Scanning Probe Microscopy Studies On Several Ferroic And Topological States

In recent years,scanning probe microscopy,as a powerful imaging technique in materials science,continues to develop,introducing more measurement modes with better performances,which enables us to observe physical and chemical properties in a microscopic scale.Scanning probe can also apply force,electric and magnetic field for the local manipulatation of the sample,so that it has been widely used as a micromachining tool.Perovskite oxides exhibit a wide range of tunable magnetic and electronic properties due to the interaction among lattice,orbit,charge and spin degrees of freedom.Recently,complex ferroelectric domain structures and multi-functional domain walls have been observed in perovskite ferroelectric thin films.Investigations on polarization configurations,formation mechanisms,and precise manipulations of domain structures,are significant for the development of high density,low power consumption ferroelectric memories and novel domain wall devices.On the other hand,the discovery of ferromagnetism,ferroelectricity,superconductivity and novel topological states in two dimensional materials expands the understanding of low dimensional materials and provides a platform to explore low-dimensional physics and corresponding novel devices,and therefore,attracts a great deal of attention.In light of the above-mentioned progress,we perform various scanning probe microscopy measurements to study ferroic and topological states in perovskite ferroelectric thin films and two dimensional materials.Major achievements in this thesis include:1.High quality Pb Zr_0.1Ti_0.9O₃/Sr Ru O₃（PZT/SRO）heterostructures have been deposited on Dy Sc O₃（DSO）or Sr Ti O₃（STO）substrates by pulsed laser deposition（PLD）.On DSO substrates,in addition to the conventional c and a domains,piezoresponse force microscopy（PFM）reveals complex domains with network,labyrinth and bubble structures The out-of-plane polarization in these complex domains is opposite to the film matrix.Across the domain walls,in-plane PFM signals with a 180°phase change can be observed.These domains are sensitive to both electric and mechanic bias loaded on the cantilever probes and can be written or erased in a controlled way.However,PZT films grown on STO substrates show a uniform c monodomain structure.It is proposed that a domains lower the enery barrier between+c and–c domains,facilitating the reversal of c domains to form complex domain patterns in certain circumstances.There could be two polarization models for these domains.The first one,there is no in-plane polarization at 180degree out-of-plane domain walls,and such in-plane signal is caused by the shear effect d₃₅.For the second possible model,there is indeed in-plane polarization at 180degree out-of-plane domain walls to form polar vortex or flux closure structures.2.High quality Bi Fe O₃（BFO）films,30 nm in thickness,has been deposited on La Al O₃（LAO）substrates by PLD.Both X-ray diffraction（XRD）and PFM measurements demonstrate the coexistence of compressive strain induced tetragonal-like（T-like）phase and strain relaxation induced rhombohedral-like（R-like）phase.PFM reveals all the four types of T-like domains and discovers that morphotropic phase boundaries can enhance the out-of-plane piezoresponse of BFO.There are also in-plane polarization signals at morphotropic phase boundaries.In addition,T-like phase and R-like phase have distinct mechanical properties:R-like phase is softer and has a stronger adhesion.By loading mechanical force via cantilever probes,we achieve the transformation from T-like to R-like phase,accompanied by a large mechanical strain and resulting in in-plane polarization signals at newly created morphotropic phase boundaries.3.In Kelvin probe force microscope（KPFM）,the electrostatic coupling between conducting probes and ambient stray electric field is a major source of error in the measurements,resulting in incorrect data interpretation.We used specific cantilever probes with metal shielding layers to make measurements at various tip-sample distances.By proper fitting procedures,we are able to effectively eliminate the electrostatic coupling.Employing this improved KPFM technique,we have studied the chemical potential change in a function of carrier density in monolayer graphenes at zero magnetic field,extracting the Fermi velocity as 1.1×10⁶ m/s.Due to the electron-electron interaction,Fermi velocity drops by 10%at higher carrier densities from the value at the charge neutral point.Under 9 T out-of-plane magnetic field,graphene’s band transforms to a series of discrete Landau levels,as observed by KPFM with the lift of the four-fold degeneracy of zero order Landau level and the negative compressibility of the carriers.Finally,both the width and the band gap of each Landau level decrease with the decrease of magnetic field.These results highlight the sensitivity and accuracy of the improved KPFM technique.4.We have developed a novel magnetic force microscope（MFM）technique to measure van der Waals,electrostatic and magnetic forces between the cantilever probes and samples simultaneously,preventing the interference to MFM signals from sample topography and electrostatic potential.Employing this improved MFM,the reversal process of magnetic moments in Cr I₃ samples in different thicknesses has been observed and analyzed semi-quantitatively.It is a two-stage switching.At the first stage,MFM signals change abruptly around 2 T,implying the switching of hard magnetic layers.At the second stage,soft magnetic layers switch gradually at lower fields.Cr I₃ samples in different thicknesses show a similar hard layer thickness,around 25 nm.The Curie temperature of the hard layers is 45 K,while that of the soft layers is 60-70 K.We discussed the correlation between the magnetic order and the interlayer stacking order of Cr I₃ samples and proposed a model of the magnetic structure:there is a 12 nm-thick hard layer at each surface of the sample,in monoclinic phase at low temperature,and the adjacent layers are strongly antiferromagnetic coupled.The soft inner layers are in rhombohedral phase at low temperature,while the adjacent layers are weakly ferromagnetic coupled.In samples of intermediate thicknesses,these two types of layers can coexist to form natural magnetic heterostructures.5.We used scanning microwave impedance microscopy（s MIM）to image the conductivity of monolayer WTe₂ and found that the interior of the sample is insulating while the conductive states strictly follow the outer physical edges.When tuning the chemical potential via electrostatic gating,bulk conductivity changes greatly while the conducting edge states remain unchanged,suggesting its nontrivial nature.The application of 9 T out-of-plane magnetic field can break the time reveral symmetry and suppress the edge conductivity.These phenomena reveal the quantum spin hall（QSH）nature of the conductive edges.In addition to the outer sample edges,we find other QSH states in the interior of the sample including cracks,degradation and oxidation induced topological boundaries,for the first time.Higher the temperature is,harder the separation of the bulk and the edge conductivity.By measuring at higher frequency,the conducting edge states can be observed at 77 K and even 100 K.We also find that the properties of bilayer WTe₂ are related to the angle between adjacent layers.When it is 5°,bilayer WTe₂ can be regarded as two independent monolayers with each layer behaving as a topological insulator.When the angle is 180°,it is consistent with a natural bilayer,where the coupling between layers cannot be neglected.