Controllable Synthesis And Multiferroic Properties Exploration Of Two-Dimensional ε-Fe₂O₃

The discovery of two-dimensional（2D）multiferroics characterized by cross-coupling of ferroelectric order and magnetic order,provides a material cornerstone for constructing high-density information storages and low-energy-consumption spintronic devices.The strong interactions between various order parameters are crucial for realizing multifunctional applications.Nevertheless,due to the mutually exclusive physical origin of ferromagnetism and ferroelectricity,it is difficult to achieve this goal in classical 2D materials at room-temperature（RT）.To solve the above scientific problems,this paper designs an ingenious space-confined chemical vapor deposition（CVD）strategy to synthesize atomically thin nonlayeredε-Fe₂O₃ single crystals and discloses the room-temperature long-range ferrimagnetic and ferrielectric order,respectively.More significantly,the robust room-temperature magnetoelectric（ME）coupling is uncovered by controlling the magnetism with electric field and verifies the multiferroic feature in atomically thinε-Fe₂O₃.This work not only represents a substantial leap in terms of the controllable synthesis of 2D multiferroics with robust magnetoelectric coupling,but also provides a material basis for the practical applications in low-energy-consumption electric-writing/magnetic-reading devices.Then,the details of this article are presented as follows:The first chapter briefly summarizes the classification and research development of multiferroics.Besides,three significant mechanisms of magnetoelectric coupling in single-phase multiferroics,the conventional synthesis methods of 2D multiferroics and their potential applications in multifunctional devices are introduced,respectively.What’s more,the major scientific problems in the development of magnetoelectric multiferroics and the innovation significance of this paper are elaborated.In Chapter II,the reagents and characterization instruments involved in this work are listed.In addition,the fabrication of back-gate field effect transistors（FETs）and ferroelectric capacitor devices is introducted,and the ferroelectric and ferromagnetic characterization methods are emphasized.In Chapter III,a unique space-confined chemical vapor deposition method is designed to grow large size,ultra-thin,pure phase and non-layeredε-Fe₂O₃ single crystals by regulating the carrier gas flow rate and growth temperature,which provides a new idea for the synthesis of novel non-layered 2D materials.In detail,the phase,crystalline quality,atomic structure,element composition and chemical environment of the samples are analyzed by Raman,XRD,XPS,EDS and TEM/STEM.In addition,ε-Fe₂O₃ is a non-layered orthogonal structure,with the space group of Pna21.Notably,Fe³⁺ions occupy the centers of oxygen octahedrons and tetrahedrons,respectively.Ultimately,the size effect of Fe³⁺ion and its spontaneous swapping between tetrahedral and octahedral sites should result in the formation of ferroelectricity.In ChapterⅣ,the physical properties ofε-Fe₂O₃ nanosheets are comprehensively characterized.Firstly,the semiconductor behavior is confirmed by the electrical transport measurement.Then,PFM characterizations unambiguously prove the strong ferroelectricity and its switching behavior,accompanied with an anomalous thickness-dependent coercive voltage.Besides,the remnant polarization ofε-Fe₂O₃ is calculated to be～770.7μC/cm² using the polarization-voltage（P-V）loop method.SQUID/MOKE/MFM characterizations indicate thatε-Fe₂O₃ has the ferromagnetic order and the thickness-dependent ferrimagnetism at room temperature.Notably,the Curie temperature（T_c）is up to～291 K,and the anisotropic ferrimagnetism and in-plane easy axis in 2Dε-Fe₂O₃ with the single magnetic domain are confirmed.In addition,the electric field-assisted MFM measurements verify the robust magnetoelectric coupling in atomically thinε-Fe₂O₃ and the control ability of electric field on magnetism is confirmed,which provides a significant reference for the exploration of magnetoelectric coupling in single-phase multiferroics.ChapterⅤsummarizes the controllable synthesis and the structural characteristics of non-layeredε-Fe₂O₃ nanosheets.In addition,the electrical,ferroelectric,ferromagnetic and magnetoelectric coupling properties of 2Dε-Fe₂O₃ are discussed.Finally,the scientific problems and promising development opportunities in the controllable synthesis of 2D non-layered materials,the physical properties research of new two-dimensional single-phase multiferroics and the application of multifunctional devices are further elaborated.