization. Mn ions are electrically neutral in II—VI compounds, but act as an acceptor in III—V semiconductors due to the large density of states in the valence band [76].

Ferromagnetic 5 Some Basic Spintronics Concepts interactions between the localized spins in Ga,Mn As are believed to be due to holes originating from these Mn acceptors [77, 78]. In spite of the lower mobilities and shorter lifetimes as compared to electrons, holes can still provide a successful spin injection [79]. Usually, III—V semiconductors are easier doped with electrically active impurities, in contrast to the II—VI compounds that can sustain greater concentrations of transition metals [81]. Techniques such as molecular beam epitaxy or laser ablation have been used below room temperature to introduce transition metals into a semiconductor matrix without the formation of undesired phases, as for instance MnAs [85].

Some researchers found high-temperature ferromagnetism in Ga,Mn N [86, 87] with conclusive proof that the ferromagnetic properties are not a result of secondary magnetic phases. This material became of interest in photonics and high power electronics, as well as integrated systems that contain both processing of information and data storage in one unit. Spin injection has also been observed in spin-LEDs containing Ga,Mn As as an emitter [91], while a Ga,Mn As emitter has been employed in resonant tunneling structures [92].

Light generates excess holes in the In,Mn As layer 5.

From Bulk to Nano : The Many Sides of Magnetism -

Their n-type counterparts can be paramagnetic [95]. These results may be applicable to the integration of ferromagnetic semiconductors with nonmagnetic III—V semiconductor devices such as lasers. These jumps appear to be independent of sample size or geometry. Nevertheless, the smaller specimens display Barkhausen jumps Fig. Employing semiconductors is expected given that they can be processed to high purity, the equilibrium carrier densities can be varied through doping, and electronic properties are tunable by gate potentials.

The latter is severe for ohmic contacts between ferromagnetic metals and nonmagnetic semiconductors []. As already mentioned, one alternative for bypassing the problem was suggested by Rashba and consists of employing relatively high impedance magnetic tunnel junctions [72]. Due to recent advances in high-quality magnetic semiconductors [], and in the optimization of interfaces [], semiconductor materials are increasingly preferred over metals.

Higher percentages in injection [] may be achieved by injection from a semiconductor, than by direct injection from metals. Nevertheless, if interface scattering is absent, the spin accumulation density may be too small in the semiconductor [], and hence the spin-polarized current through the interface may also be too small. Novel physical phenomena have been discovered [], launching the development of previously unknown spin injection devices [].

Spin-polarized electrons or holes maintain their polarization as long as they do not come across a magnetic impurity [] or interact with the lattice via spin—orbit coupling. Collisions with magnons cause momentum transfer between spin channels, a process often referred to as spin mixing [29]. To maintain spin polarization, the interaction between spin and the transport environment needs to be controlled [, ].

Metals and inorganic semiconductors have momentum scattering lengths shorter than spin scattering lengths, leading to unwanted diffusive electron transport []. For this reason, organic semiconductors have recently been under increased scrutiny [, ] as potential replacements for metal and inorganic semiconductor spacers. It was observed that organic semiconductors display unique optical as well as spintronic properties, showing promise for a whole range of signal processing and communication applications [].

Particularly appealing to spintronics is the weak spin—orbit interaction in organics, leading to long spin relaxation times []. Additionally, the reduced size, which can be obtained for devices incorporating organic semiconductors, allows integration into opto-spintronic chips [].

This is due to the fact that conjugated polymers are quasi-one-dimensional systems resulting in weakly screened electron—electron interactions. On the other hand, electrons and the lattice are strongly coupled; therefore, charge carriers in these materials are positive and negative polarons, rather than holes and electrons [].

The trilayer nanowires were fabricated in porous alumina obtained by anodization 5. These encouraging results may be due in part to the La0. A recent experiment [] with spin valves composed of nanowires with organic semiconductor spacers suggested that the primary spin relaxation mechanism in the organic material is the Elliott—Yafet mode [] for which carrier scattering and velocity changes result in spin relaxation.

These results were observed in trilayer nanowire spin valves consisting of cobalt, Alq3 tris 8-hydroxy-quinolinolato aluminum , and nickel, fabricated in 50 nm alumina pores. The fabrication process of the trilayers involves alumina pores obtained by anodizing an electropolished 0. Also, the height of the pores is adjusted by varying the anodization time, or by subsequent etching using dilute phosphoric acid []. The spin polarization extends over a few lattice constants.

This implies that the spin relaxation occurs at a higher rate in nanowires. Furthermore, carrier mobility is reduced in the Alq3 layer because of the additional Coulomb scattering caused by the charged surface states []. All things considered, the Elliott—Yafet [] spin relaxation mechanism is suggested as being dominant in the Alq3 nanowires, giving rise to long [] spin relaxation times that range from a few milliseconds to over 1 s at 1.

These unique properties are partly due to the construction of carbon nanotubes. A single walled nanotube SWNT consists of only one graphene sheet wrapped around to form a 1—3 nm diameter cylinder. On the other hand, multiwalled nanotubes contain several concentric cylinders reaching diameters of up to 80 nm.

The chirality of the wrapping in a SWNT determines whether it behaves as a semiconductor or a metal. Carbon nanotubes have been incorporated in spintronic devices with the intent of studying their spin transport properties. An example of a GMR response is shown in Fig. In TMR, an electrical current passes by quantum mechanical tunneling through an insulating tunnel barrier between two ferromagnetic layers []. Two-terminal spin tunneling junctions have been built exhibiting large changes in tunnel resistance, depending on the relative orientations of the magnetizations in the ferromagnetic layers [27].

The diameter of the nanotube is 30 nm and the conducting channel length is nm reprinted from [] copyright with permission from the American Institute of Physics Fig.

