Manipulation Of The Ferromagnetism In N-type Diluted Magnetic Semiconductor Ba（Zn,Co）₂As₂

Diluted magnetic semiconductors(DMSs)that combine the properties of charge and spin become important research area in spintronics.In 1990s,the successful synthesis of(Ga,Mn)As and(In,Mn)As thin films based on MBE method arouse much research interest in DMSs.Among them,the prototypical(Ga,Mn)As has been deeply investigated.In(Ga,Mn)As,the ferromagnetic coupling between localized spins is mediated by carriers,Mn2+substitution for Ga3+introduces both spins and holes,the highest TC reaches～200K when Mn2+ doping level is～12%.In recent years,some new bulk form DMSs that are iso-structure to the iron-based superconductors have been reported.In these new materials,carriers and spins can be introduced through different doping,which makes it possible to manipulate them and investigate their unique contributions to the ferromagnetism separately.Meanwhile,compared with the thin films,bulk samples also offer convenience for some experimental techniques,such as Neutron Scattering、NMR and μSR.But,till now,there is no bulk form DMSs with Curie temperature higher than 300K and almost all of them are P-type materials;only one N-type DMS Ba(Zn,Co)2As2(with TC～45K)has been reported.With the purpose of searching for new DMS materials with higher transition temperature and considering the fact that,in bulk form DMSs,N-type materials are rarely reported and show much lower TC,our researches focuse on the regulation of ferromagnetism in N-type DMS Ba(Zn,Co)2As2 through chemical pressure effect and carriers.Meanwhile,we also keep exploring new DMS materials and find a new "1111" type DMS SrF(Zn,Mn,Cu)Sb.The researches about DMSs in this thesis can be divided into the following four parts:(1)Manipulating the ferromagnetism in N-type DMS Ba(Zn,Co)2As2 through chemical pressure.Using solid state reaction method,we successfully dope Ba into Sr-site and Sb into As-site to introduce positive and negative chemical pressure,respectively.The magnetization measurements show that the ferromagnetic transition temperature decreases with negative chemical pressure while increases with positive chemical pressure.Hall effect measurements reveal that the dominant carriers are still electrons for Sr-doped samples.(2)Manipulating the ferromagnetism in N-type DMS Ba(Zn,Co)2As2 through extra carriers.We successfully synthesized Ba(Zn,Co,In)2As2 and Ba(Zn,Co,Cu)2As2 samples by solid state reaction method.Hall effect measurements confirm that In-doping introduces more ntype carriers while Cu-doping introduces p-type carriers.The magnetization measurements show that the ferromagnetic behavior are suppressed in Cu-doped samples;while with increasing In doping level,the ferromagnetic transition temperature increases first and then slightly decreases.When In doping level is 0.02,the Weiss temperature rises from 56K to 63K.Our work indicates that the ferromagnetism in N-type DMS Ba(Zn,Co)2As2 can be controlled by carriers.(3)The synthesis and properties of a new "1111"-type DMS SrF(Zn,Mn,Cu)Sb.We successfully prepared SrF(Zn,Mn,Cu)Sb through solid state reaction method,which is the first"1111"-type DMS with SrF layers.The co-doped Mn and Cu in Zn-sites introduce spins and carriers,separately.The magnetization measurements show that the ferromagnetic transition temperature can be as high as～40K when doping level is 0.1.(4)Attempt and exploration of new possible DMS materials.Except the materials mentioned above,we have also synthesized and investigated many other potential DMS materials,include some N-type materials,some Mn and Cu co-doped materials and Mg-based materials.Some of them can hope to become new DMS matrials.To conclude,our work contribute to the understanding of ferromagnetism in N-type DMS Ba(Zn,Co)2As2 and provide some effective methods for searching new DMS materials with higher ferromagnetic transition temperature.Our work can also be good examples for theoretic investigations about the mechanism of the ferromagnetic ordering in bulk form DMSs.