Crystal Synthesis & Physical Properties Research In "BZA" Based Diluted Magnetic Semiconductors

Diluted magnetic semiconductors?DMSs?,one branch of semiconductor spintronics,is full of investigation and debate due to its basic research and application value in our modern information society.It is a long require creating magnetic semiconductors that work at room temperature since the concept put forward in 1960s.The past decades saw so many achievements in this area,especially to?Ga,Mn?As in III-V group DMSs for the high compatibility of the traditional semiconductor GaAs with high mobility.Unfortunately,the highest Curie temperature?T_C?only reaches 200K recently,which restrict its application though a lot of prototype device have been fabricated,such as spin-LED,spin-FET,etc.Since 2011,Prof.Changqing Jin in Institute of Physics,Chinese Academy of Sciences firstly fabricated and named two new material systems,the“111”type Li?Zn,Mn?As and the“122”type?Ba,K??Zn,Mn?₂As₂ DMSs.?Ba,K??Zn,Mn?₂As₂ DMSs is usually called“BZA”based DMS due to the parent compound is BaZn₂As₂.The highest T_C could reach 230K which was selected“EurekAlert!Science News”by the American Association for the Advancement of Science.Compared with?Ga,Mn?As,another big advantage in?Ba,K??Zn,Mn?₂As₂ is the independent charge and spin doping mechanism via Ba²⁺/K¹⁺&Zn²⁺/Mn²⁺replacement.In principle,the bulk“BZA”based DMS could achieve p type and n type doping which is quite important for the p-n junction.As far as we know,?Ba,K??Zn,Mn?₂As₂ still keeps the highest T_C record.Therefore,in this thesis,we focus on this“BZA”materials in two ways,one is to explore new“BZA”based DMSs,the other is to achieve“the whole chain”research from material to prototype devices.These four works could be illustrated in three aspects.Firstly,we report the synthesis and characterization of a new bulk polycrystalline“BZA”?BaZn₂As₂?based Diluted Magnetic Semiconductor?Ba,Rb??Zn,Mn?₂As₂,in which charge and spin are doping independently via Ba/Rb and Zn/Mn substitutions,respectively.It crystalline in tetragonal ThCr₂Si₂ structure at room temperature,which shares the same“122”structure with the iron superconductor?Ba,K?Fe₂As₂,antiferromagnet BaMn₂As₂,the previous DMS?Ba,K??Zn,Mn?₂As₂&the parent compound BaZn₂As₂.Moreover,the lattice constants in the ab-plane match within about 5%.These features could provide distinct advantage to generate functional devices based on junctions of various combinations of these states.We observed a T_C up to 143 K which is comparable to the initial T_C of 180K in(Ba_0.70K_0.30)(Zn_0.90Mn_0.10)₂As₂.34%negative magnetoresistance was shown at 2 K with the external field H up to 8T,which hits the record of all of this new kind of DMSs with the T_C beyond the liquid nitrogen of 77K.More importantly,the muon spin relaxation measurements suggest the exchange interaction between Mn moments in?BaRb??ZnMn?₂As₂ may be different compared to most of the earlier DMS systems,which offers a new platform for ferromagnetism mechanism research.Secondly,we successfully grow?Ba,K??Zn,Mn?₂As₂ single crystal for the first time.The crystal shows big magnetic anisotropy.The carrier density is determined from the anomalous Hall Effect from 2.82×10²⁰ to 4.80×10²⁰ cm^-3 as the temperature increases from 2 to 130 K.More significantly,the Andreev reflection junction from the selected large single crystal fabricating to testify spin polarization degree of BZA,and 66%spin polarization reaches.The success on Andreev reflection junction paves a solid route for fabricating multilayer junctions based on this DMS.Then we carry on high-pressure electrical transport measurements.We find that upon compression,the resistivity initially decreases in the entire temperature range,but it turns to increase when pressure?P?>1.2 GPa.Combining the synchrotron XRD and first principle calculations,we know that this phenomenon is due to the competition between enhanced density of states at Fermi energy by pressure with the ditorred MnAs₄ tetrahedra and the increased interlayer As-As bonding.Finally,we do much more careful and systematic research on(Ba_1-xK_x)(Zn_1-yMn_y)₂As₂single crystals with different doping.We find that the long ferromagnetic order only occurs when doping carrier and spin simultaneously.This kind of DMSs shows big magnetic anisotropy,especially for the high doping“BZA”DMSs.The highest T_C could reach 120K meanwhile about 60%spin polarization could achieve.In summary,we make some Research breakthroughs including new materials research and“the whole chain”research from material to prototype devices in this kind of new DMS with independent spin&charge doping.