Tunable permalloy-based films for magnonic devices

Abstract

Using both broadband ferromagnetic resonance (FMR) spectroscopy and ab initio calculations, we study the magnetodynamic properties of permalloy $(Py,{\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20})$ and ${\mathrm{Py}}_{100\ensuremath{-}x}{\mathrm{M}}_{x}$ films with M as platinum (Pt), gold (Au), or silver (Ag). From the uniform FMR mode, we extract the saturation magnetization $({M}_{S})$, damping $(\ensuremath{\alpha})$, and inhomogeneous broadening $(\mathrm{\ensuremath{\Delta}}{H}_{0})$; from the first perpendicular standing spin-wave (PSSW) mode, we extract the exchange stiffness $(A)$. ${M}_{S}$ and $A$ are found to decrease with increasing alloying, most strongly for Au and less so for Pt. On the other hand, $\ensuremath{\alpha}$ increases rapidly with both Pt and Au content, while being virtually independent of Ag content. The physical origins of the observed trends in $\ensuremath{\alpha}$, ${M}_{S}$, and $A$ are analyzed and explained using density functional theory calculations in the coherent potential approximation. The calculated trends quantitatively agree with the experimental observations. The drastically different impacts of Pt, Au, and Ag on the various fundamental magnetodynamic properties will allow for significant design freedom, where different properties can be varied independently of others through careful combinations of the Pt, Au, and Ag contents of ${\mathrm{Py}}_{100\ensuremath{-}x}{\mathrm{M}}_{x}$ films. By empirical approximations of each property's concentration dependence, we can dial in any desired combination of magnetodynamic properties within this parameter space. As a proof-of-principle demonstration we design a set of ${\mathrm{Py}}_{100\ensuremath{-}x\ensuremath{-}y}{\mathrm{Pt}}_{x}{\mathrm{Ag}}_{y}$ films, where the saturation magnetization stays constant throughout the set and the damping can be tuned by a factor of 4.