*Corresponding author. fax: #81 298 59 5010.E-mail address: hideyuki@nrim.go.jp (H. Shinagawa).

Physica B 298 (2001) 580}584

Development of variable-temperature scanning probemicroscope for high magnetic "elds

H. Shinagawa*, T. Takamasu, G. Kido

Physical Properties Division, National Research Institute for Metals, 3-13 Sakura, Tsukuba-shi, Ibaraki 305-0003, Japan

Abstract

We have developed a new type of high magnetic "eld scanning probe microscope (SPM), which works #exibly in highmagnetic "eld up to 7.5T at various temperatures from 10 to 600K. This SPM can be employed as a scanning tunnelingmicroscope, atomic force microscope, magnetic force microscope, scanning near-"eld optical microscope or as a combi-nation of these di!erent microscopes to study nanostructure semiconductor devices, such as quantum dots. � 2001Elsevier Science B.V. All rights reserved.

Keywords: Scanning probe microscope; Magnetic force microscope; High magnetic "eld

Introduction

Recently, many nanostructure semiconductordevices, such as single-electron transistors, haveattracted much attention for use in quantum com-puters [1}3]. Electronic states in these nanostruc-tures display strong electron correlations, whichare important in realizing macroscopic quantume!ects, especially in high magnetic "elds. Scanningprobe microscopes (SPMs) have become some ofthe most powerful tools in exploring these struc-tures.While one type of SPM, the scanning tunneling

microscope (STM), has found widespread applica-tion in high magnetic "elds, SPMs in which a canti-lever is employed have been of limited use in such"elds. Although some studies have been conducted

with high magnetic "eld SPMs [4}6], the magnetic"elds were generated by a permanent magnet orelectromagnet, which limited the maximum mag-netic "eld to at most about 2T. Until now, therehas been no SPM that has been combined witha superconducting magnet and can be #exibly usedin high magnetic "elds.At our institute, we have developed a new type of

high magnetic "eld SPM. The entire SPM includ-ing the laser head and temperature control systemis placed in the room-temperature bore of a large-bore superconducting magnet, allowing any tech-niques for the SPM to be easily applied in highmagnetic "elds with minimal modi"cations. ThisSPM can be used as an STM, atomic force micro-scope (AFM), magnetic force microscope (MFM),scanning near-"eld optical microscope (SNOM) oras a combination of these di!erent microscopes. Inthe future we plan to incorporate a magnetic reson-ance force microscope (MRFM), such as an elec-tron spin resonance (ESR) microscope.

0921-4526/01/$ - see front matter � 2001 Elsevier Science B.V. All rights reserved.PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 0 3 8 6 - 6

Fig. 1. Cross-section of our variable-temperature high magnetic"eld scanning probe microscope system.

Fig. 2. Schematic view near the sample stage. (a) Sample stage,(b) thermal exchange block, (c) multi-thin copper plate, (d) radi-ation shield, (e) He transfer tube, (f) piezoelectric actuator, (g)mechanical approach stage, (h) cantilever with tip, (i) opticalwindow. (j) Laser unit is attached here to detect the motion ofcantilever.

Our SPM should be quite useful in studyingnanostructure semiconductor devices, for example,in clarifying the magnetic structures of quantumdots that contain magnetic dopants, such as GaM-nAs or CdMnTe quantum dots [7], and in develop-ing the solid-state quantum computer [8].

2. Development and experiments

A cross-section of our high magnetic "eld SPMsystem is shown in Fig. 1. To generate high mag-netic "elds, we developed a vibration-proof cryo-cooled helium-free superconducting magnet. Thesuperconducting magnet is cooled bya Gi!ord}McMahon (GM) refrigerator througha heat conductor that consists of a stack of copperand aluminum thin "lms, i.e., multiple thin plates.The refrigerator and the magnet are enclosed inseparate vacuum containers. A bellows #exibly sup-ports the container for the refrigerator to dampenvibrations. The amplitude of the vibration of themagnet is remarkably suppressed in comparisonwith those currently in use elsewhere. A turbomolecular pump is attached with a large-bore

vibration-free tube system so that we can keep thesample space in a good vacuum.We have modi"ed a Seiko Instruments (SII)

300HV SPM to allow its insertion into the room-temperature bore of the magnet. A variable-tem-perature sample stage is placed in the vacuumchamber, in which the temperature is controlledwith a #ow of liquid helium and a heater. Thesample and cantilever are observable through anoptical window. The laser head, which detects themotion of the cantilever, and the optical micro-scope are located outside the vacuum chamber.A schematic view near the sample stage is shown

in Fig. 2. A copper heat-exchange block is placednear the sample stage, which is cooled by liquidhelium #ow down to 10K. The sample stage isconnected to the heat-exchange block with multiplethin copper plates. The cantilever is vibrated bya piezoelectric actuator, and an FM modulationtechnique is used to detect the interaction betweenthe cantilever's tip and the sample surface.

H. Shinagawa et al. / Physica B 298 (2001) 580}584 581

Fig. 3. Schematic view of (a) our MFM and (b) SNOM probe.

