Résumé:

Spin noise spectroscopy (SNS) is an attractive optical technique, which in recent years has been widely used to probe the carrier spin relaxation in various semiconductors and heterostructures [1]. Typically SNS probes spontaneous spin fluctuations by off-resonant Faraday rotation. Spin noise spectra have to be equivalent to electron spin resonance spectra, by virtue of the fluctuation-dissipation theorem, and are quite interesting for quantum non-demolition measurements of either atomic or electronic spins [2]. Here we show that spin fluctuations of magnetic impurities in semiconductors can also be efficiently detected by SNS. This is demonstrated for manganese spins diluted in CdTe. Mn ions possess an electronic spin S=5/2, and a nuclear spin I=5/2. Thus one expects that the spin noise spectra will exhibit spectral signatures associated with the hyperfine interaction, and a dependence on the magnetic field direction dictated by the point-symmetry of the Mn ion in the lattice. This is illustrated in Figure 1, where contour plots of the spin noise spectra are shown for different magnetic field orientations in the (1-10) plane of the sample, q being the angle between the applied field and the [001] crystal axis. The spectra measured on bulk Cd0:999Mn0:001Te (Fig. 1b) exhibit many of the spectral features predicted within the spin hamiltonian approximation (Fig. 1a). Fig. 1c shows that quite good agreement between theory and experiment is obtained for a Mn spin coherence time T2 = 20 ns, which is the only fitting parameter. We will present our results on two CdMnTe quantum wells grown in coherent epitaxy on either CdTe or Cd0:96Zn0:04Te substrate. The first QW is almost lattice matched to the substrate and nearly unstrained, while the other is unmatched and strained. About 4106 Mn spins are detected in the spot size of 5 μm. We will show that, because of the hyperfine coupling between an electronic spin and a nuclear spin with the same value (5/2), below 15 mT Mn ions exhibit a Zeeman effect with an effective g-factor equal 1, rather than 2. The Mn spin noise spectra appear to be quite sensitive to the presence of local strains, and gives insights into the strain distribution in the QWs. Finally, we will discuss the possible spin relaxation mechanisms, which ultimately limit the Mn spin relaxation time. This work is supported by the French ANR research project SNS (Grant N° 2011-BS04-018 01).