PNNL

Abstract

We report the synthesis of colloidal Ni2+-doped SnO2 (Ni2+:SnO2) nanocrystals and their characterization by electronic absorption, magnetic circular dichroism, X-ray absorption, magnetic susceptibility, scanning electron microscopy, and X-ray diffraction measurements. The Ni2+ dopants are found to occupy pseudo-octahedral Sn4+ cation sites of rutile SnO2 without local charge compensation. The paramagnetic nanocrystals exhibit robust high-TC ferromagnetism (Ms (300 K) = 0.8 µB/Ni2+) when spin-coated into films, attributed to the formation of interfacial fusion defects. Facile reversibility of the paramagnetic-to-ferromagnetic phase transition is also observed. This magnetic phase transition is studied as a function of temperature, time, and atmospheric composition, from which the barrier to ferromagnetic activation is estimated to be Ea = 1200 cm-1. This energy is associated with ligand mobility on the surfaces of the Ni2+:SnO2 nanocrystals. The phase transition is reversed under air but not under N2, from which the microscopic identity of the activating defect is proposed to be interfacial oxygen vacancies.