Nucleation, growth and magnetic properties of epitaxial FeAl films on AlAs/GaAs

Abstract

An investigation of the nucleation and growth as well as magnetic properties of epitaxial FexAl1 - x on AlAs/GaAs(100) is reported. In situ RHEED and UHV STM were used to characterize the surface and ex situ MOKE measurements were used to characterize the magnetic properties. We found 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 Å of FexAl1 - x before annealing. STM images taken at 1 and 3 bilayers of growth reveal a surface covered in small islands 40 Å in size and 2 Å high. After depositing 90 Å and annealing, STM images reveal a much smoother surface with atomically flat terraces greater than 100 Å in size. STM images also showed step heights corresponding to the height of an FexM1 - x, bilayer. In addition, the annealed films exhibited a (2 x 2) and/or a (5 x 5) surface reconstruction as determined by RHEED. The (2 x 2) reconstruction was seen after annealing the films to 550°C and the (5 x 5) reconstruction would begin to show up after annealing films of composition x>0.55 to 650-700°C. STM images of the (5 x 5) surface revealed a much lower step edge 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 two 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 force a bilayer growth mode to monolayer growth mode for several layers of growth. This was due to the excess Fe getting incorporated into the growth front. Magnetic measurements showed samples above x=0.7 to be ferromagnetic with magnetization in-plane. A compositional dependence on coercivity and saturation magnetization was also found where higher Fe concentrations corresponded to higher coercivities and higher saturation magnetizations.