The coupling between magnetic and electric subsystems in composites of ferromagnetic and ferroelectric phases is a product property that is facilitated by mechanical strain that arises due to magnetostriction and the piezoelectric effect in the constituent phases. Such multiferroic composites are of immense interests for studies on the physics of electromagnetic coupling and for use in a variety of applications. Here, we focus on magneto-electric (ME) coupling in nanocomposites. Particular emphasis is on core-shell particles and coaxial fibers, thin film heterostructures, and planar structures with a variety of mechanical connectivity. A brief review of models that predict strong ME effects in nanostructures is followed by synthesis and characterization. Core-shell particulate composites can be prepared by hydrothermal processes and chemical or deoxyribonucleic acid-assisted assembly. Electrospinning techniques have been utilized to prepare defect free core-shell nanofibers. Core-shell particles and fibers can be assembled into superstructures with the aid of magnetic and electric fields and characterized for possible use in advanced technologies. Chemical-vapor deposition techniques have been shown to be effective for the preparation of heterostructures of ferrites and ferroelectrics. Exotic planar multiferroic structures with potential for enhancing ME coupling strengths are also considered. Scanning probe microscopy techniques are ideal for probing the nature of direct- and converse-ME coupling in individual nanostructures. Magnetoelectric characterization of assemblies of nanocomposites can be done by ME voltage coefficient, magnetic field induced polarization, and magneto-dielectric effects. We conclude with a brief discussion on possible avenues for strengthening the product properties in the nanocomposites.

Tunneling behaviors of composite ferroelectric tunnel junctions (FTJs) with a no-polar dielectric (DE) layer thickness from 1 to 4 nm were investigated. It is found that the low-resistance state (ON state) current decreases with the DE thickness, while the high-resistance state (OFF state) current decreases first and then increases. The largest tunneling electroresistance (TER) effect is observed for the 3 nm-thick DE layer, which corresponds to the lowest OFF-state current. Studies on the electron transport mechanisms show that direct tunneling dominates the ON-state tunneling behaviors for all FTJs as well as the OFF state for the thinnest DE layer of 1 nm. While Fowler–Nordheim (FN) tunneling plays a significant role in the OFF-state electron transport for thicker DE thicknesses and reinforces its role with the increasing DE thickness, the weak FN tunneling-assisted low OFF-state current for the 3 nm-thick DE layer relative to the DE layer of 4 nm contributes to the largest TER effect.

Resonant tunneling is a quantum‐mechanical effect in which electron transport is controlled by the discrete energy levels within a quantum‐well (QW) structure. A ferroelectric resonant tunneling diode (RTD) exploits the switchable electric polarization state of the QW barrier to tune the device resistance. Here, the discovery of robust room‐temperature ferroelectric‐modulated resonant tunneling and negative differential resistance (NDR) behaviors in all‐perovskite‐oxide BaTiO₃/SrRuO₃/BaTiO₃ QW structures is reported. The resonant current amplitude and voltage are tunable by the switchable polarization of the BaTiO₃ ferroelectric with the NDR ratio modulated by ≈3 orders of magnitude and an OFF/ON resistance ratio exceeding a factor of 2 × 10⁴. The observed NDR effect is explained an energy bandgap between Ru‐t_2g and Ru‐e_g orbitals driven by electron–electron correlations, as follows from density functional theory calculations. This study paves the way for ferroelectric‐based quantum‐tunneling devices in future oxide electronics.

A model for the low-frequency magnetoelectric (ME) effect that takes into consideration the bending deformation in a ferromagnetic and ferroelectric bilayer is presented. Past models, in general, ignored the influence of bending deformation. Based on the solution of the equations of the elastic theory and electrostatics, expressions for the ME voltage coefficients (MEVCs) and ME sensitivity coefficients (MESCs) in terms of the physical parameters of the materials and the geometric characteristic of the structure were obtained. Contributions from both bending and planar deformations were considered. The theory was applied to composites of PZT and Ni with negative magnetostriction, and Permendur, or Metglas, both with positive magnetostriction. Estimates of MEVCs and MESCs indicate that the contribution from bending deformation is significant but smaller than the contribution from planar deformations, leading to a reduction in the net ME coefficients in all the three bilayer systems.