Structural and EMCD analysis of Fe/V multilayers (2012)

Superlattice heterostructures of iron and vanadium (Fe/V) are an ideal system for the investigation of magnetic behaviour in thin films for a number of reasons: they can be grown with a high degree of structural coherency and adhere strongly to single crystalline MgO substrates [1]; the spacing between the ferromagnetic layers of iron can be adjusted by changing the monoatomic layer ratio of Fe:V; and the electronic structure of the system can be reversibly tuned through the absorption of hydrogen [2]. Consequently, this system has enabled researchers to probe fundamental concepts describing interfacial magnetism and interlayer magnetic coupling. In this study, the crystalline quality of Fe/V multilayers is assessed in cross-section through the use of aberration-corrected high resolution micrographs coupled with diffraction contrast imaging. Additionally, the ferromagnetism in this system is assessed and compared to standard iron crosssections using the Electron Magnetic Circular Dichroism (EMCD) technique [3]. Fe/V multilayers were deposited using magnetron sputtering on (001) oriented MgO single crystal substrates. The monoatomic layer ratio of V:Fe was 14:2. Figure 1 shows an atomic model of this superlattice. Samples were then characterized for structure using x-ray reflectivity and diffraction and for magnetic properties using a superconducting quantum interference device. Preparation of TEM cross-sections was carried out by mechanical polishing and dimple grinding followed by a low energy Ar ion bombardment in a Gatan PIPS device. The structural integrity of the prepared TEM specimens was assessed with atomic resolution in the ANL ACAT Team/FEI Cc/Cs image-corrected microscope as well as using two-beam scattering conditions to generate strong diffraction contrast images in an FEI Tecnai F20ST microscope (figure 2) [4]. A selected area diffraction pattern of the multilayer confirms the 45° in-plane rotated epitaxial growth of the Fe/V multilayer and reveals satellite reflections around V (0 0 2) that result from the superlattice structure. This superlattice reflection is imaged in real space in figure 2 by tilting the film into the two-beam condition g = [0 0 2]V. Significantly, this frequency is observed throughout the film thickness, suggesting that the superlattice ordering is sustained during the growth process resulting in a high quality specimen. This contrast is somewhat lessened at the substrate / multilayer interface, however, and high resolution images of this region reveal a significant amount of strain due to the highly coherent growth. Moreover, a slight out-of-plane tilting of the Fe/V (0 0 2) planes is documented near atomic steps in the MgO substrate and can be correlated to the rocking curve angle. Density functional theory calculations of this system predict μB of ~2.4 per Fe atom for the multilayer system. The dynamical diffraction calculations indicate that a magnetic dichroic signal (EMCD) should be detectable on the iron core-loss L2,3 edges up to some 50 nm of through thickness in the two scattering geometries depicted in figure 1. EMCD measurements on this system are ongoing.