高灵敏磁力显微镜研制及其在18/20T低温强磁场中的实现
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摘要
磁力显微镜(Magnetic Force Microscope,简称MFM)是一种用来探测样品表面磁畴强度及分布的功能强大的表面分析仪器,从发明至今已经在超导、巨磁阻等材料学领域得到了越来越广泛的应用,在它的辅助下,科学家们取得了很多实验方面的新发现和新进展。
随着科学研究不断往深入的方向推进,科学家们对于磁力显微镜的性能也提出了各种各样更高的(甚至是苛刻的)要求,例如：更高的探测灵敏度及分辨率、可以在低温甚至极低温下工作、可以在强磁场中工作、可以对样品进行变温、变场测量等等,尤其是低温、强磁场等极端环境往往是在一个空间十分狭小的空间中实现的,这使得对磁力显微镜的设计存在更高的技术难度。这篇文章中我们分别从上述几个方面介绍了我们在研制磁力显微镜方面的实验进展。
首先在提高磁力显微镜的灵敏度及测量分辨率方面,我们先后从调频模式和调幅模式两方面做了实验改进：1.在调频模式方面,我们在传统锁相环电路的基础上,在锁相环的输入端添加了一个特别设计的2N倍频电路,将频率分辨率在原锁相环的基础上提高了好几倍。这里的倍频我们采用了先对输入信号求2倍频再对其方波信号提取N次谐波的方式,这种方式获得的2N倍频信号噪音更低,因为2倍频方波信号的相邻谐波之间间隔更大,便于提取出纯净的2N倍信号,降低总的电路噪音。2.在调幅模式方面,我们在传统的开环调幅模式基础上增加了反馈稳幅电路,利用稳幅电压来作为成像信号,在这种模式下探针悬臂的振动幅度基本维持在一个恒定值,加快了调幅模式的响应速度(传统调幅模式的太慢说白了就是探针的振幅变化耗时太久,在我们的设计里探针悬臂振幅基本不需要怎么变,故而加快了响应速度),而难得的是这个设计并没有在很大程度上降低传统调幅模式的频率分辨率(传统调幅模式的高频率分辨率是它与目前流行的调频模式的一大优势,只因其响应速度太慢才被调频模式所取代)。这一新的设计及其实验结果已被写成文章投稿到SCI二区杂志《Microscopy and Microanalysis》并被接收,审稿人给予了很高的评价：‘'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."。
在变温测量方面,我们针对自制的磁力显微镜系统特别设计了一个结构简单、调试方便的的PID控温电路,相比于较简易的让镜体自然回温的变温方式,这一控温电路可以将样品温度稳定地控制在某一个温度点进行长时间的扫描(包括大范围搜索寻找实验者感兴趣的扫描区域),避免了因自然回温时扫描可能会出现的图像变形扭曲等问题(因为扫描时样品的温度在变化,也就意味着样品表面的磁畴也可能在变化)。与商业的Lake Shore控温装置相比,它在控温精度方面略差,但达到1K的精度应该是没问题的,这对于大部分的变温测量来说已经完全满足控温精度的要求,相比之下,它的一大优点就是制作成本比较低(而Lake Shore控温装置的售价可高达数万),可以大大缓解实验室的经费压力。
在做了频率测量电路及控温电路的准备之后,我们自制了一款可在低温环境中实现变温测量的磁力显微镜系统。在显微镜镜体的机械设计方面：1.我们采用两个压电管并排放置的方式,其中一个压电管上固定有压阻探针,负责x方向快速扫描和z方向上的反馈调节(调节探针与样品之间的间距),而另一个压电管负责探针-样品间距的粗逼近和扫描样品时Y方向慢扫,必要的时候还可以进行对样品扫描区域的横向搜索；2.粗逼近机制采用了惯性马达的步进方式,简化了粗逼近过程的电子学控制,同时也降低了粗逼近电压,使得该自制显微镜在低温环境中也可以顺畅地行走(不需要很高的电压)；3.由于惯性马达的设计中,滑块的稳定性较差,在移动显微镜镜体的过程(比如讲显微镜镜体及其真空腔体从大气环境中放入液氦杜瓦的过程)中可能会出现滑块撞向探针的现象,为此我们为显微镜特别设计了一个探针保护装置,它可以确保在扫描未开始之前压阻探针与滑块不会相撞,而在需要进行扫描的时候滑块可自由地想探针方向粗逼近直至达到探针-样品作用区；4.