Abstract
Understanding strain gradient phenomena is of paramount importance in diverse areas of condensed matter physics. This effect is responsible for flexoelectricity in dielectric materials, and it plays a crucial role in the mechanical behavior of nanoscale-sized specimens. In magnetoelectric composites, which comprise piezoelectric or ferroelectric (FE) materials coupled to magnetostrictive (MS) phases, the strain gradient can add to any uniform strain that is present to boost the strength of the coupling. Hence, it could be advantageous to develop new types of functionally graded multiferroic composites (for information technologies) or magnetic-field-driven flexoelectric/magnetostrictive platforms for wireless neurons/muscle cell stimulation (in biomedicine). In MS or FE materials with non-fully constrained geometries (e.g., cantilevers, porous layers, or vertically aligned patterned films), strain gradients can be generated by applying a magnetic field (to MS phases) or an electric field (to, e.g., FE phases). While multiferroic composites operating using uniform strains have been extensively investigated in the past, examples of new nanoengineering strategies to achieve strain-gradient-mediated magnetoelectric effects that could ultimately lead to high flexomagnetoelectric effects are discussed in this Perspective.
Highlights
Magnetism and electricity have always had an intimate link
Interesting is the coexistence of magnetic and electric orders in magnetoelectric (ME) materials, which makes them able to respond, simultaneously, to external magnetic and electric stimuli: (i) electric polarization can be modulated by the external magnetic field and (ii) magnetic properties can be largely controlled with an electric field
DME effects are appealing for healthcare technologies,4 water remediation,5 energy harvesting systems,6 and sensors/actuators,7 whereas composites for energy-efficient information technologies (CME) effects can be exploited in microelectromechanical systems and to reduce energy consumption of magnetic memories and spintronic devices
Summary
Magnetism and electricity have always had an intimate link. interesting is the coexistence of magnetic and electric orders in magnetoelectric (ME) materials, which makes them able to respond, simultaneously, to external magnetic and electric stimuli: (i) electric polarization can be modulated by the external magnetic field (direct ME effect, DME) and (ii) magnetic properties can be largely controlled with an electric field (converse ME effect, CME). In conventional ME composites, the coupling between piezoelectric [or ferroelectric (FE)] and magnetostrictive (MS) constituents is mediated by homogeneous interfacial strain and, in some cases, by electric surface charge effects. DME effects are appealing for healthcare technologies, water remediation, energy harvesting systems, and sensors/actuators, whereas CME effects can be exploited in microelectromechanical systems and to reduce energy consumption of magnetic memories and spintronic devices.. It is worth mentioning that while the FE layer most likely experiences an homogeneous strain due to its relatively large thickness (strain-gradient effects are more pronounced at the nanoscale), the magnetic-field induced strain gradient in nanoporous FeGa causes a change in the piezoresponse of the adjacent ferroelectric P(VDF-TrFE) layer, as evidenced by piezo-response force microscopy These works reveal that, owing to their high mechanical flexibility, nanoporous materials offer unique opportunities for the design of strain-gradient mediated ME structures. V for further details) due to the reduced Young’s modulus of the porous metallic counterpart. In addition, in high surface area composites, the flexoelectricity caused by inhomogeneous strain generated by each MS ligament can be added to piezoelectricity leading to an enhanced ME effect
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