Switching-on superparamagnetism in diluted magnetic Fe(iii) doped CdSe quantum dots

Abstract

Chemically prepared 0.5% iron (Fe) doped CdSe (Fe:CdSe) quantum dots (QDs) with a diameter of ∼3 nm and a wurtzite crystal structure are isolated. The oxidation state of Fe was confirmed by electron paramagnetic resonance (EPR) spectroscopy, from which it is evident that iron is in the Fe³⁺ oxidation state. The magnetic properties of Fe:CdSe QDs explored using a superconducting quantum interference device (SQUID) reveal multiple magnetic transitions. The DC magnetic susceptibility plot shows a low-temperature antiferromagnetism (AFM) transition observed below 25 K 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 the 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 the CdSe host lattice.