Statistical fluctuations of the magnetization are measured on the nanometer scale. As the experimental monitor we use the characteristic photoluminescence signal of a single electron-hole pair confined in one magnetic semiconductor quantum dot, which sensitively depends on the alignment of the magnetic ion spins. Quantitative access to statistical magnetic fluctuations is obtained by analyzing the linewidth broadening of the single dot emission. Our all-optical technique allows us to address a magnetic moment of only approximately equal 100 micro(B) and to resolve statistical changes on the order of a few micro(B).

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Optical single dot studies in wide-band gap diluted magnetic CdSe/ZnMnSe quantum dots have been performed. Due to the sample design, the photoluminescence energy of the quantum dot signal is energetically below the internal Mn2+ transition, resulting in high quantum efficiencies comparable to nonmagnetic CdSe/ZnSe quantum dots. Magnetic-field- and temperature-dependent measurements on individual dots clearly demonstrate the exchange interaction between single excitons and individual Mn2+ ions, resulting in a giant Zeeman effect and a formation of quasi-zero-dimensional magnetic polarons.

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Single diluted magnetic semiconductor (DMS) quantum dots are studied by means of photoluminescence spectroscopy and magnetoluminescence. The sp-d exchange interaction between a single electron-hole pair and roughly 100 Mn spins within the dot is demonstrated to result in (i) a significant enhancement (more than one order of magnitude) of the emission linewidth and (ii) a strongly modified magnetic field dependence of the polarization degree in a single DMS quantum dot as compared to a non-magnetic reference sample.Single quantum dots (SQDs) can be regarded as 'nanolaboratories': Discrete energy levels can be occupied by a well defined number of electrons and/or holes either by electrical or optical injection [1][2][3][4][5]. It has been shown, that on one hand the interaction between particles in the dot, like Coulomb [1-4] or exchange interaction [6,7], and on the other hand the influence of the local environment, like charge carriers trapped in neighboring defect states [8][9][10][11], has to be taken into account in order to describe optical transitions of such 'artificial atoms'. The unique possibility to include magnetic ions like, e.g., Mn in the crystal lattice of II-VI materials, opens a new path to manipulate the environment of charge carriers in a SQD in a well defined manner: Electron-hole pairs are able to interact with the Mn spins via the sp-d exchange interaction [12,13]. In this contribution, we will present optical studies on diluted magnetic semiconductur (DMS) SQDs. The interaction of one single electron-hole pair with its magnetic environment is investigated directly by comparing a semimagnetic Cd 0.93 Mn 0.07 Te/ Cd 0.60 Mg 0.40 Te SQD with its electronically identical, but non-magnetic counterpart Cd 0.93 Mg 0.07 Te/Cd 0.60 Mg 0.40 Te. We mainly concentrate on the impact of the sp-d exchange interaction on the linewidth and the polarisation degree of the emission of DMS SQDs.The samples have been grown with molecular beam epitaxy on CdZnTe substrates, embedding three monolayers of Cd 0.93 Mn 0.07 Te and Cd 0.93 Mg 0.07 Te, respectively, between Cd 0.60 Mg 0.40 Te barriers. Due to spatial fluctuations in well width and/or composi-1