Ferromagnetic-resonance-induced spin pumping in Co20Fe60B20/Pt systems: damping investigation

Abstract

Microstrip line ferromagnetic resonance (MS-FMR) has been used to investigate the dependence of the magnetic damping enhancement on the thickness of Co20Fe60B20 and Pt due to spin pumping. Samples with variable thicknesses of Co20Fe60B20 (Pt) are used to determine the spin mixing conductance (spin diffusion length) via the thickness dependence of the Gilbert damping parameter α of Co20Fe60B20/Pt heterostructures. The results obtained from the analysis of the MS-FMR measurements reveal that α increases linearly with the inverse thickness of Co20Fe60B20 for films capped with a 10 nm thick Pt layer, while the variation of α versus the Pt thickness of the 4 nm thick Co20Fe60B20 layer has an exponential behaviour. The experimental data was analysed using a ballistic or a diffusive spin transport model for spin pumping, which includes the effective spin mixing conductance of the CoFeB/Pt interface and the spin-diffusion length of Pt. The estimated values are 37.5 nm−2 (spin mixing conductance) and 1.7 nm when the ballistic transport model is used, and 56.75 nm−2 and 2.2 nm when the experimental data are analysed using a more realistic model based on a diffusive transport. Moreover, MS-FMR measurements reveal that the effective magnetization varies linearly with the Co20Fe60B20 inverse thickness due to the perpendicular interface anisotropy. This anisotropy, estimated to be 1.31 erg cm−2, reinforces the perpendicular magnetization easy axis.