Coaxial Nickel-Poly(vinylidene fluoride trifluoroethylene) Nanowires for Magnetoelectric Applications

Magnetoelectric (ME) composite materials, in which a coupling between magnetostrictive and piezoelectric effects is achieved, are potential candidates for multifunctional devices where the interplay between the electrical, magnetic, and mechanical properties of these structures can be fully exploited. Nanostructured composites are particularly interesting because of the enhancement of ME coupling expected at the nanoscale. However, direct studies of ME coupling in nanocomposites by scanning probe microscopy (SPM) techniques are rare because of the complex interplay of forces at play, including those arising from electrostatic, magnetic, and electromechanical interactions. In this work, the ME coupling of coaxial nickel-poly(vinylidene fluoride trifluoroethylene) [Ni-P(VDF-TrFE)] composite nanowires (NWs), fabricated by a scalable template-wetting-based technique, is studied using a systematic sequence of SPM techniques. Individual ME NWs were subjected to an electric field sufficient for ferroelectric poling in piezoresponse force microscopy (PFM) mode, while magnetic force microscopy (MFM) was used to measure localized changes in magnetization as a result of electrical poling. Kelvin probe force microscopy measurements of the surface potential were conducted to eliminate for the effect of contact potential differences during these measurements. An inverse, static, ME coupling coefficient of ∼1 × 10^-11 s m^-1 was found in our coaxial nanocomposite NWs, comparable to other types of planar composites studied in this work, despite having an inferior piezoelectric-to-magnetostrictive volume ratio. The efficient ME coupling in our coaxial NWs is attributed to the larger surface-to-volume interfacial contact between Ni and P(VDF-TrFE) and is promising for future integration into ME composite devices such as magnetic field sensors or energy harvesters.