Thermally enhanced perpendicular magnetic anisotropy behaviors of ultrathin [Co/Pd]n multilayers via NiOx capping layer

Abstract

We report the enhanced perpendicular magnetic anisotropy (PMA) features of ultrathin [Co/Pd] 3 multilayers (MLs) employing a NiO x insertion layer at high annealing temperatures. Thermally enhanced PMA in [Co/Pd] 3 /NiO x (capping layer) MLs were achieved at a specific capping layer thickness, while no PMA responses were observed for a NiO x (buffer layer)/[Co/Pd] 3 ML, regardless of NiO x thickness. X-ray diffraction observations, including rocking curves, identified the relatively different crystalline characteristics of the NiO x capping and buffer layers. Origin of the enhanced PMAs of [Co/Pd] 3 MLs containing a NiO x capping layer is described based on the NiO x capping effect possibly providing additional Co/Oxide i-PMA under high-temperature annealing. V C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4921885]Perpendicular spin-transfer torque magnetic random access memories (p-STT MRAMs) are of great interest as the most promising alternatives to conventional memories because of their outstanding features, i.e., a fast write time of $10 nm, non-volatility, and extremely low power consumption. 1 In particular, the development of perpendicular magnetic tunneling junctions (p-MTJ) containing perpendicular magnetic anisotropy (PMA) is a key step toward their use in p-STT MRAMs beyond a feature size of 20 nm and a high thermal stability of ᭝¼ 74 and low critical current density (J c ) of J c ¼ 4.7 MA/cm 2 compared to in-plane MTJ (i-MTJ). 2 Given the material advantages PMA, various PMA frames, such as ferromagnetic metal (FM) and noble metal (NM) artificial multilayer (ML) matrices like Co/Pd, Co/Pt, Fe/Pd, Fe/ Pt, rare earth-transition metals (RE-TM) films, and L 1 0 -(001)-oriented Fe/Pt or Co/Pt films have been widely studied. 3-7 Among the various PMA systems recently considered, Cobased MLs yield simply tunable and strong PMA characteristics by alternating two layers with different thicknesses and selecting proper repeat numbers, enabling the use of Co-based MLs in p-STT MRAM devices. However, the widespread use of ultrathin Co-based MLs has been restricted by rapid PMA deterioration under high-temperature annealing. 8 The low annealing stability of PMA in ultrathin Co-based ML may be associated with undesirable atomic intermixing at the ML interfaces between FM and NM layers. 9 In addition, the Co thickness of Co-based MLs ensuring a robust interface PMA should be less than 1 nm in order to create a super-lattice structure between Co and NM. However, ultrathin Co-based MLs are highly susceptible to reduced electron spin polarization, resulting in a very small tunneling magnetoresistive (TMR) and low annealing stability. 10 As such, much effort has been dedicated toward achieving a strong annealing stability by maintaining higher PMA properties in the p-MTJs for high-performance p-STT MRAM applications. A recent approach that involves the incorporation of a proper capping or buffer layer as a promising solution has been used to protect against unexpected annealing-related atomic interdiffusion o...

Ultrathin Co-O oxide layer-driven perpendicular magnetic anisotropy in a CoO/[Co/Pd]m multilayer matrix upon annealing

“…interface spin orbit coupling between FM and NM layers1011. Meanwhile, an ultrathin ML matrix with a small repeat number ( m  = 3 in this letter) is susceptible to rapid PMA degradation induced by atomic intermixing and deterioration of the interface anisotropy in both the as-grown and annealed states12. Our previous work addressed the possibility of using an NiO x capping layer to enhance the PMA in [Co/Pd] 3 MLs, where a large number of oxygen atoms were inserted during sputtering to build up additional Co/Oxide interface PMA which remains stable even after high-temperature annealing12.…”

“…Meanwhile, an ultrathin ML matrix with a small repeat number ( m  = 3 in this letter) is susceptible to rapid PMA degradation induced by atomic intermixing and deterioration of the interface anisotropy in both the as-grown and annealed states12. Our previous work addressed the possibility of using an NiO x capping layer to enhance the PMA in [Co/Pd] 3 MLs, where a large number of oxygen atoms were inserted during sputtering to build up additional Co/Oxide interface PMA which remains stable even after high-temperature annealing12. The alternate possibility of incorporating a [CoO/Pd] 2 ML buffer layer into a thick [Co/Pd] 7 ML frame for thermally activated diffusion of oxygen atoms leading to atomic structural reconfiguration during annealing was also examined13.…”