Unusual magnetic properties of Mn-doped ThO₂ nanoparticles

We report the synthesis of Th_1–xMn_xO₂ (x = 0, 0.001, 0.002, 0.004, and 0.01 wt%) nanoparticles by the urea combustion method using thorium nitrate gel followed by heat treatment at a higher temperature (T). The obtained Th_1–xMn_xO₂ nanocrystals were characterized by x-ray diffraction (XRD), direct-current magnetization (M) measurements and electron paramagnetic resonance (EPR). XRD analysis revealed that Th_1–xMn_xO₂ crystallizes in the cubic structure (Fm3m). M measurements showed ferromagnetic ordering at room temperature for Th_0.99Mn_0.01O₂ samples annealed at 775 K. An intense and broad ferromagnetic resonance (FMR) having linewidth of ∼1200 G, was observed at relatively lower fields in the EPR spectra of Th_0.99Mn_0.01O₂ samples annealed at 775 K, indicating the presence of a ferromagnetic phase at room temperature. EPR measurements were used to estimate the number of spins involved in the ferromagnetic ordering. Out of the total Mn present in Th_0.99Mn_0.01O₂ samples, about 25% of the Mn²⁺ ions were found to be responsible for the ferromagnetic ordering. In addition to the FMR signal, a weak hyperfine sextet was observed at g = 2.0048 (⁵⁵Mn, I = 5/2), which corresponds to the −1/2 ↔ +1/2 transition of Mn²⁺ ions, suggesting its presence at thorium sites (uncoupled spins). X-ray photoelectron spectra indicated that the manganese ions exist mainly as Mn²⁺, Mn³⁺, and Mn⁴⁺. The room-temperature ferromagnetism may be attributed to the coupling between these Mn²⁺ ions at thorium sites in ThO₂ rather than due to the formation of any metastable secondary phases.