We first optimized the fabrication of self-organized templates consisting of arrays of trenches aligned along [001] on cc(110) (cc=Mo,W). These are formed upon deposition of body-centered cubic elements (bcc) at moderate temperature (~300-550K depending on bcc), taking advantage of the anisotropy of atomic diffusion at the steps and/or of the Ehrlich-Schwoebel barrier. The kinetic building of roughness is self-limited owing to the relative stability of {210} facets, not noticed previously. Each atomic terrasse of this vicinal facet is three atoms wide. Starting from a liquid-type order, this favors the ordering of the array of grooves. The order was evidenced by satellites on RHEED patterns, whose spacing is in agreement with STM. The lateral period is influenced by the nucleation temperature, while growth proceeds at 150°C. Periods in the range 4-12nm were already demonstrated, with a depth of grooves 0.8-2.5nm, respectively.

Then, upon deposition at 150°C Fe(110) grows layer-by-layer, progressively filling the grooves and thus forming magnetic wires. Auger-Electron Spectroscopy suggests that only partial wetting of the terrasses of the microfacets occurs before Fe wires are formed.

We will report the magnetic properties of Fe wires with a period 10nm, width=7nm and height=1.5nm, deposited on W grooves. The surface was capped with Mo for ex situ measurement with Squid. For W\Fe\Mo the easy axis of magnetization lies along [001], and the mean blocking temperature is 100K. We have started to engineer the anisotropy of the wires using interfacial magnetic anisotropy. When the capping of Mo is replaced with Al the magnetic anisotropy is reduced as expected from this films studies, and so does the superparamagnetic temperature. On the reverse when a Mo bi-layer is inserted between W and Fe the magnetic anisotropy of the wires along [001] is increased. This again is expected from thin films data, as W/Fe interfaces weakensthe anisotropy along [001], while Mo/Fe interfaces reinforce it. The mean blocking temperature could thus be raised to 200K.

Current efforts target first at raising the average blocking temperature above 300K by further engineering the magnetic anisotropy, and second at fabricating a 3D array of wires by sequential deposition.

STM view of W(110) trenches with (a) small and (c) large period, and associated RHEED patterns with the beam azimuth along the trenches. The half-angle of the arrows (approx.18°) reveals the tilt of the facets of the trenches, of type {210}.

Zero-field-cooled/Field-cooled magnetization curves, revealing the blocking temperature for three samples with different cappings or underlayers, as sketched in the insets.

References:
1. Kinetic self-organization of trenched templates for the fabrication of versatile ferromagnetic nanowires, B. Borca, O. Fruchart, Ph. David, A. Rousseau, C. Meyer, cond-mat/0701304

2. Growth modes of Fe(110) revisited: a contribution of self-assembly to magnetic materials, O. Fruchart, M. Eleoui, P.-O. Jubert, Ph. David, V. Santonacci, A. Liénard, F. Cheynis, B. Borca, M. Hasegawa, C. Meyer, J. Phys. : Cond. Mat. 19 (5), 053001 (2007). (topical review)

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