We investigated the effect of using a synthetic ferrimagnetic (SyF) free layer in MgO-based magnetic tunnel junctions (MTJs) on current-induced magnetization switching (CIMS), particularly for application to spin-transfer torque random access memory (SPRAM). The employed SyF free layer had a Co40Fe40B20/ Ru/ Co40Fe40B20 and Co20Fe60B20/Ru/Co20Fe60B20 structures, and the MTJs(100x(150-300) nm^2) were annealed at 300oC. The use of SyF free layer resulted in low intrinsic critical current density (Jc0) without degrading the thermal-stability factor (E/kBT, where E, kB, and T are the energy potential, the Boltzmann constant, and temperature,respectively). When the two CoFeB layers of a strongly antiferromagnetically coupled SyF free layer had the same thickness, Jc0 was reduced to 2-4x10^6 A/cm^2. This low Jc0 may be due to the decreased effective volume under the large spin accumulation at the CoFeB/Ru. The E/kBT was over 60, resulting in a retention time of over ten years and suppression of the write current dispersion for SPRAM. The use of the SyF free layer also resulted in a bistable (parallel/antiparallel) magnetization configuration at zero field, enabling the realization of CIMS without the need to apply external fields to compensate for the offset field.
@article{arxiv.0801.1355,
title = {Current-induced magnetization switching in MgO barrier magnetic tunnel junctions with CoFeB based synthetic ferrimagnetic free layers},
author = {Jun Hayakawa and Shoji Ikeda and Katsuya Miura and Michihiko Yamanouchi and Young Min Lee and Ryutaro Sasaki and Masahiko Ichimura and Kenchi Ito and Takayuki Kawahara and Riichiro Takemura and Toshiyasu Meguro and Fumihiro Matsukura and Hiromasa Takahashi and Hideyuki Matsuoka and Hideo Ohno},
journal= {arXiv preprint arXiv:0801.1355},
year = {2009}
}