Structure And Magnetic Properties Of Co,Fe Doped ZnO Diluted Magnetic Semiconductors

Diluted magnetic semiconductor (DMS), in which non-magnetic cations are randomly substituted by transition metal ions, is a novel kind of semiconductor. It have been of much interest and studied actively for the purpose of utilizing both charge and spin of electrons to apply into the advanced electronic devices. In particular, magnetic ions especially transition metal (TM) doped ZnO has attracted considerable interest for its room-temperature ferromagnetism (RTFM), which can be attributed to the exchange interaction between magnetic ions. However, the comparatively low solubility of magnetic ions in ZnO confines the promotion of magnetic properties by increasing the ratio of magnetic ions. Thus several other methods have been attempted to optimize the magnetic characterization of ZnO based DMSs. Works on influence of structural defects such as n-type and p-type dopant have been carried out. Lattice defects such as interfacial and grain boundary defects are also promising candidates to promote the magnetic characteristics.In this work, we have prepared Co,Fe doped ZnO powder material samples by sol-gel method. By exploring a large amount of fabricating process, we made a serious of analysis for the structure and valence characterization. Magnetic properties were also measured and the origin of observed room temperature ferromagnetism was discussed. P-type defects and interfacial defects have been introduced to improve the magnetization of Co doped ZnO DMS samplesFirstly, we report on the high-temperature ferromagnetism in Co,e doped ZnO polycrystalline fabricated by sol-gel method with a ameliorated pH value and low annealed temperature(≤500℃). Structural measurements showed Co ions enter into ZnO lattice and no secondary phase exists. A serials of Zn1-xCoxO samples without any impurities were synthesized and exhibit ferromagnetic behavior with a Curie temperature higher than 300 K. Magnetizations of Zn1-xCoxO samples increase with the increase of Co ions, which is attributed to the Co ions coupled with the electron trapped oxygen vacancies in ZnO. In the case of 25% Co doped ZnO, the magnetization increased sharply for the precipitate of Co clusters.Secondly, the effect of Na concentration on the room temperature ferromagnetism in Na and Co co-doped ZnO DMS (Zn0.95-xCo0.05NaxO) was investigated. The ferromagnetic state was found to be stable below 5% doping of Na due to the exchange interaction via electron trapped oxygen vacancies (F-center) coupled with the magnetic Co ions. With large Na doping of up to 10%, a sharp reduction of the magnetization was observed, showing that the oxygen vacancy mediated antiferromagnetic state becomes predominant.DMS of Fe and Na co-doped ZnO (Zn0.95-xFe0.05NaxO) powder samples were also investigated. Structural characterizations revealed that Fe and Na ions enter into ZnO lattice without any secondary phase. The strong ferromagnetic behaviors at room temperature were found in all samples, which can be attributed to the exchange via electron trapped oxygen vacancies (F-center) coupled with magnetic Fe ions. With the increase of the Na concentration, the oxygen vacancy mediated ferromagnetic state is enhanced. The observed correlation between the Na concentration, the carrier concentration and the magnetization revealed the role of the defect in tuning the ferromagnetism in the ZnO-based DMS system.At last, we explored the correlation between magnetization and oxygen acaneies in Zn0.95Co0.05O nanoparticles. Enhanced magnetizations were found in SiO2 nanopowders and carbon nanotubes (CNTS) treated Zn0.95Co0.05O, which are attributed to minimizing nanoparticle size and increasing oxygen vacancy concentration. After oxygen annealing, the magnetization of both non-treated Zn0.95Co0.05O and CNTS treated Zn0.95Co0.05O decreased sharply with the filling of the oxygen vacancies, while the SiO2 treated Zn0.95Co0.05O was influenced little as the amorphous SiO2 shell prevents the diffusion of oxygen into magnetic particles. It demonstrated that the ferromagnetism comes from the interfacial oxygen deficiency and is tunable by changing the oxygen vacancies.