Dynamics-based Atomic Force Microscopy For Electro-magneto-mechanical Couplings Of Multiferroics At The Nanoscale

Multiferroic materials possess multiple ferroic orders simultaneously,rendering them interesting multi-field coupling,which makes them have broaden application prospects in the fields of sensors,memories,and spintronic devices.In recent years,nanostructured multiferroics have received extensive attention due to their possibility of promoting the multifunction,integration,and miniaturization of electronic devices.The study of the magneto-electro-mechanical coupling at the nanoscale can not only deepen the understanding of complex physical and mechanical phenomena of multiferroicity,but also provide a basis for optimization device design and the performance of multiferroic materials.However,current multi-field coupling characterization techniques in nanoscale have deficiencies.Therefore,various techniques based on dynamic strain AFM are developed to characterize ferromagnetic,ferroelectric,mechanical properties and magnetoelectric coupling of multiferroic materials at the nanoscale.The principles of several technique are explained,and part experimental setups are designed,and then the feasibility of the techniques is demonstrated on different samples.The main research work of this thesis are summarized as follows:(1)Multiferroic composite nanofibers PZT-CFO with different compositions have been synthesized by sol-gel based electrospinning,and their Young's modulus has been quantitatively measured by LE PFM,validated by good agreement with GE CR-AFM,nanoindentation,as well as micromechanical rule of mixture prediction.It is observed that the Young's modulus of the composite nanofibers increases with the increased CFO content,while their decreased piezoelectricity has been simultaneously mapped by LE PFM.(2)CFO-BFO core-shell nanofibers have been synthesized by coaxial electrospinning in combination with sol-gel process,and their microstructures have been measured by XRD and TEM.Multiferroic properties of core-shell nanofibers have been verified,their ferroelectricities are confirmed by PFM mappings and switching butterfly loop,and ferromagnetism is confirmed by magnetic hysteresis loop.Magnetoelectric coupling of core-shell nanofibers has been observed by using a novel SPM based technique,obvious changes in PFM mapping and switching butterfly loop are shown,which are induced by an in plane external magnetic field.(3)Capacitive excitation PFM(ce-PFM)is developed to probe and image the intrinsic piezoresponse at the nanoscale with minimized artifacts and cross-talks,overcoming one of the main difficulties in conventional PFM that often exhibits apparent piezoresponse in non-piezoelectric materials.We accomplish this technique by using nonconductive probe to detect the piezoelectric vibration of the sample,while the excitation is realized through capacitive effect using a metal disc underneath.This technique provides an effective tool to distinguish intrinsic piezoelectricity from non-piezoelectric mechanisms,and it can be easily implemented in conventional atomic force microscope(AFM)setup to probe intrinsic piezoelectric materials at the nanoscale.(4)Piezomagnetic force microscopy is developed to quantitatively characterize ferromagnetic properties at the nanoscale.The working principle and experimental set-up of the technique are introduced.The magnetic excitation module is designed,and its DC and AC magnetic field strengths are corrected.The results show that the difference between the value for designed and measured is less than 1%.We selecte ferromagnetic material Tefenol-D and non-magnetic material glass as the typical two kinds of sample,and confirm the response we measured using piezomagnetic force microscopy derived from the intrinsic piezomagnetic response of material.Finally,the ferromagnetic properties of the multiferroic Bi5Ti3FeO15 film are characterized by PmFM,and the magnetic domains of the film are written and read successfully.