7th Int. Symp. "Nanostructures: Physics and Technology" QW/SL.09p St Petersburg, Russia, June 14-18, 1999 © 1999 loffe Institute Damping of manganese spin precession in the presence of free carriers in CdMnTe quantum wells C. Camillerit, D. Scalbertt, J. Allgret, M. Dyakonovwt, M. Nawrocki§, J. Cibert¶[, A. Arnoult¶[ and S. Tatarenko¶[ T Groupe d'Etude des Semiconducteurs, UMR 5650 CNRS-Universit6 Montpellier 2, Place Eugene Bataillon, 34095 Montpellier Cedex, France -: Laboratoire de Physique Math6matique, UMR 5825 CNRS-Universit6 Montpellier 2, Place Eugene Bataillon, 34095 Montpellier Cedex, France § Institute of Experimental Physics, Warsaw University, Hoza 69, 00-681 Warsaw, Poland 9[ Laboratoire de Spectrom6trie Physique, UMR 5588 CNRS-Universit6 Joseph-Fourier Grenoble, BP87, 38402 St Martin d'Hbres Cedex Abstract. The transverse relaxation time T 2 of the Mn spins has been measured by time-resolved photo-induced Kerr rotation in n-type and p-type CdMnTe quantum wells, with a magnetic field applied parallel to the quantum well plane. The dependencies of T2 on the magnetic field and on the carrier density indicate the influence of carriers on T2. A model, akin to dielectric relaxation, is proposed to explain qualitatively these results. Introduction Spin relaxation processes in quantum confined semiconductor structures are currently a subject of intense work. In particular, in heterostructures containing a dilute magnetic semiconductor the magnetic ions constitute a new channel for electrons and holes spin re- laxation. This possibility has been considered theoretically [ I and explored experimentally through time-resolved luminescence polarization [ I and time-resolved circular dichroism []. Conversely, the spin dynamics of Mn ions in presence of a gas of electrons or holes has received little attention until now. Recent progress in the fabrication of modulation doped magnetic quantum wells of high quality should motivate such studies, where the opportunity to have a high density 2D gas of carriers strongly interacting with Mn spins is of primary importance. As an example K. Kavokin [ ] has predicted profound changes of the Mn spin dynamics in the vicinity of the ferromagnetic transition recently observed in p-doped QWs [ ]. Here we look for the influence of the electron or hole gas on the phase relaxation time of Mn spins T 2 . The T 2 time was measured by time-resolved photo-induced Kerr rotation using a pump-and-probe setup. The gist of the method is to produce a rapidly varying effective magnetic field Beff, acting as a tipping pulse for the Mn magnetization M initially aligned along the external magnetic field B 0 1 to the QW plane. Betf is produced by the spin polarized carriers, mainly the holes, which are photo-created by a circularly polarized pump pulse. Beff (t) will decay with the hole spin polarization on a time scale of a few ps, and M slightly tipped with respect to B 0 will continue to precess freely around Bo for hundreds of ps. The component of M along the growth axis oscillates back and forth and manifests itself through the Kerr rotation of the polarization plane of the probe beam (Fig. 1). 364