Ultramicroscopy 147 (2014) 133–136
Contents lists available at ScienceDirect
Ultramicroscopy journal homepage: www.elsevier.com/locate/ultramic
A compact high field magnetic force microscope Haibiao Zhou a,b, Ze Wang a,b, Yubin Hou a, Qingyou Lu a,b,n a High Magnetic Field Laboratory, Chinese Academy of Sciences and University of Science and Technology of China, Hefei, Anhui 230031, People's Republic of China b Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, People's Republic of China
art ic l e i nf o
a b s t r a c t
Article history: Received 24 April 2014 Received in revised form 12 June 2014 Accepted 24 July 2014 Available online 9 August 2014
We present the design and performance of a simple and compact magnetic force microscope (MFM), whose tip-sample coarse approach is implemented by the piezoelectric tube scanner (PTS) itself. In brief, a square rod shaft is axially spring-clamped on the inner wall of a metal tube which is glued inside the free end of the PTS. The shaft can thus be driven by the PTS to realize image scan and inertial stepping coarse approach. To enhance the inertial force, each of the four outer electrodes of the PTS is driven by an independent port of the controller. The MFM scan head is so compact that it can easily fit into the 52 mm low temperature bore of a 20 T superconducting magnet. The performance of the MFM is demonstrated by imaging a manganite thin film at low temperature and in magnetic fields up to 15 T. & 2014 Elsevier B.V. All rights reserved.
Keywords: Piezo inertial motor Scanning probe microscope High magnetic field Magnetic force microscope
1. Introduction Even though various kinds of scanning probe microscopes (SPMs) have been invented since the first demonstration of the atomically resolved scanning tunneling microscope (STM) [1], there are still instrumentation progresses in the SPM field on the daily basis. A well-established SPM that has been broadly used is the magnetic force microscope (MFM) [2,3], which has played an important role in the development of the atomically resolved frequency-modulated non-contact atomic force microscope (AFM) [4], the observation of magnetic domain behaviors in a large variety of magnetic materials [5], as well as in the accomplishment of the magnetic exchange force microscopy with atomic resolution [6]. Nowadays, commercial MFMs that work in ambient conditions are very common due to their use in magnetic recording industry. But MFMs at low temperatures and in strong magnetic fields are still sort of rare, which are important since many materials show transitions at low temperatures and in magnetic fields. Building a low temperature high field MFM is not an easy job since complex and tough requirements in low drift, high precision, rigidity and compactness, etc. must be met. Similar to any other SPMs, the core structure of a MFM is the coarse approach motor that can bring the tip close to the sample
n Corresponding author. Permanent address: Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, No. 96 Jinzhai Road, Hefei, Anhui 230026, People's Republic of China. Tel.: þ 86 551 63600247. E-mail address: [email protected] (Q. Lu).
http://dx.doi.org/10.1016/j.ultramic.2014.07.011 0304-3991/& 2014 Elsevier B.V. All rights reserved.
surface at the nanometer scale. After the coarse approach is done, a piezoelectric tube scanner (PTS) regulates the tip-sample separation and an image is taken when the tip or sample is scanned with respect to each other. Many types of piezoelectric motors have been developed for SPMs, but the MFM has its own characteristics. One example is that it needs a large scan area. Thus, its piezo tube scanner needs to be rather long [7–10], which might be sufficient to serve as the driving piezo of an inertial stepping motor. Here, we present a novel, ultra-simple and ultra-compact MFM scan head in which the traditional four-quadrant tube scanner is also used to drive the coarse approach by inertial stepping. We call it SpiderDrive in which the four outer electrodes of the PTS are driven by independent ports of the controller respectively with the exactly same waveform. This will enhance the inertial force produced by the same PTS, which is needed to allow the motor to work at liquid helium temperature. The MFM images taken in fields up to 15 T will be presented in this paper.
