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Submitted on 1 Jan 1978
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NEW APPLICATIONS OF NUCLEAR COOLING IN
NUCLEAR ORIENTATION STUDIES
G. Wilson, K. Bures, W. Brewer, J. Boysen
To cite this version:
NEW APPLICATIONS OF NUCLEAR COOLING IN NUCLEAR ORIENTATION STUDIES
G.V.H. Wilson, K. Bures, W. Brewer, J. Boysen,
Fachbeveieh Phyeik dev Freien Univevsitat Berlin, 1 Berlin S3, W. Germany.
Abstract.- The application of adiabatic "cooling" and "wanning" of the radioactive oriented probe nuclei to relaxation measurements is demonstrated. Also it is shown how the conventional CMN dema-gnetization cryostats used in such studies may be extended to the 1-2 mK region by nuclear cooling of the copper heat link.
We have recently carried out nuclear orienta-tion studies of the hyperfine interacorienta-tions and spin-lattice relaxation of 1*8V nuclei in iron and co-balt hosts. A full account of these measurements will be published elsewhere ; we report here on so-me novel applications of nuclear cooling developed during these experiments. A description of some nu-clear orientation-magnetic resonance results is gi-ven in an accompanying paper /l/.
A standard CMN demagnetization cryostat was used, having a main salt pill with about 300 g. of CMN powder in glycerin slurry. Contact to the sam-ple was made through the 3000 cm2 area of silver foils which were attached by a screw joint to a heat link of 0.3. mm dia. Cu wires with cross
sec-tional area of 0.3 cm2 and 15 cm length. A 4 T
com-pensated superconducting solenoid was used to pola-rize 'the sample ; with the maximum polarizing field, about the lower 10 cm of the heat link was subjected to a field of at least 2 T
The hBVFe_ sample used was as in /l/ and was
mounted with a 6"CoFe thermometer. The 1*8VCo
sam-ple was prepared in a similar manner using 4N Co foil of 25 ym thickness ; it was mounted with a
6°CoNi thermometer which permitted greater accuracy in the lower temperature (T<3 mK) experiments. Two Ge(Li) gamma detectors were used together with an on-line computer for the data collection.
The nuclear orientation at thermal equilibrium is a function of nuclear parameters, which are all known for *eV, and of the hyperfine splitting which
was determined in the resonance studies III. Thus, measurements of the gamma-ray anisotropy as a func-tion of time after or during temperature changes
may be used to study the spin-lattice relaxation of the '*8V nuclei. For V in cobalt the magnetic hyper-fine field is knowti to be ± 4.9 T /2/ ; our results show that the negative sign is correct.
In the temperature range used, the spin-lat-tice relaxation times for l*8V in Fe and Co are
2-20 h., and because of the relatively fast cooling of the sample after a demagnetization, it is possi-ble simply to monitor the gamma anisotropy as a function of time after cooling. Analysis using the usual angular distribution function then yields the
hSV nuclear orientation. :Such "fast" cooling expe-riments have1 previously'been carried out on ZnFe with TjT = 14.7+2.5 sK /3/ and on soCoFe with TXT
= 2 sK /4,5/. These experiments, together with those employing pulsed heating, have considerable advanta-ge over resonant methods because of the more accura-tely known initial conditions for the relaxation /6/.
In the present experiments, because of the long relaxation times at millikelvin temperatures, and because the hyperfine fields are comparable wi-th available applied fields, it is also possible to control the V nuclear spin temperature in a known way, after thermal equilibrium with the lattice is
attained, by varying the applied field. This has two advantages : it enables relaxation data to be obtained for both polarities of the initial tempe-rature difference between the nuclei and the lattice and it permits relaxation measurements in high ap-plied fields, where, because of the negative hyper-fine fields, the equilibrium gamma anisotropies would be small. This is illustrated in figure 1 ;
at t=0 the demagnetization of the CMN was completed thereafter the spin temperature of the V nuclei JOURNAL DE PHYSIQUE
Colloque
C6,
supplément au n°
8,
Tome
39,
août
1978,
page
C6-1203
Résumé.- Nous décrivons l'application du J'chauff âge" et du "refroidissement" adiabatique de noyaux radioactifs à l'étude de la relaxation nucléaire en orientation nucléaire. Nous montrons aussi l'amé-lioration des performances d'un cryostat à désaimantation adiabatique du CMN, par désaimantation successive des noyaux de cuivre dans le porte-échantillon, permettant d'atteindre une température de 1 à 2 mK pendant quelques heures.
