Ultralow Gilbert damping in ${\mathrm{CrO}}_2$ epitaxial films

In this study, we report the observation of ultralow Gilbert damping in epitaxial ${\mathrm{CrO}}_2$ films. The dynamic properties of (100)- and (110)-oriented ${\mathrm{CrO}}_2$ epitaxial films grown on ${\mathrm{TiO}}_2$ substrates were studied using ferromagnetic resonance measurements based on a resonant cavity and a coplanar waveguide in a large frequency range. The Lande $g$ factor was found to be 1.98, and it was independent of film orientation and thickness. The effective damping constant rapidly increased with film thickness when the film thickness was smaller than 50 nm, which might be attributed to magnon scattering. Extremely low damping was observed in the (110)-oriented ${\mathrm{CrO}}_2$ film with a thickness of 364 nm, and the damping constant was obtained as $\left(6.2\pm 0.4\right)\times {10}^{-4}$. This value is about half an order of magnitude lower than that of ultralow-damping CoFe systems $[\left(1.3\text{–}2.0\right)\times {10}^{-3}]$ and is comparable with the lowest value observed recently in some Heusler alloy systems. The extremely low damping behavior in the ${\mathrm{CrO}}_2$ system is strongly correlated with its half-metallic nature.

Figure 1

Schematic of FMR measurement with in-plane and out-of-plane scan mode.

Figure 2

XRD patterns of (110)-oriented (a) and (100)-oriented (b) ${\mathrm{CrO}}_2$ films epitaxially grown on ${\mathrm{TiO}}_2$ substrates with different thicknesses. The vertical dashed lines represent the peak positions of bulk ${\mathrm{CrO}}_2$.

Figure 3

(a) Resonances curve of ${\mathrm{CrO}}_2$ films, where the black curves represent the experimental results. Here, the contributions of both absorption (red circles, antisymmetric Lorentzian function) and dispersion (blue circles, symmetric Lorentzian function) are considered. (b) Frequency dependence of resonance field and fitting results based on Eq. (7)), where $f_{\mathrm{top}}$ denotes the upper limit of frequency used for fitting. (c), (d) Dependence of $f_{\mathrm{top}}$ on $g$ factor for (110)- and (100)-oriented ${\mathrm{CrO}}_2$ films, respectively.

Figure 4

Dependence of resonance field and fitting results (solid lines) on the azimuth angle of external field for (a) (110)-oriented and (b) (100)-oriented ${\mathrm{CrO}}_2$ films with 10-GHz ac field. (c),(d) Dependence of resonance on the polar angle for (110)- and (100)-oriented ${\mathrm{CrO}}_2$ films, respectively. The theoretical results obtained from in-plane fitting parameters are shown by solid lines with the ac field of 8 GHz for (100)-oriented films and 10 GHz for (110)-oriented ones.

Figure 5

Hysteresis loops of hard axis [[010] for (100)-oriented film and [1–10] for (110)-oriented film] and easy axis ([001] for both films) for (a) 52-nm-thick (110)-oriented and (b) 24-nm-thick (100)-oriented films.

Figure 6

Linewidths extracted from resonance absorption curves as a function of resonance frequency for (a) (110)-oriented and (b) (100)-oriented films with different thicknesses. (c), (d) Dependence of thickness on the effective damping constant extracted from linewidth-frequency linear fitting.

Figure 7

Dependence of linewidth on the resonance frequency of 364-nm-thick film for two measurement directions: [1–10] in-plane and [110] perpendicular to the film.