High spin mixing conductance and spin transparency in ion-beam sputtered Ta/Co60Fe20B20 bilayers on Si (100)

Abstract

Ta/CoFeB heterostructure has been considered as a promising structure for spin torque-based memory applications. Here, a systematic investigation of the magnetic (static and dynamic) properties of ion-beam sputtered Ta/Co60Fe20B20(CFB) bilayer thin films has been done. In the series, the thickness of CFB (tCFB) layer is varied from 2 to 24 nm, keeping the buffer Ta layer thickness fixed at 5 nm as a spin sink reservoir. The structural studies reveal a mixed phase (α+β) growth of Ta layer, while CFB is amorphous, and the interface between them is quite smooth with extremely low interface roughness (<3Å). The saturation magnetization is found to be increased from 957(±30) kA/m to 1097(±19) kA/m with tCFB while the coercivity lies in the range of 0.18-0.75 mT, suggesting the highly soft magnetic nature of the CFB films. Ferromagnetic resonance spectroscopy was performed to study the spin pumping response of the Ta/CFB bilayer thin films. From the variation of effective magnetization with tCFB, surface anisotropy energy of the CFB film is calculated to be 0.24±0.07 erg/cm2. From the effective damping change with tCFB, the effective spin mixing conductance (geff↑↓) and transparency (T) of the Ta/CFB interface are extracted and found to be 11.74±1.14 nm−2 and 60.61±5.88 %, respectively. The origin of large geff↑↓ and T lies at the smooth interface between Ta and CFB. Such a large values of these parameters in Ta/CFB heterostructures are essential requirements for spin device applications.