G. Sreenivasulu

Flexural deformation in a compositionally stepped ferrite and magnetoelectric effects in a composite with piezoelectrics

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.

Piezoelectric single crystal langatate and ferromagnetic composites: Studies on low-frequency and resonance magnetoelectric effects

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.

Enhancing the sensitivity of magnetoelectric sensors by increasing the operating frequency

We present a field modulation technique that increases the operating frequency of magnetoelectric (ME) sensors so that it can match the mechanical resonance frequency of the sensor. This not only improves the sensitivity but also reduces the effect of 1/f noise that is inherent at low frequencies. The technique, which is shown to apply to both symmetric and asymmetric ME sensors, relies on the strong, nonlinear magnetic field dependence of the magnetostriction. The combination of a lower 1/f noise and enhanced response at resonance has increased the signal to noise ratio of a symmetric sensor by two orders of magnitude. The detection limit of this sensor was lowered from 90 to 7 pT/Hz at 1 Hz in a magnetically unshielded environment.

Controlled self-assembly of multiferroic core-shell nanoparticles exhibiting strong magneto-electric effects

Ferromagnetic-ferroelectric composites show strain mediated coupling between the magnetic and electric sub-systems due to magnetostriction and piezoelectric effects associated with the ferroic phases. We have synthesized core-shell multiferroic nano-composites by functionalizing 10–100 nm barium titanate and nickel ferrite nanoparticles with complementary coupling groups and allowing them to self-assemble in the presence of a catalyst. The core-shell structure was confirmed by electron microscopy and magnetic force microscopy. Evidence for strong strain mediated magneto-electric coupling was obtained by static magnetic field induced variations in the permittivity over 16–18 GHz and polarization and by electric field induced by low-frequency ac magnetic fields.

A permendur-piezoelectric multiferroic composite for low-noise ultrasensitive magnetic field sensors

Low-frequency and resonance magnetoelectric (ME) effects have been studied for a trilayer of permendur (alloy of Fe-Co-V) and lead zirconate titanate (PZT). The high permeability and high magnetostriction for permendur, key ingredients for magnetic field confinement, and ME response result in ME voltage coefficient of 23 V/cm Oe at low-frequency and 250 V/cm Oe at electromechanical resonance (EMR) for a sample with PZT fibers and inter-digital-electrodes. Theoretical ME coefficients are in agreement with the data. Measured magnetic noise floor of 25 pT/√Hz at 1 Hz and 100 fT/√Hz at EMR are comparable to best values reported for Metglas-PZT fiber sensors.

Resonance mixing of alternating current magnetic fields in a multiferroic composite

Theory for nonlinear mixing of harmonic magnetic fields in a ferromagnetic-ferroelectric composite structure has been developed and compared with data. In the voltage response of the composite, the model predicts a dc voltage proportional to the magnetostriction λ and its second derivative p with respect to the bias field H, an ac voltage due to linear magnetoelectric effect that is proportional to the piezomagnetic coefficient q, and a third term due to nonlinear mixing of the ac magnetic fields that is proportional to p. Doubling of the frequency and generation of voltages with sum and difference frequencies are expected due to nonlinearity of λ (H). The theoretically predicted effects are investigated in a sample of amorphous ferromagnetic film FeBSiC and a bimorph of lead zirconate titanate. Both the efficiency of frequency doubling and nonlinear mixing of the ac magnetic fields are found to be proportional to p. The effects discussed here are of interest for magnetic field sensors and signal processi...

Nonlinear resonant magnetoelectric interactions and efficient frequency doubling in a ferromagnetic-ferroelectric layered structure

Mechanical strain mediated non-linear magnetoelectric (NLME) coupling is studied in layered composites of ferromagnetic FeBSiC and piezoelectric lead zirconate titanate (PZT) bimorph. The NLME manifests as frequency doubling in the voltage response of the sample to an applied ac magnetic field. It is shown that NLME is strong (i) in the absence of DC magnetic bias, (ii) when the frequency of h is tuned to half the frequency for bending oscillations, and (iii) a PZT bimorph (instead of a single layer of PZT) is used. A model is discussed for the non-linear magnetoelectric coupling that is of interest for RF frequency doublers.

Enhanced sensitivity of magnetoelectric sensors by tuning the resonant frequency

The sensitivity of magnetoelectric (ME) sensors is more than an order of magnitude higher at their mechanical resonant frequency fr. By applying a restoring torque to an asymmetric ME sensor, we have increased its effective stiffness and, thus, fr by 20% while maintaining the enhanced sensitivity at resonance. The torque was dependent on both the tensile force from a suspended weight and the length of the wire attaching it. This provides two alternative routes for tuning fr to optimize performance. We have detected fields below 10 pT at both the shifted and unshifted fr of 132.2 Hz.

Shifting the operating frequency of magnetoelectric sensors

A method is presented for increasing the operating frequency of symmetric and asymmetric magnetoelectric (ME) sensors so that the operating frequency can be equal to the mechanical resonance frequency of the sensor. This increase improves the signal to noise ratio of a symmetric sensor by at least two orders of magnitude because it mitigates the effect of 1/f noise and the sensor has an increased response at its resonant frequency. The method is based on the strong, nonlinear magnetic field dependence of the magnetostriction. Our method has lowered the detection limit to 4 pT/Hz at 1 Hz in a magnetically unshielded environment.

Magnetic field assisted self-assembly of ferrite-ferroelectric core-shell nanofibers and studies on magneto-electric interactions

Core-shell nanofibers of nickel ferrite and lead zirconate titanate have been synthesized by electrospinning, assembled into superstructure in uniform or non-uniform magnetic fields, and have been characterized in terms of ferroic order parameters and strain mediated magneto-electric (ME) coupling. The core-shell structure was confirmed by electron microscopy and scanning probe microscopy. Studies on magnetic field induced polarization P in assembled samples showed a decrease or increase in P, depending on the nature of fibers and strengthening of ME coupling with change in remnant-P as high as 32%. Strong ME interactions were evident from H-induced variation in permittivity at 20–22 GHz.