Fabrication and characterization of perpendicular magnetic tunnel junctions with [CoPd] synthetic antiferromagnetic structures

• Main Authors: Yao-Jen Chang, 張耀仁
• Other Authors: Yang-Hua Chang
• Format: Others
• Language: en_US
• Published: 2014
• Online Access: http://ndltd.ncl.edu.tw/handle/15313386491061503871

博士 === 國立雲林科技大學 === 工程科技研究所 === 102 === MgO-based perpendicular magnetic tunnel junctions (p-MTJ) with Ta/CoFeB magnetic electrodes was studied in this dissertation. In the first part, I investigated the effect of a thin Ta layer on the perpendicular magnetic anisotropy (PMA) of composite FM1/Ta/FM2 magnetic structures, where FM1 represents the subsystem MgO/CoFeB, and FM2 denotes a [Co/Pd]6 multilayer. I found that the stack structure without a Ta spacer layer shows no PMA. However, once a Ta layer is inserted between the thin CoFeB layer and the [Co/Pd]6 multilayer, PMA is observed. To corroborate this observation, I systematically studied the magnetic properties of CoFeB/Ta(x)/[Co/Pd]6 structures using different Ta thicknesses x. The perpendicular magnetization loops of these structures show squareness ratios close to unity, indicating the presence of almost complete perpendicular anisotropy when the Ta was inserted. These hysteresis loops also show sharp switching characteristics, indicating that the MgO/CoFeB bilayer and the [Co/Pd]6 multilayer are ferromagnetically coupled together. The coercive field Hc of the composite structure increases as Ta thickness increases. My results show that Ta layer is essential for integrating MgO/CoFeB and [Co/Pd]6 into a composite magnetic structure with perpendicular anisotropy. Once the PMA was demonstrated in the above structures, I fabricated p-MTJs with synthetic antiferromagnetic (SAF) pinned layers. The p-MTJs stack has the structure bottom contact/Free Layer CoFeB (1.0)/MgO (1)/Pinned Layer CoFeB (1.0)/Ta Spacer layer/ SAF/Ru Cap Layer/Top Contact (the units in parenthesis are in nanometers). The SAF pinned layer consisted of two Co/Pd multilayers separated by a thin Ru spacer layer. I optimized the SAF by changing the repetition period n in one of the Co/Pd multilayers as well as varying the Ru thickness in order to produce PMA with AFM coupling. The magnetic studies show that all magnetic films, including the Ta/CoFeB layers, are perpendicularly magnetized. The two Co/Pd MLs in the SAF are AFM coupled when n > 2. One of the important results I found was that the interlayer exchange coupling (IEC) strength Jiec can be tailored by controlling the Ru thickness. In addition, I found that the Jiec vs. Ru thickness curve exhibits a simple exponential decay. The electrical properties of the full p-MTJ with SAF were also investigated, showing a low RA value of 44.7 Ωµm2 and a TMR ratio of 10.2%. These results demonstrate the successful integration of Co/Pd MLs into MTJ with perpendicular anisotropy.