Manipulation of Gilbert Damping Parameter by Annealing Sputtered Deposited Co/Pt Bilayer Thin Films

Ferromagnet/ heavy metal (FM/HM) systems have attracted considerable attention due to the presence of current-induced spin-orbit torques (SOT) at their interface. The SOT allows the manipulation of magnetization of the ferromagnet when sufficient high current density is used [1, 2]. An important parameter for potential applications of these heterostructures is the Gilbert damping constant of the FM, which can be affected by the details of the interface. Here we study the damping constant of Co thin films in Co/Pt multilayers as a function of post-growth annealing. Such post-growth treatment is expected to affect the SOT and hence knowledge of the damping constant as a function of annealing temperature $( \mathrm {T}_{A})$ is useful for applications. Co (10 nm)/Pt (5 nm) bilayer structure was prepared using DC magnetron sputtering on a Si (100) n-type substrate at 3 mTorr working pressure with base pressure of $< 1 \times 10 ^{-7}$ mbar. The Si substrates were cleaned using RF cleaning with RF bias of 50 W for 15 min. We used 3-inch sputter targets with 99.99% purity for better uniformity over a large area of the substrate. We also performed pre-sputtering for 2 minutes before every deposition. Co and Pt thin films were sputtered at a very low growth rate of 5Å/min and 25Å/min, respectively. Structural characterization of as-deposited Co/Pt bilayers was performed using X-ray Diffraction (XRD) and X-ray reflectivity (XRR) measurements (Fig. 1). XRD spectra show the polycrystalline growth of Co/Pt bilayer thin film. XRR measurements show the high-quality growth of thin film with roughness $< 1$ nm, this is further confirmed by atomic force microscopy (AFM). After growth, samples were annealed at zero field in a high vacuum chamber for 300, 400 and 500 ଌ with vacuum $< 5 \times 10 ^{-6}$ Torr for 1hour. Figure 1 (c)–(e) shows AFM images for as-deposited and post-annealed samples. The AFM measurements show an increase of grain size with increase in annealing temperature, $\mathrm {T}_{A}$, which predict high inter-diffusion of Pt and Co thin films with increase in $\mathrm {T}_{A}$ [3, 4] Figure 2 (a) shows ferromagnetic resonance (FMR) spectra of as-grown Co/Pt bilayer thin films for 9 GHz RF frequency, the red line is fitted with derivative of the Lorentzian equation, fitting provides the value of linewidth and resonance field for a particular fixed frequency. Similar measurements are repeated for a frequency range of 6–12 GHz. The FMR linewidth vs. frequency was fitted to determine the Gilbert damping constant $( \alpha _{eff})$. Frequency vs. resonance field was fitted with Kittle’s formula to determine effective saturation magnetization $( \mathrm {M}_{eff})$ [5]. Gilbert damping parameter is found to increases with increase in $\mathrm {T}_{A}$ while the effective saturation magnetization shows opposite behavior [Fig. 2(b)]. No FMR signal was observed for the sample annealed at $\mathrm {T}_{A}= 500 ^{circ}\mathrm {C}$. These results can be understood by possible intermixing of Pt and Co at the interface. This study predicts that Gilbert damping constant of Co/Pt bilayer thin films can be manipulated by annealing the heterostructures at high temperature. We will show a detailed study of interface and inverse spin Hall effect measurements for understanding the possible effect on SOT due to annealing.