Static and Dynamic Studies of Charge- and Strain-Mediated Magnetoelectric Coupling in Ferromagnetic / Piezoelectric Oxide Heterostructures Using Polarized Neutron Reflectometry and Transmission Electron Microscopy

Steven R. Spurgeon (Department of Materials Science and Engineering, Drexel University)

Thin film magnetoelectric oxide heterostructures are among the most promising materials for a new generation of spintronic devices, in which spin transport is used to convey information in the solid state. However, prevailing theories of magnetoelectric coupling have failed to fully describe the switching dynamics and behavior of these composites, particularly in thicker heterostructures. Polarized neutron reflectometry (PNR) and transmission electron microscopy (TEM) offer ideal probes to correlate local atomic structure to magnetization in these systems and achieve predictive control of device performance.

Here we present evidence for a crossover from charge- to strain-mediated interfacial coupling in heterostructures of the piezoelectric Pb(Zr0.2Ti0.8)O3 (PZT) and the half-metallic ferromagnet La0.7Sr0.3MnO3 (LSMO). PNR reveals the presence of a graded magnetization that is associated with local strains and interfacial charge-transfer screening directly measured by aberration-corrected TEM. We perform density functional theory calculations, which show that strain acts to locally suppress magnetization through a change in Mn-eg orbital polarization. Our results suggest a radically new model of coupling in these materials, which may be tuned by selecting appropriate layer geometries.

We then present recent electric field studies in which we explore direct electric-field tuning of magnetization using bulk magnetometry and depth-resolved PNR. We demonstrate two approaches to these measurements: namely, the use of a single large contact with an in situ electric field and an array of contacts with ex situ poling. We discuss the implications of these results for the design of magnetoelectric devices, as well as ways to extend our biasing methods to other electrically-active systems.

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