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▲ [14p-2J-6] Fabrication of p-SAF Structure with Strong Interlayer Exchange Coupling
Keywords:p-MTJ
A perpendicularly magnetized MgO-based magnetic tunnel junction (p-MTJ) is a promising candidate for use as a memory cell in spin-transfer-torque (STT) switching-type magnetoresistive random access memory (STT-MRAM). For steady read/write operations in a p-MTJ, the reference layer should have an identical magnetization direction for all memory cells, and a synthetic antiferromagnetic (SAF) structure has been widely applied. From the viewpoint of STT-MRAM applications, introducing the higher antiferromagnetic (AF) exchange coupling field of a perpendicularly magnetized SAF (p-SAF) structure has been considered preferable to prevent any read/write disturbance. SAF coupling manifests itself as a feature of the interlayer exchange coupling (IEC) effect through a thin spacer layer made of Ru, Rh, or Ir, for example. Its energy density (Jex) oscillates as a function of the spacer layer thickness. For Ru, Parkin et al. demonstrated that the highest Jex occurred at the “first peak” of the oscillation, where the Ru thickness (tRu) was less than 0.5 nm [1]. In the study, we fabricated [Co/Pt] superlattice [2] based p-SAF structures with tRu ranging from 0.34 to 1.05 nm to attain highly stable p-SAF structure with the 1st IEC peak.
We first fabricated p-SAF structures with the following stacked structure: Si-O substrate / Ta (5.0) / Ru (8.0) / Pt (2.0) / [Pt(0.16)/Co(0.24)]5(2.0) / Ru (tRu) / [Co(0.24)/Pt(0.16)]5 (2.0) / Pt(2.0) / capping (unit in nm), where tRu = 0.34–1.05 nm. We obtained large exchange field (Hex) of up to 10.0 kOe and 6.5 kOe, for the as-deposited and the annealed (Ta = 400ºC, 1h) sample, respectively. The maximum Jex was obtained at tRu = 0.43 to be 2.2 erg/cm2, which was three times higher than that at 0.95 nm (~0.7 erg/cm2). We also fabricated top-free-type p-MTJs with a newly developed p-SAF structure that exhibited strong AF coupling (Ta = 350ºC, 1h). We attained a two-times-larger Hex (~5.5 kOe) with a wide AF-coupled plateau compared with those of previous samples with a thicker Ru spacer corresponding to the 2nd peak [3]. At the same time, we achieved a high MR ratio of 150% at an RA product of 5.3 ohm-squm. The use of p-SAF coupling at the 1st IEC peak is advantageous for achieving a highly stable reference layer for every STT-MRAM generation.
This study was partly supported by the Normally-Off Computing Project of NEDO.
References:
[1] S.S.P. Parkin, Phys. Rev. Lett. 67 (1991) 3598.
[2] K. Yakushiji et al., Appl. Phys. Lett. 97 (2010) 232508.
[3] K. Yakushiji et al., Appl. Phys. Express 6 (2013) 113006.
We first fabricated p-SAF structures with the following stacked structure: Si-O substrate / Ta (5.0) / Ru (8.0) / Pt (2.0) / [Pt(0.16)/Co(0.24)]5(2.0) / Ru (tRu) / [Co(0.24)/Pt(0.16)]5 (2.0) / Pt(2.0) / capping (unit in nm), where tRu = 0.34–1.05 nm. We obtained large exchange field (Hex) of up to 10.0 kOe and 6.5 kOe, for the as-deposited and the annealed (Ta = 400ºC, 1h) sample, respectively. The maximum Jex was obtained at tRu = 0.43 to be 2.2 erg/cm2, which was three times higher than that at 0.95 nm (~0.7 erg/cm2). We also fabricated top-free-type p-MTJs with a newly developed p-SAF structure that exhibited strong AF coupling (Ta = 350ºC, 1h). We attained a two-times-larger Hex (~5.5 kOe) with a wide AF-coupled plateau compared with those of previous samples with a thicker Ru spacer corresponding to the 2nd peak [3]. At the same time, we achieved a high MR ratio of 150% at an RA product of 5.3 ohm-squm. The use of p-SAF coupling at the 1st IEC peak is advantageous for achieving a highly stable reference layer for every STT-MRAM generation.
This study was partly supported by the Normally-Off Computing Project of NEDO.
References:
[1] S.S.P. Parkin, Phys. Rev. Lett. 67 (1991) 3598.
[2] K. Yakushiji et al., Appl. Phys. Lett. 97 (2010) 232508.
[3] K. Yakushiji et al., Appl. Phys. Express 6 (2013) 113006.