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The growth of Fe(110) revisited for the fabrication of self-organized nanostructures

We have revisited the epitaxial growth of Fe(110) on refractory metals such as Mo(110) and W(110), with a view to explore extensively the possibilities of self-organization. Various dots and wires with a size ranging from the micron-scale to below ten nanometers have been achieved.

Fe is a body-centered-cubic (bcc) material. Fe deposited on surfaces of refractory metals, mostly W along the (110) orientation, is one of the most-studied systems in surface science for magnetic materials. W(110) was chosen because of its high surface energy favoring wetting, and its immiscibility with magnetic materials. The first studies down to the monolayer range were pioneered by U. Gradmann and collaborators, as early as the beginning of the 80’s. Self-organization has become an active topic of research over the past ten years. With the view of producing nanostructures we have revisited the epitaxial growth modes of Fe on bcc(110) surfaces. Our deposition technique is Pulsed-Laser Deposition (PDL) performed under Ultra-High Vacuum (UHV) conditions, also sometimes called laser-MBE. All samples are fabricated on metallic buffer layers deposited on commercial Sapphire wafers, which can therefore be used later for ex situ measurements, lithography etc.

Upon deposition at high temperature Fe grows on W(110) and Mo(110) following the so-called Stranski-Krastanov growth mode : after the perfect wetting of one single atomic layer as explained by surface energy arguments, subsequent Fe atoms contribute to the growth of 3D islands so as to reduce the area of contact between Fe and W (or Mo) where an energy-costy interfacial network of dislocations is found to accomodate the lattice misfit. The dots are regularly facetted in relation with their crystallographic orientation. Magnetic features of interest at this micron scale are magnetic domains and domain walls, along with their magnetization processes.
FIG 1 : Fe/Mo(110) micron-size dot self-assembled at 450°C under UHV conditions by Pulsed-Laser Deposition.
FIG 2 : Arrays of Mo(110) trenches
Upon deposition at modereate temperature bcc(110) materials can develop an anisotropic roughness during growth. This instability can be tailored to yield rather well-defined arrays of nanotrenches on the surface, with one single orientation of the array on the whole wafer. These arrays of trenches can be used for the fabrication of magnetic nanowires, which grow at the bottom of the trenches when deposited under suitable conditions. A challenge in nanosized self-organized systems is to retain their ferromagnetic character at room temperature. Most self-organized systems are rather superparamagnetic.

­­Contributors

  • MNM : O. Fruchart, F. Cheynis, A. Rousseau, B. Borca

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Corresponding author : O. Fruchart

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