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Submitted on 1 Jan 1978
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NMR MEASUREMENTS OF THE MAGNETIZATION
OF 3He IN GRAFOIL
H. Bozler, T. Bartolac, K. Luey, A. Thomson
To cite this version:
JOURNAL D E PHYSIQUE
Colloque
C6,
supplement au n°
8,
Tome
39,
aout
1978,
page
C6-283
3
+
NMR MEASUREMENTS OF THE MAGNETIZATION OF H
eIN GRAFOIL/
H.M. Bozler, T. Bartolac, K. Luey and A.L. Thomson t
Department of Physios, University of Southern California, Los Angeles, California 90007, U.S.A.
Résumé.- On présente l'observation de la susceptibilité magnétique du liquide 3He en grafoil. La couche de surface a une aimentation à la Curie-Weiss équivalente environ à celle d'une solide cou-che atomique. Apparemment cette aimentation s'approcou-che de la transition de phase ferromagnétique à 0.38 mK avec une aimentation de saturation moins grande que prévue.
Abstract.- We present measurements of the susceptibility of 3He in grafoil. The surface layer
exhi-bits a Curie-Weiss magnetization equivalent to about one solid monolayer. This magnetization appears to approach a ferromagnetic phase transition at 0.38 mK with lower than expected saturation magneti-zation.
Recently Ahonen et al. IM, observed that the magnetic susceptibility of 3He in contact with both mylar and lamp black graphite, follows a Curie-Weiss Law.
x = ^
+^ e
( , )The constant term A is attributed to the susceptibi-lity of the bulk liquid and the temperature dependent term is due to a surface layer.
We have performed similar measurements using grafoil which has been prepared by baking in vacuum at 1000° C for several hours. Two samples in two coils were used with the nominal orientations of the grafoil planes perpendicular and parallel with respect to the static field H. The 3He susceptibility was
measured using pulsed N.M.R. techniques with tipping pulses of between 0.5° and 1°. We have experimental-ly determined that the free induction decay amplitu-de M(t) = M(0) exp (-a tl*s) than by an exponential
shape at temperatures below the superfluid
transi-tion HI.
Figure 1 shows typical data for x l v s- T*
The solid line in figure 1 is a best fit to eq. (1) assuming 8 = 0.4 mK and excluding points below the superfluid transition (Tc). A least squares analysis
of this data only considering data for T > T and al-lowing 9 to be a free parameter gives values of 6 = 0 . 4 2 ± 0.05 mK. Below T we see the apparent
va-c
lue of x 1 drop below the fit curve. In the B phase
the signal drops more rapidly than can be explained ^Supported by the National Science Foundation Grant No. DMR77-02160, and Research Corporation
tPermanent address : Department of Physics, Univer-sity of Sussex, Falmer, Brighton, BN1, 9QH, England.
by the change in liquid susceptibility /3/ due to extreme line broadening of the liquid component caused by the non-uniform geometry. Also at tempe-ratures below 0.8 mK significant deviation from the Curie-Weiss behaviour occurs.
Fig. 1 : Inverse susceptibility vs. temperature. The line represents the best fit to the data assu-ming 6 = 0.4 mK (see eq. 1 ) . B/A = 3.04 mK
By using the limiting value of the suscepti-bility /4/ of the normal liquid at temperatures well below T,,, we can deduce from the ratio B/A
r
the ratio of the amounts of sample having the two different susceptibilities of eq. (1). For P=3.74 bar the limiting value for the susceptibility of
the liquid is 3.37 C where C is the Curie constant and thus the B/A ratio implies that for this parti-cular pressure 95 times as many atoms have the cons-tant susceptibility as opposed to the Curie-Weiss one. The surface volume ratio for our sample was determined in a separate series of adsorption
therms. We found that the first monolayer was com- pleted on the grafoil with an amount 11.8 cm3 STP/g of 3 ~ e . Also we determined the internal v o l e o f the grafoil so that the amount of liquid both inside and around the grafoil could be calculated. The result of these calculations is that for this pressure (3.74 bar) 0.89 of the monolayer appears to possess the Curie-Weiss behaviour. For pressures 0, 6.23 and
26.0 bar, this number becomes 0.86, 0.95 and 1.19 respectively. Thus we have a small monotonic increa- se in this effective number of layemwith pressure.
Figure 2 shows the low temperature behaviour of the susceptibility for P = 3.74 bar with three
different values of
H,.
The symbols plotted are x'T whereX'
x/B. As can be seen, the higher values of magnetic field depress the values of x'T in the re-gion below 0.8 mK with x'T values for the lowest field 13.4 mT.
Fig. 2 : Normalized susceptibility, x'T vs. T for three values gf x'T using eq. 3. With 8 = 0.38 mK, b = 1.25 x 105 ~ 3 / m ~esla'. The calculated liquid contribution has been included. The dashed line re- presents unsaturated Curie-Weiss behaviour.
3 0 0 -
2 5 0 .
2 00
+ - X 1.50
A possible way of interpret'ing those date, is to use a Landau-Ginzburg model for a magnetic phase transition. Then if a E 2 %/y2@'
F = a ( T
-
8) M'+
2 ba3~' - M H (2) F is minimized if1 = M ' (T
-
8)+
bM ' ~ H ~
(3) where M' is the magnetization normalized so that M'-.
$
6s T + W. In our case, M' =9
with A redu-ced to its calculated superfluid value. The solid
lines on figure 2 represent the best fit of eq. (2) to the data shown below l mK. (6 = 0.38 mK,
b = 1.25 X 10-~rnIZ~/m ~esla~). The values for
A
and B were obtained from the high temperature fit. Only',
H+O P = 3 74 bar A H. = 13 4 mT x H. = 18.7 mT + H. = 24 1 .nT - A A U A A A A A - * x X 4 * X r x X X x S + X + + + + .8 and b were variables for the low temperature fit. The ratio of the saturation magnetization (derived from eq. 2) to the magnetization of fully aligned ' ~ e spins is (8/b)'/2 (yW/2kg)
=
0.15.Possible mechanisms to produce ferromagnetic ordering have been suggested by Beal-Monod and Doniach /5/ involving the liquid near the surface. At this time it does not appear that there is enough temperature dependent magnetization to involve more than one monolayer unless the surface itself does not exhibit the magnetization of solid 3 ~ e . Guyer
/6/ has suggested that vacancies in the "solid" monolayer may induce ferromagnetism.
/l/ Ahonen, A.I., Kodama, T., Krusius, M., Paalanen, M.A., Richardson, R.C., Schoepe, W. and Takano, Y., J. Phys. C
2
(1976) 1665 ;Ahonen, A.I., et al., Quantum fluids and Solids, Eds.B. Trickey E.D. Adams, and J.W. Dufty,
(Plenum Press. New York) 1977 p. 171
/2/ Typically this experimental decay rate fits the free induction decay signal to within 2 % for 0.12 ms
2
t5
1 ms/3/ Wheatley, J.C., Rev. Mod. Phys.
2
(1975) 415 /4/ Ramm, H., Pedroni, P., Thomson, J.R. and Meyer,H., J. Low Temp. Phys.
2
(1970) 539/5/ Beal-Monod, M.T., Doniach, S., S. Low Temp. Phys.
