Quantum phase transition from superparamagnetic to quantum superparamagnetic state in ultrasmall Cd(1-x)Cr(II)(x)Se quantum dots?

Weiwei Zheng, Pushpendra Kumar, Aaron Washington, Zhenxing Wang, Naresh S Dalal, Geoffrey F Strouse, Kedar Singh
Journal of the American Chemical Society 2012 February 1, 134 (4): 2172-9
Despite a long history of success in formation of transition-metal-doped quantum dots (QDs), the origin of magnetism in diluted magnetic semiconductors (DMSs) is yet a controversial issue. Cr(II)-doped II-VI DMSs are half-metallic, resulting in high-temperature ferromagnetism. The magnetic properties reflect a strong p-d exchange interaction between the spin-up Cr(II) t(2g) level and the Se 4p. In this study, ultrasmall (~3.1 nm) Cr(II)-doped CdSe DMSQDs are shown to exhibit room-temperature ferromagnetism, as expected from theoretical arguments. Surprisingly, a low-temperature phase transition is observed at 20 K that is believed to reflect the onset of long-range ordering of the single-domain DMSQD.

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