In this paper, we present further study on the interlayer exchange coupling of [Pt/Co]n/MgO/[Co/Pt]2 perpendicular magnetic tunnel junctions. Antiferromagnetic interlayer couplings in [Pt/Co]n/MgO/[Co/Pt]2 are observed. The strength of antiferromagnetic coupling oscillates irregularly with the repetition number n, that may be related to the Ruderman-Kittel-Kasuya-Yosida (RKKY)-type ferromagnetic interlayer coupling existing in the [Pt/Co]n hard layer. The interlayer coupling of [Pt/Co]9/MgO(22 Å)/[Co/Pt]2 magnetic tunnel junction reaches a maximum at 200 K, and decreases gradually with increasing temperature. This thermal behavior of interlayer coupling may be related to the enhanced perpendicular magnetic anisotropy of hard layer with decreasing temperature.
The microstructure development of TC21 alloy after double α/β solution and aging heat treatment was investigated. Results show that primary α phase morphology is determined by the first solution treatment at the top of α/β portion. Varying the cooling rates can significantly influence the volume fraction of retained β phase and secondary α phase. Higher cooling rate results in greater retention of the β phase and transformation of the phase into secondary α phase upon aging. This higher amount of fine secondary α phase causes higher strength and lower toughness. Second α/β heat treatment temperature remarkably influences the heat treatment response of the microstructure. Higher heat treatment temperature keeps greater amounts of β with fine transformed α in the final microstructures. Lower α/β heat treatment temperature results in less responsive retained β during aging process. Aging treatment leads to decomposition of retained β phase, particularly in larger retained β regions that exhibit lower stability.
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