Magnetic Ordering in Doped Cd1-xCoxSe Diluted Magnetic Quantum Dots

Abstract

In this study, we report structural, vibrational, and magnetic data providing evidence of random ion displacement in the core of CdSe quantum dots on the Cd²⁺ sites by Co²⁺ ions (between x = 0 and 0.30). Structural evidence for core doping is obtained by analyzing the powder X-ray diffraction (pXRD), data which exhibits a linear lattice compression with increasing Co²⁺ concentration, in accord with Vegard's law. Correlated with the pXRD shift, a hardening of the CdSe longitudinal optical phonon mode and a new local vibrational mode are observed which track Co²⁺ doping concentration. Consistent with the observed core doping, superconducting quantum interference device (SQUID) measurements indicate a surprising increase for the onset of spin glass behavior by an order of magnitude over bulk Co:CdSe. Correlation of SQUID results, pXRD, and Raman measurements suggests that the observed enhancement of magnetic superexchange between Co²⁺ dopant ions in this confined system arises from changes in the nature of coupling in size-restricted materials.