Engineering of Metallic Multilayers and Spin Transfer Torque Devices

This dissertation explores in detail the nanofabrication and characterization of ultrathin metallic multilayers designed for the study of several electronic, magnetic, and optical phenomena. Several topics are covered, but the primary focus lies in the investigation and optimization of various sputter deposited thin-film structures with perpendicular magnetic anisotropy (PMA) for their incorporation into MgO-based magnetic tunnel junctions (MTJ). MTJs with these materials systems can then be ultimately applied towards functional spin-transfer-torque magnetoresistive RAM (STT-MRAM) technologies. As multiple topics are to be covered, this dissertation is roughly sub -divided into two parts: (i) full film materials investigation and (ii) device level characterization. (i) Full films materials investigation involved the exploration and optimization of multilayer systems such as Co/Pt, Co/Ni, and Co/Pd as well as crystalline alloys such as CoFeB and CoCrPt. Once identified, these materials were used in the development of perpendicular synthetic antiferromagnet (pSAF) systems. A separate study was performed to evaluate the free layers in pMTJ thin film stacks, focusing on substrate, deposition, and capping effects that contribute to interfacial PMA in MgO/CoFeB- like systems. (ii) A variety of magnetotransport techniques were implemented to study current-induced magnetic reversal of both in-plane and perpendicular MRAM MTJs. The set of magnetotransport tools developed includes both quasi-static field/current methods as well as high- speed transport techniques and have been used to probe key parameters of MRAM cells such as switching voltage, energy barrier, bit-error rates, and spin-torque efficiency. A population of MTJs was studied to analyze the behavior of normal and tail-bit devices. It was discovered through low -temperature magnetotransport measurements that these tail bit MTJs are prone to nucleation of metastable intermediate states and have poor spin-filtering capabilities. Several sizes of perpendicular MTJs were characterized by electrical stress and field sweeping techniques and found to have a variety of defects based on poor MgO/CoFeB interfacial anisotropy. The importance of proper MgO/CoFeB interface engineering is also emphasized in this section as it affects so many of the measurable metrics by which we gauge STT-MRAM as a viable technology