Magnetoelectric coupling characteristics of layer structured piezoelectric and magnetostrictive composites

Abstract

The focus of this dissertation is to synthesize magnetoelectric (ME) composites constituted of piezoelectric and magnetostrictive layers and characterize their direct ME coupling behavior under low frequency (off-resonance) condition. This work addresses a challenge related to the processing of piezoelectric films on thermally sensitive magnetostrictive substrates. The objectives of this thesis are to realize high magnitude of ME coupling, self-biased ME characteristics, and to employ eco-friendly lead-free piezoelectric materials as alternatives to lead-based ones in ME composites. This study involves the fabrication of bilayered ME film composites of $Pb(Zr,Ti)O_3 (PZT)/Metglas (FeBSi)$ and PZT/Nickel (Ni) and trilayered ME bulk composites of $Ni/ BaZr_xTi_(1-x)O_3- Ba_(1-x)Ca_xTiO_3 (BZT-BCT)$ single crystals /Ni and Ni/PZT/Ni.

Multiferroic magnetoelectric materials are attractive for various electrically and magnetically cross-coupled devices in sensing, transduction, and memory applications. The ME coupling facilitates the magnetic field control of electric polarization or electric field control of magnetization. The ME response is quantified in terms of the ME coefficient ($\alpha_{ME}$ ), which is considered as the figure of merit for the strength of ME coupling. It represents the coupling efficiency between the electric and magnetic fields. ME composites formed with piezoelectric and magnetostrictive constituents exhibit orders of magnitude larger outputs than those found in single phase ME compounds at room temperature. The ME voltage coefficient can be simply expressed as: $\alpha_{ME} = \partial E / \partial H = k_c d_{ij} q_{ij}$ , where $d_{ij} = \partial∂E / \partial S$ and $q_{ij} = \partial S / \partial H$ where E is the induced electric voltage, H is the magnetic field, S is the strain, $k_c$ is the coupling factor ($0 \leq k_c \leq 1$ ) between the constituent phases, $d_{ij}$ is the piezoelectric constant, $q_{ij}$ is the piezomagnetic coefficient, and $\lambda_{ij}$ is the magnetostriction. Therefore, a high piezoelectric coefficient and a high piezomagnetic coefficient along with a good coupling factor will result in a large $\alpha_{ME}$ .

In chapter 1, an overview of the motivation and background for the work chosen in this thesis and the direction it followed is presented. In chapter 2, a literature review on ME composites - factors influencing the ME coupling, ME materials, their fabrication and characterization, and prior reported work on PZT film-based as well as lead-free bulk ME composites - is discussed. In this work, polycrystalline PZT and single crystal BZT-BCT ceramics are chosen as the piezoelectric components and amorphous alloy Metglas and base metal Ni are chosen as the magnetostrictive components to prepare the ME composites. The bilayered PZT/Metglas and PZT/Ni film-composites were synthesized by room temperature deposition of PZT thick films by granule spray in vacuum (GSV) technique on Metglas and Ni substrates and annealing of the films was carried out by a continuous-wave (CW), visible (532 nm and 560 nm) laser radiation. The trilayered bulk composites of Ni/BZT-BCT/Ni and Ni/PZT/Ni are prepared by epoxy bonding of the squared-shaped and sub-millimeter-thick plates of Ni, BZT-BCT and PZT.

In chapter 3, the feasibility of laser annealing for the fabrication of PZT/Metglas film composites was studied. A $4 \mu m$-thick PZT film deposited 25-um thick Metglas foil by GSV method, followed by its annealing with a 560 nm Yb laser radiation. This longer-wavelength laser radiation was able to anneal the whole of thick PZT film layer without any deteriorative effects such as chemical reaction and/or atomic diffusion at the interface and crystallization of amorphous Metglas substrate. Greatly enhanced dielectric and ferroelectric properties of the annealed PZT are attributed to its better crystallinity and grain growth induced by laser irradiation. As a result, a high ME voltage coefficient ( $~ 3 V/cm \cdot Oe$ ) that is two orders of magnitude larger than previously reported output from PZT/Metglas film-composites is achieved.

In chapter 4, optimization of the laser annealing conditions (laser power/fluence) of the PZT/Metglas film composites is demonstrated. A 532 nm Nd:YAG laser with varying fluences (210, 255, 310, 345, and $390 J/mm^2$ ) was utilized to anneal the GSV deposited $2 \mu m$-thick PZT film on Metglas. It was found that the dielectric and ferroelectric properties of the PZT film are strongly affected by the exposure to laser fluence. The ME voltage coefficient of PZT/Metglas increased with the fluence up to $345 J/mm^2$ , reaching a maximum value of $0.88 V/cm \cdot Oe$ . The electrical and ME properties were correlated with the changes observed in crystallinity and grain size of the PZT film as well as with the alterations in microstructure and magnetic behavior of Metglas.

In chapter 5, an investigation on the enhancement and optimization of the magnetoelectricity in PZT/Metglas film composites is carried out by controlling the thickness ( $2-11 \mu m$ ) of the PZT films with the already optimized laser processing prameters (560 nm Yb laser, laser fluence ( $190 J/mm^2$ ) and sample scan speed (0.03 mm/s)). The dielectric, ferroelectric, and ME properties of the PZT/Metglas composites are found to be dependent upon the thickness of the PZT film. The changes in optical properties of the PZT films with varied thicknesses influenced their annealing behavior during laser irradiation. A giant and near-theoretical ME coupling of $7 V/cm \cdot Oe$ was observed in the composite at a moderate thickness of $6 \mu m$ of the PZT film.

In chapter 6, a study on the self-biased ME characteristics in the PZT/Ni film composites is presented. Here, the GSV deposited PZT films ($3 \mu m$ -thick) were annealed using 560 nm Yb laser radiation with varied laser powers (800, 855, 900, 935, and 965 mW) by fixing sample scanning speed at 0.03 mm/s. Similar dielectric, ferroelectric and ME properties were observed under all laser power conditions. Interestingly, the laser annealed PZT/Ni film composites showed a rather strong self-biased ME coupling ( $3-4 V/cm \cdot Oe$ ). The results can be further correlated with microstructural features observed in the laser annealed PZT/Ni.

In chapter 7, trilayered ME laminates consisting of [001]- and [011]-oriented and thickness polarized piezoelectric $BaZr_xTi_(1-x)O_3-Ba_(1-x)Ca_xTiO_3$ (BZT-BCT) single crystals and magnetostrictive base metal Ni were fabricated by epoxy bonding and their ME performance was evaluated and compared with that of Ni/PZT/Ni. The ME voltage coefficient was found to be strongly dependent on the crystallographic cut directions of BZT-BCT single crystals and also on applied magnetic field directions. The composite with [011]-oriented BZT-BCT single crystal exhibited a superior ME coupling of $1 V/cm \cdot Oe$ owing to the strong anisotropy in its transverse piezoelectric properties.