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Understanding the sticking coefficient $\sigma$, i.e., the probability of anadatom sticking to a surface, is essential for controlling the stoichiometryduring epitaxial film growth. However, $\sigma$ on monolayer graphene-coveredsurfaces and its impact on remote epitaxy are not understood. Here, usingmolecular-beam epitaxial (MBE) growth of the magnetic shape memory alloyNi$_2$MnGa, we show that the sticking coefficients for metals ongraphene-covered MgO (001) are less than one and are temperature and elementdependent, as revealed by ion backscattering spectrometry (IBS) and energydispersive x-ray spectroscopy (EDS). This lies in stark contrast with mosttransition metals sticking on semiconductor and oxide substrates, for which$\sigma$ is near unity at typical growth temperatures ($T<800\degree$C). Byinitiating growth below $400 \degree$ C, where the sticking coefficients arecloser to unity and wetting on the graphene surface is improved, we demonstrateepitaxy of Ni$_2$MnGa films with controlled stoichiometry that can beexfoliated to produce freestanding membranes. Straining these membranes tunesthe magnetic coercive field. Our results provide a route to synthesizemembranes with complex stoichiometries whose properties can be manipulated viastrain.

Control of ternary alloy composition during remote epitaxy on graphene