All-optical study of the influence of Co and W stacking sequence on magnetization precession and damping and magnetic anisotropy in asymmetric Pt/(W,Co)/Pt heterostructures

The influence of the Co- and W-layer stacking sequence and the Co-layer thickness ${d}_{\mathrm{Co}}$ on the magnetization precession and damping and magnetic anisotropy is investigated using the all-optical time-resolved magneto-optical Kerr (TRMOKE) effect in asymmetric Pt/W/Co/Pt and Pt/Co/W/Pt heterostructures prepared by molecular beam epitaxy. It is found that the reversing of the stacking sequence of the Co and W layers leads to significant changes in the magnetic-field $H$ dependencies of the magnetization precession frequency $f$ and the effective damping parameter ${\ensuremath{\alpha}}_{\mathrm{eff}}$ for different ${d}_{\mathrm{Co}}$ thicknesses. A quantitative analysis of the $f(H)$ dependencies is carried out within the macrospin precession model, and the effective magnetic anisotropy constants ${K}_{\mathrm{eff}}$ are determined. It is shown that, in addition to the first-order coefficient ${K}₁$, also the second-order ${K}₂$ uniaxial anisotropy constant must be taken into account for the Pt/Co/W/Pt stack, while ${K}₂$ is insignificant for the reversed Pt/W/Co/Pt stack. From the ${K}_{\mathrm{eff}}({d}_{\mathrm{Co}})$ dependencies, the interface ${K}_{\mathrm{s}}$ and volume ${K}_{\mathrm{v}}$ anisotropy contributions for all heterostructures are obtained. The value of ${K}_{\mathrm{s}}=1.24\phantom{\rule{4pt}{0ex}}\mathrm{mJ}/{\mathrm{m}}²$ for the Pt/Co/W/Pt stack is found to be over two times larger as compared to ${K}_{\mathrm{s}}=0.55\phantom{\rule{4pt}{0ex}}\mathrm{mJ}/{\mathrm{m}}²$ for the Pt/W/Co/Pt one. The interfacial anisotropy induced at the bottom Pt/Co interface in the Pt/Co/W/Pt heterostructure is shown to be significantly larger than that induced at the top Co/Pt interface in the Pt/W/Co/Pt stack. From the effective damping parameter ${\ensuremath{\alpha}}_{\mathrm{eff}}(H)$ dependencies, the intrinsic damping ${\ensuremath{\alpha}}₀$ is extracted, and the interface-induced damping contributions to ${\ensuremath{\alpha}}₀$ within the spin-pumping model are derived. It is found that the effective spin-mixing conductance parameter ${g}_{\mathrm{eff}}^{\ensuremath{\uparrow}\ensuremath{\downarrow}}$ for the Pt/Co/W/Pt structure exceeds $100\phantom{\rule{0.16em}{0ex}}{\mathrm{nm}}^{\ensuremath{-}2}$ and is about two times larger than for the Pt/W/Co/Pt one, indicating the dominant Pt/Co-bottom interface contribution to the spin-pumping damping. A correlation between the intrinsic damping ${\ensuremath{\alpha}}₀$ and anisotropy constant ${K}₁$ for all layered structures studied is found and discussed in terms of the dependence of both quantities on the spin-orbit coupling. It is shown that the magnetic anisotropy parameters determined from the dynamic TRMOKE method agree well with those obtained from complementary measurements performed using the static magneto-optical Kerr effect.