The Robustness of the Interlayer Exchange Coupling of the Wedge-Type Perpendicularly Magnetized Pt/Co/Pt/Ru/Pt/Co Multilayers

Magnetic tunnel junctions (MTJs) with perpendicular magnetic anisotropy (PMA) attract much interest since they allow to scale down the dimension of spintronic devices to the latest technological node while keeping sufficient thermal stability [1]. Among the various types of magnetoresistive (MR) based spintronic devices, the MgO-based tunneling magnetoresistance (TMR) can provide a very high MR ratio and high field sensitivity to detect small changes in magnetic field. MTJs with PMA (p-MTJs) use a perpendicular synthetic antiferromagnet (SyAF) layer which consists of two ferromagnetic layers antiferromagnetically coupled through a nonmagnetic spacer such as Ru. The stray field generated from the SyAF on the storage layer can be greatly reduced, leading to two low and high resistance states with equivalent thermal stability and critical switching current [2]. Furthermore, SyAF structure is extensively studied for antiferromagnetic spintronics such as a spin-torque oscillator [3], spin-orbit torque device [4], and a magnetic domain wall motion device [5]. The SyAF is the consequence of the interlayer exchange coupling (IEC), which is essentially a Ruderman-Kittel-Kasuya-Yosida (RKKY) coupling. RKKY coupling originates from spin-dependent Friedel-like spatial oscillations in the spin density of the nonmagnetic spacer induced by the adjacent ferromagnet.<br/>Here, we systematically investigate IEC in the perpendicular SyAF structure of Pt/Co/Pt layers. The structure of SyAF are Ta(4)/Pt(4)/Co(1.2)/Pt(0.2)/Ru(0 - 5.6)/Pt(0.2)/Co(1.3)/Pt(3.0) and Ta(4)/Pt(4) /Co(1.2)/ Pt(0.2)/ Ru(0.9) /Pt(0.2)/Co(0 – 2.0)/Pt(3.0) (thickness in nm). The polar magneto-optic Kerr effect (MOKE) measurements were employed to measure the Ru thickness-dependent magnetic hysteresis curve. The maximum exchange field is 360 mT at 0.9 nm Ru for the calculated maximum exchange energy is 0.49 mJ/m2 for the Ru wedged sample. We strongly anticipate that our observations can improve the SyAF-based spintronic devices.<br/>[1] S. Mangin, <i>et al</i>., Nature Mater., <b>5</b>, 210-215, (2006).<br/>[2] J. L. Leal and M. H. Kryder J. Appl. Phys. <b>83</b>, 3720 (1998).<br/>[3] A. Bergman, <i>et al</i>. Phys. Rev. B <b>83</b>, 224429 (2011).<br/>[4] Y.-C. Lau, D. Betto, K. Rode, J. M. D. Coey, and P. Stamenov, Nat. Nanotech. <b>11</b>, 758–762 (2016).<br/>[5] R. Lavrijsen, <i>et al</i>. Nature <b>493</b>, 647–650 (2013).