Low-Temperature Activation and Deactivation of High-Curie-Temperature Ferromagnetism in a New Diluted Magnetic Semiconductor: Ni2+-Doped SnO2

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 pseudooctahedral Sn⁴⁺ cation sites of rutile SnO₂ without local charge compensation. The paramagnetic nanocrystals exhibit robust high-Curie-temperature (T_C) ferromagnetism (M_s(300 K) = 0.8 μ_B/Ni²⁺, T_C ≫ 300 K) when spin-coated into films, attributed to the formation of interfacial fusion defects. Facile reversibility of the paramagnetic−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 (E_a) is estimated to be 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.