Multiferroic And Improper Ferroelectric Properties In Transition Metal Oxides

In recent years,the miniaturization of multifunctional device has become highly desirable.Magnetoelectric materials,owing to their potential applications on high-density data storage and magnetoelectric sensor,have attracted extensive attentions around the world especially on the type-II multiferroics.Although these materials manifest unique physical mechanism and strong magnetoelectric cross-coupling,the low ferroelectric/magnetic transition temperatures and relatively small electric polarizations are the main blocks.In this thesis,the performance improvement and future application of these multiferroic materials are taken as the driving force.On one hand,we try to improve and optimize the performance of these systems with modern epitaxial thin film technology.On the other hand,we deeply explore and understand the physical connotation in these systems.Therefore,the arrangement of this thesis is as follows:In the first chapter,we give a review on multiferroics,mainly focusing on the clarification of several type-II multiferroic and improper ferroelectric mechanisms with their research progress,in addition to the application of epitaxial thin film technology in altering the multiferroic properties.The second chapter discusses film sample preparation method,structural characterizations and various kinds of electrical/magnetic properties' measurements.In terms of thin film preparation,the adopted technology of pulsed laser deposition is introduced.In terms of structural characterizations,various scanning modes of X-ray diffraction in addition to the reciprocal space mapping(RSM)are discussed in detail,as well as the electron microscope technologies(including SEM and TEM).Moreover,The PPMS and MPMS low-temperature systems,referring to the measurements of electrical and magnetic properties,are briefly introduced.We focus on the thin film electrodes production and ferroelectric representations including the PUND method and pyroelectric current method.In the third chapter,structural characteristics and multiferroic properties of the orthogonal GdMnO3/SrTiO3 thin films are systematically investigated.In this work,the high-quality(001)orientation GdMnO3 epitaxial film was deposited on the substrate of(001)SrTiO3 using the pulsed laser deposition technology.Detailed high resolution TEM analysis found the presence of self-assembled nano-twinned domains in the plane of the film,which releases the large in-plane lattice mismatch to a great extent.During the ferroelectric and magnetic measurements,we found that the film shows exotic multiferroic behaviors,including high-Tc(?75 K)ferroelectric state,large spontaneous polarization(Pa?4900 ?C/m2)and relatively strong ferromagnetism emerging at?105 K.We propose a possible ab-plane spiral-spin-order phase to be responsible for the large spontaneous polarization in the films,which can only be stabilized by relatively high magnetic field H>6 T in the bulk crystals.It is suggested that the nano-scale twin-like domain structure is essential for the high temperature ferroelectricity and ferromagnetism of the thin films.In the fourth chapter,we carefully performed the pyroelectric current measurements of polycrystalline GdMn2O5 to investigate the possible influence of FE phase above room temperature on the magnetically induced ferroelectric phase at low temperature.It is revealed that with different poling processes we performed,the Mn-Mn short-range magnetic correlation(Local Spin Order)induced by stuctrally driven polarization raised.With the enhancent of the local spin order,the low temperature polarization Pmm associated with the Mn3+-Mn4+-Mn3+ symmetric exchange striction,and PGM associated with the Gd3+-Mn4+-Gd3+ symmetric exchange striction,could be parallel or antiparallel depending on the poling electric fields in different T-range,leading to a larger or a positive-negative total polarization P=PGM±PMM,respectively.In the fifth chapter,we study the structural characteristics and ferroelectric properties of hybrid improper ferroelectric Ca3Ti2O7/SrTiO3 epitaxial thin films.Hybrid improper ferroelectrics have their electric polarization generated by two or more combined non-ferroelectric structural distortions,such as the rotation and tilting of Ti-O octahedral in the Ca3Ti2O7 family.In this work,we prepare the high quality(010)-oriented Ca3Ti2O7 thin films on(110)SrTiO3 substrates by pulsed laser deposition.The good epitaxial growth of the Ca3Ti2O7 thin films on the substrates with the interfacial epitaxial relationship of[001]CTO//[001]STO and[100]CTO//[-110]STO is revealed.The in-plane ferroelectric hysteresis unveils an ultralow coercive field of?S kV/cm even at low temperature,nearly two orders of magnitude lower than that of bulk single crystals.The TEM images across the Ca3Ti2O7-SrTiO3 interface and the first principles calculations suggest that the in-plane tension and out-of-plane contraction of the lattice may not be the reason for the big difference of the coercive fields between the thin films and bulk crystals.These variations can be the consequence of lattice imperfections in the thin films other than substrate induced lattice strains,suggesting high sensitivity of the ferroelectric properties to lattice defects.The sixth chapter is the summary and prospect of this thesis.