2. Coarse approach motor and MFM scan head The core of our MFM is the SpiderDrive which is an inertial stepping motor consisting of a PTS, a guiding tube, a square rod as the sliding shaft and a bent spring strip. The guiding tube and the sliding shaft are made of non-magnetic stainless steel and tantalum, respectively. A schematic of the SpiderDrive is shown in Fig. 1. The PTS has four external and one internal electrodes with dimensions 52.8 mm length by 6.35 mm outer diameter by 0.5 mm wall thickness (EBL 3# from EBL Products, Inc.). The
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Fig. 2. The measured average step size at room temperature of the SpiderDrive as a function of the voltage amplitude. (Inset) The driving signal applied to the four outer electrodes of the PTS for coarse approach. VP denotes the voltage amplitude.
Fig. 1. Three-dimensional model (left) and inner structure (right) of the Spider Drive. (1) Square rod as the sliding shaft (sample holder), (2) guiding tube, (3) PTS, (4) phosphor bronze spring strip, and (5) sapphire base.
guiding tube is 18 mm long whose inner wall is well polished. Its outer diameter is slightly larger at one end which can fit and be glued with the inner wall of the free end of the PTS (the other end of the PTS is called the mounting end and will be mounted on the bottom of the MFM frame as will be discussed shortly). The sliding shaft is 28 mm long and has a square cross-section which can well fit into the guiding tube. A bent spring strip is inserted into the gap between the outer flat surface of the sliding shaft and the inner wall of the guiding tube so that the shaft is spring held. We use a R9 controller from RHK technology to drive this inertial stepping motor, which is capable of outputting 215 V high voltage signals with arbitrary waveforms. The waveform we use is shown in the inset of Fig. 2, which is from Refs. [11–13]. To further enhance the driving inertial force, the four external electrodes of the PTS are connected to the four independent high voltage output ports of the controller which simultaneously output the identical driving signals. The inner electrode of the PTS is applied 0 V when the PTS is used to drive the inertial motor but is applied a proper voltage VZ for fine Z-direction adjustment when the PTS is used as the scanner. The measured room temperature step size as a function of the amplitude of the driving signal is shown in Fig. 2. The SpiderDrive can work even with a voltage amplitude of 60 V and the step size shows nearly a linear dependence on the voltage amplitude. Besides, we have tested it at low temperatures. A small amount of helium gas was released to the vacuum chamber to cool down the MFM scan head and the lowest temperature we can get is 6.5 K. The SpiderDrive can walk with voltage amplitudes of 120 V and 200 V at 78 K and 6.5 K, respectively.
Fig. 3. Structure of the MFM scan head with the wiring not shown.
Here, we can make a comparison between SpiderDrive and some other piezo inertial motors. Unlike many other inertial motors where a slider is usually placed on a guiding rail by its weight [13] or magnetic force [14,15], the SpiderDrive is more rigid since the slider is held by the spring. Compared with some other rigid motors such as the Pan-style walker [9,10,16] or its variables [17,18], and the attocube drive [19], the SpiderDrive is most compact and simplest in structure. The greatest advantage is that the PTS can also act as a scanner after coarse approach [20]. A possible drawback, though not observed so far, is that during scanning the sliding shaft may slip if the sample's topography is very rough, but this effect can be minimized by slowing down the scan speed. Building a MFM from the SpiderDrive is extremely easy, as shown in Fig. 3. Just co-axially mount the SpiderDrive on the
H. Zhou et al. / Ultramicroscopy 147 (2014) 133–136
bottom of the MFM frame which is a segment of capped stainless steel pipe. The sample is fixed on the top side of the shaft and the cantilever holder is mounted underneath the crossbeam. The cantilever is driven to oscillate by a dither piezo plate.
135
cantilever is lifted at a typical height of 100 nm to image the magnetic signal. The tip is coated with 5 nm thick Cr film first as the buffer layer followed by 50 nm thick Co film. These are done by electron beam deposition. The tip is then magnetized perpendicular to the cantilever with a permanent magnet before loaded into the MFM.
