Elucidation of the enhanced ferromagnetic origin in Mn-doped zno nanocrystals embedded into a SiO₂ matrix

The origin of the enhanced room temperature ferromagnetism in Mn-doped ZnO (ZnO:Mn) nanocrystals is investigated. ZnO:Mn nanocrystals, which were fabricated by using a laser irradiation method with a 248-nm KrF excimer laser, exhibited two-times increase in the spontaneous magnetization (~0. 4 emu/cm³ at 300 K) compared to the ZnO:Mn thin film (~0. 2 emu/cm³ at 300 K). The increased exchange integral of J₁/k_B = 51. 6 K in ZnO:Mn nanocrystals, in comparison with the ZnO:Mn thin film (J₁/k_B = 46. 9 K), is indicative of the enhanced ferromagnetic exchange interaction. This is attributed to the large number of acceptor defects in the SiO₂-capped ZnO:Mn nanocrystals. Namely, the holes bound to the acceptor defects form microscopic bound-magneticpolarons with Mn ions; hence, long-range ferromagnetic coupling is enhanced. The results suggest that ferromagnetism in ZnO-based dilute magnetic semiconductors can be controlled by modulating the density of native point defects, which can be chemically and thermodynamically modified during the material synthesis or preparation.