| The magnetic force microscope (MFM) is one kind of scanning probe microscopes that can be used to image the magnetic microstructures on the sample surfaces. Since its invention in the end of the eighties of last century, MFM has achieved great progresses in both the techniques and applications. For example, the frequency-modulated mode that was firstly developed in MFM enables the AFM to achieve true atomic resolution. Besides, the vortices in superconductors and magnetic domains in ferromagnets have been both successfully imaged. Magnetic field is an essential parameter that can be used to tune the properties of magnetic materials, thus, MFMs in high magnetic fields are of high significance. However, the reported highest field MFM images are taken at8T, which is still not strong enough since the transitions in many materials occur in higher fields. Thus, it’s an urgent need to construct a MFM with higher magnetic fields.Based on these considerations, we designed and constructed a MFM in a20T superconducting magnet. The MFM is based on a home-designed scan head called SpiderDrive, whose largest advantage is the the piezoelectric tube scanner responsible for approach can also be used for scanning. Thus it is very compact and can be inserted into the bore of high-field magnets. The details of the design and construction work will be introduced in Chapt3. The test of the MFM on samples suggests it is very stable, with low noise and drift in high fields. It can work properly even in17.6T. This work has been published in Ultramicrosocpy.The perovskite manganites are typical stronlgy correlated electron materials. The phase separation involving ferromagnetic metal phase (FMM) and antiferromagnetic chare-ordered insulator phase (COI) has been one of the central topics in the physics of manganites. We imaged the phase separation and its evolution in a post-annealed La0.67Ca0.33MnO3/NdGaO3(001) thin film and Pr1/2Ca1/2MnO3thin film using our home-built MFM.In the post-annealed La0.67Ca0.33MnO3/NdGaO3(001) thin film, the melting tran-sition of COI in increasing magnetic fields shows strong anisotropy, suggesting the important role of the anisotropic epitaxial strain. If the field is decreased after the COI is fully melted, the COI will reenter in a certain part of the phase diagram. We found that the microstructures during reentrance strongly depend on temperatures. The COI reap-pears as droplets, stripes and irregular puddles at decreasing temperatures, reflecting the competition between the anisotropic epitaxial strain and Jahn-Teller distortions.The Pr1/2Ca1/2MnO3is a typical manganite that possesses a COI ground state. The melting field of the COI in Pr1/2Ca1/2MnO3is very high. Due to the lack of imaging instruments in sufficiently high fields, the transitions in Pr1/2Ca1/2MnO3have seldomly been imaged. We took the advantage of our20T MFM and systematically studied the transitions in a Pr1/2Ca1/2MnO3thin film. The results reveal that the FMM(COI) is firstly formed in areas close to the structural line defects during melting(reentrance) of COI. This dual role is possibly due to the strain variations, making a certain area favorable for FMM while a nearby area favorable for COI. |
