Influence of Iridium Sputtering Conditions on the Magnetic Properties of Co/Pt-Based Iridium-Synthetic Antiferromagnetic Coupling Reference Layer

We investigated the effects of sputtering conditions for the deposition of an Iridium (Ir) layer in a [Co/Pt] _m /Co/Ir/[Co/Pt] _n /Co/W/CoFeB synthetic antiferromagnetic reference layer (Ir-SyF) on the magnetic properties and tunnel magnetoresistance ratio (TMR ratio) of magnetic tunnel junctions (MTJs) stacks annealed at 400 °C for 1 h. The exchange coupling field ( $H_{\mathrm {ex}}$ ) of Ir-SyF was improved by reducing the energy of Ir recoil ions and two times larger than that with Ru-SyF. Energy dispersive X-ray (EDX) spectrometry line analysis revealed greater interlayer diffusion in Ir when Ir was sputtered by using a conditions with large recoiled energy. This could cause the deterioration of the $H_{\mathrm {ex}}$ of the Ir-SyF. Despite the larger $H_{\mathrm {ex}}$ , the TMR ratio of the MTJ with Ir-SyF is smaller than that with Ru-SyF. The $m$ – $H$ curve of MTJ with Ru-SyF showed a large plateau region around zero magnetic field, whereas that with Ir-SyF did not. These results indicated the degradation of perpendicular magnetic anisotropy (PMA) in the top part of the Co/Pt multilayer with CoFeB reference layer and a large biquadratic coupling effect in the thin Ir layer. This causes the deterioration of the TMR ratio of the MTJ with Ir-SyF. TEM image of the Co/Pt layer in the MTJ with Ir shows some lattice defects. The EDX line analysis revealed that a large amount of Pt in the top Co/Pt layer diffused toward CoFeB reference layer in the Ir-SyF, resulting in the degradation of PMA. The structural analysis by X-ray diffraction showed the lattice spacing of CoPt (111) in Ir-SyF to be larger than that in Ru-SyF, indicating the occurrence of strain relaxation at the Co/Pt interface. These crystallographic changes in Ir-SyF might be related to a larger Pt diffusion. Suppression of Pt diffusion as well as low damage Ir deposition in the reference layer is crucial to utilize Ir-SyF.