We study the magnetic and optical properties of diluted magnetic semiconductor CdMnTe quantum dots (QDs) using various photoluminescence (PL) techniques. We use spatially resolved photoluminescence imaging to study single magnetic QDs. A solid immersion lens is combined with the confocal microscopy to achieve a high spatial resolution (~ 200nm) for PL imaging. We observe formation of exciton magnetic polarons (EMPs) in magnetic QDs for non-resonant excitation at B = 0T and T = 7K. However, the spin alignments of individual EMPs are distributed randomly resulting in zero global magnetization. Moreover, the spin alignment in the magnetic QD persists as long as the excitation is continued. This persistent magnetization is because of a long spin relaxation time of Mn ions. We estimate the relaxation time to be of the order of 1ms in CdMnTe QDs.
For resonant excitations, CdMnTe QDs exhibit a strong PL polarization (40%) at B = 0T and T = 7K. The measurements show predominant &sigma +(&sigma -) –polarized PL emission for &sigma +(&sigma -) –polarized excitations that create spin polarized excitons. This suggests that one can control the spin alignment of magnetic impurities in magnetic CdMnTe QDs optically by using selectively polarized excitations. The magnetization created by such spin polarized excitons persists up to 170K. We attribute this robust persistent magnetization to strong three dimensional confinements of excitons in smaller CdMnTe QDs.
In a low Manganese density QD sample, we observe a mixture of PL emission lines of magnetic and non-magnetic QDs. The results demonstrate that the Zeeman splitting of such non-magnetic QDs depends on the polarization of excitation. We believe that this excitation dependent Zeeman splitting is due to coupling between magnetic and non-magnetic QDs through the exchange interaction
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