Low Relaxation Rate in Epitaxial Vanadium-Doped Ultrathin Iron Films

The longest relaxation time and sharpest frequency content in ferromagnetic precession is determined by the intrinsic (Gilbert) relaxation rate $G$. For many years, pure iron (Fe) has had the lowest known value of $G=57\mathrm{MHz}$ for all pure ferromagnetic metals or binary alloys. We show that an epitaxial iron alloy with vanadium (V) possesses values of $G$ which are significantly reduced to $35\pm 5\mathrm{MHz}$ at 27% V. The result can be understood as the role of spin-orbit coupling in generating relaxation, reduced through the atomic number $Z$.

Figure 1

Measurement of intrinsic damping parameter $\alpha$ and relaxation rate $G$ in $\mathrm{MgO}/{\mathrm{Fe}}_{1-x}{\mathrm{V}}_x(8\mathrm{nm})$, $x=0$ and 0.27. Inset: swept-frequency FMR linewidths $\Delta f$; lines show model calculations for different levels of inhomogeneous broadening.

Figure 2

Angular dependence of field-for-resonance $H_{\mathrm{res}}$ at 10 GHz, cavity measurement. Lines are fits to extract cubic anisotropy constants $K_1$ after 19. Inset: $K_1$ values.

Figure 3

Extraction of spectroscopic factor $g_{\mathrm{eff}}$ from variable-frequency FMR measurement, $\mathrm{MgO}/{\mathrm{Fe}}_{1-x}{\mathrm{V}}_x$ (8 and 50 nm) films. Inset, top left: VSM measurement of $4\pi M_s$. Inset, lower right: extracted $g_{\mathrm{eff}}$ values for 8 and 50 nm films.

Figure 4

Compositional dependence of (a) moment $4\pi M_s$, (b) gyromagnetic ratio $g_{\mathrm{eff}}$, and (c) Gilbert relaxation rate $G$ for $3d$ transition metal ferromagnetic substitutional alloys. Solid line: atomic spin-orbit coupling parameter $\xi$. (d) Relaxation $G$ as a function of spin-orbit coupling ${\xi }^2$. Data for FeV films, this work, for bcc or fcc ${\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x$, Ref. 16; for ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}:\mathrm{Cu}$ films, [20]; for Ni whiskers, [15]. The Heusler compound NiMnSb data [7] are included for comparison.