59Co NMR analysis of CoFeB-MgO based magnetic tunnel junctions

Abstract

59Co Nuclear Magnetic Resonance (NMR) and X-Ray Diffraction (XRD) has been used to investigate annealing effects on crystallographic structure and magnetic properties of ferromagnetic CoFeB layer in pseudo spin valves (P-SV): Ta/CoFeB/MgO/CoFeB and in an exchange bias spin valves (EB-SV): PtMn/CoFe/Ru/CoFeB/MgO/CoFeB magnetic tunnel junctions. It was found, that the as-deposited CoFeB layers in the P-SV and EB-SV are amorphous. The annealing treatment at 340 °C and 360 °C induces in all studied systems progressive phase segregation in CoFeB layer to areas of bcc CoFe regions displaying different degree of B2 order and to areas of an amorphous magnetic reminder with a higher B content. In the CoFeB layers grown on Ru (EB-SV) and Ta (P-SV) and covered by MgO in the upper interface, after annealing at 500 °C a state of substantially full phase segregation to B2 ordered bcc CoFe phase has been observed. The annealing at 500 °C of the CoFeB layers grown on MgO and covered by Ru in P-SV and EB-SV triggers extensive thermal diffusion and induces mixing at the CoFeB/Ru interface which transforms the CoFeB into a non-magnetic layer. Moreover, it has been found that CoFeB crystallizes better into the bcc CoFe (100) texture when deposited on Ta than on a Ru layer. XRD profiles show that while CoFeB layer is growing on Ru it crystallizes into bcc CoFe (110) oriented texture, and the one grown on Ta crystallizes into bcc CoFe (200) when annealed at 500 °C. 59Co NMR restoring field analysis (Hrest - magnetic stiffness) showed that the insertion of the antiferromagnetic PtMn layer into the EB-SV system increases the magnetic hardness of the CoFeB layer by an order of the magnitude. However, adding CoFeB/MgO layers between PtMn/CoFe/Ru and CoFeB layers in the EB-SV decouples the CoFeB from the hard-magnetic system making it magnetically soft.