Magnetoelectric Heterostructures with Reduced Substrate Clamping Effect

Abstract

Due to their ability to produce electric fields by applying external magnetic fields, magnetoelectric (ME) materials have attracted considerable interest for many applications. In particular, they have an instrumental role in biomedicine by enabling wireless precision drug delivery, sensing or wireless cellular/neural electrostimulation via the application of an externally-applied magnetic field. The increased interest in magnetoelectric composites has brought forward issues which were not previously effectively dealt with; namely the substrate clamping effect which can reduce the effectiveness of composite systems. Particularly, the incorporation of flexible substrates or nanopatterned structures, can potentially minimize this effect. On this basis, the work presented in this PhD thesis is focused on the development of novel magnetoelectric composite systems which will incorporate two different methods to tackle the problem of substrate clamping, for biomedical applications._x000D_ _x000D_ Firstly, a highly flexible magnetoelectric composite film was developed by embedding a magnetoelectric Au/Ti/FeGa/ZnO heterostructure within a polydimethylsiloxane (PDMS) elastomer layer. The film was fabricated using a combination of physical vapor deposition and hydrothermal synthesis techniques. The hydrothermally-grown hexagonal ZnO nanosheet layer in the composite film exhibited a piezoelectric character, with a piezoelectric coefficient of d33 = 11.2±0.3 pm·V-1. The composite film also showed in-plane magnetization with a moderate coercivity of Hc = 200 Oe. To evaluate the magnetoelectric response, the film’s electric field-induced methylene blue degradation under a magnetic field was studied. The most significant degradation was observed under a 200 Oe magnetic field at a frequency of 100 Hz, indicating the generation of the largest electric fields. Biocompatibility testing demonstrated that the heterostructure was compatible with living cells. Cell proliferation studies revealed a 42% enhancement in cell growth after 7 days of 1-hour daily magnetoelectric stimulation, suggesting the potential for biomedical applications._x000D_ _x000D_ Secondly, magnetoelectric Au/FeGa/BaTiO3 suspended microdisc heterostructures were fabricated using physical vapor deposition, colloidal lithography, and reactive-ion etching. The properties of these structures were compared to non-suspended microdiscs and continuous films with the same composition. The suspended microdiscs exhibited a ferroelectric character and in-plane magnetization, with similar coercivity to the other morphologies. However, the saturation field differed among the three structures. The suspended microdiscs demonstrated significantly improved magnetoelectric performance due to reduced substrate clamping. Mechanical response measurements showed that the edges of the microdiscs exhibited stronger responses compared to the clamped centers and the continuous film. The piezoelectric coefficients of the continuous film, non-suspended microdiscs, and suspended microdiscs were calculated as 3.7±0.3 pm·V-1, 7.7±0.4 pm·V-1, and 13.2±0.4 pm·V-1, respectively. The magnetoelectric coupling coefficient was highest for the suspended microdiscs, measuring 730±70 V·cm-1·Oe-1 at the center and 1040 ± 70 V·cm-1·Oe-1 at the edge of the microdiscs, compared to 260±70 V·cm-1·Oe-1 for the continuous film. The superior performance of the suspended microdiscs was further demonstrated in catalytic experiments, where the magnetoelectric-induced electric field exhibited a significant effect on methylene blue degradation under a magnetic field of 200 Oe at 100 Hz. Biocompatibility testing confirmed that all three configurations were compatible with living cells. Preliminary results also indicated the ability to stimulate bone cells cultured on the suspended magnetoelectric microdiscs using a 200 Oe alternating magnetic field, as observed through confocal fluorescence microscopy. Overall, the highly flexible magnetoelectric composite film and suspended microdisc heterostructures hold great potential for diverse applications, particularly in biomedicine._x000D_ _x000D_ In summary, the work of this thesis has shown that the performance of magnetoelectric systems can be enhanced by the implementation of two different approaches to reduce the substrate clamping effect: (i) an elastomer substrate or (ii) by nanopatterning the composite into free-standing structures. The improved magnetoelectric response of both approaches makes this type of systems appealing for applications in biomedicine.

Date of Award	27 Jun 2023
Original language	English
Supervisor	Borja Sepúlveda Martínez (Director) & Josep Nogués Sanmiquel (Director)