The spin degree of freedom of charge carriers has recently found increasing attention in the perspective of spin or magneto electronics [1]. The operation of a spin transistor or of a spin-valve device depends essentially on the lifetime of a spin-polarized electron (or hole). Taking the spin transistor [2] as paradigm spin relaxation is related to the faith of carriers travelling along the conducting channel at a semiconductor heterointerface. As known from extensive studies for bulk semiconductors [3] all mechanisms of spin relaxation can be traced back to spin-orbit interaction, which comes into play in different ways:
 

1. For electrons, occupying states with predominantly s character, the spin-relaxation rates are much smaller than for holes in p states.

 
2. The momentum change connected with a scattering process (with phonons, impurities, ...) results in a change also of the spin orientation of the carrier, which is related to its momentum.

 
3. Spin precession in an effective magnetic field, caused by band structure effects or by the confinement potential at the heterointerface (Rashba effect), can be hindered by momentum scattering due to random changes of this effective magnetic field.

In semiconductor systems with reduced dimension spin relaxation is modified due to confinement [4] and interface effects [5] and depends on the material parameters of the system with the tendency, that the relaxation rates increase with a decrease of the gap energy.

The dependence of the spin precession (Dyakonov-Perel mechanism) on the crystallographic orientation of the heterointerface (or quantum well) allows to discriminate between different spin relaxation mechanisms [6].

The talk gives a survey on the mechanisms of spin relaxation in bulk material and on their modification due to reduced dimensionality in semiconductor heterostructures.
 
 

1. G.A. Prinz, Physics Today, April 1985 p. 58.
2. S. Datta, B. Das, Appl. Phys. Lett. 56, 665 (1990).
3. Optical Orientation, edited by F. Maier and B.P. Zakharchenya, (Elsevier Science Publ., Amsterdam 1984).
4. M.I. Dyakonov, V.Yu. Kachorovskii, Sov. Phys. Semicond. 20, 110 (1986).
5. T. Guettler et al., Phys. Rev. 58, R10179 (1998).
6. Y. Ohno et al., Proceedings EP2DS-13, Ottawa 1999; Physica E in print.