We have previously reported on fully perpendicular Co/Pd multilayers (ML)-based CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJ’s)1, 2. However, Co/Pd ML-based MTJ’s have rarely exhibited TMR ratios greater than about 10%. This has been attributed to the inability to pull a sufficiently thick CoFeB layer perpendicular on top of MgO, as well as the incomplete bcc templating of CoFeB from MgO owing to the adjacent fcc Co/Pd ML’s3. Recent results3-7 have generated great interest in MTJ’s with pinned perpendicular synthetic antiferromagnets (SAF), of the form AP1/Ru/AP2 where AP1 and AP2 are Co-based multilayers, for instance, Co/Ni or Co/Pd. We report on fully perpendicular MTJ’s with a thin CoFeB free layer and a Co/Pd(Pt) ML-based SAF pinned layer. For Co/Pd ML SAF’s, strong antiferromagnetic coupling was seen at tRu of 1.1nm, with a coupling strength of 0.017 mJ/m2. For Co/Pt ML SAF’s the optimum antiferromagnetic coupling was found at slightly higher Ru thickness of 1.3 nm, with a coupling strength of 0.013 mJ/m2. Improved MTJ properties are expected from using a thin Ta-seeded CoFeB bottom free layer, along with a thin, amorphous Ta layer used to transition from bcc CoFeB to fcc Co/Pd(Pt) for the top pinned layer6. The full stack is of the form: bottom lead/Ta (2)/CoFeB(1)/MgO(1.6)/CoFeB(0.8)/Ta(0.3)/[{Co(0.3/Pd(1)}5/Co(0.3) or {Co(0.5)/(Pt(2)}5/Co(0.5)/Ru1.1 or 1.3/[Co(0.3/Pd(1) or Co(0.5)/(Pt(2)]9/top lead. CIPT measurements indicated TMR values as high as 20% for as-deposited stacks. Magnetometry of blanket stacks showed a large separation in the switching fields of free and pinned layers, with free layer switching close to zero field and pinned layer switching at 0.8-1.8 kOe (Fig.1). This symmetric extended plateau of constant magnetization offers a large dynamic range over which the magnetic configuration remains stable7. The stacks were patterned into MTJ’s, annealed at 240◦ C in an in-plane field of 0.5 T, and characterized magnetically and electrically.

Acknowledgements:

 

This work is partially supported by a U.S. Department of Defense DARPA-MTO STT-RAM Universal Memory contract, and Grandis Technology, Milpitas. Dr. David Abraham of IBM is gratefully acknowledged for CIPT measurements.

References:

1. Z. R. Tadisina et al., J. Vac. Sci. Technol. A 28, 973 (2010).

2. Z. R. Tadisina et al., J. Appl. Phys. 107, 09C703 (2010).

3. K. Mizunuma et al., Appl. Phys. Lett. 95, 232516 (2009).

4. H.He et al., IEEE Trans. Magn. 46, 1327 (2010).

5. D. C. Worledge et al., _Proc. Int’l. Electron. Dev. Mtg. 10-296, (2010).

6. D. C. Worledge et al., Appl. Phys. Lett. 98, 022501 (2011).

7. J. Sort et al. Appl. Phys. Lett. 83, 1800 (2003).