Growth, structure, and magnetic properties of epitaxial NixMn100-x single layers and Co/NixMn100-x bilayers on Cu3Au(100)

The growth and structure of single-crystalline ultrathin NixMn100-x films on Cu3Au(100), and also the magnetic properties of Co ultrathin films on NixMn100-x/Cu3Au(100) have been investigated by x-ray photo-electron and Auger electron spectroscopy, low-energy electron diffraction, reflection high-energy electron diffraction, and magneto-optical Kerr effect (MOKE). For the investigated concentration range (10 <= x <= 77), our results reveal good epitaxial layer-by-layer growth at a substrate temperature of 300 K. A weak c(2 x 2) superstructure was observed at the surface of all NiMn films. The average perpendicular interlayer spacing of the NiMn alloy films decreases from 1.88 angstrom at x = 10 to 1.69 angstrom at x = 77. The lattice parameters of equiatomic NiMn films were determined to be 3.52 angstrom perpendicular to the sample surface and 3.76 angstrom in the sample plane, indicating a face-centered tetragonal (fct) structure as expected for the L1(0) NiMn phase with the c axis along the film normal ("c-axis growth"). For the Co/NiMn bilayers (23 <= x <= 55), MOKE hysteresis loops show a thickness-independent coercivity, suggesting no magnetic coupling at the Co/NiMn interface. Although the structural results indicate the formation of fct NiMn in the equiatomic concentration range, we have no indication of antiferromagnetism for the NiMn films epitaxially grown on Cu3Au(100) at room temperature. This is contrary to the observations on Co/Ni50Mn50 bilayers on Cu(100), where "a-axis growth" of NiMn occurs.