Thin films consisting of alloys of immiscible metals display a unique form of compositional ordering not observed in the bulk. Utilizing scanning tunneling microscopy and low energy electron diffraction, we have examined thin films of Co h Ag 12h and Fe h Ag 12h grown on Mo (110) surfaces. Both of these systems are observed to form a compositionally ordered alloy of alternating, contiguous stripes of Co (or Fe) and Ag with the long axis of the stripe coinciding with the Mo[001] direction in the plane of the substrate. The average stripe periodicities are on the order of 2.0 to 2.5 nm along the Mo͓110͔ (perpendicular to the stripes) direction depending on film stoichiometry. [S0031-9007(98)06982-8] PACS numbers: 68.35. -p, 61.16.Ch, 68.55. -aThe ability to induce lateral superlattices via selfassembly has been recently demonstrated for several semiconductor systems [1] which undergo spontaneous composition modulation with lateral length scales on the order of 10-100 nm. In bimetallic systems, no such behavior has been reported to date although a variety of methods for artificially generating such long period structures has been investigated [2,3]. A step toward the formation of compositionally modulated metal structures was taken by Hwang [4] who demonstrated that submonolayer films consisting of two immiscible metals (Co 1 Ag) grown on the Ru(0001) surface remain in phase separated structures to reduce the strain of the overlayer induced by the substrate. Tersoff [5] examined similar segregation effects for surfaces composed of two immiscible metals with large atomic size mismatch. It was observed that the reduction in strain energy gained by forming a dispersed alloy in the surface layer is opposed by the increase in interface energy arising from the heterogeneous bonds formed between the two immiscible metals. If the interface energy is large enough, lateral segregation occurs in the surface.In this paper, we demonstrate the ability to form compositionally modulated structures through self-assembly of immiscible metals when grown on the twofold symmetric surface of Mo(110) single crystals. Furthermore, we show the extension of this composition modulation into three-dimensional structures up to a thickness of four atomic layers. Specifically, the mixtures Co h Ag 12h and Fe h Ag 12h on Mo(110) spontaneously form a stable composition modulation in which stripes of Ag and of Co or Fe are aligned with the Mo[001] direction and alternate with a period that varies from 2.0 to 2.5 nm. Using scanning tunneling microscopy (STM) and low energy electron diffraction (LEED), we show that these stripe phases are stabilized by the reduction of strain energy and that they extend to multiple atomic layers, with Ag growing predominantly on Ag, and Co on Co (or Fe on Fe), in the second and third layers.The STM and reverse view LEED measurements were carried out in an ultrahigh vacuum system with a base pressure of P ഠ 1 3 10 210 mbar. Sample contamination and stoichiometry were determined by quantitative Auger electron spectrosco...

We report on the martensitic fcc-bcc phase transformation for Fe films grown at room temperature on Cu(100) in a thickness range from 10 to 20 monolayers. bcc(110) grains are highly elongated along (011)t" II [111]b, and exhibit characteristic island growth with steps along [001]b". The martensitic (i.e., sudden, collective, nondifFusive) transformation in the bulk of the film is demonstrated by STM topography, including the behavior of steps at grain boundaries and the presence of tilted surfaces arising from fcc-bcc interfaces.

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