Engineering Nano-Structured Multiferroic Thin Film

Multiferroics exhibit remarkable tunabilities in their ferromagnetic, ferroelectric and magnetoelectric properties that provide the potential in enabling the control of magnetizations by electric field for the next generation non-volatile memories, antennas and motors. In recent research and developments in integrating single-phase ferroelectric and ferromagnetic materials, multiferroic composite demonstrated a promising magnetoelectric (ME) coupling for future applications. Atomic layer deposition (ALD) technique, on the other hand, allows fabrications of complex multiferroic nanostructures to investigate interfacial coupling between the two materials.;In this work, radical-enhanced ALD of cobalt ferrite (CFO) and thermal ALD of lead zirconate titanate (PZT) were combined in fabricating complex multiferroic architectures in investigating the effect of nanostructuring and magnetic shape anisotropy on improving ME coupling. In particular, 1D CFO nanotubes and nanowires; 0D-3D CFO/PZT mesoporous composite; and 1D-1D CFO/PZT core-shell nanowire composite were studied. The potential implementation of nanostructured multiferroic composites into functioning devices was assessed by quantifying the converse ME coupling coefficient.;The synthesis of 1D CFO nanostructures was realized by ALD of CFO in anodic aluminum oxide (AAO) membranes. This work provided a simple and inexpensive route to create parallel and high aspect ratio (~55) magnetic nanostructures. The change in magnetic easy axis of (partially filled) CFO nanotubes from perpendicular to parallel in (fully-filled) nanowires indicated the significance of the geometric factor in controlling magnetizations and ME coupling.;The 0D-3D CFO/PZT mesoporous composite demonstrated the optimizations of the strain transfer could be achieved by precise thickness control. 100 nm of mesoporous PZT was synthesized on Pt/TiOx/SiO2/Si using amphiphilic diblock copolymers as a porous ferroelectric template (10 nm pore diameter) for ALD CFO growth. The increased filling of CFO decreased the mechanical flexibility of the composite for electric field induced strain, hence the converse ME coupling was mitigated. The highest converse ME coefficient of 1.2 10--5 Oe-cm/mV was achieved with a 33% pore filling of CFO, in compare to 1 x 10--5 Oe-cm/mV from mesoporous CFO filled with 3 nm of PZT in literature (Chien 2016).;Highly directional 1D-1D PZT-core CFO-shell composite in AAO demonstrated the magnetic shape anisotropy could be modulated. The CFO shell thickness allowed the tuning of magnetic easy axis and saturation magnetizations; whereas the PZT volume allowed the optimization of electric field induced strain of the composite. Enhanced converse ME coupling of 1.3 x 10--4 Oe-cm/mV was realized by 5 nm CFO shell on 30 nm of PZT core.;In summary, the work has demonstrated nanostructuring of multiferroic composite is an effective pathway to engineer converse ME coupling through optimizations of magnetic shape anisotropy and interfacial strain transfer.