Contents lists available at ScienceDirect Physica B: Condensed Matter journal homepage: www.elsevier.com/locate/physb Influence of annealing on spin pumping in sputtered deposited Co/Pt bilayer thin films Akash Kumar a , Nidhi Pandey b , Dileep Kumar b , Mukul Gupta b , Sujeet Chaudhary a , Pranaba Kishor Muduli a,* a Department of Physics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India b UGC-DAE Consortium for Scientific Research, University Campus, Khandwa Road, Indore 452001, India ARTICLE INFO Keywords: Ferromagnetic resonance Co/Pt bilayers High temperature annealing Spin mixing conductance ABSTRACT We report on the ferromagnetic resonance study of the Co/Pt thin films as a function of the post growth an- nealing. We found a significant increase in the spin mixing conductance, Gilbert damping parameter and in- homogeneous linewidth broadening with increase in annealing temperature. This behavior is correlated to the structural degradation of Co layer and in particular formation of Co x Pt 1-x alloy at the interface, which gives rise to increase in dissipation of spin current at the interface. The results are important for understanding of spin- orbit torque in Co/Pt bilayer structures. 1. Introduction Ferromagnet/Heavy metal (FM/HM) systems have attracted con- siderable attention due to the presence of spin-orbit torques at the in- terface [1,2]. The spin-orbit torque allows the manipulation of mag- netization of the ferromagnet when sufficiently high current density is used [1,3,4,5]. Hence understanding of spin-orbit torque is of con- siderable technological interest for magnetic random access memories [6,7] as well as for high-frequency microwave devices that uses auto- oscillation of magnetization [8–13]. One of the basic requirements of such applications is a large density of spin current, which can be gen- erated by methods such as spin pumping [14–16], spin Hall effect [17–21], spin Seeback effect [22] etc. In the case of spin pumping, a spin current is generated when the magnetization of the FM in contact with the HM is driven into ferromagnetic resonance (FMR). The mag- nitude of injected spin current depends on the so-called “spin-mixing conductance” [15]. Recent reports suggest that spin pumping and hence spin mixing conductance can be affected by the details of the interface structure [23,24]. In this work, we study the spin mixing conductance of Co in Co/Pt bilayers as a function of post-growth annealing temperature. Such post- growth treatment is expected to affect the spin-orbit torque and hence knowledge of the damping constant and spin mixing conductance as a function of annealing temperature (T A ) will be very useful for various technological applications. We show that the high temperature an- nealing can alter the interface between Co and Pt, which can change the Gilbert damping and spin mixing conductance. We correlate these changes with the structural degradation of the bilayer and in particular the formation of Co x Pt 1-x alloy at Co/Pt interface, which results in dissipation of spin current at the interface and decrease in spin back flow. 2. Experimental details Co/Pt bilayer structures were prepared using DC magnetron sput- tering on n-type Si (100) substrate at working pressure of 3 × 10 -3 mbar. The base pressure of the sputtering system was less than 1 × 10 -7 mbar. The Si substrates were first cleaned using RF bias of power 50 W for 15min. We used 3-inch sputter targets with purity of 99.99% for better uniformity over a large area of the substrate. We also performed pre-sputtering for about 15 min before every deposition to remove any impurity from the surface of the target. Substrate was kept masked during this cleaning/pre-sputtering process. The Co and Pt thin films were deposited at growth rates of 0.074 Å/s and 0.41 Å/s, re- spectively. This bilayer system is then annealed at 300 °C, 400 °C and 500 °C for 1h in high vacuum (< 5 × 10 -6 mbar) in a separate chamber. Grazing incidence X-Ray diffraction (GI-XRD) measurements are performed for the determination of crystalline phase of as-deposited bilayer structure using Cu-K α radiation. The thickness and interface roughness of the samples are obtained using X-ray reflectivity (XRR) measurements. Surface topography and average roughness has been determined using non-contact mode of atomic force microscope (AFM). https://doi.org/10.1016/j.physb.2019.06.048 Received 31 December 2018; Received in revised form 17 June 2019; Accepted 21 June 2019 * Corresponding author. E-mail address: muduli@physics.iitd.ac.in (P.K. Muduli). Physica B: Condensed Matter 570 (2019) 254–258 Available online 21 June 2019 0921-4526/ © 2019 Elsevier B.V. All rights reserved. T