Temperature dependence of bulk and interface contributions to the magnetic damping of Permalloy thin films

The magnetic damping of <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) as a function of the Py layer thickness for two different thickness series, Al/Py/sapphire and Al/Py/Al/sapphire, respectively. Additional magnetic and structural characterization is done by superconducting quantum interference device magnetometry as well as x-ray reflectivity and transmission electron microscopy. The frequency dependence of the FMR linewidth allows us to separate the Gilbert-like contributions from non-Gilbert-like ones like two-magnon scattering (TMS) processes. The thickness dependence allows us to separate the derived magnetic parameters such as saturation magnetization <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"> <b:msub> <b:mi>M</b:mi> <b:mi mathvariant="normal">s</b:mi> </b:msub> </b:math> , TMS contribution <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"> <d:mi mathvariant="normal">Γ</d:mi> </d:math> , and Gilbert damping parameter <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"> <f:mi>α</f:mi> </f:math> into their respective bulk and interfacial contributions. While the bulk contribution <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"> <g:msup> <g:mi>α</g:mi> <g:mi>bulk</g:mi> </g:msup> </g:math> monotonously decreases with temperature from 0.0061(1) down to 0.0054(1) for both samples series, the interfacial contribution <h:math xmlns:h="http://www.w3.org/1998/Math/MathML"> <h:msup> <h:mi>α</h:mi> <h:mi>interface</h:mi> </h:msup> </h:math> shows a subsequent increase at low temperature. This is rather pronounced for the Al/Py/sapphire series, caused by a temperature-dependent TMS contribution. For the Al/Py/Al/sapphire series, the increase at low temperatures is much less pronounced, and a temperature-independent TMS contribution is only found for the thinnest sample due to a wavy Py film morphology. Correcting for the TMS contributions allows us to extract <i:math xmlns:i="http://www.w3.org/1998/Math/MathML"> <i:mrow> <i:msup> <i:mi>α</i:mi> <i:mi>bulk</i:mi> </i:msup> <i:mrow> <i:mo>(</i:mo> <i:mi>T</i:mi> <i:mo>)</i:mo> </i:mrow> </i:mrow> </i:math> which reflects the intrinsic magnetic damping properties of sputtered Py thin films.