r e l a x e s siowly towards TL. r Time lh) F i g . 1 : Time dependence of t h e a x i a l y- a n i s o t r o - py 11-W(0)
I
f o r"v&
and of TL. A t t = 5.7 h , a f t e r t h e n u c l e i a r e i n e q h l i b r i u m w i t h t h e l a t t i c e , t h e p o l a r i z i n g f i e l d on t h e sam- p l e i s swept from 0.18 T t o 2.54 T d u r i n g 20 min. During t h i s time t h e 4 8 n u c l e i may be assumed t o ~ remain t h e r m a l l y i s o l a t e d because of t h e long T ;a s t h e i r r e s u l t a n t f i e l d i s lowered from 8.53 t o 6.17 T t h e y w i l l c o o l from 4.3 mK t o 3.1 mK. They then r e l a x towards t h e lower o r i e n t a t i o n expected i n t h e 2.54 T p o l a r i z i n g f i e l d . At t = 8.7 h t h e f i e l d was swept back t o 0.18 T t o produce warming of t h e 4 8 n u c l e i . F i g u r e 1 shows how t h e combina- ~ t i o n of " f a s t " demagnetization c o o l i n g p l u s n u c l e a r c o o l i n g of t h e probe n u c l e i may be used t o s t u d y r e l a x a t i o n i n b o t h d i r e c t i o n s and i n l a r g e a p p l i e d f i e l d s . P r e l i m i n a r y v a l u e s of t h e Korringa c o n s t a n t s f o r Fe and Co h o s t s , r e s p e c t i v e l y , a r e 45
+
5 sK and 80+
10 sK.The time v a r i a t i o n of t h e l a t t i c e tempera- t u r e a s i n f i g u r e 1 i n d i c a t e d some unexpectedly l a r - ge e f f e c t s of t h e f r i n g i n g f i e l d from t h e p o l a r i - z i n g c o i l on t h e n u c l e i of t h e copper h e a t l i n k . L a t e r experiments confirmed t h a t n e i t h e r t h e s t r a y f i e l d on t h e CMN n o r eddy c u r r e n t s during f i e l d sweeps produced s i g n i f i c a n t h e a t i n g ; t h e dominant e f f e c t s come from n u c l e a r a d i a b a t i c m a g n e t i z a t i o n and demagnetization of t h e copper. A t low tempera- t u r e s , t h e thermal c o n t a c t between t h e CMN s l u r r y and t h e copper i s s u f f i c i e n t l y poor t h a t v e r y long time c o n s t a n t s a r e observed f o r thermal r e l a x a t i o n
i n v e s t i g a t e d t h e p o s s i b i l i t y of enhancing t h e per- formance of t h e (unmodified) c r y o s t a t hy u t i l i s i n g t h e n u c l e a r c o o l i n g of t h e copper h e a t l i n k .
I n t h e s e experiments t h e CMN was demagnetized i n t h e usual manner b u t w i t h a 4 T p o l a r i z i n g f i e l d on t h e sample from t h e s t a r t . The time dependence f o r t h e temperature of t h e 6 0 ~ o s thermometer f o r two d i f f e r e n t experiments a r e shown i n f i g u r e 2.
F i g . 2 : Time dependence of T w i t h d e m a g n e t i z a t i o n s a s i n d i c a t e d , x 4 t o 2 T ; eE t o 0.5 T.
As above, t h e v e r y slow i n i t i a l c o o l i n g i s a t t r i b u t e d t o t h e high f i e l d a c t i n g on t h e copper and t h e accompanying l a r g e h e a t c a p a c i t y of t h e h e a t l i n k . A t t h e p o i n t s marked with arrows t h e p o l a r i z i n g f i e l d was lowered a t a r a t e of ca. 0.1 T per minute t o t h e f i n a l v a l u e s i n d i c a t e d . The r e - s u l t s show t h a t i n t h i s very simple manner t h e low temperature performance of t h e c o n v e n t i o n a l n u c l e a r o r i e n t a t i o n c r y o s t a t has been s i g n i f i c a n t l y exten- ded t o ~ r o v i d e temperatures below 2 mK f o r about
1 h. u s i n g low f i n a l f i e l d s o r f o r more extended o p e r a t i o n i n t h e r e g i o n around 3 mK using l a r g e f i e l d s ; i n t h e l a t t e r c a s e , u s e f u l low temperature o p e r a t i o n of t h e c r y o s t a t i s p o s s i b l e o v e r a period of 24 hours a f t e r t h e c o o l i n g og t h e CMN. Modifi- c a t i o n s t o f u r t h e r improve t h e performance a r e now i n p r o g r e s s u s i n g a simple Sn h e a t switch o p e r a t e d by t h e f r i n g i n g f i e l d of t h e p o l a r i s i n g c o i l .
References
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/2/ Asayama, K., Kontani, M., I t o h , J . , ' J . Phys. Soc. Japan
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