New Materials for Spintronics
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New Materials for Spintronics
Scott A. Chambers and Young K.Yoo, Guest Editors Abstract This article introduces the October 2003 issue of MRS Bulletin on “New Materials for Spintronics.” As a result of quantum mechanics, the carriers in ferromagnetic metals such as Fe, Co, and Ni are spin-polarized due to an imbalance at the Fermi level in the number of spin-up and spin-down electrons. A carrier maintains its spin polarization as long as it does not encounter a magnetic impurity or interact with the host lattice by means of spin-orbit coupling. The discovery of optically induced, long-lived quantum coherent spin states in semiconductors has created a range of possibilities for a new class of devices that utilize spin. This discovery also points to the need for a wider range of spinpolarized materials that will be required for different device configurations. In this issue of MRS Bulletin, we focus on three classes of candidate spintronic materials and review the current state of our understanding of them: III–V and II–VI semiconductors, oxides, and Heusler alloys. The field of spin-polarized materials is growing very rapidly, and the search for new magnetic semiconductors and other suitable spin-injection materials with higher Curie temperatures is bringing spintronics closer to the realm of being practical. Keywords: ferromagnetic semiconductors, spin-polarized transport, spintronics.
As a result of quantum mechanics, the carriers in ferromagnetic metals such as Fe, Co, and Ni are spin-polarized due to an imbalance at the Fermi level in the number of spin-up and spin-down electrons. An extreme limit of this spin imbalance is half-metallicity, which occurs when one of the spin bands is partially occupied at the Fermi level and the other spin band is either completely full or completely empty, and at an energy different from that of the Fermi level. In this case, conduction comes from carriers of one spin only. A carrier maintains its spin polarization as long as it does not encounter a magnetic impurity or interact with the host lattice by means of spin-orbit coupling. It became clear to researchers early on that spin-polarized currents can be preserved and utilized for various device applications. Giant magnetoresistance (GMR) was discovered in thin-film structures consisting of alternating ferromagnetic and nonmagnetic metals; alignment of the ferromagnetic layers governs the scattering of spins and, consequently, the resistance of the layered structure. GMR devices quickly found large-scale commercial application as magnetic-field sensors in the
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read heads of magnetic recording disks. Spin-dependent tunneling, in which an insulating layer is sandwiched between two thin ferromagnetic metal films, is slowly moving toward industrial application in magnetic random-access memory (MRAM). So far, the application of spinpolarized materials has been limited primarily to ferromagnetic metals as contact electrodes in switching devices. The discovery of optically induced, long-lived quantum coherent spin states in s
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