Low-frequency and resonance magnetoelectric effects in piezoelectric and functionally stepped ferromagnetic layered composites

The strain mediated magnetoelectric (ME) coupling is studied in bilayers of lead zirconate titanate (PZT) and a ferromagnetic layer that is functionally stepped. Nickel with negative piezomagnetic coefficient $q$ and Metglas with positive $q$ are bonded to achieve the desired step in $q$ for the ferromagnetic phase. Samples of PZT-Ni-Metglas and PZT-Metglas-Ni are used for measurements of ME voltage coefficient (MEVC) at low frequencies and at frequencies corresponding to bending resonance. It is shown that at low frequencies the bending moment due to stepped-$q$ counteracts the asymmetry-related flexural strain in the sample and enhances the strength of ME coupling. The MEVC is found to be as high as in symmetric trilayers in which flexural deformation is absent, and it ranges from 0.4 to 3.4 V/cm Oe depending on the grading scheme and the thickness of Metglas. A resonance enhancement of the MEVC to 40--220 V/cm Oe is measured at bending modes for samples clamped at one end. Samples of PZT-Ni-Metglas show a higher MEVC than for PZT-Metglas-Ni both at low frequencies and at bending resonance. The $q$-stepped composites are of importance for ultra-sensitive magnetic field sensors.