整个显微镜镜体的设计尽可能地简洁和小型化(圆柱形镜体外径只有30mm),使得其可以工作于极端环境的狭小空间中,镜体设计所使用的材料几乎全部是无磁材料(探针和样品除外),以便于未来将其置于强磁场环境中经行样品的测量。在显微镜的电子学部分：a.采用有源电桥电路来感知压阻探针阻值的变化,相较于无源电桥,有源电桥对于下一级电路的驱动能力更强；b.在频率检测方面采用了自主设计的反馈稳幅测力梯度电路,具备更高的频率分辨率以及足够的响应速度,且使用这种电路在实际测量时压阻探针不易失振；c.引入了Q-control技术,使得压阻探针的Q值可人为控制,便于在实际测试时调节系统的响应速度及灵敏度；d.自制PID反馈控制电路,实现了自制磁力显微镜的恒力(梯度)扫描,一方面它对扫描过程中的压阻探针针尖起到了一定的保护作用,另一方面也可以更真实地反映出被测样品的表面形貌等特征；e.将自制控温电路应用于该磁力显微镜,使得它具备了在低温环境下的变温测量能力。在隔音减震方面,我们采用了弹簧悬吊、橡胶垫减震等多级减震方式并自制隔音箱以消除外界噪音干扰,实际测试显示我们的隔音减震系统可以有效地隔绝外界的振动及声音干扰,显微镜探针-样品结所受到的外界干扰非常微弱。
在成功搭建了一款低温环境中可变温的磁力显微镜系统之后,本人参与了实验室一个重大仪器项目——SMA组合显微镜系统的研制。SMA组合显微镜系统的研制目的是要将目前流行的功能强大的三种显微镜合为一体,这三种显微镜分别是：扫描隧道显微镜(STM)、磁力显微镜(MFM)、原子力显微镜(AFM),SMA正是这三种显微镜缩写首字母的组合,其中,扫描隧道显微镜测量的是样品表面的电子态密度分布,原子力显微镜测量的是样品表面的真是结构(尤其是真是的原子排布),而磁力显微镜测试的是样品表面的磁畴分布,这三种显微镜各有其强大的地方,但也各有不足,SMA组合显微镜系统的研制就是要集合三种显微镜与一体,相互之间弥补不足,同时对样品进行三方面特性的观测,以便更全面地研究样品本身的特性。SMA组合显微镜系统项目在陆轻铀教授的指导下由其三个学生具体负责：李全锋(博士在读,主要负责扫描隧道显微镜部分)、侯玉斌(博士,主要负责原子力显微镜部分)、施益智(博士在读,主要负责磁力显微镜部分)。
SMA组合显微镜系统的设计要求是可以工作在18/20T的超导磁体中。这个设计要求对于显微镜的研制来说有三个难点：1.显微镜的环境温度非常低,超导磁体内部处于液氦温度,在这一温度下压电材料的压电系数会急剧减小；2.强磁场环境,电磁作用会引起很多在零场环境中不会出现的问题；3.空间限制,我们的18/20T超导磁体购自牛津公司,内孔径52.8mm,要想放入其中,三合一显微镜的镜体体积必须足够小。针对上述难点,我们在SMA的磁力显微镜部分做了如下设计：1.镜体中部分采用了非对称设计,负责扫描的压电管选择一个细长型的,以保证显微镜在液氦下具有足够的扫描范围；负责步进的压电管选择短粗型的,以保证对滑块拥有足够的推力,此外为进一步增强马达的推力,还将负责步进的压电管内电极轴向分割为二部分,在步进的时候分别于外电极连接。2.几乎所有磁力显微镜镜体部件全部采用无磁材料加工而成(探针和样品除外),避免了镜体本身受到磁场的作用力,同时也避免了镜体本身影响磁体内磁场分布的可能。3.镜体的设计尽可能地简化与小型化,以便最终可以将其植入到超导磁体中使用,我们的SMA显微镜镜体被设计为一个圆柱形,其外径只有281mm。
至于SMA中磁力显微镜部分的测量及控制电路与之前所述自制可变温磁力显微镜的电路基本相同,除了两点差别：1.为了减少镜体上前置放大器电路的连线,更换了压阻探针在有源电桥中的位置；2.为了简化镜体设计(如连线、探针座设计等),这里没有使用Q-Control技术,而是往显微镜真空腔体内通入一定量的氦气来调节真空度。
SMA组合显微镜系统的隔音减震设计与前面所述低温环境下可变的磁力显微镜隔音减震设计大体相同,整套系统(连同超导磁体)被置于一个大水泥池内,水泥池与周围土壤之间由5cm厚的海绵隔开：
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.
