Functionally Graded Piezomagnetic and Piezoelectric Bilayers for Magnetic Field Sensors: Magnetoelectric Interactions at Low‐Frequencies and at Bending Modes

In a piezomagnetic-piezoelectric bilayer the interaction between the magnetic and electric subsystems occurs through mechanical forces. This study is on magneto-electric (ME) interaction in a bilayer of PZT and ferromagnetic layer in which the piezomagnetic coefficient q is graded. The grading is accomplished with the use of Ni (negative q) and Ni-Fe alloy or Permendur (positive q). At low-frequencies the ME coefficient shows a four-fold increase compared to homogeneous Ni-PZT bilayer. Bending resonance and consequent enhancement in ME coupling occurs at the lowest frequency, 800-1200 Hz, for a bilayer that is fixed at one end and free at the other end. The peak ME coefficient at resonance is in the range 12-15 V/cm Oe and is 10% higher for graded samples compared to Ni-PZT. The graded bilayers are promising candidates for use in ultrasensitive magnetic field sensors. INTRODUCTION The magneto-electric effect is the dielectric polarization of a material in an applied magnetic field H, or an induced magnetization in an electric field E [1]. The polarization P is related to //by P = aH, where a is the ME-susceptibility. Most single phase compounds show weak ME interactions [2]. In a composite of piezomagnetic-piezoelectric phases the magnetoelectric coupling occurs through mechanical deformation and a is proportional to the product of the piezomagnetic coefficient q and the piezoelectric coefficient d [3]. Efforts so far have focused on ME interactions in bulk and layered composites of ferrites, manganites, or transition metals/alloys for the ferromagnetic phase, and lead zirconate titanate (PZT) or lead magnesium niobate-lead titanate (PMN-PT) for the piezoelectric phase [3]. This work is on ME interactions in composites consisting of functionally graded ferromagnetic and piezoelectric phases. Ferroics (ferromagnetic or ferroelectric) form an essential sub-group of functional materials whose physical properties are sensitive to changes in electric and magnetic fields. Recent works on functionally graded ferroics have resulted in the discovery of important phenomena including internal potentials, induced anisotropies, and spontaneous strain [4-8]. Since the piezoelectric (d) and piezomagnetic (q) coefficients are the critical parameters that determine the strength of ME coupling, grading one or both parameters will have substantial influence on a. The work was motivated by our recent theory that predicts a minimum of 50-60% stronger ME interaction in q and d graded ferrite-ferroelectrics as compared to homogeneous systems [9-11]. Here we discuss results of our studies of strain mediated ME coupling on bilayers of PZT and ^-graded magnetic phase. Samples of homogeneous bilayer of PZT-Ni and ^-graded samples of PZT-Ni and 60%Ni-40%Fe alloy and PZT-Ni-Permendur (P) were investigated. Nickel has negative q, whereas Ni-Fe alloy and Permendur, an alloy with 49% Fe, 49% Co and 2% V, both have positive q. Measurements of ME coefficients at low frequencies and at bending resonance reveal enhancement of ME coefficients compared to homogeneous bilayer of PZT-Ni. The grading of q perpendicular to the sample plane gives rise to a bending strain in a magnetic field that counteracts the flexural deformation due to asymmetry associated with 223 Advances and Applications in Electroceramics Edited by K. M. Nair, Quanxi Jia and Shashank Priya Copyright © 2011 The American Ceramic Society Functionally Graded Piezomagnetic and Piezoelectric Bilayers for Magnetic Field Sensors bilayers and an increase in ME susceptibility. The strong ME coupling is of significance for use of graded composites as magnetic field sensors. EXPERIMENT Commercial samples of 99.9% Ni and Permendur were used. Alloys with 60%Ni and 40% Fe (Ni/Fe), however, were prepared by electrodeposition. Films of thickness 100 u.m were deposited on Cu using the procedure described below. The Cu plate was first cleaned, one side was covered with lacquer and was immersed in a plating solution for the deposition of Ni/Fe. Details on the plating bath and process parameters are listed in Table I. Nickel sulfamate was used for stabilization of the plating solution and to achieve rapid deposition and low internal stress. Nickel chloride facilitates dissolution of the Ni anode. Boric acid acted as a buffer agent to stabilize the pH of the plating bath. Saccharin in the Ni-Fe bath acts as a strain reducing agent. The pH value of the solution was maintained to about 3.5 using sodium hydroxide and amidosulfonic acid. The surfactant sodium lauryl sulfate was added to the solution to avoid pinholes in the films. The thickness of the Ni-Fe layer was 100 u.m for a deposition time of 6 hours. Following the deposition, the Cu plate was dissolved in ammonium hydroxide and the Ni/Fe film was annealed in N2H2 environment for 1 hour at a temperature of 400 °C. Table I. Plating bath and process parameters for Ni-Fe electrodeposition Nickel sulfamate tetrahydrate (g/1) 480 Iron (II) sulfate heptahydrate (g/1) 104.8 Nickel (II) chloride hexahydrate (g/1) 20 Boric acid (g/1) 20 Sodium lauryl sulfate (g/1) 1.5 OBenzoic sulfimide sodium salt hydrate (Saccharin) (g/1) 3.0 pH value 3.5 Temperature (°C) 60 Cathode current density (A/cm) 0.03 Magnetosrtiction (X) of Ni, Ni/Fe alloy and P were measured on 20 mm x 3 mm platelets using a strain gage and stain indicator. With the sample plane defined as (1,2), in-plane X was measured for two orientations of the bias magnetic field H: along the field direction, termed Xii and perpendicular to H, termed XnFigure 1 shows A vs. H data for Ni. The parameter Xii is negative, increases rapidly with H and shows saturation for H > 300 Oe. The magnetostriction perpendicular to H, Xn = Xii/2 and is positive. The piezomagnetic coefficient q = qn+qi2 = dXii/dH + dXn/dH was estimated from data in Fig.land its H dependence for Ni is shown in Fig.2. A large negative-^ is evident for low bias fields and the magnitude of q decreases with increasing H. Figure 3 shows similar q vs. H data for Ni/Fe and P. It is clear from the data that both Ni/Fe and P have positive piezomagnetic coupling with Permendur showing a higher q than Ni/Fe. Bilayer laminates were made by bonding the magnetic layers to PZT plates of dimensions 20 mm x 3 mm * 0.2 mm that were poled in an electric field. A 2 |im thick slowdry epoxy was used for bonding. Following the application of epoxy between PZT and magnetic layers, the sample was to 60°C for 4-5 hrs. 224 ■ Advances and Applications in Electroceramics Functionally Graded Piezomagnetic and Piezoelectric Bilayers for Magnetic Field Sensors Figure 1. In-plane magnetostriction for Ni parallel (Xi i) and perpendicular (Xn) to the bias magnetic field H. Figure 2. In-plane piezomagnetic coefficient 500 Oe. The //-dependence in Fig.4 essentially tracks the variation of magnitude of the piezomagnetic coefficient q with H that is shown in Fig.2. The peak value for MEVC is much smaller than reported values for a symmetric structure of Ni-PZT-Ni [11]. A bilayer, due to structural asymmetry, will have a flexural deformation in addition to the in-plane deformation due to magnetostriction. The out-of-plane bending deformation counteracts the in-plane strain, leading to weakening of the ME coupling.