The Research On Structural Design And Performance Of Multiferroic Bismuth-Based Manganese Oxide Thin Films

With the advent of the era of big data and artificial intelligence,the exponential growth of data has put forward higher demands on storage technology.The realization of higher speed,higher density and lower power consumption in data storage is imminent.However,the storage density and writing efficiency of current mainstream magnetic storage are still low.Although ferroelectric storage overcomes the above defects correspondingly,they are limited by the need for a destructive read and reset operation.Multiferroic materials,which show ferromagnetism and ferroelectricity simultaneously,provide a unique platform for realizing the compatibility of the two.As the most widely studied lead-free multiferroic materials,bismuth-based multiferroic perovskite oxide thin films show attractive prospects of the applications of storage technology.In this thesis,the correlations between the multiferroic properties and external factors such as strain,special freestanding structure and superlattice interface of multiferroic BiMnO₃ thin films are studied.Firstly,by adjusting the lattice mismatch of films and substrates,the multiferroicity of single-phase BiMnO₃ thin films is tuned.Then,the flexible freestanding BiMnO₃ membranes with stable structure and multiferroic properties are prepared using water-soluble sacrificial layer to release the strains of the substrate.Finally,by tunning the periodic thickness parameters,the coexistence of strong ferroelectricity and ferromagnetism have realized in BiMnO₃-based superlattice systems.Our researches mainly include the following three parts:Single-phase BiMnO₃ thin films with different epitaxial strains are designed to tune their ferroelectric and ferromagnetic properties.High-quality BiMnO₃ thin films with different epitaxial strains were fabricated using pulsed laser deposition by selecting different single crystal substrates.Through improving the structural asymmetry in the out-of-plane direction of the films,compressive strains produce larger lattices tetragonality in BiMnO₃ films,which effectively enhance the ferroelectric properties of the films.Meanwhile,it is found that the ferromagnetic state of BiMnO₃ films is less sensitive to the epitaxial strains.By increasing the proportion of Mn²⁺ions,tensile strains change the Mn ion e_g orbital electron occupancy and magnetic exchange to increase the magnetic coercive field of BiMnO₃ films.Overall,compressive strains are more conducive to realizing the potential magnetoelectric coupling of BiMnO₃ films.This work has important guiding significance of exploring the realization of high-performance multiferroic materials.By choosing single crystal LaAlO₃ with smaller lattice constant as the substrate,the effect of larger compressive strain on the structure,piezoelectric and magnetic properties of BiMnO₃ films are further explored.The results show that the lattice mismatch of-3.65%contributes to form the special layered supercell structure of BiMnO₃.By adjusting the growth oxygen partial pressure,layered supercells with two different Bi-O structures were fabricated.Both the supercells show typical piezoelectric responses.Due to the different Bi-O structures of the out-of-plane direction,two supercells show difference in the magnetic super-exchange interaction between Mn ions,resulting in different saturated magnetizations of the two supercells.Based on broadening the applications of materials,BiMnO₃ films with flexible freestanding structures are designed to explore its multiferroic properties.Using pulsed laser deposition,water-soluble Sr₃Al₂O₆ thin films were fabricated to serve as the intermediate sacrificial layers for achieving the flexibility of BiMnO₃ films.The freestanding BiMnO₃ membranes are verified to show high quality with stable structure.Due to the high stability of lattices,the ferroelectricity and ferromagnetism of the transferred freestanding BiMnO₃ membranes are preserved well.Moreover,the freestanding BiMnO₃ membranes exhibit novel super-flexibility,which originates from the dynamic evolution of ferroelectric nanodomains of the BiMnO₃ membranes.In BiMnO₃-based composite system,the superlattice structure of BiFeO₃/BiMnO₃ is designed with different periodic thickness parameters to tune its multiferroicity.By precisely adjusting the growth parameters using pulsed laser deposition,high-quality BiFeO₃/BiMnO₃ superlattices with pure phase and clear interfaces were obtained.Through adjusting the periodic parameters of the superlattices,we find that the polarization of BiMnO₃ layers is induced by the strong ferroelectric polarization field of the thicker BiFeO₃ layers.Moreover,the introduction of antiferromagnetic BiFeO₃ leads to the interfacial antiferromagnetic super-exchange coupling of Fe-O-Mn,which tends to kill the net magnetic moment from BiMnO₃ layers.By means of obtaining an appropriate ratio of periodic thickness between the BiFeO₃ layers and the BiMnO₃ layers,the coexistence of strong ferroelectricity and ferromagnetism have achieved finally at low temperature in BiFeO₃/BiMnO₃ superlattices.