3. Circuits, control and operation The cantilevers we use are commercially-available self-sensing piezoresistive cantilevers (PRC400 from Hitachi High-Tech Science Corporation, Japan) [21]. The preamplifier, which has been described in Ref. [22], is a home-made active Wheatstone bridge followed by a second stage amplifier with a gain of 1000 to detect the resistance change of the cantilever. We use the built-in phaselocked loop (PLL) of the R9 controller to extract the frequency shift signal. The topography is imaged in tapping mode which allows us to compensate the slopes in the two scan directions. Then the
4. Chamber, magnet and vibration isolation A schematic of the whole system is shown in Fig. 4. MFM scan head is housed in a long tubular chamber similar to the one described in Ref. [23]. The chamber is two meters long and consists of three parts, which are all made of non-magnetic stainless steel. The bottom part that houses the MFM scan head is a capped tube and it is vacuum sealed with the middle part via a specially designed narrow CF flange as described in Ref. [24]. The middle part is a long and thin pipe and it is connected to the top part through a normal CF flange. The top part contains a three-way vacuum tee and a six-way vacuum cross. The former is connected to the preamplifier through the electric feedthrough at the middle flange. A vacuum gauge and a valve for pumping the chamber are connected to the six-way vacuum cross via its side flanges. The magnet is a 20 T superconducting magnet (from Oxford Instrument) with a cold bore of 52.8 mm diameter. The volume of the magnet dewar is 77 L and the filling cycle is about 4 days. The magnet is housed in a large aluminum box whose inner walls are covered with soft sponges for sound isolation. The box is lifted by heavy duty springs which are hung from two stacks of cement bricks interleaved with soft rubber slabs. The whole system is installed in a big cement pool which is isolated from the surrounding soil with 5 cm thick sponge sheets for further vibration isolation.
5. Performance test
Fig. 4. Schematic view of whole MFM system.
To show the performance of this MFM, we present the topography and magnetic images of a 25 nm thick Pr0.55(Ca0.75 Sr0.25)0.45MnO3 (PCSMO) thin film on (LaAlO3)0.3(SrAl0.5Ta0.5O3)0.7
Fig. 5. Topography and MFM images of the 25 nm thick PCSMO thin film at 175 K and in fields up to 15.0 T at the sample position. The size of the images is 6 mm 6 mm. The color scales of these images are 200 nm (topo), 0.4 Hz (0 T, 3.5 T, 4.5 T, 5.5 T, 10.0 T and 15.0 T) and 0.56 Hz (4.0 T). The line time is 1.6 s and the time to take an image is about 13 min. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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(LSAT) (110) substrate at 175 K and in fields up to 15 T. After the MFM was inserted into the magnet, a small amount of helium gas was released to the chamber for heat exchange and the vacuum was at 3 10 4 Torr. The topography and magnetic images are shown in Fig. 5. At this temperature, the sample is a paramagnetic insulator (PI), and there is nearly no contrast in the MFM image except the weak signals from the sample surface at zero field. In 3.5 T, small dark droplets appear. These are ascribed to ferromagnetic