On exceeding the solubility limit of Cr⁺³ dopants in SnO₂ nanoparticles based dilute magnetic semiconductors

The paper investigates the magnetic behavior of chromium doped SnO₂ Dilute Magnetic Semiconductor (DMS) nanoparticles, through structural, spectroscopic, and magnetic studies. A non-equilibrium solution combustion method is adopted to synthesize 0–5 at. % Cr doped SnO₂ nanoparticles. The detailed spectroscopic studies on the system using micro-Raman spectroscopy, x-ray photoelectron spectroscopy, and electron paramagnetic resonance spectroscopy along with the structural analysis confirm the presence of Cr in 3+ oxidation state, which substitutes at Sn⁴⁺ site in SnO₆ octahedra of the rutile structure. This doping is found to enhance the defects in the system, i.e., oxygen vacancies. All the synthesized SnO₂ nanoparticles (with or without dopants) are found to exhibit Room Temperature Ferromagnetism (RTFM). This occurrence of RTFM is attributed to the magnetic exchange interaction through F-centers of oxygen vacancies as well as dopant magnetic impurities and explained through the Bound Magnetic Polaron (BMP) model of DMS systems. Nonetheless, as the doping of Cr is further increased beyond 2%, the solubility limit is achieved. This antiferromagnetic exchange interaction from interstitial Cr dopants dominates over the BMP mechanism and, hence, leads to the decrease in the net magnetic moment drastically.