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

• Published in: IEEE transactions on magnetics Vol. 58; no. 8; pp. 1 - 5
• Main Authors: Honjo, H., Naganuma, H., Nishioka, K., Nguyen, T. V. A., Yasuhira, M., Ikeda, S., Endoh, T.
• Format: Journal Article
• Language: English
• Published: New York IEEE 01-08-2022; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Annealing; Antiferromagnetism; Cobalt; CoFeB-MgO; Coupling; Couplings; Crystal defects; Crystallography; Degradation; Deposition; Deterioration; Diffusion layers; double interfaces structure; interfacial anisotropy; Interlayers; Iridium; Magnetic anisotropy; Magnetic fields; Magnetic properties; magnetic tunnel junction; Magnetic tunneling; Magnetism; Magnetoresistivity; Multilayers; perpendicular anisotropy; Platinum; Recoil ions; Ruthenium; spin-transfer-torque magneto-resistive random access memory (STT-MRAM); Sputtering; Strain relaxation; Structural analysis; Thermal stability; Tunnel junctions; Tunnel magnetoresistance
• ISSN: 0018-9464; 1941-0069
• DOI: 10.1109/TMAG.2022.3151562

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 (<inline-formula> <tex-math notation="LaTeX">H_{\mathrm {ex}} </tex-math></inline-formula>) 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 <inline-formula> <tex-math notation="LaTeX">H_{\mathrm {ex}} </tex-math></inline-formula> of the Ir-SyF. Despite the larger <inline-formula> <tex-math notation="LaTeX">H_{\mathrm {ex}} </tex-math></inline-formula>, the TMR ratio of the MTJ with Ir-SyF is smaller than that with Ru-SyF. The <inline-formula> <tex-math notation="LaTeX">m </tex-math></inline-formula>-<inline-formula> <tex-math notation="LaTeX">H </tex-math></inline-formula> 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.