A magneto-optical study of Heusler alloy Co$_{2}$MnGe films grown on Ge(111) substrates

We report an extensive study of magneto-optic properties and magnetic anisotropy of ternary alloys of Co, Mn and Ge over a wide range of composition near the Heusler alloy compound Co$_{2}$MnGe grown on a single Ge(111) substrate by combinatorial molecular-beam epitaxy. The scanning magneto-optic Kerr effect (MOKE) study reveals a range of room temperature ferromagnetic phases near the Heusler alloy composition. A ridge of higher MOKE intensity near the Co/Mn atomic ratio of two is found, which appears to correlate with the structural perfection of the epitaxial film. The MOKE hysteresis loops in this composition range exhibit asymmetries that are associated with the presence of a large second-order magneto-optic effect. The magnetic anisotropy in the same composition range can be described by a combination of a six-fold and a uniaxial term. A single domain model in conjunction with the second-order MOKE is used to explain the magnetic anisotropy in these alloys. The model reveals a systematic change of magnetic anisotropy and the second-order MOKE coefficients as a function of composition and structural and chemical ordering of the films.