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Submitted on 1 Jan 1978
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BLOCKING OF PAIRS AND ”SPIN GLASS
FREEZING” IN EuxSr1-xS
J. Tholence, F. Holtzberg, H. Godfrin, H. Löhneysen, R. Tournier
To cite this version:
JOURNAL DE PHYSIQUE Colloque C6, supplément au n° 8, Tome 39, août 1978, page C6-928
BLOCKING OF PAIRS AND "SPIN GLASS FREEZING" IN E u
xS
rS
1-x
J.L. Tholence, F. Holtzberg , H. Godfrin , H.V. Lohneysen and R. Tournier . S.N.C.I. and C.R.T.B.T., C.N.R.S., B.P. 166 X, 38042 Grenoble, France.
+IBM T.J. Watson Research Center, Xorktown Heights, N.Y. 10598, U.S.A.
^Centre de Recherches sur les Ires Basses Temperatures, C.N.R.S., BP 166 X. 38042 Grenoble Cedex, France.
Résumé.- Nos mesures de susceptibilité en alternatif, montrent pour la première fois, l'effet de la fréquence v sur la température Tp de blocage des paires ferromagnétiques dans un composé isolant : Eu Sr _ S, lorsque x 6 0,03. Tp ne dépend pas de la concentration et sa variation avec la fréquence suit unXloi d'Arrhénius. Dans le régime verre de spin observé pour x > 0,07, les variations de Tf avec la concentration et la fréquence suivent des lois d'échelle(^j = f(Ln v)) ce qui démontre le rôle de 1'anisotropie dipolaire sur le gel des verres de spins.
Abstract.- Through ac susceptibility measurements, we show for the first time, the effect of the frequency v on the blocking temperature T_ of ferromagnetic pairs in an insulating dilute compound : Eu Sr,_xS, with x £ 0.03. T is concentration independent and its frequency dependence obeys an Arrhenius law. In the spin-glass regime observed for x S 0.07, the existence of scaling laws
•=r- = f (Ln v) for the concentration and frequency dependence of the freezing temperature Tf clearly indicates the role of the dipolar anisotropy on the freezing of spin-glasses.
INTRODUCTION.- The scaling laws in spin glasses observed e.g. for magnetization and specific heat are due to the 1/r3 dependence of the RKKY
interac-tion / ) / . The remanent magnetizainterac-tion, too, shows scaling which is attributed to an anisotropy inte-raction, also decaying as I/r3 /2/. Hence, although much smaller than the RKKY interaction, the dipolar coupling is supposed to play an important role in the existence of the remanence of spin-glasses. Mo-reover, the average size of magnetic clouds is pro-portional to the ratio of the RKKY and dipolar in-teractions. We have undertaken a study of Eu Sr, S
J x 1-x to look for the occurence, with increasing concen-tration, of a spin glass like freezing in a system without RKKY interaction. Earlier measurements have shown the existence of a remanence proportional to x2, rapidly decreasing with the time, and zero above M O O mK in dilute systems (x<0.03)/3/. This was in-terpreted as due to the blocking of independent ferromagnetic pairs of Eu nearest neighbours /3/. For more concentrated samples, a spin glass like freezing is inferred from susceptibility and rema-nence A / . We present here preliminary ac suscepti-bility measurements which might allow a better un-derstanding of the different behavior of Eu ions depending on their concentration in Eu Srj_ S. EXPERIMENTAL RESULTS.- Figure 1 shows the thermal dependence of the ac susceptibility x of Eu Sr S for x = 0.017 and 0.03, measured in a field
£ 0 . 1 Oe and with an ac frequency of 7.2 Hz.
Fig. 1 : ac susceptibility vs. temperature for two Eu Sr _ S alloys at several frequencies.
For both samples, x displays a maximum or a pla-teau around the same temperature of 100 mK. In ad-dition, for the x = 0.03 sample the ac susceptibi-lity for several other frequencies is also shown. For x = 0.03, the maximum is displaced to higher temperatures and becomes less pronounced when the frequency increases. For x = 0.017 the plateau is not as marked at higher frequencies, the
tempera-ture where it appears also increases with frequen-cy. Below the plateau or a minimum, the susceptibi-lity increases with decreasing temperatures but much more slowly than the Curie law observed above
100 mK. The depression in x,c compared to the high
temperature Curie law increases more rapidly than the concentration x.
