Quantum Phase Transition from Superparamagnetic to Quantum Superparamagnetic State in Ultrasmall Cd<sub>1–<i>x</i></sub>Cr(II)<sub><i>x</i></sub>Se Quantum Dots?
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Abstract
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<sub>2g</sub> 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