metallic phase (FMM) because the attractive force on the tip causes a negative frequency shift which corresponds to a dark color. In 4.0 T, maze-like FMM phase domains are observed and in higher fields, the areas of ferromagnetic phase grow and they gradually occupy the sample. But some PI droplets persist even in 10 T and they disappeared in 15 T. To the best of our knowledge, this is the highest field MFM image up to now. 5. Conclusions We have presented a home-built MFM that fits in a 20 T superconducting magnet and can operate at low temperatures. Its PTS is used as the scanner as well as the driving piezo of a four-way driven inertial motor capable of vertical approach even at 6.5 K. Its rigidity, simplicity and compactness make it a distinctive and powerful MFM, whose performance is shown by the clear magnetic images of a manganite thin film in fields up to 15 T. Acknowledgment We sincerely thank Prof. Z.G. Sheng for providing the PCSMO thin film. This work is supported by the National Natural Science
Foundation of China (Grant nos. U1232210, 11374278 and 11204306), the project of Chinese National High Magnetic Field Facilities and the Fundamental Research Funds for the Central Universities (Program no. WK2340000035). References [1] G. Binnig, H. Rohrer, Rev. Mod. Phys. 59 (3) (1987) 615–625. [2] Y. Martin, H.K. Wickramasinghe, Appl. Phys. Lett. 50 (20) (1987) 1455–1457. [3] J.J. Saenz, N. Garcia, P. Grutter, E. Meyer, H. Heinzelmann, R. Wiesendanger, L. Rosenthaler, H.R. Hidber, H.J. Guntherodt, J. Appl. Phys. 62 (10) (1987) 4293–4295. [4] F.J. Giessibl, Science 267 (5194) (1995) 68–71. [5] Q.Y. Lu, C.C. Chen, A. de Lozanne, Science 276 (5321) (1997) 2006–2008. [6] U. Kaiser, A. Schwarz, R. Wiesendanger, Nature 446 (7135) (2007) 522–525. [7] H.J. Hug, B. Stiefel, P.J.A. van Schendel, A. Moser, S. Martin, H.J. Guntherodt, Rev. Sci. Instrum. 70 (9) (1999) 3625–3640. [8] M. Roseman, P. Grutter, Rev. Sci. Instrum. 71 (10) (2000) 3782–3787. [9] C. Israel, C. Hyun, A. de Lozanne, S. Phark, Z.G. Khim, Rev. Sci. Instrum. 77 (2) (2006) 023704. [10] T.M. Chuang, A. de Lozanne, Rev. Sci. Instrum. 78 (5) (2007) 053710. [11] C. Renner, P. Niedermann, A.D. Kent, O. Fischer, Rev. Sci. Instrum. 61 (3) (1990) 965–967. [12] W.F. Smith, M.C. Abraham, J.M. Sloan, M. Switkes, Rev. Sci. Instrum. 67 (10) (1996) 3599–3604. [13] Y. Hou, J. Wang, Q. Lu, Rev. Sci. Instrum. 79 (11) (2008) 113707. [14] F. Mugele, A. Rettenberger, J. Boneberg, P. Leiderer, Rev. Sci. Instrum. 69 (4) (1998) 1765–1769. [15] F. Mugele, C. Kloos, P. Leiderer, R. Moller, Rev. Sci. Instrum. 67 (7) (1996) 2557–2559. [16] S.H. Pan, International Patent No. WO93/19494, 30 September 1993. [17] S.H. Kim, A. de Lozanne, Rev. Sci. Instrum. 83 (10) (2012) 103701. [18] A.Z. Stieg, P. Wilkinson, J.K. Gimzewski, Rev. Sci. Instrum. 78 (3) (2007) 036110. [19] 〈www.attocube.com〉. [20] R.T. Brockenbrough, J.W. Lyding, Rev. Sci. Instrum. 64 (8) (1993) 2225–2228. [21] H. Takahashi, K. Ando, Y. Shirakawabe, Ultramicroscopy 91 (1–4) (2002) 63–72. [22] Y.Z. Shi, Q.Y. Lu, Microsc. Microanal. 18 (5) (2012) 1016–1020. [23] Q. Li, Q. Wang, Y. Hou, Q. Lu, Rev. Sci. Instrum. 83 (4) (2012) 043706. [24] Z.Q. Pang, F. Wu, Y.Z. Shi, Y.B. Hou, Q.F. Li, Q.Y. Lu, IEEE Trans. Appl. Supercond. 20 (3) (2010) 668–671.