For x 2 0.07, only one rounded maximum in the
x
(T) curves is observed. The temperatures, Tf, ofac
the maxima are 0.16 K and 0.235 K for x = 0.07 and
0.10 respectively. These values are in rather good agreement with values found for similar concentra- tions by Maletta et al. 141. In this concentration
range (x 5 0.07) where Tf increases linearly with
x, and where the remanent magnetization decreases with in t, the magnetic properties look like those
of classical spin glasses, although scaling laws are not obeyed due to the high Eu concentration.
DISCUSSION.- The above results suggest that the ma-
gnetic behavior in E U ~ S ~ ~ _ ~ S changes markedly when
passing from dilyte (x 6 0.03) to moderate (xh0.07)
Eu concentration. For the latter, spin glass like
behavior exists as reported previously for x
>
0.1141. In the dilute regime, a quite differente beha-
vior is observed in susceptibility and remanent ma- gnetization :
The temperature of the maximum (or plateau)
of the susceptibility around 100 mK (figure 1) is
independent of concentration.
xac
starts to increa-se again when the temperature decreases well below
100 mK. The susceptibility curve for x = 0.03 and
V = 1640 Hz (figure 1) suggests that a second maxi-
mum could appear below 20 mK. As illustrated on fi-
gure 2, the frequency dependence of the anomaly in
the susceptibility is the same for x = 0.03 and
x = 0.017. On the contrary, in the spin glass re-
gime (x
>
0.07). The variation ATf corresponding toa variation
A
in V is proportional to the concentra-tion x.
A remanent magnetization appears below 100 mK
for x 6 0.03 and it varies as x2. This remanent
magnetization saturates to a value which corres- ponds exactly to the value given by the number of ferromagnetic pairs of nearest neighbour Eu atoms 131. Difficulties in measurement of the time depen- dence of the remanent magnetization are due to the fact that the sample is cooled when the field is suppressed. Nevertheless the time dependence is more rapid than that observed in classical spin- glasses.
The properties for x
<
0.03 may be understoodby assuming that every ferromagnetic pair of nea- rest neighbours of Eu atoms is blocked on its ani-
sotropy axis at a temperature of the order of 100
mK, independent of the concentration and only cha-
racteristic of the anisotropy energy of pair. This causes the observed depression in the
susceptibility around 100 mK and its increase at
lower temperatures is presumably due to unblocked impurities. Considering a pair with a relaxation
time T and anisotropy energy E such that :
E P
T = T ~ ~ $
it is blocked when T > T ~ , the measuring time.
Hence, the blocking temperature T of ferromagnetic
P 1
pairs depends on the measuring frequency V = - m
.
For the dilute samples, one observes the same
AT variation for a given A 9.n v (cf. figure 2).
P
Fig. 2 : Pair blocking temperature Tp (for x & 0.03)
and freezing temperature Tf(for x>, 0.07) of E U ~ S ~ ~ - ~ S
as function of measuring frequency. Universal curves
(concentration independent) are observed for Tp =
f(9.n v) (blocking of pairs) and for = f (9.n V )
(spin glass regime). T f
Since T does not depend on x, this shows that E
P P
is also concentration independent as it should be for a blocking of pairs of nearest neighbour Eu ions. From the slope of the I/T vs. In V curve
E P
calculate T~ = 6 x 10-'s and
9
= 1.7 K, in goodk
ted to be proportional to x, in a picture where Reference the biggest clouds of the distribution are frozen
around T 121. Then ATf/A in f V must be proportional /1/ Souletie, J. and Tournier, R., J. Low Temp.
to x as it is indeed observed in figure 2. In this Phys.
1
(1969) 95.case a better representation is given by the dia- 12/ ThO1ence, J.L. and TOurnier, R., J.
35 (1974) C4-229
-
gram x/Tf = f(En V ) (dashed lines in figure 2)
/ 3 / Holtzberg, F., Tholence, J.L. and Tournier, R.,
where scaling laws for the concentration and f e-
E
Amorphous Magnetism 11, (Pergamon New-York andquency dependence of Tf are observed (Tf s x, 3 London) 1977, p. 155.
k
= 0.9 K/%). /4/ Maletta, H., Felsch, W. and Tholence, J.L.,
J. Magn. Mag. Mat. (to appear). To conclude, the present study of the frequency
dependence of the susceptibility helps to reveal
two distinct concentration regimes of Eu ions in
insulating (Eu,%)S which display quite different
magnetic properties. For x
<
0.03, the magnetic be-havior is dominated by the blocking of pairs which appears at a much higher temperature (100 mK) than
a possible spin glass freezing ( < 20 mK). Above a
certain concentration threshold value (x % 0.05)