随着科学研究不断往深入的方向推进,科学家们对于磁力显微镜的性能也提出了各种各样更高的(甚至是苛刻的)要求,例如：更高的探测灵敏度及分辨率、可以在低温甚至极低温下工作、可以在强磁场中工作、可以对样品进行变温、变场测量等等,尤其是低温、强磁场等极端环境往往是在一个空间十分狭小的空间中实现的,这使得对磁力显微镜的设计存在更高的技术难度。这篇文章中我们分别从上述几个方面介绍了我们在研制磁力显微镜方面的实验进展。
首先在提高磁力显微镜的灵敏度及测量分辨率方面,我们先后从调频模式和调幅模式两方面做了实验改进：1.在调频模式方面,我们在传统锁相环电路的基础上,在锁相环的输入端添加了一个特别设计的2N倍频电路,将频率分辨率在原锁相环的基础上提高了好几倍。这里的倍频我们采用了先对输入信号求2倍频再对其方波信号提取N次谐波的方式,这种方式获得的2N倍频信号噪音更低,因为2倍频方波信号的相邻谐波之间间隔更大,便于提取出纯净的2N倍信号,降低总的电路噪音。2.在调幅模式方面,我们在传统的开环调幅模式基础上增加了反馈稳幅电路,利用稳幅电压来作为成像信号,在这种模式下探针悬臂的振动幅度基本维持在一个恒定值,加快了调幅模式的响应速度(传统调幅模式的太慢说白了就是探针的振幅变化耗时太久,在我们的设计里探针悬臂振幅基本不需要怎么变,故而加快了响应速度),而难得的是这个设计并没有在很大程度上降低传统调幅模式的频率分辨率(传统调幅模式的高频率分辨率是它与目前流行的调频模式的一大优势,只因其响应速度太慢才被调频模式所取代)。这一新的设计及其实验结果已被写成文章投稿到SCI二区杂志《Microscopy and Microanalysis》并被接收,审稿人给予了很高的评价：‘'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."。
在变温测量方面,我们针对自制的磁力显微镜系统特别设计了一个结构简单、调试方便的的PID控温电路,相比于较简易的让镜体自然回温的变温方式,这一控温电路可以将样品温度稳定地控制在某一个温度点进行长时间的扫描(包括大范围搜索寻找实验者感兴趣的扫描区域),避免了因自然回温时扫描可能会出现的图像变形扭曲等问题(因为扫描时样品的温度在变化,也就意味着样品表面的磁畴也可能在变化)。与商业的Lake Shore控温装置相比,它在控温精度方面略差,但达到1K的精度应该是没问题的,这对于大部分的变温测量来说已经完全满足控温精度的要求,相比之下,它的一大优点就是制作成本比较低(而Lake Shore控温装置的售价可高达数万),可以大大缓解实验室的经费压力。
在做了频率测量电路及控温电路的准备之后,我们自制了一款可在低温环境中实现变温测量的磁力显微镜系统。在显微镜镜体的机械设计方面：1.我们采用两个压电管并排放置的方式,其中一个压电管上固定有压阻探针,负责x方向快速扫描和z方向上的反馈调节(调节探针与样品之间的间距),而另一个压电管负责探针-样品间距的粗逼近和扫描样品时Y方向慢扫,必要的时候还可以进行对样品扫描区域的横向搜索；2.粗逼近机制采用了惯性马达的步进方式,简化了粗逼近过程的电子学控制,同时也降低了粗逼近电压,使得该自制显微镜在低温环境中也可以顺畅地行走(不需要很高的电压)；3.由于惯性马达的设计中,滑块的稳定性较差,在移动显微镜镜体的过程(比如讲显微镜镜体及其真空腔体从大气环境中放入液氦杜瓦的过程)中可能会出现滑块撞向探针的现象,为此我们为显微镜特别设计了一个探针保护装置,它可以确保在扫描未开始之前压阻探针与滑块不会相撞,而在需要进行扫描的时候滑块可自由地想探针方向粗逼近直至达到探针-样品作用区；4.整个显微镜镜体的设计尽可能地简洁和小型化(圆柱形镜体外径只有30mm),使得其可以工作于极端环境的狭小空间中,镜体设计所使用的材料几乎全部是无磁材料(探针和样品除外),以便于未来将其置于强磁场环境中经行样品的测量。在显微镜的电子学部分：a.