Strain‐structure‐property relation in Co‐super tetragonal BiFeO3 heterojunctions

Abstract

Bismuth ferrite (BiFeO 3 ) has received a lot of attention as a rare multiferroic material showing simultaneous ferroelectric and antiferromagnetic orders at room temperature. Strain engineering offers a new avenue to tune the coupling between various orders, and provides access to some new crystalline phases, which are not stable in bulk BiFeO 3 (BFO). The super tetragonal phase (T‐phase) with a giant axial ratio (c/a~1.27) is one such strain engineered phases, obtained through a biaxial compressive strain on BFO (>‐4%) [1]. This phase exhibits a large polarization (~150 µC/cm 2 ) that was exploited for applications such as ferroelectric tunnel junctions with colossal electroresistance [2]. The interface between ultrathin films of T‐phase BFO and the bottom electrode of (Ca,Ce)MnO 3 has been thoroughly investigated recently [Marinova et al. Nano Lett. 15, 2533‐2541 (2015)]. However, the interface between the ferroelectric and the top electrode of Co used in devices remains to be investigated. In this context, we synthesized Co‐BFO‐CCMO heterostructures using pulsed laser deposition on the (001) surface of three different substrates (NdGaO 3 , LaSrGaO 4 , YAlO 3 ). Structural details were probed and compared through high resolution STEM imaging in both annular bright field (ABF) modes to obtain information about lighter atoms, and in high angular annular dark‐field modes. Furthermore, by correlating local chemical information obtained from STEM EELS, with ABF STEM images, we detect features that are consistent with the existence of oxygen vacancies in ultrathin T‐phase BiFeO 3 . We also investigated the interface quality between Co and T‐BFO using EELS and EDX, and correlate these observations with our transport measurements.