Thickness dependent perpendicular magnetic domain patterns in sputtered epitaxial FePt(001) L1₀ films

The present paper discusses the magnetic anisotropy and magnetic domain structure of highly ordered epitaxial FePt(001) films grown on Pt seeded MgO(001) substrates. These films were grown by dc-magnetron sputtering from a Fe₅₀Pt₅₀ alloy target at a substrate temperature of 550 °C during deposition. Thicknesses were varied between 15 and 170 nm. The presence of the highly anisotropic face centered tetragonal L1₀ crystal structure with a maximum long range chemical ordering of 95% and a low degree of misorientations was confirmed by specular and grazing incidence X-ray diffraction measurements. For film thicknesses ≥ 50 nm in-plane and out-of-plane hysteresis measurements indicate large perpendicular magnetic anisotropy and at the same time low remanent magnetisation. Magnetic force microscopy reveal highly interconnected perpendicular stripe domain patterns. From their characteristic width, which is strongly dependent on the film thickness, a value of the dipolar length, D₀, of 50 ±5 nm is derived. Assuming an exchange constant of 10^-6 erg/cm, this value is consistent with an anisotropy constant K_U approx. 1.10⁸ erg/cc.