采用有源电桥电路来感知压阻探针阻值的变化,相较于无源电桥,有源电桥对于下一级电路的驱动能力更强；b.在频率检测方面采用了自主设计的反馈稳幅测力梯度电路,具备更高的频率分辨率以及足够的响应速度,且使用这种电路在实际测量时压阻探针不易失振；c.引入了Q-control技术,使得压阻探针的Q值可人为控制,便于在实际测试时调节系统的响应速度及灵敏度；d.自制PID反馈控制电路,实现了自制磁力显微镜的恒力(梯度)扫描,一方面它对扫描过程中的压阻探针针尖起到了一定的保护作用,另一方面也可以更真实地反映出被测样品的表面形貌等特征；e.将自制控温电路应用于该磁力显微镜,使得它具备了在低温环境下的变温测量能力。在隔音减震方面,我们采用了弹簧悬吊、橡胶垫减震等多级减震方式并自制隔音箱以消除外界噪音干扰,实际测试显示我们的隔音减震系统可以有效地隔绝外界的振动及声音干扰,显微镜探针-样品结所受到的外界干扰非常微弱。
在成功搭建了一款低温环境中可变温的磁力显微镜系统之后,本人参与了实验室一个重大仪器项目——SMA组合显微镜系统的研制。SMA组合显微镜系统的研制目的是要将目前流行的功能强大的三种显微镜合为一体,这三种显微镜分别是：扫描隧道显微镜(STM)、磁力显微镜(MFM)、原子力显微镜(AFM),SMA正是这三种显微镜缩写首字母的组合,其中,扫描隧道显微镜测量的是样品表面的电子态密度分布,原子力显微镜测量的是样品表面的真是结构(尤其是真是的原子排布),而磁力显微镜测试的是样品表面的磁畴分布,这三种显微镜各有其强大的地方,但也各有不足,SMA组合显微镜系统的研制就是要集合三种显微镜与一体,相互之间弥补不足,同时对样品进行三方面特性的观测,以便更全面地研究样品本身的特性。SMA组合显微镜系统项目在陆轻铀教授的指导下由其三个学生具体负责：李全锋(博士在读,主要负责扫描隧道显微镜部分)、侯玉斌(博士,主要负责原子力显微镜部分)、施益智(博士在读,主要负责磁力显微镜部分)。
SMA组合显微镜系统的设计要求是可以工作在18/20T的超导磁体中。这个设计要求对于显微镜的研制来说有三个难点：1.显微镜的环境温度非常低,超导磁体内部处于液氦温度,在这一温度下压电材料的压电系数会急剧减小；2.强磁场环境,电磁作用会引起很多在零场环境中不会出现的问题；3.空间限制,我们的18/20T超导磁体购自牛津公司,内孔径52.8mm,要想放入其中,三合一显微镜的镜体体积必须足够小。针对上述难点,我们在SMA的磁力显微镜部分做了如下设计：1.镜体中部分采用了非对称设计,负责扫描的压电管选择一个细长型的,以保证显微镜在液氦下具有足够的扫描范围；负责步进的压电管选择短粗型的,以保证对滑块拥有足够的推力,此外为进一步增强马达的推力,还将负责步进的压电管内电极轴向分割为二部分,在步进的时候分别于外电极连接。2.几乎所有磁力显微镜镜体部件全部采用无磁材料加工而成(探针和样品除外),避免了镜体本身受到磁场的作用力,同时也避免了镜体本身影响磁体内磁场分布的可能。3.镜体的设计尽可能地简化与小型化,以便最终可以将其植入到超导磁体中使用,我们的SMA显微镜镜体被设计为一个圆柱形,其外径只有281mm。
至于SMA中磁力显微镜部分的测量及控制电路与之前所述自制可变温磁力显微镜的电路基本相同,除了两点差别：1.为了减少镜体上前置放大器电路的连线,更换了压阻探针在有源电桥中的位置；2.为了简化镜体设计(如连线、探针座设计等),这里没有使用Q-Control技术,而是往显微镜真空腔体内通入一定量的氦气来调节真空度。
SMA组合显微镜系统的隔音减震设计与前面所述低温环境下可变的磁力显微镜隔音减震设计大体相同,整套系统(连同超导磁体)被置于一个大水泥池内,水泥池与周围土壤之间由5cm厚的海绵隔开：
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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