Our MFM and SNOM probes are schematicallyshown in Fig. 3. Magnetic "elds are applied perpen-dicular to the cantilever and the sample surface.A Si micro-cantilever with a magnetic-coated tip isused for the MFM. In this system, the cantilever isplaced at the center of the magnet, where themagnetic "eld is homogeneous, so that the fer-romagnetic cantilever is mechanically stable in highmagnetic "elds. For the SNOM, a metal-coated"ber with a conical tip is used as a cantilever. The"ber is vented near the tip and works as a cantileverfor the AFM, also.

To check the system, we have observed the AFMand MFM images of a magnetic "lm with theMFM probe.

3. Results and discussion

AFM and MFM images are stably observed atvarious temperatures between 60 and 600K inmagnetic "elds up to 7.5T. The change of thepiezoelectric coe$cient is less than 10% in thistemperature range. The resonant frequency of thecantilever critically depends on the temperature sothat we had to wait to achieve thermal relaxation ofthe system after the temperature was changed. Theresonant frequency of the cantilever varies little inmagnetic "elds so the cantilever works stably up to7.5T. The laser path moves a little depending onthe applied magnetic "eld, which suggests a kind ofmechanical distortion of the system. It would becaused by some magnetic parts that are present inthe laser head.Fig. 4 shows some MFM images made at room

temperature in various magnetic "elds of the sur-face of a magnetic "lm used in a commercial harddisk drive, and the macroscopic magnetizationcurve of the "lm. Magnetic bit patterns are re-corded from the upper side to the lower side asindicated by the arrow in Fig. 4(a). Three tracks areshown in the observed area. When magnetic "eldsare applied perpendicular to the sample surface, themagnetic domains fade out with an increasing mag-netic "eld, and consequently no magnetic bit pat-terns are shown above 1.0T. Longitudinal stripesthat were observed in higher magnetic "elds re#ectthe rugged surface structure of the sample, which isin good agreement with the AFM images of thesample [9].The magnetization curve that is presented in Fig.

4(d) is obtained with a SQUID for a fragment of thehard disk, with the component of the magnetiz-ation due to the substrate of the disk subtractedout. The magnetic patterns in the MFM imagesfade out around the point on the curve where thehysteresis in the magnetization curve vanishes, near0.8T.The MFM image of the surface of CoCr high-

density magnetic recording media is presented for

582 H. Shinagawa et al. / Physica B 298 (2001) 580}584

Fig. 4. MFM images for a magnetic film used in a hard disk drive at room temperature for (a) 0T, (b) 0.4 T and (c) 1.0T. (d) Macroscopicmagnetization curve of the sample.

0 and 0.05T in Fig. 5(a) and (b), respectively.Magnetic recording bits are observed in themarked area, and the pro"les presented in Fig. 5(c)and 5(d) are taken along the arrow indicated inFig. 5(a). The magnetic domains fade out in highermagnetic "elds, while it is observed that the res-olution of the image is substantially improved at0.05 T compared with that at 0 T. The MFM tiphas a magnetic moment that consists of only resid-ual magnetism at zero magnetic "eld, while in

magnetic "elds the tip is strongly magnetized bythe external "eld signi"cantly increasing theinteraction between the tip and the sample. Thisfrees us to choose magnetic material for the MFMtip that does not have large residual magnetiz-ation, greatly expanding our choice of materials.Furthermore, it is likely that the magnetic domainin the tip that spoils the resolution at zero mag-netic "eld fades away in an applied magnetic"eld.

H. Shinagawa et al. / Physica B 298 (2001) 580}584 583

Fig. 5. MFM images of the CoCr high-density magnetic recording media at room temperature for (a) 0 T and (b) 0.05T. The pro"lealong the recording direction for (c) 0T and (d) 0.05T.

More details of the results on the magnetic "lmwill be presented in another paper [9]. SNOMdetection of GaAs quantum dots is now inprogress.We hope to make many signi"cant contri-butions to the exciting and growing "eld of nanos-tructure semiconductor physics with our newdevice in the coming years.

Acknowledgements

We are grateful to Prof. Y. Nakamura for pro-viding CoCr high-density magnetic recordingmedia.

References

[1] G.P. Berman et al., Phys. Rev. B 61 (2000) 14694.[2] G. Jin, J.L. Liu, S.G. Thomas et al., Appl. Phys. Lett. 75

(1999) 2752.[3] Y. Nakamura, Yu.A. Pashkin, J.S. Tsai, Nature 398 (1999) 786.[4] R. Proksch, E. Runge, P.K. Hansma, S. Foss, B. Walsh,

J. Appl. Phys. 78 (1995) 3303.[5] D. Gomez, E.R. Burke, I. Mayergoyz, J. Appl. Phys. 79

(1996) 6441.[6] G. Merton, G.D. Skidmore, J. Schmidt, E.D. Dahlberg,

H. Wan, B. Pant, J. Appl. Phys. 85 (1999) 4601.[7] N. Takahashi, K. Takabayashi, I. Souma, J. Shen, Y. Oka,

J. Appl. Phys. 87 (2000) 6469.[8] G. Kido et al., in these Proceedings, Physica B 298 (2001).[9] H. Shinagawa, T. Takamasu, G. Kido, J. Electron Microsc.,

to be submitted.

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