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.Comment: 10 pages, 5 figures, submitted to Applied Physics Letter