TWO PRACTICAL MAGNETIC THERMOMETERS FOR USE BELOW 30 K

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TWO PRACTICAL MAGNETIC THERMOMETERS FOR USE BELOW 30 K

B. Mangum, W. Bowers

To cite this version:

B. Mangum, W. Bowers. TWO PRACTICAL MAGNETIC THERMOMETERS FOR USE BELOW 30 K. Journal de Physique Colloques, 1978, 39 (C6), pp.C6-1175-C6-1176.

�10.1051/jphyscol:19786520�. �jpa-00218006�

TWO PRACTICAL MAGNETIC THERMOMETERS FOR USE BELOW 30 K

B.W. Mangum and W.J. Bowers

National Bureau of Standards, Washington, D.C. 20234

Abstract.- We have measured the magnetic susceptibility of Gd(P03)3(GP) and of Nd(C2H5SOO3.9H20 (NES) over the range 2 to 27 K and found these salts to be good magnetic thermometers, although NES is less suitable for accurate work over this entire range. By fitting the magnetic data to recently calcu- lated temperatures (Tac) determined acoustically by Plumb and Catalan, a magnetic temperature scale is developed and it is compared to other scales.

INTRODUCTION.- We report magnetic susceptibility results obtained with vitreous Gd(P03)3(GP) and sin- gle crystal Nd(C2H5S0i,) 3.9H20 (NES), which we have used to smooth through the temperatures in the ran- ge 2-27 K obtained by Plumb and Cataland /I/ with their acoustical interferometer. This gives a ma- gnetic temperature scale which is compared with other scales. GP and NES are shown to be good ther- mometers .

NES has been identified as a possibly very useful thermometer /2/ because experimental data

3 +

indicated that the interactions of the Nd ions are limited to dipolar mechanisms /3/, because the first two dipole-dipole parameters in the equation

X = (C/T)(l+A/T+6/T2 +y/T3 +... ) (1) have been calculated /4/ (and recently confirmed

experimentally / 5 / ) , and because the first excited doublet is sufficiently high that below 20 K the ground state doublet is well isolated. Previous in- vestigations /6/ indicated GP likely to be a good magnetic thermometer and since it is vitreous, it can easily be made into any desired shape.

EXPERIMENTAL DETAILS.- The susceptibilities of sphe- rical samples of NES and GP were measured with an ac mutual inductance bridge operating at 270 Hz.

The primary and secondary coil systems, each of which consisted of modified Helmholtz coils, were kept in liquid helium at a constant temperature.

The field produced at a sample was about 2 gauss.

This gave a sensitivity at 20 K for GP of 3.5 yK.

Instabilities, however, reduced the reproducibili-

ty to about ± 0 . 1 mK.

The temperature was measured with a germa- nium resistor (GER), which had a long history of high stability and which had been calibrated against the NSB acoustical thermometer 11/, using an ac bridge operating at 26 Hz. The useful sensitivity of the GER at 20 K was about ± 0.1 mK. Thermal con- tact between the sample and the GER mount was pro- vided by 144 •//• 36 formvar-insulated copper wires tied to the sample with cotton thread and liberally coated with stopcock grease. The entire sample as- sembly and GER were located in a vacuum space.

RESULTS AND DISCUSSION.- The susceptibility data were analyzed in terms of acoustically determined

temperatures, T , which were provided to us by Plumb IM, who has recently calculated them, ta- king into account at each isotherm the term qua- dratic in pressure. His new calculations use the same gas constant used previously 111. A least squares fit of the NES data gave a value of A in eq. (1) in agreement with that calculated theoreti- cally/4,5/, and with that measured by Fisher et al 15/. Figure 1 shows the fit of the NES data to T

ac where we have used the values of A and 6 determined experimentally 15/. The scatter in the NES data is worse than that in the GP data, also shown in fi- gure 1, since its sensitivity is some 30 times less than that for GP. A fit of third degree in 1/T was also used for GP but the coefficients were deter- mined by a least squares fit of the data to 1/T and not calculated theoretically. The susceptibili- ty is related to the mutual inductance bridge set- JOURNAL D E PHYSIQUE Colloque C6, supplément au n° 8, Tome 39, août 1978, page C6-1175

Résumé.- Nous avons mesuré la susceptibilité magnétique de Gd(PO3) 3 (GP ) et de Nd(C2H5S0i,) 3. 9H2O (NES) dans le domaine de température allant de 2 à 27 K et nous avons trouvé que ces sels sont de bons thermomètres magnétiques, bien que le NES ne soit pas convenable pour des mesures précises dans la totalité du domaine considéré. En ajustant les données magnétiques aux températures récemment déter- minées par des mesures acoustiques de Plumb et Cataland, nous avons développé une échelle magnétique de température ; et nous avons comparé cette échelle avec d'autres.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19786520

ting by

ax

N - ^NW

A/T + B / T ~

C / T ~

(2) where x

magnetic susceptibility, a

constant, N

bridge setting, Nm

bridge setting with sam- ple removed from the coils or at infinite tempera- ture, and A, B, and C

constants.

Fig.

Comparison of the GP and the NES tempera- ture scalesfTGp or TNES) obtained from a least squares fit of the data to TaC, with the measured temperature on Tac as a functlon of temperature.

Although measurements of GP and NES were made at the same temperatures, the data points are displa- ced in temperacure for clarity.

The best fit of the GP data to Tac was (in arbitra- ry units) N

-257.1421 + 11351.461T - 900.7162./T2 1016.844/~~. The corresponding fit in eq. (I) for NES is N

264.7431 -

T~). Performing a least squares fit of the suscep- tibility data to T gives a magnetic temperature

ac

scale, TGp or TNES which smoothes T and which, ac

through the use of published scale differences, can be compared with -other temperature scales. Such a comparison for Tep ?s shown in figure 2. As shown there, there appears to be.an error in the acous- tically determined value of 2.8 K (either Tac qr T~~~ 2-20t7/) .

On extrapolating the GP data, fitted to Tac,

the boiling point of neon, we obtain a value about-13 mK larger than that obtained by fitting to TXAc1/8/. This does not seem unreasonable since Tac and TXAcI appear to be diverging linearly be- low 20 K and if they were to continue in the same way, a difference of about 8 mKwould be expected at 27 K.

Fig. 2

Comparison of various temperature scales with T G ~ , the GP temperature scale obtained from a least squares fit of the data to Tac, the newly derived acoustic scale. Measurements were made wi- thin a few mK of those acoustic points defining the NBS 2-20 scale/7/.

In conclusion, NES and GP appear to be good magnetic thermometers with GP having the higher sensitivity and thus being more useful at the hi- gher temperatures. In our experiments, NES was use- ful up to only about 16 K but it has the advantage that measurements at only two rather than four tem- peratures are required for its calibration.

References

/I/ Plumb,H., private communication. These tempe- ratures are newly derived from all of the acoustical interferometer data by taking into account the 2nd degree term in pressure.

/2/

Note 830 (1974) 18

/3/ Daniels,J.M., Proc.Phys.Soc. London (1953) 673

/4/ Meyer,H., Phil.Mag. 2 '(1957) 521

/5/ Fisher,R.A., Hornung,E.W., Brodale,G.E. and Giauque,W.F., J.Chem.Phys.65 (1976) 4724

/6/ Arnbler,E., Mangum,B.W., Pfeiffer,E.R. and Utton, D.B., Phys.Lett. (1974) 249

/7/ Plumb,H. and Cataland,G.,Metrologia 2(1966)127 /8/ Besley,L.M. and Kemp,W.R.G., Metrologia 12

(1977) 35