Perpendicular Magnetic Tunnel Junctions With Four Anti-Ferromagnetically Coupled Co/Pt Pinning Layers

We developed perpendicular magnetic tunnel junctions (MTJs) with four synthetic anti-ferromagnetically coupled Co/Pt layers (quad-SyF) and investigated their magnetic and transport properties. The quad-SyF comprised four Co/Pt layers and three 0.9 nm-thick Ru coupling layers, which consisted of Co/[Co/Pt] ${}_{a}$ /Ru/Co/[Co/Pt] ${}_{b}$ /Ru/Co/[Co/Pt] ${}_{c}$ /Ru/Co/[Co/Pt] ${}_{d}$ from top to bottom. The exchange coupling field ( $H_{\mathrm {ex}}$ ) reached a maximum of 1 T when the values of $a$ , $b$ , $c$ , and $d$ were 1, 2, 2, and 1, respectively. The tunnel magnetoresistance ratio of the MTJ with the quad-SyF and the second-peak conventional double-SyF increased as the annealing temperature was increased up to 400 °C, whereas that of the MTJ with the first-peak conventional double-SyF degraded at temperatures of more than 350 °C in blanket films. A 55 nm diameter MTJ with quad-SyF was found to be stable even against an external magnetic field up to 300 mT. On the contrary, in the conventional double-SyF, the reference-layer (RL) magnetization direction flips at around 250 mT. The shift magnetic field of the MTJ with quad-SyF becomes approximately zero when the values of $a$ , $b$ , $c$ , and $d$ are 1, 4, 1, and 2, respectively. No back-hopping of the MTJ with quad-SyF was observed even for the write pulsewidth ( $t_{W}$ ) down to 10 ns. On the contrary, an MTJ with conventional double-SyF exhibited back-hopping. In the patterned MTJ with conventional double-SyF, as the MTJ size decreases, the coercive field of Co/Pt significantly increases and $H_{\mathrm {ex}}$ decreases, causing the $m$ – $H$ curve of the RL to cross the zero magnetic field. This enables both parallel and antiparallel configurations for the top and bottom Co/Pt layers in double-SyF at the zero magnetic field, which could induce back-hopping. However, the $m$ – $H$ curves of the RL in the patterned MTJ with quad-SyF are far from the zero magnetic field owing to the high $H_{\mathrm {ex}}$ and low $H_{c}$ , which could lead to the suppression of back-hopping.