Selective Study of Fe Atoms at the Interfaces of an Fe/Ir(100) Superlattice by means of Diffraction Anomalous Fine Structure

Abstract

Studies of superlattices are important in order to synthesize 3D metals in a new crystalline structure which may exhibit exotic magnetic properties. The challenge is to relate these properties to the details of the chemical gradient and the local strain. We report the use of diffraction anomalous fine structure (DAFS) spectroscopy and a crystallographic analysis of the DAFS data to separate the x-ray-absorption fine structure (XAFS)-like information about Fe atoms located at different crystallographic sites of an Fe͞Ir(100) superlattice. We obtained the first and the second (in the plane of growth) nearest-neighbor distances of Fe atoms at the Ir-Fe interface. [S0031-9007 (97)02789-0] PACS numbers: 61.10.-i, 07.85.Jy, 78.70.DmDiffraction anomalous fine structure (DAFS) spectroscopy provides a site-selective x-ray-absorption fine structure (XAFS)-like information about the local atomic environment and the chemical state of the "edge atom" through resonant x-ray scattering and diffraction. Several years after a few pioneering works [1], Stragier et al. [2] presented an elegant demonstration of this method on a copper single crystal and later, several groups applied it on different types of samples like thin films, superlattices [3,4], powders [5], and single crystals [6]. Most results were obtained with materials of which structural information could be extracted with single site-selective Bragg reflections. Site-selective information with real samples was successfully extracted in the DANES (diffraction near edge structure) region of the spectrum; however, due to insufficient statistics no information in the EDAFS (extended diffraction anomalous fine structure) region could be obtained [6]. Regarding superlattices, only the spatial selectivity of the DAFS has been used. The first DAFS experiments on several pseudomorphic Fe͞Ir(100) superlattices were carried out at the Ir edge to determine separately the Ir crystallographic parameters in the buffer and the superlattice [4] (spatial selectivity). We present here a crystallographic-based site-selective EDAFS study of an Fe͞Ir(100) superlattice to separate the XAFS-like information about Fe at the Ir-Fe interfaces.Fe͞Ir superlattices were synthesized to stabilize Fe in new crystalline structures with the aim of obtaining new magnetic properties. Different strained Fe͞Ir structures may be synthesized by varying the thicknesses of the Fe and Ir layers [7]. It has been shown that Fe relaxes into a bcc structure (as bulk Fe) in thick Fe layers (more than five atomic planes) grown at most on three Ir atomic planes, whereas in thin Fe layers (less than five atomic planes) Fe has a body centered tetragonal structure (bct) with a c͞a ratio near 1.25 [8] (pseudomorphic regime). For intermediate thicknesses with thick Fe and Ir layers(more than five atomic planes each) the Fe structure is still not accurately known. High resolution electron microscopy experiments showed that the superlattices are damaged with the disappearance of 2D growth and the presence of a large amount of...

Resonant elastic X-ray scattering: Where from? where to?

“…DAFS methods have evolved towards applications dedicated to materials sciences, such as element selective observations of interfaces [61]. Similarly, it has been shown that DAFS spectroscopy combined with MAD can be exploited to obtain structural information of nanostructures, in particular nano-islands or nanowires grown on Si (001) [62].…”

Diffraction anomalous fine-structure spectroscopy at beamline BM2 at the European Synchrotron Radiation Facility

“…Because of this, DAFS may distinguish the valence and local environment of atoms of the same atomic number located on different crystallographic sites. This has been used, for example, to recover the Fe scattering anomalous terms of the tetrahedral and octahedral sites in magnetite [Pickering et al, 1993a], the local environment of Cu1 and Cu2 sites in the superconductor Y Ba 2 Cu 3 O 7 [Cross et al, 1997, Cross et al, 1998b, and the interface structure of an Ir/Fe superlattice [Renevier et al, 1997]. In the special case of magnetite, there exist Bragg reflections strictly sensitive either to the octahedral or tetrahedral sites and without contribution from the other site.…”

X-ray resonant powder diffraction

“…However the EDAFS technique (combined or not with DANES) seem to be far more powerful when applied to thin films, to determine the local environment of Cu sites in the superconductor YBa 2 Cu 3 O 7 [45,46] or to investigate the interface structure of an Ir/Fe superlattice [47].…”