Radiation Effects on Magnetic Tunnel Junctions and Novel Magnetic thin Films

Spintronics, which utilizes spin polarized currents in memory and logic device, promises new paradigms for information processing and storage. However, the spin- related disordering effect caused by the irradiation has not been systematically investigated for ferromagnetic materials. The first generation of Magnetoresistive Random Access Memory (MRAM) is known to be “Rad hard”. However, advances in the magnetic nanostructures and new materials for the scalability of MRAM and other potential applications require a re-evaluation of their radiation hardness.

In this dissertation, as the key elements for the MRAM technology, the Spin Transfer Torque – Magnetic tunnel junction (STT-MTJ) devices with perpendicular magnetic anisotropy along with novel magnetic materials have been examined. Various radiation sources, including protons, Neon ions, and x-rays are used to irradiate the MTJ devices followed by the characterization of critical metrics for the device performance such as TMR, the device retention, and the switching currents to identify plausible failure mechanisms in MTJ devices associated with the irradiation species. The STT-MTJ shows “Rad Hard” properties after irradiation with the fluence and total ionization doses (TID) beyond CMOS threshold standard. The localized annealing, which is a by-product of the ionization process, improves the crystallinity of the MgO tunnel barrier that results in the small but appreciable increase of TMR after the accumulated irradiation of 10keV-energy x-ray with TID of 1 Mrad. The MTJ devices also maintain their normal functions after exposing to 2 MeV-energy Neon ions and protons separately. However, after proton irradiation, two of 60 nm devices exhibited unstable behavior during the retention test, which may be caused by the trapped charges.

Proton radiation affects the highly ordered L10 MnAl system by which the modification of magnetic properties results from the combined effect of irradiation- induced thermal annealing and atomic displacement. A highly ordered pre-rad MnAl is more susceptible to displacement damages which manifests itself in a continuing reduction of chemical ordering with irradiation doses going from S ~ 0.97 to ~ 0.8 to ~ 0.72 at a total fluence ~ 1x1015 H+/cm2. After the final irradiation with the total fluence 2x1015 H+/cm2, the change in the chemical ordering reverses and S becomes ~ 0.81, which results from the thermal annealing induced by the high fluence proton beam.
In amorphous TbFeCo materials, both the displacement and ionization damage are observed. The displacement damages are displayed through small enhancements of the coercive field, and an increase of resistance as the material becomes more porous. Meanwhile, the changes of the magnetic properties such as the magnetization, the compensation temperature can be attributed to the ionization damages.