Engineered magnetoelectric (ME) composites with increased coupling efficiency between the constituent materials have led to the development of sensitive, low-noise, ME magnetic field sensors, and conversely, to the development also of voltage driven magnetic field generators. (Fiebig, 2005; Spaldin and Fiebig, 2005; Lenz and Edelstein, 2006; Chen et al., 2010; Geiler et al., 2010; Fitchorov, T., Chen, Y. et al., 2011, and Fitchorov, T., Yajie, C. et al., 2011). An ideal ME device would require no external power supply and no external conditioning circuitry, would exhibit stable room-temperature operation, and would be relatively inexpensive to fabricate. Current generation ME devices exhibit several of these characteristics and, as such, have tremendous potential to compete with existing flux-gate, Hall-effect, SQUID (superconducting quantum interference device), and magnetoresistive magnetometers in a variety of applications.
Numerous geometries and topologies of ME composites have been investigated, such as bulk heterostructural laminates, thick- and thin-film devices, and more recently a quasi-one dimensional tube topology (Ma et al., 2011; Chen et al., 2011). The ME phenomenon occurs in these composites via transfer of stress energy between magnetostrictive and piezoelectric phases. Due to the nature of stress-coupled magnetization in a magnetostrictive material, and stress-coupled polarization in a piezoelectric material, elastically-bonded composites having both materials have the ability to transduce a voltage response from an applied magnetic field and vice-versa (Nan et al., 2008).
Among the various parameters of ME composite devices under investigation, e.g., topology, bonding, amplification, and sensing techniques, the magnetostrictive and piezoelectric materials used in fabrication are of particular importance (Li et al., 2011; Gillette et al., 2011; and Dong et al., 2005). Typically, piezoelectric materials, exhibiting high piezoelectric constants, such as PZT (lead zirconte titanate) and PMN-PT (magnesium lead niobium, and lead titanium), are desirable for generating large strain-induced charge separation. However, in a magnetostrictive material, a large value of saturation magnetostriction alone does not always make it an optimal material choice. Other factors such as magnetization process, magnetic hysteresis, and magnetic anisotropy also play an important role. Further, the slope of the magnetostriction curve (dλ/dH) has a significant influence on ME coupling. The sensitivity of ME magnetic field sensors can be increased by applying an optimal external DC magnetic bias field. Peak magnetoelectric sensitivity typically occurs when the magnitude of the external magnetic bias field corresponds with the peak of the derivative of the magnetostriction curve, a maximum in dλ/dH, but can be offset due to factors such as magnetic hysteresis, shape anisotropy, and demagnetization. Optimal external magnetic bias field magnitude can range from tens to thousands of oersted (Oe), requiring the use of bulky permanent magnets or electromagnets.
Advances in ME composite materials offer opportunities for developing miniature, lightweight, highly-sensitive, low-noise ME magnetic field sensors that require little to no external magnetic bias for deployment in various magnetometry applications.
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