Strain Effect On The Properties Of The Spintronic Devices

In recent years,with the application of spintronics devices in the field of data storage and processing,the construction and regulation of spintronic devices have attracted more and more attentions.As an important regulation method,strain plays an important role in the electrical and magnetic properties modulation on spintronic devices.Therefore,it has become the focus of research to find more effective and stable strain sources for spintronic devices.At present,the most popular strain sources mainly include the strain using the piezoelectric properties of piezoelectric material,the strain originating from the phase transition of shape memory alloy,the strain generated by the flexible substrate and the lattice strain arising from the lattice mismatch between the substrate and the epitaxial films.By these methods,the researchers have successfully controlled the performance of spintronic devices,which plays an important role in accelerating the development of spintronic devices.Although strain-mediated spintronic devices have been reported in recent years,the magnitude and reversibility of the regulation still need to be further improved,new physical phenomena need to be discovered,and the fundamental physical mechanisms need to be clarified.Therefore,this thesis focuses on the strain engineering by employing the shape memory alloys,flexible substrates,piezoelectric materials and lattice strain as the strain sources.The performance of magnetic multilayers and Hall bar devices has been studied including the giant magnetoresistance,perpendicular magnetic anisotropy,perpendicular exchange bias,as well as the spin-orbit torque-based current induced magnetization switching.The aim of this thesis is to illustrate the mechanisms of the strain regulation and explore the physical essence.The main innovative achievements of this thesis are as follows.(1)Reversible regulation on the giant magnetoresistance effect of spin valve by using the strain arising from the shape memory alloys during the phase transition.After depositing the spin valve(CoFe/Cu/CoFe/IrMn/Pt)structure on the NiTi and NiTiNb shape memory alloys,we applied cyclic heat treatment on the samples.The results show that shape memory alloys can change the magnetic anisotropy of the pinned layer and free layer by one-way or two-way shape memory effect,and then modulate the giant magnetoresistance,realizing the one-way or reversible control on the magnetoresistance of the spin valve.The change amplitude of GMR can be up to 17%in the one-way regulation and 15%in the two-way regulation.(2)The regulation of perpendicular magnetic anisotropy of multilayer structure fabricated on the flexible substrates.We deposited the multilayer(Ta/Pt/Co/Pt)structure with perpendicular magnetic anisotropy and the multilayer(Ta/Pt/Co/IrMn/Pt)structure with perpendicular exchange bias effect on the polyimide(Kapton)and polyethylene terephthalate(PET)flexible substrates.The compressive and tensile strain was transferred to the multilayers through bending the flexible substrates.Basing on the good bendability of the flexible substrates and good adhesion performance of the films,we achieved the control on the perpendicular magnetic anisotropy and perpendicular exchange bias of the multilayers.(3)Strain-mediated perpendicular exchange bias effect of magnetic multilayer films by piezoelectric materials.The Hall bar device(Ta/Pt/Co/IrMn/Pt)with perpendicular exchange bias effect was fabricated on the piezoelectric substrate Pb(Mgi/3Nb2/3)O3-PbTiO3(PMN-PT).The tensile stress or compressive stress was generated by the inverse piezoelectric effect of PMN-PT through applying different in-plane electric fields on PMN-PT.It was found that the strain can reversibly regulate the perpendicular exchange bias field of the Hall bar through the study of anomalous Hall magnetoresistance under different strain states,and the range of regulation is about 20%.In addition,we also study the influence of the perpendicular exchange bias on the process of current-induced perpendicular magnetization switching.(4)Strain-mediated spin-orbit torque induced magnetization switching process of magnetic multilayers based on piezoelectric materials.The Hall bar device with a stacking structure of Ta/Pt/Co/Pt with good perpendicular magnetic anisotropy was fabricated on the PMN-PT piezoelectric substrate.After applying an in-plane electric field on PMN-PT,the tensile or compressive strain generated by the piezoelectric effect of PMN-PT was used to realize the reversibly regulation on the magnetization switching field,the domain wall motion and the current driven magnetization switching process of the Hall bar.By applying different electric field,we can get three different resistance states(0.4 Ω,0.6 Ωand 0.8Ω)and design the electric field controled logic devices.(5)The strain-mediated Neel vector of NiO controlled by lattice strain.We deposited epitaxial NiO antiferromagnetic insulator films on different substrates with different lattice constants.The lattice strain generated by lattice mismatch between the NiO layer and the substrate was used to change the orientation of the Neel vector of NiO.Hall bar devices were further fabricated with a stacking structure of substrate/NiO/Pt/Co/Pt.It was found that different Neel vector orientations would affect the spin polarization direction of the spins at the NiO/Pt interface.The perpendicular component of the spin polarization of the spin current would drive the field-free current-induced perpendicular magnetization switching.When the thickness of NiO layer is thin,the Hall bar on the MgO substrate can achieve about 60%switching at zero-field,while the the Hall bar on STO substrate can only achieve 20%field-free switching due to the lattice mismatch.