Contents lists available at ScienceDirect
Ultramicroscopy journal homepage: www.elsevier.com/locate/ultramic
A compact high field magnetic force microscope Haibiao Zhou a,b, Ze Wang a,b, Yubin Hou a, Qingyou Lu a,b,n a High Magnetic Field Laboratory, Chinese Academy of Sciences and University of Science and Technology of China, Hefei, Anhui 230031, People's Republic of China b Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, People's Republic of China
art ic l e i nf o
a b s t r a c t
Article history: Received 24 April 2014 Received in revised form 12 June 2014 Accepted 24 July 2014 Available online 9 August 2014
We present the design and performance of a simple and compact magnetic force microscope (MFM), whose tip-sample coarse approach is implemented by the piezoelectric tube scanner (PTS) itself. In brief, a square rod shaft is axially spring-clamped on the inner wall of a metal tube which is glued inside the free end of the PTS. The shaft can thus be driven by the PTS to realize image scan and inertial stepping coarse approach. To enhance the inertial force, each of the four outer electrodes of the PTS is driven by an independent port of the controller. The MFM scan head is so compact that it can easily fit into the 52 mm low temperature bore of a 20 T superconducting magnet. The performance of the MFM is demonstrated by imaging a manganite thin film at low temperature and in magnetic fields up to 15 T. & 2014 Elsevier B.V. All rights reserved.
Keywords: Piezo inertial motor Scanning probe microscope High magnetic field Magnetic force microscope
1. Introduction Even though various kinds of scanning probe microscopes (SPMs) have been invented since the first demonstration of the atomically resolved scanning tunneling microscope (STM) [1], there are still instrumentation progresses in the SPM field on the daily basis. A well-established SPM that has been broadly used is the magnetic force microscope (MFM) [2,3], which has played an important role in the development of the atomically resolved frequency-modulated non-contact atomic force microscope (AFM) [4], the observation of magnetic domain behaviors in a large variety of magnetic materials [5], as well as in the accomplishment of the magnetic exchange force microscopy with atomic resolution [6]. Nowadays, commercial MFMs that work in ambient conditions are very common due to their use in magnetic recording industry. But MFMs at low temperatures and in strong magnetic fields are still sort of rare, which are important since many materials show transitions at low temperatures and in magnetic fields. Building a low temperature high field MFM is not an easy job since complex and tough requirements in low drift, high precision, rigidity and compactness, etc. must be met. Similar to any other SPMs, the core structure of a MFM is the coarse approach motor that can bring the tip close to the sample
n Corresponding author. Permanent address: Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, No. 96 Jinzhai Road, Hefei, Anhui 230026, People's Republic of China. Tel.: þ 86 551 63600247. E-mail address: [email protected] (Q. Lu).
http://dx.doi.org/10.1016/j.ultramic.2014.07.011 0304-3991/& 2014 Elsevier B.V. All rights reserved.
surface at the nanometer scale. After the coarse approach is done, a piezoelectric tube scanner (PTS) regulates the tip-sample separation and an image is taken when the tip or sample is scanned with respect to each other. Many types of piezoelectric motors have been developed for SPMs, but the MFM has its own characteristics. One example is that it needs a large scan area. Thus, its piezo tube scanner needs to be rather long [7–10], which might be sufficient to serve as the driving piezo of an inertial stepping motor. Here, we present a novel, ultra-simple and ultra-compact MFM scan head in which the traditional four-quadrant tube scanner is also used to drive the coarse approach by inertial stepping. We call it SpiderDrive in which the four outer electrodes of the PTS are driven by independent ports of the controller respectively with the exactly same waveform. This will enhance the inertial force produced by the same PTS, which is needed to allow the motor to work at liquid helium temperature. The MFM images taken in fields up to 15 T will be presented in this paper.
