ar X iv : c on d - m at / 0 00 63 69 v 1 2 3 Ju n 20 00 Issues , Concepts , and Challenges in Spintronics

We review from a theoretical perspective the emerging field of spintronics where active control of spin transport and dynamics in electronic materials may provide novel device application possibilities. In particular, we discuss the quantum mechanical principles underlying spintronics applications, emphasizing the formidable challenges involving spin decoherence and spin injection facing any eventual device fabrication. We provide a critical assessment of the current status of the field with special attention to possible device applications. INTRODUCTION Spintronics (sometimes also referred to as 'magneto-electronics' although we prefer the 'spintronics' terminology because a magnetic field or the presence of a magnetic material is not necessarily essential for manipulating spins) is the emerging field [1] of active control of carrier spin dynamics and transport in electronic materials (particularly, but not necessarily limited to, semiconductors). In some sense, existing technologies such as GMR-based memory devices and spin valves are elementary spintronic applications where the role of spin, however, is passive in dictating the size of the resistance (or tunneling current) depending on the spin direction controlled by local magnetic fields. Spintronics is projected to go beyond passive spin devices, and introduce applications (and possibly whole new technologies) based on the active control of spin dynamics. Such active control of spin dynamics is envisioned to lead to novel quantum-mechanical enabling technologies such as spin transistors, spin filters and modulators, new memory devices, and perhaps eventually quantum information processing and quantum computation. The possibility of monolithic integration on a single device of magnetic, optical, and electronic applications, where magnetic field and polarized light control spin dynamics, is an exciting new spintronic prospect for creating novel magneto-electro-optical technology. The two important physical principles underlying the current interest in spintronics are the inherent quantum mechanical nature of spin as a dynamical variable (leading to the possibility of novel spintronic quantum devices not feasible within the present-day charge-based electronics) and the inherently long relaxation or coherence time associated with spin states (compared with the ordinary momentum states). The fact that carrier spin in semiconductors can be easily manipulated noninvasively by using local magnetic fields, by applying external electric fields through controlled gates, and even by shining polarized light is an important impetus for developing spintronics applications. In spite of the great current interest in the basic principles and concepts of spintronics a large number of obstacles need to be overcome before one can manufacture spintronics applications. For example, a basic spintronics …