Chemical-order-dependent magnetic anisotropy and exchange stiffness constant of FePt (001) epitaxial films

Anomalous Hall voltage was measured for FePt ${L1\mathrm{}}_0$ films having very high magnetic anisotropy. The magnetic anisotropy $K_1$ and $K_2$ were determined with high accuracy by analyzing the magnetization curves obtained from the Hall voltage measurement. The saturation magnetization $M_s$ of the samples with different chemical-order parameter (S) exhibits a different temperature dependence, implying that the Curie temperature weakly depends on S. The first-order anisotropy $K_1$ gradually increases with S, while the second-order anisotropy $K_2$ remains almost constant of about $5\times {10}^6\mathrm{e}\mathrm{r}\mathrm{g}/\mathrm{c}\mathrm{c}.$ The temperature dependence of $K_1$ is correlated with S, that is, $K_1$ with a small S is more temperature dependent than that with a large S. These behaviors are quite similar to the temperature dependence of $M_s$ with different S, and can be explained by the conventional model based on thermal spin fluctuations. The domain wall energy ${\sigma }_w$ evaluated by the theoretical analysis of the stripe-domain structure tends to increase linearly with S, in a similar manner as that of $K_1,$ whereas the exchange stiffness constant A of about $1\times {10}^{-6}\mathrm{e}\mathrm{r}\mathrm{g}/\mathrm{c}\mathrm{m}$ deduced from ${\sigma }_w$ and $K_u{(=K}_1{+K}_2)$ hardly depends on S.