Low-coverage nonpseudomorphic and metastable structures of magnetic Fe on Cu(001)

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Abstract
The detailed structure of a 3-ML Fe film grown epitaxially at 100 K and annealed to room temperature on a Cu(001) substrate was determined by high-resolution medium-energy ion scattering. Each layer of the Fe film has large glide distortions along fcc 〈110〉 directions which are precursors to the martensitic shear transformation of the fcc Fe to bcc Fe. The glide causes vertical expansions of each Fe layer and significant local atomic disorder of the Fe atoms in the film and the Cu substrate atoms near the interface. Unlike previous studies it is found that the Fe in-plane nearest-neighbor distance is about 1.5% smaller than the Cu(001) substrate. These structural features demonstrate the strong energetics that drive the Fe towards a bcc structure. The structural model derived from the ion scattering data is compared to the model obtained from a recent low-energy electron-diffraction study. The structural results have a strong correlation with the measured magnitude of the magneto-optic Kerr effect hysteresis loops at magnetic saturation.