TEMPERATURE AND CONCENTRATION
DEPENDENCE OF THE SPECIFIC HEAT OF
(ErxY1-x) Co2
N. Pillmayr, G. Hilscher, E. Gratz, V. Sechovsky
To cite this version:
N. Pillmayr, G. Hilscher, E. Gratz, V. Sechovsky. TEMPERATURE AND CONCENTRATION DEPENDENCE OF THE SPECIFIC HEAT OF (ErxY1-x) Co2. Journal de Physique
Colloques, 1988, 49 (C8), pp.C8-273-C8-274. <10.1051/jphyscol:19888121>. <jpa-00228265>
HAL Id: jpa-00228265
https://hal.archives-ouvertes.fr/jpa-00228265
Submitted on 1 Jan 1988
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JOURNAL DE PHYSIQUE
Colloque C8, Suppl6ment au no 12, Tome 49, d6cembre 1988
TEMPERATURE AND CONCENTRATION DEPENDENCE OF THE SPECIFIC
HEAT OF (ErzY1-O) Co2
N. Pillmayr (I), G. Hilscher (I), E. Gratz (I) and V. Sechovsky (')
(I) Inst. fiir Experinaentalphysik, TU Vienna, A-1040 Vienna, Austria
(2) Inst. fiir Festk&perforschung, KFA Jiilich, 0-5170 JCfich, F.R. G.
Abstract. - We report on specific heat measurements of (Er,Y1-,) Con in the temperature range from 1.5 to 60 K. The
variation of the electronic, phonon and magnetic contribution to the specific heat as well as the magnetic entropy will be
discussed in terms of spin-freezing effects and the onset of itinerant 3d-magnetism induced by the 4f-molecular field.
Recently a great deal of progress was made in
understanding the exceptional physical properties appearing in the pseudobinary intermetallic cubic Laves
phase compounds (RE, Y) Con (RE = rare earth element), such as specific heat [I, 21, magnetization and
thermal expansion [3]. DyCo2, HoCoz and ErCoz show
in contrast to the other boundary RECo2-compounds
a first order magnetic phase transition which changes
into a second order type by substituting the RE-atoms
by Y. Freezing phenomena of the 4f-localized magnetic moments, observed in specific heat and magnetization measurements as well as pronounced minima
in the electrical resistivity and thermopower [4] occur in a rather large concentration range. The aim
of the present paper is to discuss temperature and
concentration dependent heat capacity measurements
of (Er,Yl-,) Cop (0 < x 5 1) in terms of spinfluctuations and a possible Kondu-like screening of the localized RE-moments.
The magnetic phase diagram of (Er,Y)Con is
presented in figure 1. The ferrimagnetic Curietemperatures Tcand the spin-freezing temperatures Tf
were determined by magnetization, ac and dc susceptibility measurements. In the long range ordered regime
(0.6 5 x 5 1)the magnetic transition is of a fist order
type whereas for x < 0.7 this transition changes into
second order accompanied by freezing effects of the 10calized moments. The dilution of the Er-moment by
Y reduces the ordering temperature continuously.
The low temperature specific heat can be analysed
by using the expression C,,=yT @ T+~Cn+Cm with
7 the electronic and @ the phonon coefficient, Cm the
magnetic and Cn the nuclear contribution to the specific heat. The latter dominates in the lowest temperature range and arises as a Schottky type anomaly
when the (21 1)-fold degeneracy of the nuclear states
, v
: spin of the Er-nucleus) is in general removed
I =2
by a magnetic hyperfine field Herr and/or a quadrupole
moment [5]. To separate the magnetic contribution
C, from the total specific heat Cp we subtracted the
nuclear term (assuming that the hyperfine field remains almost unchanged over the whole concentration range) and the heat capacity of the nonmagnetic
YCo2-compound.
Figure 2 demonstrates in a (Cp C,) / T vs. T ~ diagram, that the magnetic contribution dominates
the specific heat in the spinglass-regime with increasing Er-concentration, in particular near the transition
from long-range ordering to spin-freezing of the Ermagnetic moments (0.1 g x g 0.5).
