Spin Injection and Transport in Micro- and Nanoscale Devices

Abstract

Experiments to explore the transfer of a spin-polarized electric current within small devices have been ongoing for nearly 30 years. But attaining the same level of exquisite control over the transport of spin in micro- or nanoscale devices, as currently exists for the flow of charge in conventional electronic devices, remains elusive. Much has been learned since the time of the first demonstrations of spin polarized tunneling by Tedrow and Meservey. Dur- ing this period we have witnessed the transformation of spin-based electronic devices from laboratory experiments to the realm of commercially available products. This has been driven especially, just in this past decade, by the robust phenomena of giant magnetoresistance (GMR) 1. Even more re- cently, magnetic tunnel junction devices, involving transport of spin polar- ized electrons across interfaces, have proceeded to commercial development 2. Meanwhile, spin injection devices - and by injection we here denote transferal of spin-polarized carriers into an otherwise nonmagnetic conductor (or semiconductor) have not reached a similar, commercially viable, state of maturation. In fact, it is fair to say that, at present, even the fundamental physics and materials science of the spin injection process remains in need of significant elucidation. Yet the problem of spin injection continues to capture the focus of many researchers including ourselves. Ongoing interest in spin electronics espe- cially in semiconductors of late is, in part, motivated by the expectation that, in the near term, this field may lead to the large scale integration of semiconductor micro- and nanodevices capable of performing very high speed logic and memory operations, such as performed via conventional charge-based electronics, but at a fraction of the power. In the long term, surveying the spectrum of other possible solid-state embodiments, many researchers an- ticipate that the spin degree of freedom may provide the most robust founda- tion upon which practical realizations of qubits and quantum computers may ultimately be constructed. All of these prospects clearly require us to under- stand how to transfer electron spins across interfaces, and how to preserve their polarization during this traversal. In this chapter we shall review some of the most important developments in the field of spin injection that have emerged since the earliest experiments. With apologies at the outset to many important contributors to this area whose work may not be included, we strive herein more to develop a coherent overall perspective, rather than for absolute completeness of coverage. Below we shall attempt to summarize the evolution of thinking about spin injection by describing, in part, the succession of research that has ensued.