The metastable bcc phase of ultra-thin Ni layer on Fe(001) studied by scanning tunneling microscopy

The interest in magnetic materials has been spurred by the ability to realize metastable phases experimentally through epitaxial growth on single-crystal substrates acting as templates. Examples include face-centered-cubic (fcc) iron and body-centered-cubic (bcc) nickel. Phase diagrams derived from total-energy calculations for these bulk metastable structures predict a stable magnetic state at some lattice constant. In the ultrathin-film region [one to two monolayers (ML)], interesting properties, such an enhanced moments, have also been predicted. In the past, the structure of the bcc phase of nickel grown epitaxially on Fe(001) substrates has been studied using low-energy electron-diffraction (LEED) and reflection high-energy electron-diffraction (RHEED) techniques. The bcc-fcc Ni transition is interpreted, at the present state-of-art, in terms of a not-continuous Bain's path or equivalent more detailed transformations. It means that, up to about six layers, the nickel is found to grow in a bcc manner, and above this thickness a slight distortion occurs, eventually leading to c(2X2)-like LEED and RHEED patterns. The latter is consistent with a distortion and rotation of the Ni super-structure, respect to the Fe(001) substrate. In this talk we show, for the first time, the morphology evolution of Ni ultrathin and thin films (up to 40ML) on clean Fe(001) at RT. We used an in-situ scanning tunneling microscopy (STM) to check the surface evolution during each step of Ni growth in our molecular beam epitaxy (MBE) chamber. We observe an intermediate new phase transition between the Ni bcc (1x1) and the fcc c-(2x2). The latter has been interpreted in terms of a Ni surface cell distortion not followed by the rotation. Our findings can help the current debate about the microscopic changes that occur in the formation of the Ni/Fe interface.