Au/Fe thin-film magnetic multilayer materials: A layer-specific structural analysis using medium-energy ion scattering

Abstract

This paper presents an account of the application of medium-energy ion scattering (MEIS) to the investigation of thin-film metallic multilayers grown using molecular-beam epitaxy. MEIS can provide high resolution compositional and structural information as a function of depth in the near surface region (0--250 \AA{}); these parameters are inextricably linked with the magnetic properties exhibited by materials of this type. Amongst the information available from MEIS is the accurate determination of the layer spacings, structural information from individual layers (even at thicknesses close to a monolayer), and high sensitivity to disorder in the layers. MEIS therefore provides additional information above that provided by in situ reflection high-energy electron diffraction monitoring during growth and ex situ x-ray diffraction measurements so that it represents an ideal complementary technique for the analysis of thin-film magnetic multilayer materials of this type. An Au/Fe multilayer sample of a type previously shown to exhibit giant magnetoresistance (GMR) was analyzed. Individual gold layers were clearly resolved and a measurement of the bilayer spacing obtained; this parameter determines the magnitude of the exchange coupling and GMR. Au/Fe/Au trilayer samples grown on both MgO(100) and sapphire(112\ifmmode\bar\else\textasciimacron\fi{}0) substrates were also analyzed for a series of Fe layer thicknesses between 2 and 16 \AA{}. The MgO(100) grown samples showed unusually high second-layer Au signal consistent with atomic layer spacings in the Fe layers that lead to enhanced illumination of the second-layer Au. This effect could be modeled using bcc(100) layer spacings thus confirming the structure to be bcc(100) Fe between fcc(100) Au layers. In the sapphire-grown samples, twinned fcc(111) structure was observed in the individually resolved Au and Fe layers. The amplitude of the Fe blocking features was reduced with increasing Fe layer thickness indicating a reduction in crystallinity until for the highest thickness there was little indication of structure within the layer. The maximum layer thickness for fcc(111) Fe growth was seen to lie between 8 and 16 \AA{}.