Material Exploration for Spin transfer torque - Magnetic Random Access Memory

Spin Transfer Torque Magnetic Random Access Memory (STT-MRAM) has become increasingly important for the memory industry due to its projected scalability and performance. This key component of the memory is called a magnetic tunnel junction (MTJ), consisting of a trilayer of two magnetic and one insulating film whose resistance depends on the relative orientation of the magnetic layers. When writing the memory, one of the magnetic layers is fixed and the magnetization of the free layer can be rotated using a torque exerted by a spin polarized current. However, the critical polarized current density in conventional CoFeB-based MTJs is still too large, preventing the STT-MRAM size from scaling down. The objective of this dissertation is to examine new ferromagnetic materials as candidate electrodes in MTJs, with properties that can reduce the critical current density, and thus realize a practical STT-MRAM.
Materials of interest in this study include CoFeCrB, Co2FeAl and MnAl, each showing advantages over CoFeB for STT-MRAMs in different ways such as reduced magnetization, low damping parameter and large perpendicular magnetic anisotropy. For CoFeCrB, as-deposited films were amorphous, but were α-bcc crystallized after annealing. The addition of Cr reduced the magnetization moment due to the antiferromagnetic coupling between Cr and Co/Fe. This work has shown the effect of Cr and B content on the crystallization and magnetic properties. With a high chemical ordering, Heusler alloy Co2FeAl demonstrates a damping parameter at least three times lower than that of CoFeB. The efforts are devoted into achieving a very high film quality with high chemical ordering, which is critical in obtaining a low damping parameter. Besides, an interfacial perpendicular magnetic anisotropy of ~ 1.9 x 106 erg/cc was introduced in Co2FeAl/MgO interfaces annealed at 350 ˚C without an external magnetic field. These results indicate the significant benefits of using Co2FeAl in reducing the critical current density of a perpendicular STT-MRAM. Ordered binary intermetallic τ-MnAl also demonstrates a large perpendicular magnetic anisotropy ~ 5.34 x 106 erg/cc. The magnetization of τ-MnAl is much smaller than that of CoFeB, but robust enough for the STT-MRAM applications. Layer-by-layer deposition approach is developed to improve the degree of the chemical ordering in τ-MnAl. Different seeding conditions are studied to manipulate the crystal strains thus to optimize the perpendicular magnetic anisotropy. The damping parameter of τ-MnAl in my study is ~0.033, lower than that of most perpendicular systems, yet not comparable to in-plane systems such as CoFeB.
This dissertation also explores spintronic devices including spin valves and magnetic tunnel junctions incorporating Co2FeAl thin films. In pseudo spin valves (Co2FeAl/Cr/Co2FeAl), bilinear and biquadratic couplings are observed. The bilinear coupling shows an oscillatory behavior with the spacer thickness, while the biquadratic coupling demonstrates a significant dependence on the temperature. Co2FeAl-based MTJs were developed. A well defined magnetic switching was achieved after proper optimizations of the junction structure and growth conditions.