Enhanced Thermal Stability in Magnetic Random-Access Memory Cells With Free Layer Composed of Multilayer Co/Pt Coupled to Co2Fe6B2 With Interfacial Perpendicular Magnetic Anisotropy

A novel perpendicular spin-transfer torque magnetic random-access memory spin valve with a memory-cell size below 20 nm × 20 nm and a thermal stability factor Δ of ~77 (10-year retention time) was designed by ferromagnetically coupling a multiple free layer [Co/Pt]n to Co
₂
Fe
₆
B
₂
having interfacial perpendicular magnetic anisotropy (i-PMA) instead of coupling to a conventional double i-PMA free layer (Δ = 33). Thermal stability (Δ) increased with an increase of n in the [Co(0.47nm)/Pt(0.23 nm)]n multiple free layer. In particular, Δ = 80 could be achieved with n = 4 for a 15 nm × 15 nm memory-cell size. However, the tunneling magnetoresistance (TMR) ratio, which should be above 150% to assure a reasonable sensing margin, rapidly decreased from 190% to 98% with an increase in n from 0 to 4. This decrease was associated with W and Pt atomic diffusion into the MgO tunneling barrier. Improvement in the crystallinity of the MgO tunneling barrier increased the TMR ratio to 144% for n = 4.