NUCLEATION, GROWTH AND MAGNETIC-PROPERTIES OF EPITAXIAL FEAL FILMS ONALAS GAAS/

An investigation of the nucleation and growth as well as magnetic prop erties of epitaxial FexAl1-x on AlAs/GaAs(100) is reported. In situ RH EED and UHV STM were used to characterize the surface and ex situ MOKE measurements were used to characterize the magnetic properties. We fo und that epitaxial films can be grown over a broad composition range, 0.5<x<0.8, provided the appropriate nucleation procedure is used, most important of which is the deposition of more than 90 Angstrom of FexA l1-x before annealing. STM images taken at 1 and 3 bilayers of growth reveal a surface covered in small islands 40 Angstrom in size and 2 An gstrom high. After depositing 90 Angstrom and annealing, STM images re veal a much smoother surface with atomically flat terraces greater tha n 100 Angstrom in size. STM images also showed step heights correspond ing to the height of an FexAl1-x bilayer. In addition, the annealed fi lms exhibited a (2 x 2) and/or a (5 x 5) surface reconstruction as det ermined by RHEED. The (2 x 2) reconstruction was seen after annealing the films to 550 degrees C and the (5 x 5) reconstruction would begin to show up after annealing films of composition x>0.55 to 650-700 degr ees C. STM images of the (5 x 5) surface revealed a much lower step ed ge kink density than the (2 x 2) surface. When growing FexAl1-x on an annealed FexAl1-x surface, RHEED oscillations were found to occur in t wo distinct modes, monolayer and bilayer, where monolayer growth would occur at x>0.7 and bilayer growth at x<0.7. Excess Fe on the annealed surface, such as what is encountered on a (5 x 5) surface, could forc e a bilayer growth mode to monolayer growth mode for several layers of growth. This was due to the excess Fe getting incorporated into the g rowth front. Magnetic measurements showed samples above x=0.7 to be fe rromagnetic with magnetization in-plane. A compositional dependence on coercivity and saturation magnetization was also found where higher F e concentrations corresponded to higher coercivities and higher satura tion magnetizations.