Microwave Properties and Damping in [Pt/Co] Multilayers with Perpendicular Anisotropy

Magnetic tunnel junctions with perpendicularly magnetized elements are being considered for next-generation, non-volatile, magnetic random access memory (MRAM) elements due to their large thermal stability, low Gilbert damping constant α, and very low critical current density Jc required for spin-Transfer-Torque (STT), current-induced, magnetization switching. Here we study, by means of static magnetization and field-swept ferromagnetic resonance (FMR), [Pt/Co] multilayers with reduced Pt and Co layer thicknesses, ranging between 0.2 nm and 0.32 nm. Such materials are known to exhibit strong perpendicular magnetic anisotropy due to low interdiffusion, even after annealing up to 500°C. We analyzed our data on FMR frequency versus applied magnetic field with the appropriate Kittel formulas and obtained a high anisotropy field Hk and a very low Gilbert damping constant α. This improvement makes the [Pt/Co] system with low overall Pt and Co content a promising candidate for STT-MRAM.