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Submitted on 1 Jan 1978
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HEAT TRANSFER BETWEEN SAPPHIRE (Al2O3 :
Fe3+) AND DILUTE 3He AT VERY LOW
TEMPERATURE
M. Chapellier, M. Chiba, M. Rotter
To cite this version:
JOURNAL D E PHYSIQUE
Collogue C6, supplement au n" 8, Tome 39, aout 1978, page C6-271
HEAT TRANSFER BETWEEN SAPPHIRE
( A I203: F
e 3 +) AND DILUTE 3H
eAT VERY LOW TEMPERATURE
M. Chapellier, M. Chiba , M. Rotter*
Wh-G/PSRM - CEN-SACLAI - Botte Postale n°2 - 91190 Gif s/Tvette, France
Résumé.- Le comportement thermique d'un ensemble de spins nucléaires et électroniques plongés dans un bain de 3He dilué est étudié par désaimantation adiabatique. En champ
faible les spins nucléaires sont fortement couplés aux spins électroniques. L'ensemble relaxe lentement vers la température du bain, le temps de relaxation dépend de la taille de l'échantillon, ha température de spin minimum est d'environ 2_,5 mK. Des expériences complémentaires sur ce système sont suggérées.
Abstract.- The thermal behaviour between electronic spins, nuclear spins and a cold bath of dilute 3He have been studied by adiabatic demagnetization. In weak field, the
coupling between nuclei and electronic spins is very fast. The total heat reservoir relaxes slowly to the bath, the relaxation time being size dependent. The lowest spin temperature obtained is around 2.5 mK. Further experiments on this promising system are proposed.
In order to use dynamically polarized dielectrics to cool He /l/ one needs a sample in which nuclei have a quadrupolar splitting in zero field. Sapphire is peculiarly well suited because of the high density of Al, the Fe electronic system being potentially interesting by the three crossings of the lowest two levels (Figure 1, H=0, HA, HB) .
In a preliminary study we have measured the time constant of equilibrium between electrons, nuclei and the bath which consist in the 3He
dilu-te phase in a mixing chamber of a dilution refri-gerator at temperatures between 6 mK and 300 mK. A single crystal of size 7x5x3 mm3 and powder of
grain size between 0.1 and 0.2 mm were used. Sam-ples are prepared by Verneuil techniques and are doped with 150 ppm of F e3 +. The single crystal is
oriented with c-axis parallel to H produced by a superconducting coil. The temperature was measured by Pt nuclear resonance thermometer. The tempera-ture of the electronic spins was measured by either usual low frequency susceptibility or by radio fre-quency susceptibility (300 to 800 kHz) which allows us in the same time to observe the quadrupolar resonance of .27A1 /2/. For the latter technique a
Robinson box (auto-oscillator) was used at a low level excitation of the coil. The system was pre-pared at a lower temperature than the bath by
adiabatic demagnetization from fields in the range 28 to 2000 G, to zero field (earth field).
E 1 Ho//C v, = 12.00 GHz yS v, = 19.26 GHz yS HB X 1 10 GHz 1 1 , - 5 / 2 1 , - - 3 / 2 • .1/2 ^ - . 3 / 2 ^ • 5 / 2 Ho(kG)
Fig. 1 : Energy level diagram of F e3 + in AI2O3 with magnetic field H. //c axis /4 /. There is 3 different fields with a low level crossing label-led H = 0 H., H„. Micro-wave transition at 10 GHz
A B are indicated.
RESULTS.- We have observed a very rapid mixing (T<0.1S) between the quadrupolar reservoir of 27A1 and the reservoir of the spin-spin of F e3 + in
fields below 28 G which is of the order of the local field of Fe ions. The main heat capacity of
19 tOn leave from Institute of Atomic Energy, Kyoto
University, Kyoto, Japan.
On leave from Charles University, Prague,Czechoslo-vakia.
the system is probably the 2 7 ~ 1 one (~=3.83~10-* T - ~ erg/mole) which prevents the Fe system to cool at very low temperature. The observed minimum was around 2.5 mK.
The relaxation of the single crystal (Fi- gure 2) was measured only with demagnetization from high field (2000 gauss). It follows a T - ~
law from 180 P I to 70 mK and continues to increa- se to very high values (2 60 hours at 6.8 mK) but measurements become very difficult
.
Fig. 2 :Relaxation time of the coupled nuclear and electronic spins at H=O for a single crystal and powder
The powder relaxes in a much shorter time, which depelds on the initial field.It is not described by a single exponential constant. The shaded area
(Figure 2)indicates the limits of these time constants. We have not any explanation of this behaviour for the moment, and contrary to Ce salts
/3/ the severe bottleneck expected (due to high Debye temperature) is not present.
CONCLUSION.- The quadrupolar resonance observed could be used as a thermometer, provided that the time constant of equilibrium will reduce as the size of the powder (or by magnetic coupling with very pure 3~e). Dynamic
polarization
is under trial with different micro-wave transitions la- belled on the figure 2 and if good polarization is obtained on 2 7 ~ 1 , the coupling of the system with pure 3 ~ e will be tried.For Kapitza resistance studies this com-
pound offers the possibility of magnetic coupling in three different fields which could help to sol- ve the microscopic nature of the coupling.
References
!l/ Chapellier, M. in liquid and solid He Procee- ding of the EPS Haifa 1975,'h.wile-q 8 Sons, New York).
/2/ Chiba, M., Chapellier, M., Rotter, M., this Conference.
/3/ Chapellier, M., this Conference.
