Magnetic damping in Fe$_{x}$Co$_{1-x}$ thin films

VELÁZQUEZ RODRÍGUEZ, DANIEL; AVILÉS-FÉLIX, LUIS; GÓMEZ, JAVIER E.; LEIVA, L.; SILVINA BENGIO; ETIENNE GOOVAERTS; A. BUTERA

Bariloche

Simposio; XXIII Latin American Symposium on Solid State Physics; 2018

One of the limiting factors in spintronic devices is the damping coefficient ($alpha$) of the ferromagnetic material. In the special case of ferromagnetic/non magnetic bilayers (FM/NM), designed for the generation and detection of spin currents, a very low damping coefficient is always desired. In this way it is possible to maximize the voltage signal at the ends of the non magnetic metal due to the Inverse Spin Hall Effect (ISHE).In this work we present an experimental study of the static and dynamic properties of the ferromagnetic alloy Fe$_{z}$Co$_{1-z}$ (FeCo) for different values of $z$ close to $z=0.75$. For this purpose we have fabricated, using dc magnetron sputtering at room temperature, two series of bilayers on single crystal Si substrates of the following structure: Si/FeCo(x)/Pt(y) y Si/FeCo(x)/Ta(y), with $x=$3, 5, 7, 9 nm and $y=$4 nm. Samples have been structurally and magnetically characterized using X-ray diffraction, photoelectron spectroscopy, fluorescence, dc magnetization (VSM and Kerr), magnetic force microscopy and ferromagnetic resonance (FMR) in X, K, Q and W bands (9.5, 24, 35 and 94 GHz). From the experimental data we have determined that there is a dependence between the effective anisotropy of the FeCo films with its thickness $d_{FeCo}$, arising probably from the presence of an interfacial anisotropy that tends to align the magnetization perpendicular to the film plane, as deduced from the $1/d_{FeCo}$ dependence. We have also observed that the surface anisotropy is almost twice as large in the case of FeCo/Pt than in FeCo/Ta, which is expected because Pt is often used to induce a perpendicular anisotropy in multilayers.From FMR measurements at different frequencies we have estimated the damping coefficient. In FeCo/Ta bilayers we have found values of $alpha ~ 5 $imes$ 10^{-3}$, which are relatively low for ferromagnetic conductors. Even lower values have been observed in films deposited at higher temperatures. In FeCo/Pt bilayers the values of $alpha$ are considerably larger, which indicate the presence of an additional relaxation mechanism through the interface due to the high value of the spin Hall angle of Pt.