Electron carrier concentration dependent magnetization in ZnO:Co and ZnO:Mn thin films
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1035-L06-06
Electron carrier concentration dependent magnetization in ZnO:Co and ZnO:Mn thin films Zheng Yang1, Maurizio Biasini2, Leelaprasanna J Mandalapu1, Zheng Zuo1, Ward P Beyermann2, and Jianlin Liu1 1 Quantum Structures Laboratory, Department of Electrical Egineering, University of California at Riverside, Riverside, CA, 92521 2 Department of Physics and Astronomy, University of California at Riverside, Riverside, CA, 92521 ABSTRACT Co and Mn ions were implanted into n-type ZnO thin films with different electron carrier concentrations. X-ray diffraction measurements show that the ZnO:Co and ZnO:Mn thin films are of high crystallinity. From magnetization measurements, ferromagnetism was observed in both n-type ZnO:Co and n-type ZnO:Mn thin films with Curie temperatures well-above room temperature. Furthermore, the electron carrier concentration dependence of the saturated magnetization was measured in both types of thin films, and our results support an electronmediated mechanism for ferromagnetism in ZnO:Co, as predicted by theory. However, our measurements seem to contradict theory for ZnO:Mn, which only predicts long-range ferromagnetism for p-type mediated material. INTRODUCTION In recent years, ZnO-based diluted magnetic semiconductor (DMS) materials have attracted great interest [1, 2] due to theoretical predictions of above room-temperature Curie temperatures [3, 4]. The free carriers play an important role in the DMS materials [3-5], and based on first principles calculations, the Co-doped ZnO will be ferromagnetic in an electronrich condition while the Mn-doped ZnO will be ferromagnetic in a hole-rich condition [4]. These theoretical efforts led to extensive experimental studies on transition-metal doped ZnO materials, such as Co [6-13] and Mn [14-21] doped ZnO. However, the experimental results are controversial, because both ferromagnetic and non-ferromagnetic phases were observed in ZnO:Co and ZnO:Mn materials. Most recently, ferromagnetism was observed in ZnO DMS codoped with either Co or Mn together with some donor or acceptor impurities. Some examples include Al [22-25] and Zn interstitials [26-28] for n-type ZnO and N [29-32], P [33, 34], and As [35] for p-type ZnO. This seems to be consistent with the theoretical predictions. However, ferromagnetism has also been observed in n-type ZnO:Mn [16-18]. So whether ferromagnetism occurs only with ZnO:Co in an n-type environment and with ZnO:Mn in a p-type environment is still debatable. So far, there is no systematic study on the magnetic properties in the ZnO with different free carrier concentrations. In this paper, the ferromagnetism of ZnO:Co and ZnO:Mn thin films with different electron carrier concentrations is reported. Above room-temperature ferromagnetism and the carrier concentration dependence of the saturated magnetization were observed in both ZnO:Co and ZnO:Mn n-type thin films. EXPERIMENT ZnO thin films were grown on r-plane sapphire substrates using plasma-assisted molecular-beam epitaxy (MBE). The electron carrier concentrations of
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