Spin Transport Properties Based On High Spin-Polarized Heusler Alloys

Moore's Law tells us that the number of transistors on an integrated circuit will double every eighteen months and computer performance will increase accordingly.However overheating and electric leakage are always inevitable.As the transistor circuit approaches performance envelope,the International Technology Roadmap for Semiconductors has begun to slow.So,scientists have developed spintronics to keep Moore's Law rolling.Spintronic devices might speed up the development of information technology,because they have the advantages of high integration,low energy consumption and faster speed compared to conventional semiconductor devices such as the diode,the bipolar transistor and the field-effect transistor.High spin polarization,comparability with semiconductors,and long relaxation time are required in spintronic devices.Therefore,as high spin-polarized materials,spin gapless semiconductors and half-metallic fully compensated ferrimagnets have become research focus in recent years.In this thesis,for the recently synthesized three classes of typical spintronic materials,ferromagnetic spin gapless semiconductor CoFeMnSi,fully compensated ferrimagnetic half-metal Mn₃Al and fully compensated ferrimagnetic spin gapless semiconductor Ti₂MnAl,we investigate their electronic structures,magnetic and spin transport properties by using the density functional theory combined with nonequilibrium Green's function.The main results are as follows:Spin gapless semiconductor CoFeMnSi is ferromagnetic and the magnetic moment is mainly consisted of Co,Fe and Mn atom,especially the large exchange splitting of Mn 3d orbitals results in the large magnetic moment in Mn atom.The heterostructure of GaAs/CoFeMnSi?001?exhibts spin filtering and spin diode effects.The magnetic tunnel junction of CoFeMnSi/GaAs/CoFeMnSi?001?has large tunneling magnetoresistance.These excellent characteristics make it feasible to fabricate the spintronic devices based on spin gapless semiconductor CoFeMnSi.The half-metallic fully compensated ferrimagnet Mn₃Al has zero magnetic moment and total DOS around the Fermi level mainly originates from the Mn atomic 3d orbitals.The rectification ratio of heterostructure Mn₃Al/GaAs?001?reaches 1.1×10⁶ at 0.8 V and this is a great feature for diode.Due to the special transport spectrum,the magnetic tunnel junction of Mn₃Al/GaAs/Mn₃Al?001?displays the double spin filter effect in the antiparallel magnetization configuration.At the same time,the unilateral conductivity and spin filtering effect appear in the parallel magnetization configuration.The band structure of half metal also make transport mechanism of Mn₃Al different from ferromagnetic metal,so that we can get large tunneling magnetoresistance in magnetic tunnel junction.These results indicate half-metallic fully compensated ferrimagnet Mn₃Al is a promising spintronic material.For compensated ferrimagnetic spin gapless semiconductor Ti₂MnAl,the top of valence band and the bottom of conduction band are tangent to the Fermi level in the majority-spin?spin-up?channel,indicating gapless semiconductivity like graphene,meanwhile,the Fermi level is located at a band gap in the minority-spin?spin-down?channel,showing semiconductivity.The trend of tunneling current produces large TMR in the magnetic tunnel junction of Ti₂MnAl/InAs/Ti₂MnAl?001?.The current spectrum distribution and the difference of Fermi-Dirac distribution at different temperatures results in the spin Seebeck effect and the high spin polarization.Therefore,Ti₂MnAl is feasible used in spin caloritronic devices.