Interfacial Effects In Fecob Based Magnetic Tunnel Junctions And Spin Hall Effect Bi-Layer Structures
The transfer of spin angular momentum from a spin-polarized current to a ferromagnet can generate sufficient torque to reorient the magnet's moment, thus this effect has prospective applications in spintronics. In this dissertation, the main theme of my work has been the study of spin-transfer-driven ferromagnetic resonance (STFMR) signals to understand what is the interfacial effects on the spin transfer torque (STT) in magnetic tunnel junction (MTJ) devices with a nanopillar geometry and bilayer spin Hall effect structures. In the first part of my work, I have studied the bias dependence of the spin transfer torques in the as-grown and annealed FeCoB/MgO/FeCoB MTJs by ST-FMR measurement. The motivation of this work was to understand the important mechanisms contributing to the in-plane torque with a view to optimizing the switching current of devices. I have found the annealing reduces the inelastic tunneling component of the junction conductance, and creates or enhances a peak at about 0.15V above the Fermi level in the density of states of the minority band of the electrodes, which may be the reason for the asymmetry of the bias dependence of the in-plane torque. This peak location is essentially the same as the peak of the minority band states that has previously observed on the surface of bcc (100) Fe. The existence of these interfacial states can also explain the asymmetry of the TMR, and the bias dependence of both G AP and GP . As a function of temperature, my experimental STTs can be explained by magnon contributions. My results show that with a relative increase of 50% in tunneling MR, however, both in-plane torque and perpendicular torque have been depressed at low temperature, indicating that magnon assisted tunneling, which results in a spin-flip process for the tunneling electron, has a negative contribution to the spin polarization, but plays a positive role in STT, with the latter consistent with recent theoretical predictions. In the second part of my work, I have studied the spin-torque effects on the FeCoB/W bi-layer structures with different W phases. The motivation of this work was to investigate the spin Hall and spin pumping effects in this system. I have observed a greatly enhanced magnetic damping coefficient for the Fe40Co40B20 layer with [alpha]-W, and I tentatively attribute these results to a significantly enhanced spin pumping effect in [alpha]-W, relative to that in [beta]-W. Magnetization measurements indicate that the two different types of Fe40Co40B20/W bilayers also have substantially different interfacial magnetic anisotropy coefficients.
Buhrman, Robert A
Van Dover, Robert B.; Ralph, Daniel C
Ph.D. of Applied Physics
Doctor of Philosophy
dissertation or thesis