Construction of a simple lowfield solenoid for the Quantum Design© SQUIDmagnetometerL.Q. Wang, M. S. M. Minhaj, J. T. Chen, and L. E. Wenger Citation: Review of Scientific Instruments 64, 3018 (1993); doi: 10.1063/1.1144350 View online: http://dx.doi.org/10.1063/1.1144350 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/64/10?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Magnetic design for low-field tunability of microwave ferrite resonators J. Appl. Phys. 85, 4856 (1999); 10.1063/1.370044 A simple demonstration of the magnetic field of a solenoid Am. J. Phys. 64, 1525 (1996); 10.1119/1.18403 Simple toploading cryostat insert for a SQUID magnetometer Rev. Sci. Instrum. 60, 943 (1989); 10.1063/1.1140349 Effect of lowfrequency ambient magnetic fields on the control unit and rf head of a commercial SQUIDmagnetometer Rev. Sci. Instrum. 55, 1475 (1984); 10.1063/1.1137961 Simple, lownoise uhf SQUID magnetometer Rev. Sci. Instrum. 46, 1249 (1975); 10.1063/1.1134455

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NOTES BRIEF contributions in anyjieid of instrumentation or technique within the scope of the journal should be submitted for this section. Contributions should in general not exceed 500 words.

Construction of a simple low-field solenoid for the Quantum Design@ SQUID magnetometer

L.-Q. Wang, M. S. M. Minhaj, J. T. Chen, and L. E. Wenger Department of Physics and Astronomy, Wayne State University, Detroit, Michigan 48202

(Received 3 May 1993; accepted for publication 8 July 1993)

A simple copper-wire solenoid is constructed on the exterior of the experimental sample chamber tube of the Quantum Design MPMS@ SQUID magnetometer. This copper solenoid can generate fields of less than 20 Oe reproducibly and a field uniformity with less than a 0.1 % variation over a 4-cm length. With the addition of a mu-metal cylinder, zero fields of less than 1 mOe can also be attained.

In the earlier models of the Quantum Design MPMS@ SQUID magnetometer,’ the magnetic fields are generated by a 5.5 T superconducting magnet. At low-field values of 20 Oe or less, several experimental problems can be encountered.24 The fields are not reproducible at the 50- mOe level, the field profile over the distance that the sam- ple traverses may not be uniform especially after reducing or quenching from high fields, and zero-field cooling of samples with less than 10 mCe cannot be attained. These problems can play a significant factor in magnetization measurements which require sensitive measurements at very low fields, for example in high-temperature supercon- ductors and magnetic spin glasses.

In order to minimize these problems, we have con- structed a copper-wire solenoid on the exterior of the ex- perimental sample chamber tube of the magnetometer’ (see Fig. 1). The strings surrounding the copper-wire iso- thermal shield are removed and two layers of 40 AWG copper wire are wound over a 200-mm length. Steady mag- netic fields of 20 Oe (280 mQe/mA) can be easily attained by utilizing a constant dc current source6 even though the Cu coil resistance changes two orders of magnitude be- tween LHe temperature and room temperature. This copper-wire solenoid is positioned so its center coincides with the middle coil of the second-order gradiometer de- tection coils. Since the solenoid extends well beyond the lo-mm gradiometer coil separation distance, the field vari- ation is less than 0.1% over a distance of *20 mm from the center, our typical scan distance. This field variation will be maintained as long as the 5.5 T superconducting magnet is not energized after the initial cooldown of the magnetometer to LHe temperatures. Otherwise, nonuni- form field profiles will develop after reducing or quenching from high fields in the superconducting magnet as de- scribed in Refs. 2-4. The electrical leads from the copper solenoid are twisted and follow the heater and thermome- ter leads to the top of the sample chamber tube where extra pins are available for external connection. The two layers

of copper wire still permit clearance with the inner vacuum jacket wall for sufficient He gas to flow past the isothermal shield-copper solenoid system in order to maintain reason- able temperature regulation. Since the resistance of the Cu solenoid is nearly 600 G at room temperature, Joule heat- ing at the sample location can be significant for magnetic

Top Plate

Sample Chamber Tuk

Inner Vacuum Jacket Wall

Outer VacuumJacket Wall

/II/ Iso-Thermal Sheet wirh Heater

Copper Wire Solenoid

5.5 Tesla Superconducting Solenoid

FIG. 1. Diagram of MPMS cryostat-magnet system and position OF copper-wire solenoid on sample chamber tube.

3018 Rev. Sci. Instrum. 64 (lo), Ott 1993 0034~748/93/64(10)f30~8/2/$6.00 0 1993 American Institute of Physics 3018 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

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fields greater than 5 Oe. For example, at 300 K, Joule We have used this copper solenoid over a 1-yr period heating results in a difference of 4 K between the actual and have found the “zero” field and field calibration to sample temperature and the temperature determined by have changed less than 1% during this time. In addition, the platinum thermometer at the bottom of the experimen- the scatter in the measured data is extremely small, ap- tal sample chamber tube. Below 50 K, the Joule heating is proximately 2 X lop8 emu for measured magnetizations on negligible for fields less than 10 Oe. the order of 10V7 emu.

In order to perform zero-field cooling of samples in fields Iess than 1 mQe, the entire magnetometer dewar was placed inside a single layer, mu-metal cylinder.’ This cyl- inder attenuates the earth’s field to approximately 16 mOe along the longitudinal axis of the magnetometer and to less than 1 mQe in the transverse direction.’ By applying a small current to the copper solenoid in opposition to this residual field, the net external field at any sample location along the 40-mm scan length could be further reduced to less than 1 mOe. This zero field was determined by mea- suring the magnetization of a paramagnetic sample and fInding the current in the solenoid where (i) the magnitude of the magnetization was less than lop8 emu and (ii) the corresponding SQUID response was minimal and flat. This value can be utilized as an offset field in determining the applied magnetic field at the sample. Furthermore, the ad- justment of the current source can be incorporated into the MPMS operating software by utilizing the Quantum Design@ EDC software package.

Support of this work by the Air Force Office of Scien- tific Research and the WSU Institute for Manufacturing Research is gratefully acknowledged.

‘Model MPMS SQUID magnetometer, Quantum Design, San Diego, CA 92121-9704.

*F. J. Blunt, A. J. Perry, A. M. Campbell, and R. S. Liu, Physica C 175, 539 (1991).

‘W. Braunisch, N. Knauf, V. Kataev, S. Neuhausen, A. G&z, A. Kock, B. Roden, D. Khomskii, and D. Wohlleben, Phys. Rev. Lett. 68, 1908 (1992).

‘Technical notes, Quantum Design, San Diego, CA 92121-9704. sThe sample chamber tube can be removed from the system at room temperature by first removing the sample transport mechanism assem- bly.

6Model K220 programmable current source with IEEE interface, Kei- thley Instruments, Cleveland, OH 44139.

’ 19-in.-diam., 30-m. high mu-metal cylinder manufactured by Magnetic Shield Division, Bensenville, IL 60106.

‘The mu-metal cylinder still attenuates the earth’s field to these levels even after energizing the superconducting magnet to 5.5 T.

3019 Rev. Sci. Instrum., Vol. 64, No. 10, October 1993 Notes 3019 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

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