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Submitted on 1 Jan 1978
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ITINERANT ANTIFERROMAGNETISM IN NpIr2
M. Brodsky, R. Trainor
To cite this version:
JOURNAL D E PHYSIQUE Colloque C6, suppliment au no 8, Tome 39, aolit 1978, page C6-777
ITINERANT
ANTlFERROMAGNETlSMIN
NpIr2 T M.B. Brodsky and R.J.rain or^*
Argonne National Labomtory, Argome I L 60439, U . S . A.
R6sumC.- La chaleur spgcifique et la susceptibilit6 magn6tique ont 6t6 mesurees pour le systiime NpIr2. Les valeurs obtenues sont en accord avec le modiile de bande antiferromagngtique. Elles sont aussi en accord avec le modiile de BCS avec couplage fort.
Abstract.- Specific heat and magnetic susceptibility measurements have been made on NpIr2. The results support the model of itinerant antiferromagnetism. The properties agree a BCS-band gap model with strong coupling.
The cubic Laves phase compound NpIr2 is known to order magnetically. Mijssbauer measurements /l ,2/ show an ordering temperature of 7.5 K and a magnetic hyperfine field corresponding to an orde- red Np moment of 0.6 pB. The magnetic susceptibi- lity /2/ shows a maximum also at 7.5 K. Measure- ments of the specific heat have been made to see if the likely antiferromagnetism is itinerant in nature. This possibility was made more likely by the observation of itinerant antiferromagnetism
(IAF) in NpSn3 / 3 / . The susceptibility was remea-
sured on the specific heat sample which had been annealed at S1250 K for two weeks.
Figure I shows the electronic specific heat (C ) plotted as CE/T versus T. The lattice
E
contribution was assumed to be that of nonmagnetic ThOs2. From a law of corresponding states, the
O 'S are expected to be within 1K for NpIr2 and D
ThOsp. The sharp peak at 6.6 K corresponds to T N. The Large, and nearly constant, value of C /T
E above 10 K is y the enhanced electronic coeffi-
P'
cient in the paramagnetic phase. The large value, 234
_+
8 mJ/(mole-K~), is reasonable. when compared to other itinerant actinide systems : 242. for the IAF NpSn3; 92 for the itinerant ferromagnet NpOs2; and 142 for the spin fluctuation.compound UA12 / c c / The relatively large uncertainty for NpIr2 is due mainly to the inexact equivalence of BD for ThOs2 and NpIr2. Below TN, the data would be expected to drop more sharply (see below). It is possible that there are contributions due to a second pha- se.X Wprk supported by the U.S.Department of Energy. n Permanent Address : Lawrence Livermore Laborato-
ry, Livermore, CA 94550.
The magnetic entropy AS = {(C -y T)dT/T may
E P
be determined graphically. If it is assumed that the C /T curve extrapolates to about yo=400 mJ/
E
(mole-K~), a total excess entropy of 0.53 eu 0.38 Rln2 is obtained.
800
0
0 5 10 I5 TEMPERATURE l K)
Fig. 1 : Electronic specific heat of NpIr2. The dashed line for T<TN is CE/T a 100+1800 exp(-6.6/T).
This value is quite low for a magnetic transition involving localized moments, for which AS > 1.0 ~ l is expected. However, AS for n ~ NpIr2 is condi-
derably larger than the value of AS = 0 f 0.01 Rlnz found for NpSnj / 3 / .
Figure 2 shows the magnetic susceptibility results compared to the data of Aldred et al. / 2 / .
The maximum in X occurs at about 7.5 K here also, and the maximum of dx/dT is at a slightly lower temperature, as was found for NpSn3 and expected from the IAF model of Fedders and Martin /5/.
The break seen in x(T) at 220 K by Aldred et al. /2/ is not found here. It was probably due to NpC impurity. T = 200 K, in the earlier sample.
C
Fig. 2 : Magnetic susceptibility of NpIrp. The The solid line is for the data of Aldred et al.
(2).
If the transition is due to W , the transition involves a band gap and the thermody- namics are described by the BCS theory 151, with
Here ACE(TN) is the jump in CE at the ordering
- -
temperature.For any y < yD = 234 there are physi- cal solutions. The large value of ACE. here, 5 2 0 m ~ /
T~
(mole-K~), yields a constant value of K = 2.22 where yo = 0, and even larger values for a finite yo. In the case of a superconductor, it is possi- ble to calculate the deviation of K from the BCS value of 1.43, from reasonable estimates of the characteristic phonon frequency 161. The present case is similar to strong coupling in a supercon- ductor, but the physical si~nificance for the IAF case is unknown. NpSnS has K = 1.63, much closer to the BCS value. In addition to predicting a re- sult for ACE, the presence of the gap also pre- dicts that on the low temperature side of the peak CE/T should drop as exp(-TN/T) as T + 0. This is shown as the dashed line in Figure 1 for yo = 100 m~/(rnole-$~). The resulting curve fits the data moderately well near TN, and the strong de- viation at lower temperatures seems to support the thought that the higher experimental values for C /T are due to a second phase not seen in the
E
x-ray diffraction pattern. The lowyo = 100 (merely an estimate) yields AS = 0.27 eu = 0.19 Rln2 which is more indicative of the band nature of the antiferromagnetism.
Contrary to the situation in NpSn3, X (T) shows a strong temperature dependence, and fits a modified Curie-Weiss law with BC" = 56 K and a slope equivalent to peff = 2.4 vB/F.U. (but of a course this is not to be taken as a true local moment value).
It is not possible to accurately calculate temperature-independent contributions to X or the enhancements to y. However, some information is obtained by setting the Stoner exchange enhance- ment, S = x/yp, to the experimental values of
exP
X and expressed in density of states units and
P
taken at T This yields S = 7 for NpIr2 and
N' exP
0.9 for NpSn3. The result for NpSn3 is easily explained for an LAF. S =
E
-
IX O(Q,O,O~~-~ and for an IAF the maximum in occurs at Q if 0. Thus, the observed value of S = x0(2 = O)/y may beexp
less than unity but in reality S may be quite large 171. If the maximum occurs near but not at Q = 0, antiferromagnetism will be seen, but S > 1. For such a material, e.g., NpIr2, it is not sur- prising that AS is substantially greater than zero although much lower than a local mment value. Also, K > 1.43 is reasonable for this case.
ACKNOWLEDGEMENT.- The authors wish to thank S.D. Bader for a number of interesting discussions and C. H. Sowers for aid in taking the experimental data.
References
/l/ Gal, J., Hadari, Z., Atmony, U., Bauminger, E.R., Nowick, I., and Ofer, S., Phys. Rev.
(1973) 1901.
/2/ Aldred, A.T., Dunlap, B.D., Lam, D.J., and Nowick, I., Phys. Rev.
E
(1974) 1011. 131 Trainor, R.J., Brodsky, M.B., Dunlap, B.D.,andShenoy, G., Phys. Rev. Lett.
37
(1976) 1511. /4/ Brodsky, M.B. and Trainor, R.J. Physica(1977) 271.
/5/ Fedders, P.A. and Martin, P.C., Phys. Rev.% (1966) 245.
/6/ Kresin, V.Z. and Parkhomenko, V.P., Sov. Phys. Solid State
16
(1975) 2180./7/ Jullien, R. and Coqblin, B., J.Low Temp.Phys. 22 (1976) 437.
