Book Description

Extrinsic magnetoelectric heterostructure materials receive increased interest because of the potential to tune the magnetoelectric properties through material selection and actively, through applied electric and magnetic field. Understanding the strength of the coupling of ferroic properties in composite solids and the roles of size, shape, and arrangement of the constituent phases is central to realizing high-performance magnetoelectrics and their applications. Nanoscale magnetoelectric materials are excellent candidate systems to study the aforementioned effects of shape and finite size, to meet the growing demand for faster, more efficient, low cost, and above all smaller device components for use in advanced magnetic memories, actuators, transducers, and sensors. Nanoscale materials offer increased interfacial surface area compared with bulk, making them appealing in the design of an enhanced magnetoelectric composite because the magnetoelectric effect in a composite system is driven by interfacial coupling mechanisms. However, nanoscale (approximately 100 nm or less) ferroic materials often exhibit a dimensionality-dependent suppression of ferroic and piezoelectric properties below a critical size. By controlling e.g. the surface chemical environment, introducing strain engineering of films through epitaxy or through the shape of a nanostructure, the ferroelectric phase stability can be tuned for a given material and temperature. In this dissertation nanoscale ferroic and multiferroic properties were investigated, highlighting five characteristic systems: ferromagnetic nanoparticles, ferroelectric nanocubes, extrinsic magnetoelectric nanowires, and resonant beams and resonant membranes. An experimental study of ferromagnetic nanoparticles is presented to underscore the importance of understanding the growth and interfacial coupling mechanisms in ferromagnetic nanoparticle systems. To investigate the finite-size driven ferroelectric phase transition at the nanoscale, the functional properties of individual ferroelectric nanocubes of varying sizes were measured at elevated temperature using local ferroelectric piezoelectric amplitude and phase switching analysis. Experimental evidence of the direct magnetoelectric effect within a single integrated nanostructure is presented. The synthesis, fabrication, and functional property characterization of highly tunable magnetoelectric coupling within individual multiferroic nanowires is described. The direct magnetoelectric response is distinctively enhanced by extreme curvature of the ferroelectric shell in relation to planar heterostructures, the geometric dominance of the interface as a fraction of the total volume of the nanowire, and magnetic shape anisotropy of the ferromagnetic nanowire core. This study of geometry aided direct magnetoelectric coupling can help the development of a future study and design of a magnetoelectric proximity sensor. One solution to address the issues associated with current magnetic field sensors, such as cost, durability, and detection range, is to develop a mesoscale magnetoelectric resonator device. Magnetoelectric resonator structures have a resonant frequency which will shift in the presence of an applied magnetic field due to the magnetostrictive properties of the ferromagnetic material. The resonator detection range can be tuned by pre-straining the piezoelectric layer. Two suggested resonator designs which are promising candidates for magnetic field proximity sensors are the fixed-fixed beam design and the membrane design.