2. Coarse approach motor and MFM scan head The core of our MFM is the SpiderDrive which is an inertial stepping motor consisting of a PTS, a guiding tube, a square rod as the sliding shaft and a bent spring strip. The guiding tube and the sliding shaft are made of non-magnetic stainless steel and tantalum, respectively. A schematic of the SpiderDrive is shown in Fig. 1. The PTS has four external and one internal electrodes with dimensions 52.8 mm length by 6.35 mm outer diameter by 0.5 mm wall thickness (EBL 3# from EBL Products, Inc.). The
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Fig. 2. The measured average step size at room temperature of the SpiderDrive as a function of the voltage amplitude. (Inset) The driving signal applied to the four outer electrodes of the PTS for coarse approach. VP denotes the voltage amplitude.
Fig. 1. Three-dimensional model (left) and inner structure (right) of the Spider Drive. (1) Square rod as the sliding shaft (sample holder), (2) guiding tube, (3) PTS, (4) phosphor bronze spring strip, and (5) sapphire base.
guiding tube is 18 mm long whose inner wall is well polished. Its outer diameter is slightly larger at one end which can fit and be glued with the inner wall of the free end of the PTS (the other end of the PTS is called the mounting end and will be mounted on the bottom of the MFM frame as will be discussed shortly). The sliding shaft is 28 mm long and has a square cross-section which can well fit into the guiding tube. A bent spring strip is inserted into the gap between the outer flat surface of the sliding shaft and the inner wall of the guiding tube so that the shaft is spring held. We use a R9 controller from RHK technology to drive this inertial stepping motor, which is capable of outputting 215 V high voltage signals with arbitrary waveforms. The waveform we use is shown in the inset of Fig. 2, which is from Refs. [11–13]. To further enhance the driving inertial force, the four external electrodes of the PTS are connected to the four independent high voltage output ports of the controller which simultaneously output the identical driving signals. The inner electrode of the PTS is applied 0 V when the PTS is used to drive the inertial motor but is applied a proper voltage VZ for fine Z-direction adjustment when the PTS is used as the scanner. The measured room temperature step size as a function of the amplitude of the driving signal is shown in Fig. 2. The SpiderDrive can work even with a voltage amplitude of 60 V and the step size shows nearly a linear dependence on the voltage amplitude. Besides, we have tested it at low temperatures. A small amount of helium gas was released to the vacuum chamber to cool down the MFM scan head and the lowest temperature we can get is 6.5 K. The SpiderDrive can walk with voltage amplitudes of 120 V and 200 V at 78 K and 6.5 K, respectively.
Fig. 3. Structure of the MFM scan head with the wiring not shown.
Here, we can make a comparison between SpiderDrive and some other piezo inertial motors. Unlike many other inertial motors where a slider is usually placed on a guiding rail by its weight [13] or magnetic force [14,15], the SpiderDrive is more rigid since the slider is held by the spring. Compared with some other rigid motors such as the Pan-style walker [9,10,16] or its variables [17,18], and the attocube drive [19], the SpiderDrive is most compact and simplest in structure. The greatest advantage is that the PTS can also act as a scanner after coarse approach [20]. A possible drawback, though not observed so far, is that during scanning the sliding shaft may slip if the sample's topography is very rough, but this effect can be minimized by slowing down the scan speed. Building a MFM from the SpiderDrive is extremely easy, as shown in Fig. 3. Just co-axially mount the SpiderDrive on the
H. Zhou et al. / Ultramicroscopy 147 (2014) 133–136
bottom of the MFM frame which is a segment of capped stainless steel pipe. The sample is fixed on the top side of the shaft and the cantilever holder is mounted underneath the crossbeam. The cantilever is driven to oscillate by a dither piezo plate.
135
cantilever is lifted at a typical height of 100 nm to image the magnetic signal. The tip is coated with 5 nm thick Cr film first as the buffer layer followed by 50 nm thick Co film. These are done by electron beam deposition. The tip is then magnetized perpendicular to the cantilever with a permanent magnet before loaded into the MFM.
