Spin-pumping in Pt/Py heterostructures: The role of Al spacer layers

The magnetic damping of spin-pumping heterostructures consisting of Pt and <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:msub> <a:mi>Ni</a:mi> <a:mn>80</a:mn> </a:msub> <a:msub> <a:mi>Fe</a:mi> <a:mn>20</a:mn> </a:msub> </a:mrow> </a:math> (Permalloy, Py) thin films is studied via temperature- and frequency-dependent ferromagnetic resonance (FMR). Additional magnetic and structural characterization is done by transmission electron microscopy (TEM), x-ray absorption spectroscopy, and x-ray magnetic circular dichroism (XMCD). The frequency-dependence of the FMR linewidth allows to extract the Gilbert damping parameter <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"> <b:mi>α</b:mi> </b:math> as a function of temperature. Py in direct contact with Pt exhibits a strong enhancement of <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"> <c:mrow> <c:mi>α</c:mi> <c:mo>(</c:mo> <c:mi>T</c:mi> <c:mo>)</c:mo> </c:mrow> </c:math> , and the dependence on the thickness of the Py layer suggests this to be an interfacial effect. The enhanced <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"> <d:mrow> <d:mi>α</d:mi> <d:mo>(</d:mo> <d:mi>T</d:mi> <d:mo>)</d:mo> </d:mrow> </d:math> is accompanied by an induced magnetic polarization of the Pt as evidenced by XMCD, while the other magnetic properties of Py as measured with FMR and static magnetometry remain virtually unchanged. The increase of <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"> <e:mrow> <e:mi>α</e:mi> <e:mo>(</e:mo> <e:mi>T</e:mi> <e:mo>)</e:mo> </e:mrow> </e:math> can be efficiently suppressed by the insertion of an Al-spacer layer between Pt and Py as thin as 1 nm, which coincides with the loss of the magnetic polarization of Pt.