We describe extensive studies on a family of perovskite oxides that are ferroelectric and ferromagnetic at ambient temperatures. The data include x-ray diffraction, Raman spectroscopy, measurements of ferroelectric and magnetic hysteresis, dielectric constants, Curie temperatures, electron microscopy (both scanning electron microscope and transmission electron microscopy (TEM)) studies, and both longitudinal and transverse magnetoelectric constants α₃₃ and α₃₁. The study extends earlier work to lower Fe, Ta, and Nb concentrations at the B-site (from 15%–20% down to 5%). The magnetoelectric constants increase supralinearly with Fe concentrations, supporting the earlier conclusions of a key role for Fe spin clustering. The room-temperature orthorhombic C_2v point group symmetry inferred from earlier x-ray diffraction studies is confirmed via TEM, and the primitive unit cell size is found to be the basic perovskite Z = 1 structure of BaTiO₃, also the sequence of phase transitions with increasing temperature from rhombohedral to orthorhombic to tetragonal to cubic mimics barium titanate.

The nature of strain mediated magnetoelectric (ME) coupling is investigated in laminates of lead zirconate titanate (PZT) and compositionally stepped ferrite with grading of piezomagnetic coefficient. ME effects that could only be attributed to grading related bending strain are observed in a trilayer of ferrite and oppositely poled PZT. It is shown that in a bilayer, grading induced flexural strain counteracts bending moment due to structural asymmetry and enhances ME coupling by a factor of 2. A zero-bias field ME effect is observed in such laminates. The graded composites are of interest for self-biased magnetic field sensors.

Mechanical strain mediated magnetoelectric (ME) effects are studied in bilayers and trilayers of piezoelectric single-crystal lanthanum gallium tantalate (LGT) and magnetostrictive permendur (P). The ME voltage coefficient ranges from 2.3 V/cm Oe at 20 Hz to 720 V/cm Oe at bending resonance and is higher by an order of magnitude than in composites with ferroelectric lead zirconate titanate or lead magnesium niobate-lead titanate. The low-frequency magnetic noise for P-LGT-P is a factor of 2-10 smaller than for ferroelectrics based composites. Langatate is free of ferroelectric hysteresis, pyroelectric effects, and phase transitions up to 1450 °C and is of interest for ultrasensitive, high temperature magnetic sensors.

A magnetoelectric (ME) phenomenon in a multiferroic composite consisting of magnetization-graded ferromagnetic and ferroelectric phases is discussed. The traditional strain mediated coupling in such composites arises due to magnetostriction and piezoelectric effects associated with the ferroic phases. Such an ME effect, in general, requires a bias magnetic field $H_0$ and an ac magnetic field. This paper is on the observation and theory of ME interactions under zero bias ($H_0$ $=$ 0) in a bilayer of lead zirconate titanate (PZT) and a ferromagnetic layer in which the magnetization is graded with the use of Ni and Metglas. At low frequencies, the ME coefficient ranges from 0.3 to 1.6 V/cm Oe and depends on the thickness of the Metglas. A similar dependence is also observed for the ME coupling at bending modes. A factor of 40 increase in the ME voltage is measured at resonance. The zero-bias ME coupling is attributed to strain-mediated coupling between the transverse magnetization due to magnetization grading at the interface of Ni-Metglas and the in-plane ac magnetic field. Theoretical estimates of ME coefficients at low frequencies and bending modes are in general agreement with the data.

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\char21{}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.