Improvement of Transport Properties in Magnetic Tunneling Junctions by Capping Materials

The capping effects on the properties of magnetic tunneling junctions (MTJs) have been investigated to improve the magnetoresistive (MR) ratio [Nagamine , Applied Physics, vol. 99, p. 08K703, 2006] and thermal stability [Fukumoto , Japanese Journal of Applied Physics, vol. 45, p. 3829, 2006. In this paper, MgO/Mg and MgO capping layers for MTJs were used to replace our previously used Ta and Ru capping layer [Yen , IEEE Transactions on Magnetics, vol. 42, no. 10, p. 862, 2004]. The MTJs with NiFe and CoFeB free layers were patterned to ellipses of 0.36 mum times 0.72 mum. The MR ratios of the MTJs with an NiFe free layer were enhanced from 26% to about 33% by capping MgO/Mg bilayers. A great improvement was also found on the MTJs with the CoFeB free layer, where the MR ratio was 50.4% with the MgO cap layer enhanced from 44.7% of Ru-capped MTJs. In addition, the influences of those capping layers included Ta, Ru, MgO, and MgO/Mg on thermal stability were also discussed