Optically Controlled Nanomagnets
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1208-O01-03
Optically Controlled Nanomagnets Sebastian Mackowski1*, Tak Gurung2, Grzegorz Karczewski3, Howard E. Jackson2, and Leigh M. Smith2 1 Institute of Physics, Nicolaus Copernicus University, Grudziadzka 5/7, 87-100 Torun, Poland. 2 Department of Physics, University of Cincinnati, Cincinnati, OH 45220-0011, USA. 3 Institute of Physics Polish Academy of Sciences, 02-668 Warsaw, Poland. * electronic mail: [email protected] ABSTRACT We report on single dot photoluminescence imaging and spectroscopy at B=0T on magnetically doped CdMnTe self-assembled quantum dots with average Mn concentration of several percent. Quasi-resonant excitation with circularly polarized laser leads to formation of magnetic polarons with magnetization induced by the laser light. In this case the majority of quantum dots are polarized in the same direction. In contrast, when the dots are populated using above the barrier excitation, with randomly polarized excitons, the resultant magnetization is random and varies from dot to dot. These experiments demonstrate a way to control the magnetization of magnetically doped quantum dots by means of light excitation. In addition, they point towards extremely long spin memory times in these structures, reaching hundreds of microseconds, making CdMnTe quantum dots promising candidates for local magnetic field sources on the nanoscale. INTRODUCTION Controlling the magnetic properties of semiconductor quantum dots (QDs) doped with magnetic impurities has in recent years become an important field of research [1-4]. Experiments carried out on QDs doped with only one Mn ion have shown that the emission consists of a series of six narrow lines [2] attributed to doubly degenerated exciton levels coupled to six spin orientations of the Mn ion. Recently, optical control of single Mn ion confined to a QD has been demonstrated. On the other hand, a single CdTe QD, where several percent of Cd cations are replaced with Mn, features a broad luminescence associated with exciton magnetic polarons (EMPs) [5,6]. The formation of EMPs, where magnetic ions are aligned within an exciton Bohr radius, is energetically favorable due to strong exchange interaction of the Mn spins and the electron-hole pair. In the case of CdMnTe quantum wells, the exciton must first be localized to an impurity or potential fluctuation before the EMP is formed. On the other hand, in magnetic QDs excitons are already strongly confined in all three dimensions and, in addition, the EMPs form always within the same dots with their position precisely defined through the spatial dot distribution. The strong and immediate electronic confinement should dramatically influence both the EMP formation and magnetization dynamics in the QDs. Indeed, recent experiments on CdTe QDs doped with magnetic impurities have shown that resonant excitation with circularly polarized laser leads to strong magnetization of EMPs at zero magnetic fields [4].
In this communication we describe experiments focused on single dot photoluminescence imaging and spectro
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