Giant tunnel magnetoresistance and high annealing stability in   CoFeB/MgO/CoFeB magnetic tunnel junctions with synthetic pinned layer
  We investigated the relationship between tunnel magnetoresistance (TMR) ratio and the crystallization of CoFeB layers through annealing in magnetic tunnel junctions (MTJs) with MgO barriers that had CoFe/Ru/CoFeB synthetic ferrimagnet pinned layers with varying Ru spacer thickness (tRu). The TMR ratio increased with increasing annealing temperature (Ta) and tRu, reaching 361% at Ta = 425C, whereas the TMR ratio of the MTJs with pinned layers without Ru spacers decreased at Ta over 325C. Ruthenium spacers play an important role in forming an (001)-oriented bcc CoFeB pinned layer, resulting in a high TMR ratio through annealing at high temperatures. 
Young Min Lee, Jun Hayakawa, Shoji Ikeda, Fumihiro Matsukura and Hideo   Ohno