Atomic resolution analytical electron microscopy study of multiferroic-ferromagnetic interfaces

Abstract

Multiferroic bismuth ferrite (BFO), which possesses both ferroelectric and anti-ferromagnetic ordering, has generated significant research interest. Combining BFO with ferromagnetic materials, such as lanthanum strontium manganese oxide (LSMO), in a thin film epitaxial heterostructure provides a novel method for controlling electron spin polarization through lattice polarization, leading to new possibilities for spintronic devices. For superior device performance an atomically sharp interface, free from the presence of defects or dislocations, is a prerequisite. In this study, using a (001) strontium titanate (STO) substrate high quality epitaxial heterostructure such as STO-LSMO-BFO, STO-LSMO-BFO-LSMO were fabricated using pulsed laser deposition (PLD). Bright field transmission electron microscopy (TEM) images of STO-LSMO-BFO interfaces revealed dislocation-free interfaces. However, chemical analysis using scanning electron microscopy-energy dispersive spectroscopy (STEM-EDS) and energy filtered transmission electron microscopy (EFTEM) across the LSMO-BFO interface revealed intermixing of elements over a distance of ~ 4nm irrespective of BFO thickness (20 nm, 7 nm) and cooling rate. Scanning transmission electron microscopy (STEM)-high angle annular dark field (STEM-HAADF) was used to study the STO-LSMO-BFO-LSMO heterostructure. In the STO-LSMO-BFO-LSMO heterostructures BFO thickness was varied between 20 nm and 5 nm keeping the heterostructure cooling rate (20oC.min-1) constant. STEM-HAADF studies revealed dislocation-free and atomically sharp interfaces in both samples, while in the 5nm BFO thickness sample oxygen vacancies were observed. The STEM-electron energy loss spectroscopy (STEM-EELS) studies revealed atomically sharp LSMO-BFO and BFO-LSMO interfaces. Stacking fault defects were observed in the top LSMO when the STO-LSMO-BFO-LSMO heterostructure was cooled at a slower (5oC.min-1) rate. STEM-EELS studies across the stacking fault revealed the presence of Mn in two valence states (Mn2+ and Mn3+) rather than stoichiometric single (Mn3+) state. The co-existence of the rhombohedral and tetragonal phases in mixed phase BFO was confirmed by STEM-HAADF imaging of the lanthanum aluminate (LAO)-BFO heterostructure. STEM-EELS analysis across the R-T phases indicated a change in fine structure of the Fe L3,2, edge indicating the change in Fe-O co-ordination between the R-T phases in mixed phase BFO. STEM-EELS revealed a diffusion of La and Fe across the LAO-BFO interface over a distance of ~2 nm. Thus, high resolution chemical and physical analysis of interfaces reveal the significance of PLD processing conditions on the structural and chemical homogeneity across functional oxide interfaces.