The Construction And Application Of Scanning Probe Microscopes Based On The Water-cooled Magnet

Scanning tunneling microscope(STM)plays a crucial role in condensed matter physics due to its real-space imaging with atomic resolution and the ability to obtain the local electronic state density of a sample.However,the development of scientific research has placed increasing demands on experimental conditions such as the strength of magnetic fields.Macroscopic transport experiments have shown that the suppression of superconducting state in some unconventional superconductors requires a magnetic field lager than 30 T;the extremely large magnetoresistance in some materials remains unsaturated at magnetic fields up to 60 T;phase transitions in some materials in heavy fermion systems also require magnetic fields above 30 T,as well as lower temperatures.This puts a high magnetic field strength requirement on the use of STM for the study of these systems.After more than 40 years of development,there are commercial products for scanning tunneling microscope.However,most of its supporting superconducting magnets are limited by the critical field,and the magnetic field that can be applied is generally does not exceed 20 T,which is much lower than what is required in the systems described above.At present,water-cooled and hybrid magnets can provide more than 3 0 T of magnetic fields,and combining STM and magnetic force microscope(MFM)with them will greatly expand its application range.This paper introduces the construction of the scanning probe microscope(SPM)imaging device compatible with ultra-high steady magnetic fields based on the water-cooled magnet and hybrid magnets of the High Magnetic Field Laboratory,which can play an important role in the research of novel physical properties,magnetoresistive and superconducting materials.Coarse motors are one of the core components of scanning probe microscope.To improve the approach accuracy,a non-inertial linear piezoelectric motor is designed.The friction relations required for operation can be satisfied automatically,the assembly control is simple,but the structure is compact and the rigidity is high.The dependence of the step size on the signal amplitude and frequency is examined,as well as the dependence of the step size on the signal amplitude for different loads.The results show that the starting voltage of the motor is low,only 35 V at room temperature,and the linearity of the step size and signal amplitude is excellent,the step size is in the range of 100 nm to micron,and the step size is accurate,which is quite suitable for the approach between the probe and the sample.A motor-based scanning tunneling microscope is then assembled.Finite element analysis shows that the structure has a high natural frequency and strong rigidity.Atomically resolved images of graphite are obtained by using the new STM unit in room-temperature atmospheric environments.In this paper,the construction of STM system based on hybrid magnets is introduced.In this work,the first atomically resolved images have been successfully obtained at magnetic fields above 30 T.Considering the small diameter sample cavity of the hybrid magnet of 32 mm and the vibration of the magnet platform which is about 30 times stronger than the ordinary ground,a newly designed high rigidity STM unit was used in the work,together with a two-stage spring suspension insert with vibration isolation.The STM unit is designed with nonmetallic structure with an outer diameter is as small as 8.8 mm in order to minimize the influence of stray magnetic fields,and to avoid touching the wall to enhance the seismic resistance.The insert has a natural frequency as low as 2 Hz,which can further suppress the interference of strong vibrations on the STM imaging system.Under this scheme,the graphite atom-resolved images were successfully obtained at room temperature with a magnetic field strength of 30.1 T,which was the highest magnetic field imaging record at that time.Until now,no images at a higher magnetic field have been reported.Then a self-designed liquid nitrogen dewar is added to introduce a low-temperature environment to the system,because the sample cavity size of liquid nitrogen dewar is smaller,only 19 mm,it is easy to touch the wall and make the vibration isolation failure,the atomic resolution image at a temperature of 77 K and a magnetic field of 10 T is finally obtained.Low temperature is indispensable for improving the energy resolution of scanning tunneling microscopy spectroscopy and for controlling the properties of samples.Then,the construction of a cryogenic SPM based on a 22 T water-cooled magnet is presented.In this part,a custom-made He4 thermostat with a larger aperture is added to the watercooled magnet to achieve a temperature range of 1.5 K to room temperature.A novel compact nonmetallic STM unit is used,again with a two-stage spring suspension.In addition,the entire imaging setup and the magnet are separated by heavy cement bricks and rubber pads to attenuate the strong vibrations of the magnet.During commissioning,we collected imaging signals at zero and the highest field for frequency domain analysis and comparison,confirming the effectiveness of our vibrational isolation part and imaging setup.We also obtained atomically resolved images at 1.6 K temperature and 22 T magnetic fields,scanning tunneling spectra of FeSe bulk,and magnetic domain images of a SrRuO3 film at different magnetic fields.The MFM has also been commissioned at magnetic fields up to 35 T,which is the first MFM imaging work above 30 T based on water-cooled magnets.This paper presents the construction of a rotating magnetic force microscope(RMFM)based on superconducting magnet finally.Due to the running time of watercooled and hybrid magnets is precious,a rotating magnetic force microscope based on a superconducting magnet in laboratory is constructed to perform routine tests and screen suitable samples for high-fields experiments.The R-MFM uses a laboratory developed piezoelectric rotation and angle measurement scheme that is integrated into a vacuum chamber fitted with the magnet.The operating temperature range of the RMFM is 6 K to room temperature,the range of the magnetic field is ±12 T,the Angle between the sample and the magnetic field is 0-90 degrees controllable,the entire magnet and the imaging part are placed on the platform with shock absorber,the spatial resolution can reach 80 nm,and the frequency resolution is as low as tens of mHz.We have used this device to perform detailed tests on SrRuO3 thin films with anomalous Hall effect,giving direct evidence for a two-channel model interpretation.