Abstract: An Fe—Co—Si alloy magnetic thin film contains, in terms of atomic ratio, 20% to 25% Co and greater than 0% to 20% Si. The Fe—Co—Si alloy magnetic thin film primarily has a body-centered cubic crystal structure. Among three <100> directions of the crystal structure, one of the three <100> directions is perpendicular to a substrate surface and the other two <100> directions are parallel to the substrate surface. The Fe—Co—Si alloy magnetic thin film deposited onto MgO (100) has suitable magnetic properties, that is, a high magnetization of 1100 to 1725 emu/cc, a coercive force of less than 95 Oe, and an effective damping parameter of less than 0.001.