Identification of the Mn structure in Mn/Ag(001) superlattices

Abstract

We have examined the structure of Mn in coherent ultrathin-film (Mn/Ag) superlattices by application of conversion-electron extended x-ray absorption fine structure (CEEXAFS), which is a near-surface variant of extended x-ray absorption fine structure (EXAFS). The superlattices were grown by molecular beam epitaxy with 20 to 100 periods, the periods consisting of 1, 2, 3, 5, and 14 monolayers (ML’s) of Mn and 10–15 ML’s of Ag. Exploiting the single-crystal nature of the samples and the polarization properties of synchrotron radiation, we have found that the Mn structure for superlattices with 3 ML Mn layers, corresponding to one complete unit cell, is nearly fcc with a c/a ratio of 1.4±0.1 (for fcc, c/a=√2). For a superlattice with 5 ML Mn layers, corresponding to two complete unit cells, the out-of-plane spacing is decreased to give a body-centered-tetragonal structure with a c/a ratio of 1.13±0.06. No measurable change occurs in the Mn structure as the Mn layer thickness increases further to 14 ML’s. Above 14 ML’s, the Mn film cannot maintain the single-crystal phase and loses long-range order. A detailed analysis of the CEEXAFS data shows a continuous increase of the disorder within the films.