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A new structure for FeMnAs

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Molecular beam epitaxy (MBE) of thin films can lead to the creation of novel phases of matter, not seen in bulk materials, due to the effects both of anisotropic strain induced by a lattice mis-matched substrate, and the competing kinetics of adsorption, diffusion and condensation during the thin film growth. The team “Growth and properties of hybrid multilayer systems” at the INSP, has shown that an interfacial compound FeMnAs forms during the epitaxial growth of Fe/MnAs/GaAs(100) by MBE. The two previously determined structures of the arsenide FeMnAs are tetragonal (T, under normal bulk growth conditions) or hexagonal (H, at high pressure and temperature). In contrast, the interfacial FeMnAs compound grown by MBE has been shown to have an orthorhombic structure, never previously observed. This structural determination was made possible by the use of STEM_HAADF electronic microscopy (High Angle Annular Dark Field Scanning Transmission Electron Microscope).

The multilayer Fe/MnAs/GaAs(100) compound was synthesized by MBE. After oxide desorption under As flux, the GaAs(100) substrate was covered by a GaAs buffer layer grown at 560°C, in order to obtain a flat surface. The sample temperature was then reduced to 230 °C for growth of the MnAs layer. Finally the equivalent of 25 iron monolayers were deposited on the GaAs/MnAs template at a substrate temperature of 160 °C. The interfacial FeMnAs compound subsequently formed by partial intermixing of Fe with the underlying MnAs layer. The stabilization of this orthorhombic structure, which does not exist in bulk FeMnAs (where the T or H phases only are stable), is due to the specific conditions of epitaxial growth by MBE.

STEM_HAADF is a transmission electron microscopy technique which is becoming increasingly used due to its very high performance. This technique allows the atomic columns of crystallized materials to be visualised directly, contrary to phase contrast high resolution microscopy which only provides an image of interferences characteristic of the crystal lattice of the material. With a contrast depending on the atomic number of the observed elements, STEM-HAADF microscopy permits high and low Z elements to be distinguished and the sample images can then be directly compared with the structural models. This has allowed a new compound at the Fe/MnAs interface to be identified in the Fe/MnAs/GaAs(100) system. HAADF images were obtained on an aberration-corrected STEM (Jeol 2200FS) with field emission gun, operating at 200 kV. The probe size was 0.1nm.

GIF Figure 1
(a) STEM-HAADF cross-sectional image of Fe/MnAs/GaAs(001) along [100] axis of MnAs. Filtered images of (b) MnAs layer (c) intermediate compound (d) Fe layer.

By using XEDS (X-ray Energy Dispersive Spectroscopy) the average composition of this interfacial phase was found to be FeMnAs. Its structure was determined by comparing the STEM images (which give the positions and effective masses of the atomic columns) along the [100] and [001] MnAs axes with the arsenide structures known in the literature. A perfect agreement with an orthorhombic Co2P type structure was found, confirmed by a study of the diffraction patterns along the above-mentioned axes.

GIF Figure 2
(a) STEM-HAADF cross-sectional image of the FeMnAs compound obtained by diffusion of the iron in MnAs.
(b) Orthorhombic Co2P type structure

The STEM-HAADF investigation along the [001] MnAs crystal direction revealed the presence, in this orthorhombic FeMnAs structure, of ordered vacancies, which have not been observed in the two stable structures (T and H) of bulk FeMnAs.

GIF Figure 3
(a) STEM-HAADF cross-sectional image of the FeMnAs compound along [001] axis of MnAs. (b) Schematic orthorhombic Co2P type structure along [010] direction. Green and blue filled circles correspond respectively to the pyramidal and tetrahedral sites occupied by the metal atoms. White crosses symbolize the vacancies in the structure.
This structural work has been completed by ab initio calculations performed in collaboration with Argentinian researchers of the CNEA who have shown that this orthorhombic FeMnAs compound is antiferromagnetic. The known T and H structures of FeMnAs are respectively antiferromagnetic and ferromagnetic.

The formation of this interfacial phase might be at the origin of the very low exchange-coupling observed previously between the ferromagnetic Fe and MnAs compounds.

“Structure and Magnetism of Orthorhombic Epitaxial FeMnAs” Dominique Demaille, Gilles Patriarche, Christian Helman, Mahmoud Eddrief, Victor Hugo Etgens, Maurizio Sacchi, Ana Maria Llois, and Massimiliano Marangolo Crystal Growth Design. 13, 4279 (2013)

Dominique Demaille