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Submitted on 1 Jan 1978
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A DOUBLE SAMPLE CELL NMR TECHNIQUE FOR
MAGNETIC SUSCEPTIBILITY MEASUREMENTS
IN SOLID 3He
W. Kirk, M. Twerdochlib, D. Chesire
To cite this version:
A DOUBLE SAMPLE CELL NMR TECHNIQUE FOR MAGNETIC SUSCEPTIBILITY MEASUREMENTS
IN SOLID
3H
et
W.P. Kirk, M. Twerdochlib, and D.P. Chesire
Physios Department, Texas A & M University College Station, Texas 7784S, U.S.A.
Abstract.- A method for obtaining more reliable nuclear magnetic resonance suscep-tibility measurements in solid 3He is described. A double sample cell in conjunction
with a computer controlled NMR spectrometer provides a way to make simultaneous sus-ceptibility measurements of high and low density samples as well as measurements of susceptibility differences. This method is being used in a study of solid 3He over a
temperature range of about 3 mK to 50 mK to provide higher quality data needed for comparison with other types of thermodynamic data and with more comprehensive theo-ries of solid 3He magnetism.
Good measurements of the solid 3He
magne-tic susceptibility, x(T)> have been difficult to obtain because contributions from Curie Law de-pendence dominates the magnetic behavior over much of the temperature range that most workers have been able to achieve. A number of efforts /1,2/ over the past decade as well a few earlier ones have contributed to measurements of ^ (T)» but even in the case of the more extensive measurements /2/ the need for more accurate and extensive suscepti-bility data is clearly demanded so that further theoretical efforts can proceed effectively /3/ and
more meaningful comparisons can be made with other thermodynamic data .
In order to achieve the goal of a more reliable susceptibility measurement, we have deve-lopped a method which uses a double sample cell and a computer controlled NMR pulse spectrometer. This method provides a way to overcome some of the problems encountered in the earlier measurements. In particular, problems associated with the tempe-rature resolution and the thermal equilibrium between the thermometer and 3He have perplexed the
earlier measurements. From our past experience, we believe that better control of these problems can
be gained by using a sample cell which has two chambers as identical as possible. Each chamber can be filled independently with 3He. They are connected thermally by a bundle of fine copper wires through epoxy seals. The wires are tightly packed in the chambers to provide a large surface area for thermal contact with the He. NMR reso-nance coils (transmitting and pick-up) surround each chamber and are connected to a pulse spectro-meter so that signals from each chamber can be ta-ken out simultaneously after a proper pulse sequen-ce has been applied. A magnetic field gradient can be used to help distinguish between signals. The two signals are processed (i.e., signal-averaged, analyzed and stored) with the aid of a minicompu-ter. Clearly, for a pulsed technique, it is impor-tant to be able to do Fast Fourier Transform of the FID in order to separate the two signals; this we are able to do with our present set-up. Figure
1 shows a schematic diagram of the arrangement for the sample cell and computer controlled spectrome-ter.
The double cell arrangement is rather flexible and has several advantages. First, one of the cells can be pressurized with a higher density solid so that its exchange energy is essentially zero and the susceptibility will be very close to t This work has been supported in part by the
Na-tional Science Foundation under Grant Number DMR-76-09807.
JOURNAL
DE
PHYSIQUEColloque
C6,
supplément au n°
8,
Tome
39,
août
1978,
page
C6-1180
Résumé.- On décrit une méthode pour obtenir des mesures de susceptibilité par résonance magnétique nucléaire dans l1 3He solide, plus fiables. Une cellule à double échantillon en conjonction avec un spectromètre RMN contrôlée par ordina-teur procure un moyen de faire des mesures de susceptibilité simultanée d'échan-tillons de haute et basse densité ainsi que de différence de susceptibilité. Cette méthode est utilisée pour l'étude de l'3He solide entre 3 et 5o mK afin d'obtenir des résultats de meilleure qualité nécessaires à la comparaison avec d'autres types de données thermodynamiques et avec des théories plus complètes de magnétisme de l'3He solide.
Curie Law dependence only.. The higher density solid can be used as an N M R thermometer for measu- ring the susceptibility of a much lower density solid in the other cell.
filtering schemes. Accurate measurements of AX woukl of course be very useful because the Curie Law dependence is cast out, thus revealing more expli- citly the important higher order terms.
All the various data manipulations, spectro- meter pulse sequencing etc.. is handled by a Nic-80 minicomputer. The double cell is cooled with a '~e-
k e dilution refrigerator to about 12 mK. Additio: nal cooling down to about 2 or 3 mK is done with CMN. A sixth order compensated superconducting solenoid provides a static field for the NMR.
References
Fig. 1 : Schematic layout of computer controlled spectrometer and double cell arrangement.
3 ~ e has a strong resonance signal and hence can be used as a more effective thermometer (a fac- tor of 5 at least) than say a Cu or Pt NMR thermo- meter. By using pulse techniques one can measure
spin-lattice relaxation times of each cell's sample and obtain an idea of the thermal equilibrium time. Systematic errors and some understanding of the thermal equilibrium times can also be checked by reversing the roles (i.e., high and low density) of the two sample cells. An important point to emphasize here is that signals from each cell can be obtained simultaneously so that any thermal
fluctuations, spurious noise, etc..get added to each signal'almost equally and hence any results extracted from the data will be more immune to these types of noise problems. Another advantage offered by the double cell arrangement is that it is possible to make a susceptibility difference, A~~measurement by taking the two signals coming kom each cell during each measurement and subtrac-
ting them digitally with the computer. Of course, it is assumed that the signals have been properly normalized with respect to each other at a much higher temperature where Curie law dependence domi- nates in each sample. If both signals have close to the same noise and are not badly out of phase, the difference measurements can give rather clean data which is much better than trying to subtract the two data sets manually where the noise is car- ried along and simply suppressed by smoothing or
/I/ Sites, J.R., Osheroff, D.D., Richardson, R.C. and Lee, D.M., Phys. Rev. Letters
2
(1969)836; Pipes, P.B. and Fairbank, W.M., Phys. Rev. Letters2
(1969) 520; Phys. Rev. (1971)1590; Johnson, R.T.and Wheatley, J.C., Phys. Rev. &(1970) 1836; Bernat, I- T.P. and Cohen, H.D., Phys. Rev. (1971) 1709.
/ 2 / irk,-W.P., Osgood, E.B., and Garber, M., Phys. Rev. Letters
2
(1 969) 833.131 In particular, comments by R.A. Guyer, prepint, and Zane, L.I., in Proceedings of the Quantum Crystal Conference,Fort Collins, Colorado, ed by J.R. Sites (August 1977).