3. Circuits, control and operation The cantilevers we use are commercially-available self-sensing piezoresistive cantilevers (PRC400 from Hitachi High-Tech Science Corporation, Japan) [21]. The preamplifier, which has been described in Ref. [22], is a home-made active Wheatstone bridge followed by a second stage amplifier with a gain of 1000 to detect the resistance change of the cantilever. We use the built-in phaselocked loop (PLL) of the R9 controller to extract the frequency shift signal. The topography is imaged in tapping mode which allows us to compensate the slopes in the two scan directions. Then the
4. Chamber, magnet and vibration isolation A schematic of the whole system is shown in Fig. 4. MFM scan head is housed in a long tubular chamber similar to the one described in Ref. [23]. The chamber is two meters long and consists of three parts, which are all made of non-magnetic stainless steel. The bottom part that houses the MFM scan head is a capped tube and it is vacuum sealed with the middle part via a specially designed narrow CF flange as described in Ref. [24]. The middle part is a long and thin pipe and it is connected to the top part through a normal CF flange. The top part contains a three-way vacuum tee and a six-way vacuum cross. The former is connected to the preamplifier through the electric feedthrough at the middle flange. A vacuum gauge and a valve for pumping the chamber are connected to the six-way vacuum cross via its side flanges. The magnet is a 20 T superconducting magnet (from Oxford Instrument) with a cold bore of 52.8 mm diameter. The volume of the magnet dewar is 77 L and the filling cycle is about 4 days. The magnet is housed in a large aluminum box whose inner walls are covered with soft sponges for sound isolation. The box is lifted by heavy duty springs which are hung from two stacks of cement bricks interleaved with soft rubber slabs. The whole system is installed in a big cement pool which is isolated from the surrounding soil with 5 cm thick sponge sheets for further vibration isolation.
5. Performance test
Fig. 4. Schematic view of whole MFM system.
To show the performance of this MFM, we present the topography and magnetic images of a 25 nm thick Pr0.55(Ca0.75 Sr0.25)0.45MnO3 (PCSMO) thin film on (LaAlO3)0.3(SrAl0.5Ta0.5O3)0.7
Fig. 5. Topography and MFM images of the 25 nm thick PCSMO thin film at 175 K and in fields up to 15.0 T at the sample position. The size of the images is 6 mm 6 mm. The color scales of these images are 200 nm (topo), 0.4 Hz (0 T, 3.5 T, 4.5 T, 5.5 T, 10.0 T and 15.0 T) and 0.56 Hz (4.0 T). The line time is 1.6 s and the time to take an image is about 13 min. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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(LSAT) (110) substrate at 175 K and in fields up to 15 T. After the MFM was inserted into the magnet, a small amount of helium gas was released to the chamber for heat exchange and the vacuum was at 3 10 4 Torr. The topography and magnetic images are shown in Fig. 5. At this temperature, the sample is a paramagnetic insulator (PI), and there is nearly no contrast in the MFM image except the weak signals from the sample surface at zero field. In 3.5 T, small dark droplets appear. These are ascribed to ferromagnetic metallic phase (FMM) because the attractive force on the tip causes a negative frequency shift which corresponds to a dark color. In 4.0 T, maze-like FMM phase domains are observed and in higher fields, the areas of ferromagnetic phase grow and they gradually occupy the sample. But some PI droplets persist even in 10 T and they disappeared in 15 T. To the best of our knowledge, this is the highest field MFM image up to now. 5. Conclusions We have presented a home-built MFM that fits in a 20 T superconducting magnet and can operate at low temperatures. Its PTS is used as the scanner as well as the driving piezo of a four-way driven inertial motor capable of vertical approach even at 6.5 K. Its rigidity, simplicity and compactness make it a distinctive and powerful MFM, whose performance is shown by the clear magnetic images of a manganite thin film in fields up to 15 T. Acknowledgment We sincerely thank Prof. Z.G. Sheng for providing the PCSMO thin film. This work is supported by the National Natural Science
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