Quantum Well State Induced Oscillation of Pure Spin Currents in Fe=Au=Pdð001Þ Systems Eric Montoya, * Bret Heinrich, and Erol Girt Surface Science Lab, Department of Physics, Simon Fraser University, 8888 University Drive, Burnaby, British Columbia, Canada V5A 1S6 (Received 13 June 2014; published 24 September 2014) Spin pumping at the ferromagnetic metal (Fe)/normal metal (Au) interface and the subsequent spin transport in Au=Pd heterostructures is studied using ferromagnetic resonance. The spin pumping induced damping in the Fe=Pd structure is greatly suppressed by the addition of a Au spacer layer in the structure Fe=Au=Pd. The rapid decrease in the interface damping with an increasing Au layer thickness does not correspond to an expectation based on a simple spin diffusion theory in the Au layer. It is possible to account for this behavior by introducing a partial reflection of spin current at the Au=Pd interface. Furthermore, oscillations in the amplitude of spin pumping damping are observed in the Fe=Au=Pd structure as a function of Au thickness for thicknesses less than half the electron mean free path of bulk Au. This new effect indicates a formation of quantum well states in the accumulated spin density in the Au spacer that affect the time irreversible process of spin pumping. DOI: 10.1103/PhysRevLett.113.136601 PACS numbers: 72.25.Mk, 75.70.Cn, 76.50.+g The generation and transport of pure spin currents is an important topic in spintronics [1,2]. It allows one to transport spin current information without the presence of a net electric charge current (as opposed to spin- polarized currents), thus avoiding problems with capaci- tances, electromigration, and Joule heating. Spin pumping using microwave excitations or thermal gradients across a ferromagnetic layer (FM) allows one to create pure spin currents. For spintronics applications, understanding the propagation of pure spin currents in heterostructures involving both FMs and normal metal layers (NMs), and their associated interconnects, is vital. Furthermore, with ever shrinking device sizes, it is important to consider how quantum size effects may affect pure spin currents. For the remainder of this Letter, such pure spin currents will be referred to as spin currents. The generation and transport of spin currents in simple NM systems, such as Au, Ag, and Cu, [3–6] has been extensively studied by ferromagnetic resonance (FMR) in FM=NM and FM=NM=FM structures and is well described by the standard spin pumping and spin diffusion model. Spin pumping leads to interface damping at the FM=NM interface that can be described by Gilbert phenomenology. There are two alternative (and agreeing) theories of spin pumping using a spin dependent scattering potential at the FM=NM interface: (a) the theory by Tserkovnyak et al. [7] based on the time dependent scattering matrix formalism [8], and (b) the theory by Šimánek and Heinrich [9] based on the time retarded response of spin dependent scattering. The theory of case (b) points out that the instantaneous response of NM electrons at the FM=NM interface leads to static accumulated spin density in the NM, which results in a time reversible static interlayer exchange coupling that exhibits oscillations with the NM thickness. The oscillation length scales are given by the Fermi surface spanning k vectors in the NM [10]. The time retarded response leads to a time irreversible process resulting in interface damping. Considering that the interlayer exchange coupling and spin pumping are generated by the same mechanism and spin pumping also generates an accumulated spin density in the NM [11,12], one might expect that in some structures the spin pumping contribution to the interface damping can in principle show some degree of oscillatory behavior as a function of the NM thickness. This has not been reported, which is not that surprising for FM=NM=FM systems because in FMR with a small angle of precession, the spin current is fully absorbed at the FM=NM interfaces as the FMs act as spin sinks [11–13]. However, oscillatory behavior has not been previously found in FM=NM systems, on which we comment later. It is not obvious that the confined geometry of an ultrathin NM must lead to quantum size effects in spin pumping, considering that this is an irreversible effect. This behavior was predicted in FM=NM structures with chang- ing FM thickness by Mills [14] and Šimánek [15]. However, Zwierzycki et al. [16] showed how this effect would only be notable for an extremely thin FM. No oscillations were so far predicted with changing NM thickness, nor have they been experimentally observed. Therefore, it is challenging to explore a possibility to observe an oscillatory dependence of spin pumping with changing NM thickness. Since in our system the quantum confinement is in 1D, we refer to these effects as quantum well effects in the language of Refs. [14,15]. Since no oscillations have been so far reported in simple FM=NM structures, we have decided to investigate spin pumping in PRL 113, 136601 (2014) PHYSICAL REVIEW LETTERS week ending 26 SEPTEMBER 2014 0031-9007=14=113(13)=136601(5) 136601-1 © 2014 American Physical Society