| Magnetic force microscopy(MFM)is one of the most powerful high-resolution magnetic imaging tools for studying and analyzing local magnetic properties in surface structures at nanometer scales.It can be considered as a special mode of operation of an atomic force microscopy equipped with a ferromagnetic tip.Since its invention in 1987(by Y.Martin&H.K.Wickramasinghe),it has been widely used in directly imaging magnetic features of magnetic materials as well as the modern magnetic storage devices,such as the finding of alternating net magnetic moments at ferroelectric domain walls around vortex cores in multiferroic hexagonal ErMnO3,the directly imaging of vortices in superconductors and the observation of charge-ordered insulator phase transition from a pure ferromagnetic metal in a manganite film.Nowadays,the main developments in MFM are focused on the quantitative analysis of date,improvement of magnetic signal resolution,and the application of external fields during measurement(Schwarz&Wiesendanger,2008).Besides,with the increasing interesting on micron-scales magnetic storage devices such as patterned nanomagnetic films,a high resolution MFM which can visualization the domain structure of a certain device on a large temperature range under extremely high magnetic field is urgent demanded.Based on the above considerations,we design and build a multi-functional MFM and a lot of measurement works has also been done.Due to the different research contents,we divide this PhD thesis into two parts.In the first part,we first briefly introduce the basic information about MFM in chapter 1 and then our home-made MFM in chapter 2.In the second part(chapters 3-6),many practical measurement results based on this home-made MFM are shown.A lot of efforts have been put into improving the imaging quality and the results are carefully analyzed.In chapter 1,some basic information of MFM such as development history,principles,explanation of image contrast,and working models are introduced.In order to upgrade existing magnetic microscope technology,different materials and designs are tried and this part of work is described in chapter 2.We have successfully designed and constructed a high resolution MFM which can measure the device samples under extreme conditions.Chapters 3-6 are test results.In chapter 3,we for the first time show the real space observation of skyrmions and skyrmion clusters in ultrathin oxide materials(only 5 unit cell,-2 nm).This part of work has been accepted by the journal Nature Materials(IF-40).In chapter 4,we directly visualize the structure and phase dynamics of the Electronic Multiple Ordering(EMO)in a Pr0.5Ca0.5MnO3 thin film at the microscopic scale utilizing our high magnetic field MFM.The field needed to completely melt such EMO is as high as 17.6 T,which is the highest field record in MFM measurement.This part of word has been published in ACS Applied Materials&Interfaces(If=8.1).In chapter 5,we present new research demonstrating the controllability associated with fabricating a class of structural domain walls(DWs)in various phase-separated manganite thin films.Using MFM,we systematically explored the physical properties of the DWs,and their critical roles in the microscopic patterns of phase separation states.Specifically,this part of work presents three scientific advances:(1)The DWs form closed loops,with no preferred orientations,and exhibit no dependence on temperature or magnetic-field.(2)Moreover,through direct imaging with our home-made MFM,we also confirm that the DWs are ferromagnetic and metallic,while their surrounding domains are not.A Curie temperature as high as-280 K was experimentally determined in the DWs,which is approaching the requisite room temperature and would thus constitute a significant breakthrough for device operation.Notably,this value cannot be obtained by macroscopic measurement due to the very limited volume of the DWs.(3)Moreover,we found that the DWs can act as immobile spatial restrictions that confine the growth and emergence of the phase-separated domains.Hence,the DWs could play a deterministic role in phase competition and the evolution of phase-separated manganite films.This part of work has been accepted by the journal Advanced Materials(IF=22)and I am the first author.Two-dimensional(2D)magnetic oxides are very important for applications such as ultrasmall quantum-spintronic devices.It is necessary to find their unique magnetic properties that are different from the three-dimensional(3D)system.To do that,two extreme conditions are better met,including an extremely uniform 2D structure and an extremely unique experimental condition,i.e.,absolute low temperature near zero and very large magnetic field for the 2D characters to stand out.But such experimental studies are very challenging since it is difficult to meet the two extremes at the same time.Xiaofang Zhai et al.have demonstrated that they can grow ferromagnetic LaCoO3 ultrathin films(8-11 nm)with extremely stoichiometric composition and uniform structure from the interface all the way to the surface(D.Meng et.al.,PNAS 115,2873(2018)).Thus the uniform ultrathin LaCoO3 film is a model system for investigating the unique 2D magnetic character.In chapter 6,for the first time,we report a pure magnetic phase separation in a 2D film of uniform atom and charge distributions.Although the film is uniformly structured and extremely stoichiometric((?)0.8%oxygen vacancies,the record-high stoichiometry),we observed a stunning 50%-50%phase separation between the ferromagnetic phase and the non-ferromagnetic phase down to the lowest measurement temperature(4.5 K)and largest achievable magnetic field(13.4 T).Our findings are of fundamental and practical importance for future studies of the 2D magnetic materials. |
PDF Full Text Request