Role of antisite disorder on intrinsic Gilbert damping in $L{1}_0$ FePt films

The impact of antisite disorder $x$ on the intrinsic Gilbert damping ${\alpha }_0$ in well-ordered $L{1}_0$ FePt films is investigated by time-resolved magneto-optical Kerr effect. The variation of $x$ mainly affects the electron scattering rate $1/{\tau }_{\mathrm{e}}$, while other leading parameters remain unchanged. The experimentally observed linear dependence of ${\alpha }_0$ on $1/{\tau }_{\mathrm{e}}$ indicates that spin relaxation is through electron interband transitions, as predicted by the spin-orbit coupling torque correlation model. Measurements at low temperature show that ${\alpha }_0$ remains unchanged with temperature even for FePt with very high chemical order, indicating that electron-phonon scattering is negligible. Moreover, as $x$ decreases, the perpendicular magnetic anisotropy increases, and the Landau $g$ factor exhibits a negative shift due to an increase in orbital momentum anisotropy. Our results will facilitate the design and exploration of magnetic alloys with large magnetic anisotropy and desirable damping properties.

Figure 1

Schematic of TRMOKE measurement geometry (a), static magnetic hysteresis loops measured by VSM (b), structure characterization results of FePt thin films by XRD (c), and antisite disorder percentage $x$ as a function of growth temperature $T_g$ (d). The insets in (d) depict FePt alloy structure with low (left) and high (right) antisite disorder concentration.

Figure 2

SEM results from FePt thin films grown at $T_g=620{}^{\circ }\mathrm{C}$ (a) and $T_g=720{}^{\circ }\mathrm{C}$ (b).

Figure 3

TRMOKE data from FePt thin films with different $x$ and applied magnetic field $H=6.5\mathrm{T}$ (a) and with $x=4\%$ measured at different $H$ (b). The dependence of spin precession frequency $f$ (c) and effective Gilbert damping ${\alpha }_{\mathrm{eff}}$ (d) on $H$ obtained from Eqs. (2) and (4) for FePt thin films with different $x$. The solid lines refer to fitted results using Eqs. (2) and (3).

Figure 4

Landau $g$ factor (a), intrinsic Gilbert damping ${\alpha }_0$ (b), and ${\alpha }_0/g$ (c) as a function of antisite defect concentration $x$ for FePt films with thicknesses of 17 nm and 22 nm.

Figure 5

The $f$ and $1/\tau$ as functions of temperature $T$ with applied field $H=5\mathrm{T}$ (a) and as functions of $H$ at 20 K (b). The solid lines refer to fitted results using Eq. (3).

Figure 6

Perpendicular magnetic anisotropy $K_u$ as a function of antisite disorder percentage $x$.