Switching-on Superparamagnetism in diluted magnetic Fe (III) doped CdSe Quantum Dots

Chemically prepared 0.5% iron (Fe) doped CdSe (Fe: CdSe) quantum dots (QDs) are isolated as ~3 nm in the diameter with a wurtzite crystal structure. The oxidation state of Fe was confirmed by electron paramagnetic resonance (EPR) spectroscopy, from which it is evident that iron is in Fe+3 oxidation state. Magnetic properties of Fe: CdSe QDs explored by superconducting quantum interference device (SQUID), reveal multiple magnetic transitions. DC magnetic susceptibility plot shows low-temperature antiferromagnetism (AFM) transition observed below 25 K and followed by another event occurring between 25 and ~80 K. Further, the material shows superparamagnetic behaviour with a weak ferromagnetic exchange interaction as observed by field-dependent magnetization. At lowest dopant concentration, we do not expect dopants to cluster or to form inclusions. Therefore, the observed magnetic behaviour is believed to be arising from the random substitution of the dopant ions and their magnetic exchange interaction with CdSe host lattice.