In comparison with results recently obtained for
other (REXY1-,) Con-compounds [2] (RE = Dy, Ho)
figure 3 shows the concentration-dependent variation of 7 (Fig. 3A) and the Debye-temperature 8~
(Fig. 3B), revealing a drastic increase of 7 with growing
=content.
For (Er,Y1-,) Coz the highest yvalue
(about 200 mJmol-l~-') appears around x = 0.5.
The magnetic entropy per REatom Sm/ x, obtained
+
+
-
from S m / x =
Fig. 1. - Magnetic phase dlagram of (Er,Y) Co2
((A) Curie-temperature Tc,( 0 ) spin-freezing temperature
Tf 1.
'I
-x
Cm/ TdT, is also included in figure 3A. The loslof magnetic entropy in the lower
REconcentration range is one of the most remarkable
features in these (RE, Y) Con-systems. We note that
the decrease of the Sm/ x-values starts while diluting
the REatoms at those concentrations where 7 attains
the maximal value. The magnetic entropy decreases
from the theoretical value reached in the long range or-
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19888121
JOURNAL DE PHYSIQUE
C8 - 274
Fig. 2. - (C, - Cn)/ T
0.1 5 x 5 0.5.
vs.
~ ' - ~ r a . ~ofh (Er,Y)
s
Co2 for
dered regime (derived from Sm/ x = R 1x1(2J + 1)) of
23.6 ~mol-'K-' for Ho ( J = 8) and 23.05 ~mol-'K-l
for Dy and Er ( J = 7.5). The reduction of the magnetic entropy can possibly be due to a Kondo screening
mechanism, where the conduction electron spins partly
compensate the localized .If-moments; the compensation effect increases with decreasing RE-concentration
[8]. Above T, in all these systems a magnetic entropy
corresponding to the Co-3d-moments (resulting from
1
spin - : 5.76 ~ m o l - l ~ - lcan
) neither be observed
2
in the magnetically disordered spin-glass range nor
in the long range ordered concentration regime (Er:
x 2 0.6, Ho: x 2 0.5, Dy: x 2 0.4). This seems
to be a clear indication for the itinerant character of
the Co-3d-magnetism in the sense of the Stoner theory [6, 7. Within this theory the magnetic contribution to the specific heat can be expressed as Cm= y m ~ + ~ m: the
~ 3first negative term causes a change of
sign of the Sm(T)-curvatures from negative t o positive
while passing the concentration range where Sm/ x attains the full theoretical value. The positive curvature
increases with the rising induced Co-moment which finally reaches 1 /.AB in the long range ordered regime
[I, 81.
Acknowledgement
This work was supported by the Austrian Science Foundation ("Fonds zur .Forderung der wissenschaftlichen Forschung in Osterreichn ) under
project 6104. V.S. (permanent address: Charles University Prague, CSSR) is indebted t o the A. v. Humboldt foundation.
Fig. 3. - Variations of the specific heat coefficient 7,the
magnetic entropy contribution Sm/ a (.A) and the Debyetemperature eD (B) with the concentrationfor (Er,Y) Coz,
(Dy,Y) Co2 and (Ho,Y) C02.
[I] Hilscher, G., Pillmayr, N., Schmitzer, C., Gratz,
E., Phys. Rev. B 37 (1988) 3480.
[2] Pillmayr, N., Schmitzer, C., Gratz, E., Hilscher,
G., Sechovsky, V., J. Magn. Magn. Mater. 70
(1987) 162.
[3] Duc, N. H., Hien, T. D., Brommer, P. E., Franse,
J. J. M., J. Phys. F 18 (1988) 275.
[4] Gratz, E., Pillmayr, N., Bauer, E., Hilscher, G.,
J. Magn. Magn. Mater. 70 (1987) 159.
[5] Philips, N. E., Crit. Rev. Solid State Sci. 2 (1971)
467.
[6] Stoner, E. C., Proc. R. Soc. A 169 (1939) 339.
[7] Wohlfarth, E. P., Physica B 91 (1977) 305.
[8] Pillmayr, N., Ph. D. Thesis, Technical University
of Vianna, 1988 (unpublished).