A system of two coupled CdTe quantum dots, one of them containing a single Mn ion, was studied in continuous wave and modulated photoluminescence, photoluminescence excitation, and photon correlation experiments. Optical writing of information in the spin state of the Mn ion has been demonstrated, using orientation of the Mn spin by spin-polarized carriers transferred from the neighbor quantum dot. Mn spin orientation time values from 20 ns to 100 ns were measured, depending on the excitation power. Storage time of the information in the Mn spin was found to be enhanced by application of a static magnetic field of 1 T, reaching hundreds of microseconds in the dark. Simple rate equation models were found to describe correctly static and dynamical properties of the system.One of important research directions that may influence the future of information processing, especially of spintronics [1], is focused on physical phenomena occurring in nanoscale-size quantum objects. One of such objects, close to the ultimate limit of information storage miniaturization, is a single Mn atom in a semiconductor quantum dot (QD) [2,3]. After intensive studies of semimagnetic QD containing many magnetic ions [4,5,6,7], single Mn atoms in CdTe [8] and InAs [9] QDs have been observed in photoluminescence (PL) experiments. Many experiments supplied substantial knowledge on physical properties of single Mn atoms, especially in CdTe QDs. In particular, they revealed a strong influence of the position of the Mn atom in the QD, reflecting the symmetry of the system [10]. They demonstrated an efficient optical read-out of the Mn spin state [8]. Furthermore, the dynamics of this state has been studied in photon correlation experiments [11], revealing an important influence of photo-created carriers on Mn spin relaxation. The writing and storing of the information in the Mn spin state has received less attention so far. These issues represent the focus of the present work.In particular, we demonstrate optical writing of information in the spin state of a single Mn ion and we test the stability of this state in the time range up to 0.2 ms.CdTe QDs containing single Mn ions were grown by molecular beam epitaxy. A single layer of self-assembled QDs was deposited in a ZnTe matrix. Manganese was added by briefly opening the Mn effusion cell during the growth of the CdTe layer [12]. The opening time and the Mn flux were adjusted to achieve a large probability of growth of QDs with a single Mn ion in each dot. The selection of single QDs was done by spatial limitation of PL excitation and detection to an area smaller than 0.5 micrometer in diameter, with microscope objective immersed in pumped liquid helium. Continuous wave excitation was used either above the ZnTe barrier gap (at 457 nm) or by a tunable dye laser in the range 570 -600 nm. Well separated photoluminescence lines from individual QDs were observed in the low energy part of the PL spectrum. We were able to select numerous lines showing a PL pattern characteristic for the presence of a...
A strong influence of illumination and electric bias on the Curie temperature and saturation value of the magnetization is demonstrated for semiconductor structures containing a modulationdoped p-type Cd0.96Mn0.04Te quantum well placed in various built-in electric fields. It is shown that both light beam and bias voltage generate an isothermal and reversible cross-over between the paramagnetic and ferromagnetic phases, in the way that is predetermined by the structure design. The observed behavior is in quantitative agreement with the expectations for systems, in which ferromagnetic interactions are mediated by the weakly disordered two-dimensional hole liquid.Soon after the discovery of carrier-controlled ferromagnetism in Mn-doped III-V [1] and II-VI [2] semiconductor compounds, it has become clear that these systems offer unprecedented opportunities to exploit the powerful methods developed for tuning carrier densities in semiconductor quantum structures, in order to control the magnetic characteristics in these systems [2,3,4,5,6]. Such a control opens new prospects for information storage and processing, as well as it makes it possible to examine the behavior of strongly correlated systems as a function of externally controllable parameters. In the case of III-V magnetic semiconductors, Koshihara et al.[3] detected an enhancement of ferromagnetism by illumination of an (In,Mn)As/GaSb heterostructure, an effect assigned to the presence of an interfacial electric field that drives the photo-holes to the magnetically active (In,Mn)As layer. More recently, Ohno et al.[6] demonstrated that a gate voltage of ±125 V changes the Curie temperature T C by about 1 K in a field-effect transistor structure containing an (In,Mn)As quantum well (QW).In the case of II-VI diluted magnetic semiconductors (DMS), Mn does not introduce any carriers. Hence, holemediated ferromagnetic interactions can be induced by modulation-doping of heterostructures [7]. Due to the valence band structure, T C is typically lower in II-VI than in III-V DMS. At the same time, however, it may be expected [2,5] that, owing to the small background hole density, the strength of the carrier mediated ferromagnetic interactions can be tuned over a wider range in II-VI than in III-V DMS.In this paper, we present photoluminescence (PL) studies of modulation-doped p-type (Cd,Mn)Te QW. The (Cd,Zn,Mg)Te barriers are doped either p-or n-type, so that p-i-p or p-i-n structures are formed. The QW in these systems are ferromagnetic below about 3 K. We show that, depending of the sample layout, the ferromagnetism is either destroyed or enhanced during illumination by photons with energy greater than the band gap of the barrier material. In both cases, the switching process is isothermal and reversible. Moreover, we demonstrate that the reverse biasing of the p-i-n diode by a voltage smaller than 1 V turns the ferromagnet into a paramagnetic material. Importantly, this strong effect of light and electric field can be readily explained by considering the distributi...
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