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Pt, Au, Ir(110) and some Pt-metal alloys exhibit (1x2) or (1x3) missing row (MR) reconstructions, with a regular distribution of deep troughs. Such structures look quite attractive to grow atomic chains of magnetic species, isolated between the surface metal dense rows. The presence of another metal around Pt sites was shown to provide them with special catalytic properties connected to the number of neighbours of the other species [1]. Similar mixed distributions, easy to reproduce at the (110) surface of reconstructed metals or alloys, are likely to modify the magnetic moment, too.
The submonolayer growth of Mn on Pt(110)-(1x2) was studied by surface x-ray diffraction. At room temperature, Mn fills in the empty rows of the substrate's missing row structure. For 0.5 ML, a (1x2) MnPt surface alloy is formed, with alternating pure Pt and pure Mn dense rows. Upon annealing, another surface alloy forms with (2x1) symmetry : as in a bulk Pt3Mn(110) layer, Pt and Mn sites alternate along the dense rows. This latter ordered bulk phase forms when annealing a thicker Mn film (3 ML). The top layer of both surface alloys is corrugated, with Mn lying 0.19 Å and 0.16 Å above the Pt sites for the (1x2) and (2x1) phases respectively. These periodicities are at variance with the Mn-Ni and Mn-Cu cases where c(2x2) arrangements are found.
Our DFT calculations show that the (1x2) alloy is metastable and forms antiferromagnetic Mn chains. The stable (2x1) 2-D alloy is also antiferromagnetic, but the coupling switches to ferromagnetic at the surface of a thick Pt3Mn(110) film albeit with a quite identical surface structure. The calculated magnetic moment per surface Mn atom is close to 4 μB, in all cases. This value, one of the largest ever found in similar metal-Mn surface alloys, is directly related to the surface corrugation and to the Mn volume, as already observed for other Mn-based surface alloys.
[1] Y. Gauthier, M. Schmid et al. , Phys. Rev. Lett. 87 (2001) 36103
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