Tunable permalloy-based films for magnonic devices

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 ${\mathrm{Py}}_{100-x}{\mathrm{M}}_x$ films with M as platinum (Pt), gold (Au), or silver (Ag). From the uniform FMR mode, we extract the saturation magnetization $\left(M_S\right)$, damping $\left(\alpha \right)$, and inhomogeneous broadening $\left(\mathrm{\Delta }{H}_0\right)$; from the first perpendicular standing spin-wave (PSSW) mode, we extract the exchange stiffness $\left(A\right)$. $M_S$ and $A$ are found to decrease with increasing alloying, most strongly for Au and less so for Pt. On the other hand, $\alpha$ increases rapidly with both Pt and Au content, while being virtually independent of Ag content. The physical origins of the observed trends in $\alpha$, $M_S$, 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 ${\mathrm{Py}}_{100-x}{\mathrm{M}}_x$ 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 ${\mathrm{Py}}_{100-x-y}{\mathrm{Pt}}_x{\mathrm{Ag}}_y$ films, where the saturation magnetization stays constant throughout the set and the damping can be tuned by a factor of 4.

Figure 1

The FMR measurement setup consisting of the coplanar waveguide inside the electromagnet. Inset: Top view of coplanar waveguide, showing the directions of the static ($H$) and microwave ($h$) fields.

Figure 2

Frequency of the uniform mode (blue points) and the first PSSW mode (red points) as a function of field for (a) Py, (b) ${\mathrm{Py}}_{85}{\mathrm{Ag}}_{15}$, (c) ${\mathrm{Py}}_{85}{\mathrm{Pt}}_{15}$, and (d) ${\mathrm{Py}}_{85}{\mathrm{Au}}_{15}$. Solid lines are fits to Eq. (2). Dashed lines mark the resonance field of the FMR mode and the first PSSW modes at 9 GHz as extracted from the full field scans, which are shown in the insets. Red solid lines in the insets are fits to Eq. (1).

Figure 3

FMR linewidth $\mathrm{\Delta }{H}_F$ for ${\mathrm{Py}}_{85}{\mathrm{Pt}}_{15}$, ${\mathrm{Py}}_{85}{\mathrm{Au}}_{15}$, ${\mathrm{Py}}_{85}{\mathrm{Ag}}_{15}$, and Py films. The solid lines are fits to Eq. (3). Note that the error bars are smaller than the data points.

Figure 4

Left: experimental results for (a) damping, (b) saturation magnetization, and (c) exchange stiffness as a function of Pt, Au, and Ag concentration. Solid lines are empirical fits to second-order polynomials. Right: the same parameters calculated using DFT at room temperature. Solid symbols in (d) include both magnon and phonon contributions, while open symbols include phonons only. Solid symbols in (f) include exchange interactions between M and Py, whereas open triangles (Pt) ignore those. Solid and dashed lines in (d), (e), and (f) are visual guides.

Figure 5

AMR vs composition for ${\mathrm{Py}}_{100-x}{\mathrm{Au}}_x$, ${\mathrm{Py}}_{100-x}{\mathrm{Ag}}_x$, and ${\mathrm{Py}}_{100-x}{\mathrm{Pt}}_x$ films. Solid lines are parabolic fits. Inset: example of an angular measurement together with a fit (red line).

Figure 6

(a) Calculation of the Ag and Pt concentrations for constant values of ${\mu }_0{M}_S$ in multi-alloyed ${\mathrm{Py}}_{100-x-y}{\mathrm{Pt}}_x{\mathrm{Ag}}_y$ films. Red circles in (a) represent the compositions of ${\mathrm{Py}}_{100-x-y}{\mathrm{Pt}}_x{\mathrm{Ag}}_y$ films chosen for our experiment. The inset of (a) shows the resulting ${\mu }_0{M}_S$ of the sputtered films, as determined from FMR, where the dashed line marks the average value of ${\mu }_0{M}_S=0.724\text{T}$. (b) Measurement of the exchange stiffness $A$ and damping $\alpha$ as a function of the Pt concentration for the ${\mathrm{Py}}_{100-x-y}{\mathrm{Pt}}_x{\mathrm{Ag}}_y$ films. Solid lines are guides to the eye.