Evidence for a Thickness-Dependent Crossover from Charge- to Strain-Mediated Magnetoelectric Coupling in La0.7Sr0.3MnO3 / PbZr0.2Ti0.8O3 Thin Film Oxide Heterostructures

Abstract

Layered magnetoelectric thin film materials, which exhibit coupling between ferroelectric polarization and magnetization, have received considerable attention for use in a new generation of "spintronic" memories [1]. Coupling at the interface between the layers can occur by various mechanisms, including charge and strain transfer, which are sensitive to local microstructure, chemistry, and defects [2]. The performance of devices for applications such as spin valves and tunnel junctions hinges on quantifying and controlling the local features that mediate coupling.Here we present a comprehensive structural, chemical, and magnetic analysis of LSMO / PZT / LSMO and LSMO / PZT/ SrRuO 3 (SRO) thin film heterostructures in significantly different charge and strain states. Scanning transmission electron microscopy (STEM) and geometric phase analysis (GPA) are used to directly map local strain, while electron energy loss spectroscopy (EELS) is used to explore valence changes at the LSMO / PZT interface. These results are complemented by depth-resolved polarized neutron reflectometry (PNR) measurements that provide information about the spatial dependence of magnetization [3]. These experimental observations are combined with phenomenological models and first-principles density functional calculations, which reveal the role of local elastic strains on the electronic distribution, to understand the suppression in the LSMO Curie temperature (T C ). Figure 1 shows local strain gradients measured by GPA. It is clear that there is a significant gradient in strain that depends on the thickness of the PZT underlayer. This gradient reaches a maximum near the vacuum surface, where neutron results show the magnetization is most suppressed. Likewise, STEM high-angle annular dark field (HAADF) images and electron energy loss spectroscopy (EELS) shown in figure 2 reveal changes in PZT polarization and an induced change in Mn valence near the interface. This is associated with an interfacial magnetization measured in PNR. From the magnitude of the induced magnetization changes we find local strain dominates coupling beyond the ultrathin (~4 nm) limit of LSMO. This suggests that layer thickness is crucial parameter in determining device performance and we offer ways to tune the geometry of these structures to favor a particular coupling mode.