| Magnetic Force Microscope (MFM) is a powerful tool for analyzing the domian distribution of the sample surface. Up to now, it has been used in many fields of material science, such as superconductivity, giant magnetoresistance and so on. Scientists have achieved many progresses with this powerful tool.With the develop of scientific research, people need the MFM to have a better performance. Scientists hope that MFM can work in some extreme conditions, such as low temperature, high magnetic field and so on. They need an MFM with higher resolution and sensitivity. They need an MFM in which the temperature of sample and/or the magnetic field applied on the sample is variable. It is difficult to design such an MFM in the extreme conditions since the volumes are always small. In this article, we will report how we design such an MFM.First, we improve the frequency detecting circuit both in FM mode and AM mode MFM.1. With FM mode, we add a2N frequency multiplier to the front of a phase lock loop (PLL) which is always used in an FM mode and improve the frequency resolution significantly. The input signal is first processed by a frequency doubler to obtain a2f(f is frequency of input signal) signal which is then converted into square wave. The2Nf signal is acquired by extracting the N order harmonic of the2f square wave. In this way, the interval of adjacent harmonics of2f square wave is larger and we can extract a purer2Nf signal which means low noise of the whole frequency detecting circuit.2. With AM mode, a fast amplitude modulation detector is designed. A feedback circuit is added to the traditional open loop AM circuit. The feedback circuit keeps the piezoresistive cantilever vibrating with a constant amplitude and the feedback signal is used for imaging. This design speed up the response since the amplitude of the cantilever is kept constant (In traditional AM mode, the amplitude variation of the cantilever costs most of the response time). With this method, the frequency resolution does not decrease significantly (comparing with FM mode, high frequency resolution is AM mode’s main superiority, the long response time is the only reason that AM mode was replaced by FM mode). This work is described in a paper which has been submitted to the journal Microscopy and Microanalysis. The paper has been accepted and the reviewers’comments are excellent:"In short, I found your document extremely interesting and believe it is a great fit for the journal...The core science is great...It is of great interest to me and I am sure the rest of the AFM community.". To make a temperature-variable MFM, a temperature-control circuit is designed in which PID feedback is used. Comparing with the MFM’s naturely temperature rising (cool the MFM with LN2/Lhe, then put it in air condition), this circuit can keep the temperature of the sample at a constant value which can be set by the setpoint of PID feedback. A stable temperature of sample means that we can scan an area of sample surface several times (or searching an interesting area) to get a good image at a certain temperature (this is very important, since it is possible that the domain of the scan area varies with the temperature). This temperature control circuit can achieve a1K control accuracy at least. This accuracy is worse than the commercial temperature controller Lake Shore whose accuracy is mK level. We decide to design the temperature control circuit ourselves for two reasons:1. The1K control accuracy can satisfy most of this kind of applications in MFM scan;2. The cost is low (the price of a Lake Shore temperature controller can be tens of thousands of Yuan).With the circuits described above, we build a temperature-variable MFM system. Its mechanical structure is designed as:1. Two piezotubes are fixed in parallel, one of them with a sensor fixed on the top is for both X scan and feedback control; the other tube is used for Y scan (and searching scan area if necessary).2. An inertia motor is used for this MFM, simplifying the control and decreasing the coarse approach voltage. It can walk smoothly even in the low temperature condition.3. Since the stability of slider of a inertia motor is not very well, it is possible to crash onto the sensor while operating with the scan head or moving the MFM. To protect the sensor, a safeguard is designed. It protects the sensor before coarse approach.4. To make sure that the MFM can be put into an extreme condition whose volume is very small, we simplize and smallize the scan head as much as possible. Almost all of the components of the scan head are made of non-magnetic materials except the sensor and sample, since it will work in the18/20T superconducting magnet. The measurement and control circuit can be described as:A. An active Wheatstone bridge is used to detect the variation of the cantilever’s resistance. Compared with passive Wheatstone bridge, it has a stronger driving ability. B. We use the fast amplitude modulation detector for frequency detecting. It has high resolution and appropriate response speed. With this circuit, the vibrating of the cantilever is not so easy to be crashed. C. To adjust the sensitivity and response time, Q-control circuit is used to control the Q factor of the cantilever. D. PID feedback is used to realize constant force mode scan. This mode can protect the tip of the cantilever and give a real feature of the scan area. E. The home-made temperature control circuit is used to make this MFM a temperature-variable MFM.To ensure a stable tip-sample junction, multi-stage damping is used to isolate vibrations (including sound) from outside of the MFM, such as springs, rubber sheets and so on.After building the temperature-variable MFM, I joined in an important research project-SMA combined microscope system which combines STM, MFM and AFM together as one combined microscope. As we know, STM images reveal the distribution of density of states of the sample, AFM tells the atomic arrangement of the sample surface, and MFM detects magnetic domain distribution of the sample surface. In order to observe all the three features of a sample within the same scan area, SMA combined microscope system is built. Under the direction of Prof. Lu, three of his students are responsible for this project. They are Quanfeng Li (responsible for STM), Yubin Hou (responsible for AFM) and Yizhi Shi (responsible for MFM).We need the SMA combined microscope system work in an18/20T superconducting magneto. The obstacles are:1. the low temperature (4.2K) inside the magneto which decreases the piezoelectric coefficient of piezotubes used in MFM;2. the high magnetic field which may interact with the scan head as well as the wires;3. the small bore size of the magneto (52.8mm ID) which limits the volume of the scan head. To address the issues, we:(1) select a long and thin piezotube as the scanning tube to ensure enough scan range, a short and thick piezotube is used as a motor tube due to its strong driving force (to further strengthen the driving force, the inner electrode of the motor tube is axially splitted into two);(2) almost all components of the scan head are non-magnetic (except the sensor and sample), avoiding the possible interaction between MFM and the high magnetic field and the influence on the field distribution;(3) simplize and smallize the scan head as much as possible, the OD of this scan head is designed to be28mm.The measurement and control circuit of this combined microscope is roughly the same as the temperature-variable MFM previously mentioned, there are two differences:1. the position of the piezoresistive cantilever in the preamplifier;2. Q-control circuit is not used. The purpose is to simplify the connection on the scan head. Instead, the Q fact is regulated by releasing a certain amount of helium into the chamber of the MFM. The vibrating isolation and sound absorption system of this combined microscope is roughly the same as the temperature-variable MFM previously mentioned. The whole system is placed in a big cement pit which is separated with the earth by5cm sponge sheet for further vibrations isolation. |
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