Paramagnetism and Diamagnetism

Electrons with majority spins tunnel if the two half-metallic ferromagnets have aligned magnetizations, rendering a low resistance for the device. The resistors in each path represent the resistances the spins experience in each layer reprinted from [38] copyright with permission from the Institute of Physics 5. As shown in Fig. The high and low resistances correspond to the two spin channels.

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To gain some insight into this issue, a very simplistic approach can be taken for GMR where spin transport is considered in a ferromagnetic multilayer stack. A spin splitting of 5. Conversely, spins experience maximum scattering for the antiparallel alignment of magnetizations of neighboring magnetic layers, because spins in both channels are scattered while traversing the layers [27].

Generally, a current sourced through a ferromagnet acquires a spin polarization due to the remnant magnetization of that ferromagnet. When the current is injected into the spacer layer it induces a net polarization. Spin-polarized holes relax quickly, whereas spin-polarized electrons persist for long times [38]. Scattering at Bloch walls was suspected of being responsible for colossal magnetoresistance CMR in manganate perovskites []. It is not just up to the ferromagnet to control the spin polarization, magnetoresistance or tunneling process.

Therefore, proper understanding of spintronics is only possible by taking into account the electronic structure of both ferromagnetic and nonmagnetic spacer layers, and how they interface. For instance, the Fermi surfaces of Co and Cu match well for the majority spin electrons in Co, while the match is not good for the minority spins []. Although large TMR can be achieved, this system is very sensitive to the interface conditions between Co and Cu requiring smooth and sharp interfaces between the magnetic and nonmagnetic layers [].

TMR depends on the type of insulator material, as well as on the barrier height and width []. Additionally, TMR varies with barrier impurities, temperature, and bias voltage []. On the other hand, some opinions [, ] suggest that increases in temperature bring about a reduction in the overall magnetization in the ferromagnet because of excitations of magnons. Future applications will put these ideas to the test. This does not sound encouraging for single-spin detection.

Fortunately, there is another technique with much improved sensitivity. Magnetic resonance force microscopy MRFM has proved to have the ability to detect an individual electron spin [], because MRFM is based on detecting a magnetic force between the spins in the sample and a ferromagnetic tip. However, the force from an electron spin is only a few attonewtons, roughly times smaller than the forces detected by atomic force microscopy AFM [].

These ultrasensitive cantilever-based force sensors are the newest development in MRFM, and are made for the purpose of facilitating singlespin detection []. The custom fabricated mass-loaded silicon cantilever employed in the single-spin experiment [] had an attached nm-wide SmCo magnetic tip used to sense the force from the electron spin. For the spin measurement to occur, the vitreous silica specimens had to be irradiated with Co gamma rays.

This procedure resulted in silicon dangling bonds containing unpaired electron spins. The shift is due to the magnetic force exerted by the spin on the tip. Unfortunately, low temperatures 1. Nevertheless, the MRFM technique allows imaging of spins as deep as nm below the surface. The latter makes use of a superconductor connected to a spin asymmetric material. A small applied bias voltage leads to electron tunneling from the normal metal to the superconductor, and the formation of Cooper pairs which have zero spin. This is because electrons with given momentum and spin couple with electrons of opposite momentum and spin, so that when the latter 5 Some Basic Spintronics Concepts Cantilever Microwave coil Interferometer Magnetic tip Resonant slice Spin z y x Fig.

The spin polarization in the normal metal is related to the conductivity of the contact, allowing determination of the spin polarization []. In practical devices, not the spins themselves need to be detected, but rather changes in the spin states that lead to changes in the measured signals [, ]. The device was not realized experimentally, but is still useful as it illustrates some of the principles on which spintronic devices are supposed to work. A large current is obtained when the spins are parallel to the magnetizations in the source and drain.

Nevertheless, the prerequisites for a successful experimental implementation of spin transistors remain: There still remain many challenges to be overcome before successfully fabricating commercial devices. Future developments in spintronic devices will be largely driven by advances in spintronic materials. In the end, all-semiconductor structures are expected to take over hybrid designs with metallic ferromagnets.

But this will depend on the ability of producing high-quality interfaces with nonmagnetic materials. A , 5. B 61, 6. A 68, 7. Kane, Nature , 8. D 32, R B 43, B 58, B 62, B 54, B 69, Matter 10, Today 52 6 , 33 Awschalom, Nature , Ohno, Science , B 68, References Nguyen Van Dau, F. Today 54 5 , 39 Xiang, Nature , Ferrand, Science , Matter 2, Matter 17, L39 Ohno, Physica E 7, IEEE 91, Allen, Solid State Commun. B 62, R Dietl, Physica E 10, B 65, R Ohno, Physica E 9, Kindo, Physica B, Condens. Matter , Ohno, Physica E 16, Ohtani, Nature , Shinjo, Science , References B 57, R Awschalom, Science , B 62 8 , R B 62 12 , Molenkamp, Nature , D 35, Treger, Science , Awschalom, Physica E 10, 1 Shi, Nature , Barbanera, Solid State Commun.

Matter 62, R B 35, R. Crystal Growth 3—4 , E. Matter 15, R V. B 63, B. Ago, Physica E 6, P. B 39, G. B 84, 44 N. Seneor, Science , J. Popularity Popularity Featured Price: Low to High Price: High to Low Avg. Basics and Applications Jan 13, Available for download now. Only 1 left in stock more on the way. From Bulk to Nano: The Many Sides of Magnetism: Only 1 left in stock - order soon. Provide feedback about this page. There's a problem loading this menu right now. Get fast, free shipping with Amazon Prime. Get to Know Us. English Choose a language for shopping.

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