Abstract
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 J 1/k B = 51.6 K in ZnO:Mn nanocrystals, in comparison with the ZnO:Mn thin film (J 1/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.
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