SPINTRONICS

Inanc Adagideli, Adam Rycerz, Michael Wimmer,
Matthias Scheid, Jens Siewert, Klaus Richter

Nowadays electronics is based on the principle to generate or control electrical current by exploiting the charge nature of electrons, that is by steering the motion of the charge carriers trough their interaction with external electrical or electromagnetic fields.

Spintronics [1] or spin electronics follows the vision to employ another intrinsic feature of electrons, namely that they carry a spin. Usually electrons are unpolarized: their spins have no preferred direction. To utilize the spin nature of electrons it is thus desirable to prepare electrons in defined spin states, e.g. spin-up or spin-down states with respect to a certain (field) direction. This is achieved in magnetic metals and magnetic semiconductors. Whether this is possible in nonmagnetic semiconductors, for instance by injecting spin-polarized electrons from a metal without loosing too much spin-polarization, is a current issue, both on the experimental and theoretical side.

However, recent experimental progress [2] in creating spin-polarized charge carriers in semiconductors indicates the principle ability to perform spin electronics base on nonmagnetic semiconductors devices. This widens the field of usual magneto-electronics in metals and opens up the intriguing program of combining the rich physics of spin-polarized particles with all the advantages of semiconductor fabrication and technology, e.g. precise design of nanoelectronic devices with controllable charge carrier densities.

Since the spin relaxation times involved can be rather long – coherence of spin-states can be maintained up to scales of more than 100 µm – coherent control and quantum transport of spin states in semiconductor heterojunctions or quantum dots is attracting increasing interst, also in view of proposed future applications including spin transistors [4], filters [5] , and scalable devices for quantum information processing [6] , to name only a few.

Our research follows two main directions: (i) we study how spin states are affected (e.g. spin flips) for electrons passing through interfaces between magnetic and non-magnetic materials; (ii) we study spin effects in quantum transport in low-dimensional nanostructures, assuming spin-polarisation of the charge carriers.

(i) One aim of this research project is to investigate the tunneling magneto resistance and the effect of excitations related to spin ordering on electronic transport in magnetic tunnel junctions. This is motivated by recent experiments on spin-polarized transport through ferromagnet-semiconductor-ferromagnet hybrid systems, performed in the group of Dieter Weiss at Universität Regensburg, which have revealed insteresting anomalies in the current-voltage characteristics.

(ii) We investigate spin-dependent quantum transport in mesoscopic disordered and ballistic conductors in the presence of either Rashba (spin-orbit) interaction or external inhomogeneous magnetic fields. Spin-orbit coupling or the spin coupling to nonuniform fields may give rise to Berry-phase effects on the conductance one focus of our research [7]. In particular we consider whether adiabatic spin transport, required for the observation of geometrical phases, is achieved in ballistic and disordered mesoscopic systems.

Besides, inhomogeneous magnetic fields can be employed to manipulate electrons spins: We have recently shown that the polarization direction of spin-polarized electrons transmitted through an Aharonov-Bohm ring plus an inhomogeneous magnetic field can be coherently controlled via an additional magnetic flux [8].

The methods used are both analytical and numerical quantum approaches based on the Landauer formula as well as semiclassical approaches including the treatment of the spin degrees of freedom by bosonization techniques.

Collaborations with:

Diego Frustaglia, Universität Sevilla

Martina Hentschel, Max-Planck-Institut für Physik komplexer Systeme Dresden

Literature

[1] G. A. Prinz, 282, 1660 (1998)
[2] R. Fiederling et al., Nature 402, 787 (1999); Y. Ohno et al., Nature 402, 790 (1999)
[3] J. M. Kikkawa and D. D. Awschalom, Nature 397, 139 (1999)
[4] S. Datta and B. Das, Appl. Phys. Lett. 56, 665 (1990)
[5] M. J. Gilbert and J. P. Bird, Appl. Phys. Lett 77, 1050 (2000)
[6[ D. Loss and D. P. DiVincenzo, Phys. Rev. A 57, 120 (1998)
[7] D. Frustaglia and K. Richter, Foundations of Physics, 31, 399 (2001), cond-mat/0011161
[8] D. Frustaglia, M. Hentschel and K. Richter, Phys. Rev. Lett. 87 256602 (2001)