HAL Id: jpa-00217997
https://hal.archives-ouvertes.fr/jpa-00217997
Submitted on 1 Jan 1978
HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
HYPERFINE ENHANCED NUCLEAR MAGNETIC COOLING IN PrBe13
K. Andres, G. Eska, S. Darack
To cite this version:
K. Andres, G. Eska, S. Darack. HYPERFINE ENHANCED NUCLEAR MAGNETIC COOL- ING IN PrBe13. Journal de Physique Colloques, 1978, 39 (C6), pp.C6-1157-C6-1158.
�10.1051/jphyscol:19786512�. �jpa-00217997�
HYPERFINE ENHANCED NUCLEAR MAGNETIC COOLING IN P
rB
e13
K. Andres, G. Eska and S. Darack
Bell Laboratories, Murray Hill, N.J. 07974 (USA)
Abstract.- In Van Vleck paramagnetic PrBe , the hyperfine enhancement of the local field at the Pr nuclei is 9.8 times the external applied field. Nuclear demagnetization experiments show good ther- modynamic reversibility and yield end-'temperatures below 1 mK. The calculated nuclear ordering tem- perature is 0.46 mK.
From an analysis of magnetic susceptibility and specific heat data in the cubic compound PrBe Bucher et al. /I/ concluded that exchange interac- tions between Pr ions must be small and that magne- tic order among the nuclei of the singlet ground state Pr ions should occur only much below 1 mK. We have performed such nuclear demagnetization experi- ments and were indeed able to cool a Copper cold- finger attached to a PrBe,, sample to 0.85 mK.
The sample was prepared by repeatedly arc mel- ting the constituents in an arc furnace into the shape of an irregular cylinder of about 1 cm dia.
and 7.5 cm length, compensating for small weight losses by adding more Be during the last meltings.
The stated purities of the materials used were 99.99 % for Be and 99.9 + % for Pr. For thermal contact, the sample was first wetted in a pure He atmosphere with molten Cd by means of an ultrasonic soldering iron. Two Cu strips (cross section = 0.1 x 1.5 cm2) were then soldered with Cd onto opposite sides of the sample in a vacuum chamber.
One strip (the coldfinger) is extended to 20 cm be- low the cooling pill and its end dips into one of the two search coils of a d.c. SQUID magnetometer.
A 60CoCo hep single crystal Y~ray anisotropy ther- mometer (of dimension 0.02 x 0.3 x 0.5 cm3 and of 2 y Curie intensity) is also attached to the end of the coldfinger by means of a Tl-Hg-In eutectic solder. It serves to both monitor temperatures in fields less than 1 T as well as to calibrate the Cu nuclear susceptibility signal down to 3 mK. Such a calibration is shown in figure 1. We observe that
below 3 mK, the Y- a nis o t r oP y starts deviating from the calculated curve and saturates 5 % below the theoretical value.
Specific heat measurements indicate that the thermal relaxation time in the cooling pill around 1 mK is of order 1 h. This is in fair agreement with the time T = c d2/< = 45 min (c = specific heat per unit volume, d = sample thickness, K =
thermal conductivity) and confirms that this long relaxation time is mainly due to the poor thermal
— 6
conductivity of the sample at 1 mK ( K = 7 x 10 watt/cm K, as calculated from the residual resisti- vity p = 3.4 uft cm (p2 9 3 = 41 yfi cm)).
From the specific heat data (in H = 0.01 T) we have computed the entropy variation as a func- tion of temperature and have plotted in figure 2.
Also plotted in this figure is the cooling entropy S(0) - S(H./T.) as a function of H./T.. Two dema- Fig. 1 : Susceptibility of Cu coldfinger plotted versus the inverse of the y - anisotropy tempera-
ture. The solid line is the calculated value.
Permanent address :
TU Milnchen, 8046 Garching, Germany.
JOURNAL DE PHYSIQUE
Colloque C6, supplément au n° 8, Tome 39, août 1978, page C6-1157
Résumé.- Dans le composé PrBe , qui est un paramagnétique de Van Vleck à basse température, le ren- forcement du champ magnétique appliqué au noyau de Pr par rapport au champ externe est de 9,8. Les expériences de désaimantation adiabatique nucléaire indique une bonne réversibilité thermodynamique et on atteint des températures finales inférieures à 1 mK. La température calculée de la transition dans l'état nucléaire ordonnée est de 0,46 mK.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19786512
gnetizations and subsequent remagnetizations are the somewhat higher temperature of 0.85 mK measured
shown. at the end of the coldfinger.
Fig. 2 : Cooling entropy diagram of PrBe,, (see text).
In the upper one we started at Hi = 4 T and Ti = 20 mK, demagnetized to 0.01 T in 40 min, waited 10 min and remagnetized again in 40 min. It can be seen that the total entropy loss during this cycle was surprisingly small. The extrapolation of the entro- py curve to below I mK is based on the following estimate of the nuclear magnetic ordering tempera- ture : EPR data of Bloch et al. /2/ on the g-shift of Gd in PrBe13 yield a Gd-Pr spin coupling energy J/k = -4.64 K of the ferromagnetic sign. Summing only over nearest neighbours, this would yield a molecular field (MF) exchange constant
A
ofThis value is not as small as originally anticipated and is in fact similar to values found e.g. in PrNi
(X
= 5 molelemu) and PrCus(X
= 6.2 molelemu). It leads, in the MI? approximation for the electron-nu- ,clear coupled system, to a ferromagnetic ordering tewerature of'Tc = X C ~ ( I - + ~ ~ ) ~ / ( 1
-
AX,) = 0.46 mK,where we have used the values
xc
= 0.036 emu/mole (= crystal field-
only Susceptibility), l+Kc = 7.8 and CN = 9.5 x I 0-l emuK/mole (= Curie const. of bare Pr nuclei). Thus the thermodynamic end-tempera- ture for the upper demagnetization in figure 2 should be close to 0.5 mK. This is consistent withReferences
/ I / Bucher E., Maita J.P., Hull G.W., Fulton R.C.
and Cooper A.S., Phys. Rev. B G (1975) 440.
/2/ Bloch J.M., Davidov D., Felner I. and Shaltiel D., J. Phys. @ (1976) 1979.
