Evolution of damping in ferromagnetic/nonmagnetic thin film bilayers as a function of nonmagnetic layer thickness

The evolution of damping in Co/Pt, Co/Au, and ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$/Pt bilayers was studied with increasing nonmagnetic (NM) heavy-metal layer thicknesses in the range $0.2\mathrm{nm}\le t_{\mathrm{NM}}\le 10\mathrm{nm}$, where $t_{\mathrm{NM}}$ is the NM layer thickness. Magnetization precession was measured in the time domain using time-resolved magneto-optical Kerr effect magnetometry. Fitting of the data with a damped sinusoidal function was undertaken in order to extract the phenomenological Gilbert damping coefficient $\alpha$. For Pt-capped Co and ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$ layers a large and complex dependence of $\alpha$ on the Pt layer thickness was observed, while for Au capping no significant dependence was observed. It is suggested that this difference is related to the different localized spin-orbit interaction related to intermixing and to $d\text{−}d$ hybridization of Pt and Au at the interface with Co or ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$. Also it was shown that damping is affected by the crystal structure differences in FM thin films and at the interface, which can modify the spin-diffusion length and the effective spin-mixing conductance. In addition to the intrinsic damping an extrinsic contribution plays an important role in the enhancement of damping when the Pt capping layer is discontinuous. The dependence of damping on the nonmagnetic layer thickness is complex but shows qualitative agreement with recent theoretical predictions.

Figure 1

(a) Examples of x-ray reflectivity data and the best-fitting simulations for Co (10 nm)/Pt $\left(t_{\mathrm{Pt}}\right)$. (b) Background subtracted time-resolved magneto-optic Kerr rotation data for Co/Pt with two different Pt layer thicknesses and the best-fitting damped sinusoids.

Figure 2

[(a) and (c)] ${\alpha }_{\mathrm{eff}}$ as a function of $t_{\mathrm{NM}}$ for Co/Pt and ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$/Pt, respectively. (b) Frequency and saturation magnetization as a function of $t_{\mathrm{NM}}$ and $\sim 1.4$ kOe of magnetic field strength; it shows a similar trend in their variations. The shaded bar indicate the Pt thickness where the Pt became continuous.

Figure 3

Damping coefficient ${\alpha }_{\mathrm{eff}}$ as a function of precessional frequency for 10-nm Co films capped with 0.6-nm and 2-nm Pt. The extrinsic damping decreases with the increasing Pt thickness until it reaches to negligible extrinsic effect at $t_{\mathrm{NM}}=2$ nm.

Figure 4

Schematic illustration of the growth of discontinuous to continuous NM capping layer. (a) Experimental damping data for Co/Pt and Co/Au as a function of $t_{\mathrm{NM}}$. The circular point is a literature value for pure cobalt. (b) Theoretical variation in damping data for Co/Pt and Co/Au adapted from Ref. [3] as a function of $t_{\mathrm{NM}}$.