Defect-induced ferromagnetism in W18O49 nanowires

In this paper, strong room temperature ferromagnetism was observed in non-stoichiometric tungsten oxide nanostructure, sea urchin-like single crystalline W18O49 nanowires prepared by a solvothermal approach. The prepared W18O49 nanowires exhibited a 39-fold enhancement in saturation magnetic moment as compared to the transformed WO3 by annealing in air. By combining x-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), x-ray photoelectron spectroscopy (XPS), and electron-spin resonance (EPR) spectroscopy, a defect complex, i.e., W5+ ions accompanied by oxygen vacancies, has been identified in the prepared W18O49. The observed ferromagnetism in W18O49 is qualitatively explained through a model involving bound magnetic polarons which were induced by the coupling between the electron trapped by oxygen vacancy and the unpaired 5d electron of W5+ ion. The results provide us better insight into the defect engineering of room-temperature ferromagnetism in tungsten oxide. (C) 2019 Elsevier B.V. All rights reserved.