A study on the origin of room temperature ferromagnetism in Ni_1−xGd_xO nanoparticles

We herein report the existence of room temperature (RT) ferromagnetism (FM) in Gd-doped NiO nanoparticles prepared by hydrothermal process. The Gd-content in Ni_1−xGd_xO was varied from 0.01 to 0.1 and the resultant structural and magnetic properties due to Gd-doping were studied using X-ray diffraction (XRD), transmission electron microscopy (TEM), micro- Raman spectroscopy and vibrating sample magnetometer (VSM). The composition analysis and charge state were obtained from energy dispersive x-ray spectroscopy (EDS) and x-ray photoelectron spectroscopy (XPS), respectively. Both pure and Gd-doped NiO nanoparticles retained face centered cubic crystal structure. Formation of any secondary phases or metallic clusters related to pure Gd or Ni is not evident in the as-synthesized samples. TEM analysis revealed that the particles are spherical with sizes in the range of 8−18 nm. Micro-Raman spectra showed increase in Ni-vacancies with increase of Gd-concentration. XPS spectra of Gd-doped NiO revealed a shift in binding energy compared to the pure NiO, implies the replacement of Gd atoms in Ni-vacancies. Interesting RT-FM behavior is observed for the Gd concentrations of 7 and 10%; while the pure NiO and the other lower concentrations of Gd-doping demonstrated paramagnetic nature. The observed RT-FM can be attributed to the Ni vacancies induced by the doping of Gd³⁺ ions and this fact is explained on the basis of bound magnetic polaron model. - Highlights: • Gd-doped NiO nanoparticles of size 8−18 nm were synthesized by hydrothermal method. • Structural, magnetic and electronic properties were strongly affected by Gd-doping. • RT ferromagnetism was observed for the 7 and 10% Gd-doped NiO nanoparticles. • Maximum coercivity of 203 Oe was obtained for the Ni_0.90Gd_0.10O nanoparticles. • The origin of ferromagnetism in Gd-doped NiO is explained using BMP model