2026-09-18T20:25:20Zhttps://api.repository.cam.ac.uk/server/oai/request

oai:www.repository.cam.ac.uk:1810/3809262025-03-13T01:43:32Zcom_1810_34586com_1810_256064col_1810_205358

Voltage Control of Magnetism in Novel Structured Vertically Aligned Nanocomposite Thin Films https://doi.org/10.17863/CAM.116349 de h-Ora, Muireann Driscoll, Judith This thesis investigates voltage control of magnetism (VCM) in novel structured VAN thin films. Both magneto-ionic (MI) and magnetoelectric (ME) systems were studied. Magneto-ionic devices, which utilise voltage-driven ion migration to control magnetic properties, hold promise for energy-efficient, non-volatile magnetic memory and neuromorphic computing. A study of the current literature suggests that a high-surface interface and single crystalline structures could be advantageous for promoting large ionic migration and device durability. Epitaxial nanopillars, fabricated using a novel method based on VAN structures, can offer just that. This work demonstrates significant advancement in MI systems, achieving large changes in magnetic moment saturation (mS) and coercivity (HC) in CoFe2O4 nanopillars through voltage-induced oxygen ion migration. Notably, a rapid response time and high endurance were achieved, surpassing other magneto-ionic systems. Magnetoelectric systems exhibit a strain coupling between magnetic and electric order parameters. Research highlights the need for stronger strain coupling between phases and less electrical leakage for functional ME devices. The high surface area interface between phases in VAN structures not only enhances strain coupling but also benefits from a low-defect, single-crystalline structure that ensures high alignment of magnetic domains and minimal clamping to the substrate. This thesis delves into the ME properties of several dual-phase VAN thin films, a promising device structure with large area vertical interfaces to enhance strain coupling out-of-plane while also being unaffected by substrate strain. New material combinations were explored, specifically chosen to enhance ME coupling and reduce electrical leakage compared to previous work. Notably, a ME VAN system of CoFe2O4:PVDF-TrFE was fabricated in a novel way, to allow the low-leakage ferroelectric polymer to be integrated with large surface area, single crystal CoFe2O4 nanopillars. This versatile system was tested for both the direct magnetoelectric (DME) or converse magnetoelectric (CME) effect, achieving the highest recorded DME coefficient, αDME , for CoFe2O4:PVDF-TrFE systems. Materials with high ferroelectric and magnetostrictive properties were investigated to further enhance the range of possible materials in a ME VAN film. FeGa has the highest magnetostrictive coefficient of any non-rare earth material but is difficult to incorporate in a VAN system as it is a non-oxide. In this thesis, FeGa is grown in a VAN film alongside highly ferroelectric BaTiO3 and tested with the prospect of large ME coupling. A BaTiO3:Ni system is also investigated as a ME system, as Ni, while not as MS as FeGa, can potentially limit the electrical leakage in BTO through cross-substitution. Currently, the only widely available conducting substrate for growing epitaxial oxides, including VAN films, is Nb-doped SrTiO3. However, CoFe2O4 has a dissimilar crystal structure to Nb-doped SrTiO3, leading to less-than-ideal growth. To address this, conducting NiCo2O4 layers were grown on single-crystal MgAl2O4 as potential electrodes. Insulating spinel MgAl2O4 shares a similar lattice parameter with NiCo2O4 and is a cost-effective, readily-available substrate. The spinel structure of CoFe2O4 is expected to grow more ide- ally on NiCo2O4 than on Nb-SrTiO3. Although NiCo2O4 was previously shown to lose high conductivity at temperatures above 400 oC—temperatures required for the growth of CoFe2O4—it was stabilised on MgAl2O4 by the growth of a CoFe2O4 capping layer, maintaining its conductivity. This not only benefits CoFe2O4-based magnetoelectric devices, but it shows that NiCo2O4 could be used as an electrode material on MgAl2O4 for a wide range of other spinel materials which have interesting functionalities for magnetic devices, solid-state battery electrodes and beyond. University of Cambridge 2024-10-22 Thesis Doctoral Doctor of Philosophy (PhD) eng This work has received funding from the European Union’s Horizon 2020 research and innovation program BeMAGIC under the Marie Sklodowska-Curie Grant Agreement No. 861145. This work also received support from the EPSRC CDT in Nanoscience and Nanotechnology (NanoDTC). https://www.repository.cam.ac.uk/handle/1810/380926 https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/412f77ca-a0cb-4467-b713-3a49cb686ce6/download 5d73e5d4960090412eaa825e7518a0f2 https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/deca4ccc-6b57-4418-a33f-c1856f507bc4/download 87eda9de84448d1f82354d60eee3eb5f http://purl.org/NET/rdflicense/allrightsreserved Vertically Aligned Nanocomposites Magnetoelectrics CoFe2O4 PVDF-TrFE NiCo2O4 Magnetostrictive Ferroelectric