Electrical spin injection into semiconductors

Abstract

The basic approach to improve performance in modern semiconductor electronics to date has been to reduce the minimum feature size to increase circuit density. According to Moore's Law, which extrapolates the number of transistors per unit area due to such scaling, this approach is expected to reach practical and fundamental physical limits by the year 2010 [3.1]. Consequently, there is keen interest in developing a new paradigm for future electronic technologies. Spintronics, or the use of carrier spin as a new degree of freedom in an electronic device, represents one of the most promising candidates for this paradigm shift. Commercial success has already been realized in all-metal structures based on giant magnetoresistance (GMR), a new and entirely spin-derived functionality. The GMR effect is due to spin transport between two ferromagnetic metals separated by a non-magnetic spacer metal, and refers to the increase in resistance which occurs when the relative orientation of the magnetic moments of the two magnetic layers is switched from parallel to anti-parallel [3.2, 3]. In a simple model, this change in resistance is ascribed to the availability of states of the correct spin in the collector ferromagnet. A "majority spin" electron from the source ferromagnet, FM1 (i.e. an electron whose moment is parallel to the magnetization of FM1), is easily transmitted through the nonmagnetic spacer metal and into the collector ferromagnet, FM2, if the magnetizations of FM1 and FM2 are parallel. In this case, the electron is also a majority spin carrier in FM2 and the appropriate spin states are available. However, if the magnetization of FM2 is aligned anti-parallel to that of FM1, fewer states of the appropriate spin are available, and it is less likely that the carrier will be transmitted into FM2, resulting in a higher resistance. Applications of this remarkably simple effect include GMR-based sensors, recording heads and nonvolatile memory [3.4]. More recent work has extended this basic effect to metal/insulator/metal tunnel junctions to increase the relative change in resistance and thereby the performance of the end product [3.5-7]. GMR recording heads, first introduced in 1998, now completely dominate the hard disk industry, and are responsible in part for the remarkable performance to cost ratio (∼ 1 GB/$1) enjoyed in the consumer electronics market.