Preparation And Multi-state Storage Of Multiferroic Spin Filter Tunnel Junction

The basic unit of the traditional ferroelectric random access memorys(FeRAMs)is a ferroelectric capacitor that requires a certain amount of capacitance area to maintain sufficient charges to store information,making it difficult to achieve high density storage.Readout mode of the FeRAMs is destructive and the readout time is limited,because it necessitates the rewriting of information after readout.In recent years,there has been a new ferroelectric tunneling storage program,it's basic unit is ferroelectric tunnel junction that is a resistive storage scheme modulated by ferroelectric polarization.It is expected to realize high density and low power nonvolatile storage.Under this background,we design a multiferroic spin filter tunnel junction combining spin filter magnetic tunnel junction.It can integrate ferroelectric tunnel junction and magnetic tunnel junction function simultaneously in a storage unit,producing four non-volatile resistive states that can be read out in a non-destructive manner.At the same time,we also study the polarization relaxation behaviors of ultra-thin ferroelectric thin films.A series of important contents and results are described below.(1)A series of Pr0.8Ca0.2MnO3/BaTiO3 heterojunctions with different relative thicknesses are prepared on the SrTiO3(001)single crystal substrate.The magnetic hysteresis loop measurements show Pr0.8Ca0.2MnO3 film with a thickness of 9 cells(u.c.)is in ferromagnetic phase below 10 K,and the piezoelectric force microscope results show that 5 u.c.-thick BaTiO3 is still ferroelectric,and the optimum composite barrier is Pr0.8Ca0.2MnO3(9 u.c.)/BaTiO3(5 u.c.).(2)The LaNiO3/Pr0.8Ca0.2MnO3/BaTiO3/La0.7Sr0.3MnO3 epitaxial heterojunctions fabricated by pulsed laser deposition are defined by standard fabrication procedures including multiple lithography and ion beam milling.The test results show that the device has four distinct non-volatile resistance states,the tunneling electroresistance is about 100%,the tunneling magnetoresistance is about 24%.In addition,the ferroelectric polarization direction of BaTiO3 can modulate Pr0.8Ca0.2MnO3 spin polarization efficiency,which affects the device's tunneling magnetoresistance.(3)The BaTiO3 films with different thicknesses show different ferroelectric domain relaxation performances.As for 7 u.c.-thick BaTiO3 film,the,area of ferroelectric domain changes slowly and the polarization intensity becomes smaller.As for 40 nm-thick BaTiO3 thin film,the relaxation is completed by decreasing the area of ferroelectric domain.The temperature imposes a great influence on the ferroelectric domain retention property of Pb(Zr0.1Ti0.9)O3 thin film.The higher the temperature,the worse the domain retention.The shape of the ferroelectric domain affects the relaxation time,when the shape of the ferroelectric domain is closer to round,the slower the relaxation,the better the retention.