Characterization of Spin Hall and Magneto-ionic Devices for Logic, Memory and Neuromorphic Applications

Abstract

This thesis is divided into two parts. In the first part, my research is focused on spin-to-charge conversion in amorphous Gd (40%)-alloyed Bismuth Selenide (60%) (BSG) thin films. The spin Hall effect has emerged as a key proponent for spintronic devices. Such devices typically consist of a bilayer structure made from a spin Hall channel and a ferromagnet. Polycrystalline Bi2Se3 was discovered to have a large spin Hall effect. Spin Hall angle and spin Hall efficiency (SCE) have been key parameters for comparing spin Hall channels. However, the output voltage becomes an essential requirement for spin logic devices, which also depends on resistivity. Gd (40%) alloyed Bi2Se3, grown by sputtering, can fill these gaps for spin logic devices. The material is amorphous, ensuring good scalability. Resistivity as a function of temperature showed strong signs of 3D variable range hopping with a characteristic Mott temperature of 9.7 x 105 K and a room temperature resistivity of 60,000 µOhm.cm. With 5nm in-plane CoFeB, the spin pumping results show good symmetric peaks for different excitation frequencies. The spin to charge conversion efficiency (~ Jc/Js) increased with decreasing thickness of BSG. Second harmonic measurements were performed to characterize thermal effects. The spin-orbit torque was negligible due to the dominance of thermal effects and current shunting through the ferromagnet. Anomalous Nernst effect was found to be the dominant thermal effect. However, it couldn’t explain our spin pumping results due to the lack of BSG thickness dependence and the dominance of the first harmonic voltage. The spin pumping was concluded to originate from the inverse spin Hall effect in BSG layer. My research focuses on irreversible magneto-ionic devices for one-time-programmable memory and neuromorphic applications in the second part of the thesis. Magneto-ionic devices rely on ionic movement through a gate dielectric to manipulate the magnetic properties of a magnetic material. We use Co20Fe60B20 perpendicular magnetic anisotropy (PMA) thin films. CoFeB/MgO interfacial PMA is a consequence of orbital overlapping between Oxygen and transition metal atoms. We further engineer the device to enable field-free magnetization switching. We use an exchange bias field from an adjacent ferromagnet ([Co(0.3nm)/Pd(0.7nm)]3) separated by a non-magnetic layer (Ta), forming a [Co(0.3nm)/Pd(0.7nm)]3/Ta/CoFeB/MgO structure. Pd (111) was used as the seed layer for [Co(0.3nm)/Pd(0.7nm)]3. The final stack is given by: Substrate/Ta(5nm)/Pd(10nm)/[Co(0.3nm)/Pd(0.7nm)]3/Ta(1nm)/CoFeB(1.3nm)/MgO(2nm). XRD and HRTEM were used to characterize the film, which showed distinct layers with some interdiffusion and a polycrystalline Pd(111). This stack is then topped with an ionic gate made from 100nm sputtered SiOx. AHE minor curves showed that the two ferromagnets have weak antiferromagnetic coupling. Application of negative gate voltage decreases the coercivity of CoFeB from ~34 Oe to 16 Oe, signaling a lowered PMA. The exchange bias field magnitude increases from ~ 25 Oe to ~ 45 Oe, due to the decrease in thickness of CoFeB. Major loop measurements show no change in [Co(0.3nm)/Pd(0.7nm)]3 layer with gate voltage. Oxygen ions from SiOx move towards the interface of MgO/CoFeB interface under negative gate voltage. This creates an overoxidation of the interface and destroys the interfacial PMA of CoFeB. This makes the CoFeB layer go from a bi-stable to a monostable state, resulting in a pathway for a field-free magnetization switch.