Voltage control of magnetism in multiferroic ferromagnetic/ferroelectric heterostructures

Abstract

This Thesis dissertation tackles the design, fabrication and magnetoelectric characterization of artificial multiferroic ferromagnetic/ferroelectric heterostructures; in particular, iron-aluminum/lead-magnesium-niobate-lead titanate (FeAl/PMN-PT). These hybrid systems are intended to reduce heat dissipation and power consumption when magnetoelectrically actuated. One of the main strategies to control magnetism with voltage is the use of magnetostrictive/ferroelectric hybrid materials. However, different processes can occur simultaneously when these heterostructures are exposed to an electric field such as piezostrain-mediated effects, electronic charging, or voltage-driven oxygen migration (magnetoionics). This makes the interpretation of magnetoelectric effects not straightforward and, often, it leads to misconceptions. Because the induced strain (and variations in the magnetization) is proportional to the square of the ferroelectric polarization, the strain-mediated magnetoelectric response is usually symmetric with the sign of the applied voltage. Conversely, asymmetric responses can be obtained from electronic charging and voltage-driven oxygen migration. Here, ferromagnetic/ferroelectric heterostructures based on FeAl/[011]-oriented PMN-PT have been engineered in terms of layer thickness, composition, and microstructure to exhibit a highly asymmetric magnetoelectric response to be able to disentangle the aforementioned magnetoelectric effects. Specifically, Fe atomic % around 75 and large thicknesses (> 20 nm) allow dismissing any possible charge accumulation effect, whereas no evidence of magnetoionics is observed experimentally, as expected from the high resistance to oxidation of Fe75Al25, leaving strain as the only mechanism to modulate the asymmetric magnetoelectric response. In parallel, this approach has been scaled down to microscale patterned FeAl dots on [011]-oriented PMN-32PT substrates, using UV-lithography. Interestingly, 50 nm thick Fe75Al25 (at. %) disks grown onto 10 nm Cu/[011]-oriented PMN-32PT show a range of magnetic properties, ranging from multidomain to vortex-like behavior, as a consequence of the interplay between in-plane magnetic properties and voltage actuation.