Quantum Phase Transition from Superparamagnetic to Quantum Superparamagnetic State in Ultrasmall Cd_(1-x)Cr(II)xSe Quantum Dots?

摘要

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. Surpris-ingly, 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.