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This book is intended to be served as a textbook for researchers in spintronics, as it contains from the details of epitaxial film growth and nano-scale sample fabrication to the latest device applications and also from basic physics theory behind spin behaviors to ab initio calculations to simulate the phenomena. All the topics are carefully selected to cover the leading edges in spintronics and are written by the active pioneers in relevant subjects. The spintronics has been inspired by the discovery of giant magnetoresistance (GMR) late 1980s, which has been awarded by the Nobel Prize in physics this year. The GMR is based on the difference in the resistance induced by spin scattering at ferromagnet/non-magnet/ferromagnet layer interfaces with respect to parallel and anti-parallel magnetization configurations. This effect has been exploited in a read/write head in a hard disc drive (HDD), the most common data storage these days. The second major progress is the observation of tunneling magnetoresistance (TMR) at room temperature (RT) in 1995, derived from spin-dependent tunneling through an oxide barrier as an exchange of the non-magnetic layer in the GMR systems. The TMR offers a much larger MR ratio, improving a signal to noise ratio for higher areal density in a HDD. In concurrent with these physical discoveries, materials science has also contributed to the rapid development of spintronics. On account of recent advance in vacuum technology, these films, not only ferromagnetic films but also non-magnet including oxides, are able to be grown epitaxially, further improving the magnetic properties, such as the TMR ratios. Nano-fabrication is another key to broaden the spintronic device applications. Reduction in the junction area restricts the spin transport path into 1-dimensional pillar or wire, and also eliminates unfavorable defects at the junction interfaces, such as pinholes. Further reduction in size, even with precise atomic structure control, leads to creation of new spintronic materials, which confers unique magnetic characteristics, 100% spin polarization and large magnetic anisotropy for instance. Combination of the above physics and materials science affords a platform for spintronics, where spins, electrons and photons interact with each other, realizing spin-polarized electron transport, very large spin polarization and photo-induced spin dynamics, etc. The integration of these three fundamental particles will promote future spintronic applications, a polarization-tunable spin source, a spin transistor, spin diode and so on, each of which is anticipated to incorporate many accessible functionalities in conventional electronics. This book is divided into three parts; epitaxial growth and magnetic properties of conventional ferromagnetic films, those of half-metallic ferromagnetic films and their device applications. As an introduction, a general overview on spintronics research is provided from the view points of device applications in particular. Consequently, Part I focuses on both structural and magnetic properties of epitaxial ferromagnetic films. Systematic investigation on the evolution of both structures and magnetism is first presented. Spin dynamics of an epitaxial single film is then discussed with using magneto-optical pump-probe techniques. The epitaxial growth method is further extended to a multilayered spin-valve system, where crystalline anisotropy and interfacial effect are additionally supplied onto a conventional GMR behavior. The beauty of current-perpendicular-to-plane GMR and TMR is also theoretically presented and evaluated by first-principles calculations. Coherent tunneling in an epitaxial magnetic tunnel junction (MTJ) is experimentally observed, proving the theoretical prediction of a giant TMR ratio at RT, which has been applied for next-generation spin memories. Part II delivers a series of investigation on new materials towards the realization of RT half-metallicity as a spin source of 100% spin polarization. The very first spin-filter tunneling is observed in a MTJ with using Europium chalcogenides at low temperature. Oxide materials are also investigated for their half-metallic behaviors. Heusler alloys are the other candidates and are discussed both theoretically and experimentally particularly over the influence of atomic disorder near interfaces and surfaces. Diluted magnetic semiconductors also show half-metallic behavior at low temperature, of which fabrication and spin-polarized carrier properties are then described. In addition, an epitaxial FePt film is studied due to its high spin polarization and large magnetic perpendicular anisotropy, which are favorable to future recording media. Finally, Part III contains major spintronic device applications, most of which are currently based on a polycrystalline conventional ferromagnetic films but can be restored with epitaxial ferromagnets for better operation. A Johnson transistor is the first demonstration of a three-terminal spin-polarized device, consisting of spin injection from a ferromagnet into a non-magnetic metal and spin detection vice versa. Spin-polarized field effect transistor can be realized by substituting a non-magnetic metal with a semiconductor and by utilizing their spin-orbit interaction. Magnetic random access memory has also been developed based on GMR and TMR in order to replace semiconductor memories in the near future. Ferromagnetic nano-rings and nano-wires can also store data bits, which can be manipulated by either an external field or an electrical current. Experimental observation as well as theoretical background of this spin transfer phenomena in a nano-magnet are presented accordingly. By employing such nano-wires to form a lateral spin-valve structure, a spin current can be injected and manipulated in a non-magnetic nano-wire. These spintronic devices can be operated in greater speed, density and efficiency with using an epitaxial (half-metallic) ferromagnet instead of a polycrystalline conventional ferromagnetic layer. We sincerely wish this book offers up-to-date comprehension on spintronics and contributes to further advancement in this exciting research field. We are grateful to all the authors to spend their precious time and knowledge to submit to this spintronic book. We would also like to thank Dr S. G. Pandlai of Research Signpost for his kind offer of the opportunity to make this book accessible to general audience.
ISBN - 9788130803197
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