The heterostructures consisting of different perovskite ABO₃ oxides provide a remarkable rich platform for creating new physical state and functionalities, as it can tailor the degree of the coupling between lattices, charge, orbital and spins. The multiferroic heterostructures showing a strong coupling between electric and magnetic orders draw extensive interest because of their promising applications in the modern spintronic devices. The magnetic order is coupled with the ferroelectric order at the interface, thereby permitting reversible electric field tuning of local spin and transport properties. Current understandings of this interfacial coupling effects are still limited, pioneering works found experimental evidences for different interface-mediated magnetization models, such as charge-transfer screening effect and local strain fluctuations. [1-3]. To address current disputes, the main challenge is to map directly the local change of the ferroelectric polarization, the interfacial electronic (charge, orbital polarization) and magnetic behavior at atomic scale. In principle, all these aspects might be map using combination of STEM and EELS technique.  

Our preliminary work has been conducted on a model multiferroic system consisting of epitaxial La_1-xSr_xMnO₃/Pb(Zr,Ti)O₃ (LSMO/PZT) heterostructures grown onto STO (100) substrates (Fig.1 (a)).  The LSMO oxide shows a strong interplay between transport and magnetic properties, which can be tuned via ferroelectric polarization reversal of the PZT layer. Two focused-ion-beam (FIB) TEM lamella samples at [100] zone axis has been prepared with the two opposite polarization states of the PZT, i.e. pointing toward or away from the LSMO layer, respectively. Microstructure and charge analyses were done by using atomic resolved high angle annular dark field (HAADF) imaging, annular bright field (ABF) imaging and high energy resolved electron energy loss spectroscopy (EELS) in a Cs-corrected scanning transmission electron microscope (NION USTEM200). The recent developed ABF technique allows the simultaneous visualization of both light and heavy elements, which is ideal to precisely determine the oxygen positions and therefore the BO₆ octahedra distortions. In Fig.1(b) and (c), two ABF images of the PZT are obtained respectively from the two differently polarized lamellas, and their ferroelectric polarization were determined by the relative displacement between the position of B-site Zr/Ti cation and the center of oxygen octahedra. Two opposite polarization directions are found which confirm the well preservation of PZT polar state during the FIB preparation. The EELS spectrum extracted from the atomic planes in the vicinity of the LSMO/PZT interface are shown in Fig.2, focusing on the fine structure of the O-K and Mn-L_2,3 edges, which are sensitive to the local bonding environment. We found that in the LSMO layer, even in the middle of the layer which is 5 u.c. far away from the interface, the pre-peak of O-K edge shows differences for the two polar states, indicating a relative hole doping when the polarization is pointing toward LSMO and a relative electron doping when it is pointing away. It is also coincident with the chemical shift of Mn edges where a higher valence is found in the hole doping configuration, and vice versa. Moreover, when approaching toward the LSMO/PZT interface, an apparent reduction of the Mn valence is found in the polar down state starting from the LSMO layer 2 u.c. away from the interface and continually into the diffusion region at PZT side. At the meantime, the Mn valence in the polar up state is well maintained until the interface where a slightly reduction appears, indicating a stronger resistance for the interfacial charge transfer.

                Our preliminary results suggest a clear link between the ferroelectric polarization direction and the change of charge configuration in the interface and the LSMO layer, indicating an efficient tuning of carrier injection by the ferroelectric field, which may play an important role in the magnetization modulation. Further study in the quantitative analysis on the amount of charge transfer and local interfacial magnetization change will be carried out for a thorough understanding of the magnetoelectric coupling.

[1] Spurgeon, S.R., et al., Nat Commun, 2015. 6: p. 6735.

[2] Spurgeon, S.R., et al., ACSNano, 2014. 8(1): p. 894-903.

[3] Lu, H., et al., 2012. 100(23): p. 232904.

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EMC Abstracts - https://emc-proceedings.com/abstract/charge-strain-and-polarization-profiling-in-ferroelectricferromagnetic-epitaxial-heterostructures/