Dependence of spin pumping in W/CoFeB heterostructures on the structural phase of tungsten

Abstract

We report the systematic dependence of spin pumping in tungsten (W)/CoFeB heterostructures on the structural phase of W, which is intricately related to argon gas pressure (${p}_{\mathrm{Ar}}$) maintained during the sputter deposition. We found that with increasing ${p}_{\mathrm{Ar}}$ the structural phase of W changes from mixed ($\ensuremath{\alpha}+\ensuremath{\beta}$) phase to pure $\ensuremath{\beta}$ phase. The $\ensuremath{\beta}$-W is stabilized in films for the high thickness of 40 nm which is desirable for spin devices. Using ferromagnetic resonance measurement of W(${p}_{\mathrm{Ar}}$)/CoFeB heterostructures, it is shown that enhancement of magnetic damping (${\ensuremath{\alpha}}_{\mathrm{eff}}$) from spin pumping is more in $\ensuremath{\beta}$-W compared to ($\ensuremath{\alpha}+\ensuremath{\beta}$)-W. The effective spin mixing conductance (${g}_{\mathrm{eff}}^{\ensuremath{\uparrow}\ensuremath{\downarrow}}$) is estimated for different phases of W from the linear evolution of ${\ensuremath{\alpha}}_{\mathrm{eff}}$ with the inverse thickness of the CoFeB layer. For $\ensuremath{\beta}$-W, the ${g}_{\mathrm{eff}}^{\ensuremath{\uparrow}\ensuremath{\downarrow}}$ is found to be larger than that of ($\ensuremath{\alpha}+\ensuremath{\beta}$)-W and it is attributed to different interface structure. Thus, effective tuning of spin pumping efficiency can be achieved using different W crystal phases. We also studied the dependence of ${\ensuremath{\alpha}}_{\mathrm{eff}}$ on $\ensuremath{\beta}$-W film thickness to calculate the value of spin-diffusion length (${\ensuremath{\lambda}}_{\mathrm{SD}}$) and intrinsic spin mixing conductance (${g}_{\ensuremath{\beta}\text{\ensuremath{-}}W}^{\ensuremath{\uparrow}\ensuremath{\downarrow}}$) using both the ballistic and diffusive spin transport models.