Magnetic And Transport Properties Of Zn-based Diluted Magnetic Semiconductors

Diluted magnetic semiconductors?DMS?have attracted considerable attention due to their potential application in spintronics,which can manipulate charge and spin degrees of freedom in one material.In 2000,Dietl et al.theoretically predicted that the T_C of transition metal?TM?doped ZnO and GaN wide-bandgap semiconductors could be increased to above room temperature.The prediction has triggered the passion for the research of TM-doped ZnO and ZnS DMS.The magnetic properties of DMS are related to the synthesis method and conditions.Some DMS show ferromagnetism,some show antiferromagnetism,and some even show spin-glass state or paramagnetism.A considerable amount of researches have been done to elucidate the ferromagnetic mechanism of DMS.In this paper,the magnetic and other properties of Co-doped ZnO and ZnS nanomaterials are studied.The details are listed as following:?1?Zn_0.97Co_0.03S ultrafine nanoparticles in diameter of⁴ nm with zinc blende structure were synthesized by the chemical coprecipitation method.After annealing in H₂ atmosphere at 500°C for 2 h,Zn_0.97Co_0.03S transformed to wurtzite structure and strongly enhanced room temperature ferromagnetism has been observed.Combining the structural characterizations and density functional theory calculations,we conclude that the interstitial H⁺ions in wurtzite ZnS may provide efficient ferromagnetic mediation between the neighboring Co²⁺ions.?2?In order to clarify the origin of ferromagnetism and obtain ferromagnetic catalysts,the magnetic properties and visible-light photocatalytic properties of Zn_0.98Co_0.02O nanoparticles post-annealed under different ambient conditions?vacuum,hydrogen,vacuum followed by hydrogen?have been studied.The as-prepared and vacuum-annealed Zn_0.98Co_0.02O nanoparticles show dominant paramagnetic properties,while strong room temperature ferromagnetism is observed after the hydrogenation process.This is explained by the ferromagnetic mediation between doped Co ions by the interstitial H⁺ions.It is found that the Zn_0.98Co_0.02O annealed in vacuum exhibits the best photodegradation properties which is mainly attributed to the existence of oxygen vacancies.?3?Zn_0.9Co_0.1O films were prepared by pulsed laser deposition and followed by annealing treatment in hydrogen atmosphere.Both samples show ferromagnetic behavior and saturated ferromagnetic magnetization is significantly increased by five times after the hydrogenation treatment.Co ions in both samples have been confirmed to be bivalent as substituents.And no segregation of Co,Co oxides or any other secondary phases were detected.Furthermore,the Co 2p_3/2 peaks shift to lower energy level after hydrogenation,excluding the formation of Co-H-Co complexes.These results indicate that the oxygen vacancies play a crucial role in mediating the ferromagnetism in Zn_0.9Co_0.1O film.?4?Sulfur is easy to be incorporated into ZnO nanoparticles by the solution-combustion method.Herein,the magnetic and adsorption properties of a series of ZnO_1-xS_x?x=0,0.05,0.1,0.15,and 0.2?nanoparticles were systematically investigated.The introduction of sulfur not only modifies the bandgap of ZnO,but also impacts the concentration of Zn vacancies.The as-prepared ZnO shows weak room-temperature ferromagnetism,and the incorporation of sulfur improves the ferromagnetism owing to the increased concentration of Zn vacancies,which may be stabilized by the doped sulfur ions.The adsorption capability of ZnO_1-xS_x nanoparticles has been significantly improved which due to the active sulfate groups existing in zinc oxysulfide nanoparticles.