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Lecture Dimitrie Culcer


16 May 2008 FWN-Building 5114.0001, Nijenborgh 4, 9747 AG, Groningen
Speaker: Dr. Dimitrie Culcer
Affiliation: Advanced Photon Source, Argonne National Laboratory, USA
Title: Current-induced spin torques in ferromagnetic semiconductors
Date: Fri May 16, 2008
Start: 11.00
Location: FWN-Building 5114.0001
Host: Bart van Wees
Telephone: +31 50 363 4933


Over a decade ago, Slonczewski [1] and Berger [2] predicted that an electrical current induces a torque on the magnetization of a ferromagnetic metal. This torque is a result of the fact that a nonequilibrium spin density is established which is not collinear with the magnetization. Because they represent the effect of conduction electrons on the magnetization in the steady state, spin torques are the converse of processes such as the spin-valve effect and giant magnetoresistance (GMR). I will present a general theory of current-induced spin torques in zincblende III-V ferromagnetic semiconductors [3], demonstrating that spin torques in these materials are qualitatively different from their analog in metals, since ferromagnetic semiconductors differ in many profound aspects. Magnetism is due to localized Mn impurities, with the exchange interaction between them mediated by itinerant carriers. The carriers are holes, which are described by an effective spin-3/2 and are subject to a strong spin-orbit interaction so that the hole spin is not conserved. The combined effect of the exchange and spin-orbit interactions is not a simple additive problem. We have determined the spin density in an electric field and found that the torque on the magnetization is intimately related to spin precession under the action of both the spin-orbit interaction and the exchange field. Ferromagnetic semiconductors are often in the weak momentum scattering limit [5], in which EF t /ћ >>1, where t is a characteristic scattering time. Due to this fact, and to the fast carrier spin precession as a result of the spin-orbit interaction, the relaxation time approximation is inappropriate for describing carrier spin dynamics. Our work shows that neither scalar nor spin-dependent scattering affect the final result. We find the spin polarization excited by the electric field to be rather small, which is related to the fact that electrically-induced spin densities in bulk nonmagnetic zincblende semiconductors are forbidden by symmetry. Thus, even when the symmetry is lowered by the presence of a magnetization the effect is small.


[1] J. Slonczewski, J. Mag. Mag. Mat. 159, L1 (1996).

[2] L. Berger, Phys. Rev. B 54, 9353 (1996).

[3] Dimitrie Culcer and R. Winkler, arXiv:0802.3717.


Last modified:22 October 2012 2.30 p.m.