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Epitaxial growth of metastable phases is discussed. A linearized theory predicts new ordered compounds stabilized by the interaction of atomic size differences with epitaxial strain during growth.

We report the operation for Co-Cu alloys of a surface pump mechanism whereby bulk metastable alloys are formed during epitaxial growth. This happens when a material with a larger surface energy (Co) is grown on a film with a lesser surface energy (Cu). The specific pathway transports Cu through pinholes by surface diffusion at temperatures which are too low for the metastable alloy to decompose by bulk diffusion. [S0031-9007(96)01717-6] PACS numbers: 68.55. -a, 81.05.Bx, 81.15.HiNanostructures of successive Cu and Co layers have attracted attention as spin valve systems with giant magnetoresistances (GMR) and consequent technical potential [1]. Both as thin films and nanophases formed by mechanical milling their structural characteristics have proved interesting from a much wider perspective for the marked departures from the equilibrium phase diagram they exhibit [2][3][4][5][6]. Curious new alloying effects have also been observed in monolayer systems with Co and Cu components [6]. All these behaviors are of interest both for understanding the rich growth-related magnetic properties [1] of this system, and for the opportunities they reveal for the synthesis of new and metastable materials. The Co-Cu phase diagram [7], reproduced for the reader's convenience in Fig. 1, is of limited promise, showing only small solubilities of the components all the way from low temperatures to the solidus. Cu is fcc at all temperatures, but Co has a hcp to fcc transition near 400 ± C. The components remain largely unmixed because the bulk free energy G rises towards the 50-50 alloy as indicated in the inset [2] of Fig. 1, where the chemical potentials for both Co and Cu are also shown. Despite these clearly documented energetics, Co-Cu alloys, near 50% in composition, have been observed to form during mechanical milling [2,3], as epitaxial monolayers [6,8], and during the epitaxial growth of attempted multilayer nanostructures at temperatures above 300 ± C [4,5]. Our own attention was focused on the system by quite extraordinary observations on their epitaxial films, described immediately in what follows. This Letter reports a study of novel growth behavior and proposes an explanation for the observations. In our initial work, about 1000 Å of fcc Cu(111) was grown by molecular beam epitaxy on bcc Nb(110) near 200 ± C, using sapphire ͑1120͒ as a substrate and with equipment that is described in other publications [9]. A 500 Å layer of hcp Co (0001) was then grown on top of the Cu at the same temperature. The materials were of reasonable quality, exhibiting streaky reflection high energy electron diffraction (RHEED) characteristic of single crystals with flat surfaces. X-ray diffraction from the Cu films revealed Bragg scans ϳ0.1 ± wide and rocking curves ϳ0.3 ± wide for (111) reflections, which indicate structural coherence lengths ¿10 2 atomic spacings.After the initial Co growth the sample was then heated to 500 ± C and a further 500 Å of Co deposited. X-ray analysis quickly revealed that what had grown on top of the first, pur...

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