Sanjay Mandal

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.

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.

Flexural deformation and bending mode of magnetoelectric nanobilayer

A model is discussed for magnetoelectric (ME) coupling in a nanobilayer of ferromagnetic and piezoelectric phases on a substrate. The theory is applied to a specific case, nickel ferrite and lead zirconate titanate bilayer on strontium titanate substrate. Both low-frequency coupling and ME interactions at frequencies corresponding to bending oscillations have been considered. The influence of lattice mismatch between the substrate and piezoelectric and piezomagnetic layers on material parameters has been estimated using the Landau–Ginsburg–Devonshire phenomenological thermodynamic theory. At low frequencies, the model predicts a maximum in the ME coupling strength when the substrate is of the same thickness as the bilayer. The ME voltage coefficient drops with further increase in substrate thickness due to clamping effects. The bending oscillations in nanobilayers are expected to occur at ultralow frequencies (<5 Hz) compared to longitudinal acoustic modes at 100–200 kHz for bilayers of nominal dimensions...

Enhancement of magnetoelectric coupling in a piezoelectric-magnetostrictive semiring structure

Studies on magnetoelectric (ME) coupling in a semiring of lead zirconate titanate (PZT) with a Terfenol-D insert reveal strong ME coupling at low frequencies and two orders of magnitude enhancement in the strength at resonance associated with a unique bending mode in PZT. A model is discussed for the resonance ME coupling that arises from radial and shearing displacements and theoretical estimates are in excellent agreement with the data. The model also predicts weak ME coupling in a full ring of PZT with Terfenol-D insert in agreement with the experiment. The results are of importance for ME composite based magnetic field sensors.

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.

Bending Resonance in a Magnetostrictive-Piezoelectric Bilayer and Magnetoelectric Interactions

A ferromagnetic and piezoelectric layered composite is found to show resonance enhancement of strain mediated magnetoelectric (ME) coupling at frequencies as low as 20 Hz. Samples of Permendur-lead zirconate titanate (PZT) and Metglas-PZT show bending resonance at fr = 120–158 Hz and two-orders of magnitude enhancement in ME voltage. When a tip mass m = 1 – 20 g is placed on the cantilever, fr decreases and levels off at 20 Hz and ME voltage increases by a factor of two. These observations are in agreement with theory. The composite cantilevers are of importance for ultrasensitive low-frequency magnetometers.

Modelling of magneto-acoustic resonance in ferrite–piezoelectric bilayers

A model is discussed for magnetoelectric (ME) effects in a single-crystal ferrite–piezoelectric bilayer on a substrate. The specific focus is on coupling at magneto-acoustic resonance (MAR) at the coincidence of ferromagnetic resonance in the ferrite and thickness modes of the electromechanical resonance in the piezoelectric. The clamping effect of the substrate has been considered in determining the ME voltage coefficient and applied to a model system of a bilayer of lead zirconate titanate (PZT) and yttrium iron garnet (YIG) on a gadolinium gallium garnet substrate. The theory predicts a giant ME effect at MAR due to interaction and transfer of energy between elastic modes and the uniform precession spin-wave mode. It is shown that the ME coupling strength decreases with increasing substrate thickness. Estimates for YIG–PZT for nominal film parameters predict MAR at 5 GHz and ME coefficients on the order of 5–70 V cm−1 Oe−1. The phenomenon is of importance for the realization of multifunctional ME sensors and transducers operating at microwave frequencies.

FUNCTIONALLY GRADED MAGNETOSTRICTIVE-PIEZOELECTRIC LAYERED COMPOSITES: STUDIES ON MAGNETO-ELECTRIC INTERACTIONS

ABSTRACT Ferromagnetic/piezoelectric composites show strong magnetoelectric coupling that is mediated by mechanical forces. This work is on (i) synthesis of functionally-stepped ferrite-ferroelectric composites and (ii) investigations on the magneto-electric (ME) interactions. Studies were performed on sintered samples of nickel cobalt ferrite and lead zirconate titanate (PZT). The grading parameters of interest are the piezomagnetic coefficients, q, and the piezoelectric coefficients, d. The grading in q is accomplished by compositional variation and the grading in d by poling direction. Measurements of the ME coefficient revealed the strongest coupling in functionally graded samples and the weakest for bilayers with homogeneous q and d.