Studies of the effect of seed and capping layers on CoFeB free layers of magnetic tunnel junctions (MTJ’s) originated from reports (1) of the crystallization of the CoFeB through diffusion of the B into the cap, as well as inducing an interfacial perpendicular magnetic anisotropy in the free layer (2, 3). We have also seen thatCoFeB can be made perpendicular[3, 4] with seed layers of certain materials, such as Ta and Ru. We deposited Ta and Ru seed layers with the following stack structure: (Ta/Ru/Hf/Zr) 2/ [tCoFeBx]/MgO 0.9/TaN5 nm. The thickness of CoFeB, tCoFeB, was varied between 0.8 to 1.4 nm. Samples with the Ta seed layer showed higher perpendicular anisotropy than that of Ru, Hf and Zr because of the B diffusion into Ta after annealing. At tCoFeB = 1nm, high perpendicular anisotropy was seen, with anisotropy energy density Kut = 0.24 erg/cm2. The optimized Ta-seeded CoFeB was used as the free layer in a fully perpendicular MTJ stack with a Co/Pd multilayer synthetic antiferromagnet pinned layer[5]. These MTJ stackswere then patterned into devices with photolithography and planarized at each step of fabricationwith a novel sputtered aluminum oxide passivation layer. After fabrication, these devices were subjected to a variety of annealing conditions: a) furnace annealed with a field of 0.5T applied in the plane of the sample at 1500C for 2 hours, b) rapid thermally annealed (RTA) at 3500C, 4000C and 5000C for various time periods. Magnetometry of the minor loops indicated that, as the RTA time was increased at each temperature, the free layer became fully perpendicular at 8 minutes and then went in-plane with longer annealing times of 12 minutes. These results matched closely with the transport measurements.Increase of annealing time improved the tunneling magnetoresistance (TMR) to a maximum of 50% at room temperature (nearly 60% at 4.2K). Further increase in annealing time degraded the TMR at all temperatures tested.Thusfor the first time, we have found that magnetometry on the free layer of fully perpendicular magnetic tunnel junctions (pMTJ) can be used to optimize the annealing conditions.

1. E. Chen et al., IEEE Trans. Magn. 46, 1 (2010).

2. S. M. Watts et al., Digest FV-11, 11th Joint MMM-Intermag Conference, Washington, DC(2010)

3. D. Worledge et al., Digest HB-10, 11th Joint MMM-Intermag Conference, Washington, DC(2010)

4. D. C. Worledge, G. Hu, David W. Abraham, J. Z. Sun, P. L. Trouilloud, J. Nowak, S. Brown, M. C. Gaidis, E. J. O’Sullivan, and R. P. Robertazzi. Appl. Phys. Lett. 98, 022501 (2011).

5. A. Natarajarathinam, R. Zhu, P.B. Visscher and S. Gupta, J. Appl. Phys 111, 07C918 (2012).