The Spin-Dependent Tunneling Time In Tunnel Junctions With Ferromagnetic Insulators And Ferromagnetic Semiconductors As Barrier Layers

This thesis mainly studies the tunneling times(dwell time,phase time and transverse time)of two types of tunnel junctions: normal metal/ferromagnetic insulator/normal metal(NM/FI/NM)junctions and normal metal/ferromagnetic semiconductor/normal metal(NM/FS/NM)junctions.The effects of barrier height,barrier width,molecular field and Rashba spin-orbit coupling effect on spin-dependent tunneling time are discussed.And the relationship of spin-dependent tunneling time with the spin polarization and the angle between the molecular fields in the ferromagnetic layer and the ferromagnetic semiconductor layer are discussed also.Its main contents are as follows:In chapter 1,the concepts of the spin polarized tunneling,the Rashba spin-orbit coupling effect and spin filtering effect are introduced,and then two main models for tunneling magnetoresistance are discussed.Finally,the backgrounds and progress about the tunneling time are described briefly.In Chapter 2,based on Winful tunneling time model(dwell time and phase time),the relationships between spin-dependent tunneling time and spin polarization in NM/FI/NM and NM/FS/NM tunnel junctions are studied.For the above two tunnel junctions,the spin tunneling polarizations are derived from the spin filtering effects of the FI layer and the ferromagnetic and Rashba spin-orbit coupling effects in the FS layer,respectively.The results show that:In the NM/FI/NM tunnel junction,the dwell time and phase time of the spin-down electrons are larger than the spin-up electrons when the thickness and the molecular field of the FI layer increase and the corresponding spin polarization is always positive.For NM/FS/NM junctions,due to the Rashba spin-orbit coupling in the FS layer,the tunneling time of spin-up electrons and spin-down electrons exhibit periodic oscillations as the thickness and molecular field of the FS layer increase.The corresponding spin polarization exhibits periodic oscillations also.When the tunneling time of spin-down electron is larger than spin-up electrons,the corresponding spin polarization is negative;When the spin-down electron tunneling time is less than the spin-up electron tunneling time,the spin polarization is positive.In chapter 3,the spin-dependent tunneling time(transverse time)in the FM/FS/FM and FM/FI/FM tunnel junctions which are all composed by the ferromagnetic materials are studied and the relationships of the spin-dependent tunneling time with the angles between the magnetic moments in FM layers are discussed.The results show: when the intermediate layer is a ferromagnetic semiconductor,for different molecular fields in FS layer and for parallel or anti-parallel configuration,the tunneling time for spin-up and down electrons are all increased with the increasing of the thickness and the Rashba spin-orbit coupling strength of FS layer.But the effects on the spin-up electrons are larger than those of spin-down electrons.When the intermediate layer is a ferromagnetic insulator,for different molecular fields in FI layer and parallel or anti-parallel configuration,the tunneling times of the spin-up and down electrons increase exponentially with the barrier thickness and the barrier height.When the middle layer is a ferromagnetic semiconductor or a ferromagnetic insulator,for different thicknesses and spin-orbit coupling strengths of FS layer and for different barrier heights and thicknesses of FI layer,the variations of tunneling time difference between the spin-up and down electrons with the angle of the magnetic moment between the two FM electrodes are basically same.But when the molecular fields in FS or FI layers are increased to a certain values,the transition from the minimum value to the maximum value for the tunneling time difference between the spin-up and down electrons appears.In chapter 4,the previous research results are summarized and discussed.