Performance Extension Of Scanning Probe Microscopy Under Extreme Conditions And Characterization Of Magnetic Materials

Scanning probe microscopy(SPM)is an important invention of nanoscience,and its application has led to a paradigm shift in the understanding and perception of matter at the nanoscale.It provides insight into structural,electronic,vibrational,optical,magnetic,(bio)chemical and mechanical properties.According to the different physical quantities of the feedback,SPM includes the scanning tunneling microscope(STM)based on the tunneling signal,the atomic force microscope(AFM)based on the force,and a series of subsequent variants of the two.A typical representative is a magnetic force microscope(MFM)capable of imaging magnetic effects by being equipped with a magnetic probe.After continuous development,SPM has the ability to work under extreme physical conditions(ultra-low temperature,strong magnetic field and ultra-high pressure).While the spatial resolution of SPMs under extreme conditions has reached unprecedented atomic scales,the traditional temporal resolution of SPMs is limited by the bandwidth of the signal acquisition electronics and the resonant frequency of the scan head,typically on the order of microseconds.In addition,in terms of magnetic field control,the main focus is to pursue higher magnetic field strength,but ignores the importance of the control of the magnetic field direction for material exploration:this will undoubtedly provide a new spatial perspective for the exploration of anisotropy in materials.In view of the above background and needs,my work during the doctoral period mainly includes the following three parts:1:In order to add the control of magnetic field direction to MFM research in higher magnetic field and wider temperature range,we developed a fully piezoelectric rotatable MFM(R-MFM)system.This is the world’s first fully piezoelectric,rotatable scanning probe system.The full piezoelectric design avoids a series of problems caused by mechanical structures such as vibration,heat leakage,and angle accuracy.The R-MFM is able to use spring suspension and can work in a 10 T superconducting magnet(GM refrigerator)without any shock absorbing measures.The low temperature part is realized by a self-made liquid nitrogen Dewar and a low temperature chamber specially designed for R-MFM.We verified the rotational performance of the R-MFM by imaging commercial video tapes.Based on the experimental results,we propose a plausible.hypothesis for the change of the magnetic moment on the magnetic tip during the rotation of the MFM.Then,we observed the magnetic domain evolution of post-annealed La0.67Ca0.33MnO3/NdGaO3(100)films with intense competition under the control of magnetic field direction.It further demonstrates the new research opportunities brought by magnetic field direction regulation to magnetic materials science.2.We built a long-wavelength light-driven scanning probe microscope(LD-SPM)system.By coupling the tip-surface junction of SPMs with infrared light,terahertz pulses,visible laser light,etc,the inherent limitations of SPMs both electronically and mechanically can be overcome.In this way,it is possible to explore the ultrafast electronic behavior of materials at picosecond or even femtosecond resolution,and to establish near-field and magnetic analysis methods under long-wavelength light.We innovatively present a compact and efficient optical cryostat designed for LD-SPM testing over a wide temperature range of 4.2 K-300 K under a magnetic field.Its special multi-layer radiation shielding insert(MRSI)forms an excellent temperature gradient when filled with heat-conducting gas,so that a layer of optical window realizes the gas heat conduction cooling of the test cell.This minimizes the loss of light when it enters a low-temperature environment,while avoiding the additional mechanical vibration and thermal drift caused by other cooling methods.The design superiority of MRSI has been proved by the steady-state thermal analysis of ANSYS software and the actual temperature test.The morphology and magnetic domain images of a 45 nm thick La0.67Ca0.33MnO3 film on a NdGaO3(001)substrate under a magnetic field were obtained through the self-made LD-SPM in this thermostat.The resolution and noise spectrum during imaging reveals the temperature stability and low vibration of the complete system.The system is scalable to various calibers and types of magnets,and is suitable for various types of optical research including nearfield measurements.3.As the research on matter and materials has penetrated into the microcosm and multiple parameters(electron,charge,spin,momentum,etc.),people have become more interested in the interaction between matter at the micro-nano scale and terahertz electromagnetic waves.Based on the successful experience in the LD-SPM system,we try to build an accurate test system for advanced material physical properties in the terahertz spectrum.In the vector magnet(5-9 T for the x-z direction),we introduce the characteristic terahertz light technology and ultrafast pulse of the National Synchrotron Radiation Laboratory into a multifunctional SPM device designed for narrow spaces that can scan in a wide range.This enables high-resolution,highsensitivity,and high-throughput physical property testing and analysis of advanced functional materials in the terahertz spectrum.At the same time,the ultrafast nature of light in this system can be used to excite and explore various types of excitation and transient behavior in associated materials.We expect that this system will be able to carry out high-throughput physical property screening research in a variety of key functional materials including multiferroics,superconductivity,and giant magnetoresistance,filling the gap in this field in the world.