ELSEVIER Physica B 230-232 (1997) 208-210

High-pressure study of antiferromagnetic order in CePd2(A1,Ga)3

J. Tang*, H. Kitazawa, A. Matsushita, T. Matsumoto National Research Institute for Metals, Tsukuba, Ibaraki 305, Japan

Abstract

The pressure effect on the antiferromagnetic order in CePd2(All_ xGax)3 for x = 0.01 and 0.05 has been studied by specific heat measurements under pressure. It was found that antiferromagnetic order temperature TN increased initially with increasing pressure, then passed through a maximum and decreased at higher pressures. The pressure Pmax, which gives rise to the maximum TN, increased with the increase of Ga content. For x = 0.01, Pmax equals 1.0 GPa, while for x = 0.05, Pmax is 1.2 GPa. These results have been discussed in terms of the variation of the exchange coupling between the 4f and conduction electrons estimated from the pressure dependence of lattice constants. An interpretation has been given on the basis of Doniach phase diagram. This interpretation also indicates that the hybridization of the p electrons with 4f electrons played an important part in determining the antiferromagnetic order in the CePd2A13 compound.

Keywords: Phase diagram; High pressure; Doniach phase diagram; CePd2(A1,Ga)3; Antiferromagnetic order

CePdzA13 is a heavy-fermion (HF) intermetallic compound having the hexagonal PrNizA13 struc- ture [1]. The structure can be regarded as being composed of two layers along the c-axis: one layer contains Ce and Pd atoms and the other only A1 atoms. Annealed polycrystals show antiferromag- netric order below 2.8 K with the Sommerfeld con- stant of 380 mJ/K 2 mol [1]. The magnetic structure has been established by neutron diffraction in which the magnetic moments of Ce atoms are coupled ferromagnetically within Ce-Pd planes and antiferromagnetically along the c-axis [2]. Ac- cording to the anisotropic hybridization model by Mentink et al., the inter- and intra-planar couplings are treated independently in CePd2A13 [3]. In the HF materials as this compound, the magnetic properties are frequently discussed in terms of a competition between the indirect RKKY interac- tion and the Kondo effect through the same ex-

*Corresponding author.

change coupling Jcf. In the study of such HF mater- ials, pressure can be used as an effective parameter to control the electronic states. In fact, the antifer- romagnetic order temperature TN of CePdzA13 has been shown to have a maximum in its pressure dependence, and the behavior of TN can be under- stood in accordance with the Doniach phase dia- gram as a function of Jcf determined by external pressure [4]. However, it has not been clear which type of Jcf is predominant, the interplanar coupling based on the Ce4f-A13p hybridization or the intra planar coupling on the Ce4f-Pd5d hybridization. In order to clarify which hybridization, f -p or f-d, contributes dominantly to the magnetic properties of the CePdzA13 system, we have investigated the pressure effect on the antiferromagnetic order in CePd2(All - xGax)3.

In the report on CePd2(All-xGax)3 by Ludoph et al. [5], the substitution of A1 by Ga resulted in a continuous change of lattice parameters: enlarge- ment of the c-axis and contraction of the a-axis.

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J. Tang et al. / Physica B 230-232 (1997) 208-210 209

They also found that TN in the CePd2(All-~Ga~)3 system in most cases decreased with increasing Ga content, whereas a slight rise of TN was observed at 1% Ga. In the CePdz(A11 _~Ga~)3 system, it is con- sidered that the decrease in the interplanar coup- ling and the increase in the intraplanar coupling are induced by the changes in the c- and the a-axis, respectively. Therefore, there exist two possibilities to explain why TN decreases with increasing Ga content based on the Doniach phase relationship: (i) the system of CePd2(AI~_~Ga~)3 is located on the left side of the maximum TN in the relationship of TN versus. J~f, and (ii) the system is on the right side. Since pressure makes the lattice constants smaller when no structural change occurs, both the interplanar coupling, Jcr(f p), and the intraplanar coupling, Jcr~t4), become stronger with increasing pressure. Therefore, T N is expected to increase ini- tially with pressure for case (i), but will decrease for case (ii).

We prepared polycrystalline samples of CePdz(All_~Ga~)3 for x = 0.01 and 0.05 by Ar arc-melting the pure elements in their stoichiomet- tic ratio with continuous titanium gettering. These samples were annealed at 900°C for one week in sealed quartz ampoules. In the X-ray powder dif- fraction experiments, we did not observe any other phase except the PrNi2A13 structure, and obtained lattice constants as follows: a = 5.470(7)~,o and c = 4.217(3) A for x = 0.01, and a = 5.469(1) A and c = 4.219(3) ~, for x = 0.05. The specific heat under pressure was measured with a quasi-adiabatic method up to 1.6 GPa between 0.7 K and 10 K. A Be Cu piston and cylinder clamp were used to generate and to retain pressure which was applied at room temperature.

Results of the specific heat measurements are shown in Fig. 1 for x = 0.01. The specific heat has a peak at about 3 K, indicating an antiferromag- netic transition. The specific heat around the peak decreased gradually under increasing applied pres- sure. A similar feature was observed for x = 0.05, whereas TN is different to that for x = 0.01.

In Fig. 2, TN, defined by the peak position as a function of pressure, is plotted. Similar to the TN for CePd2A13 as given in Ref. [4], there is a maximum in the pressure dependence of TN for both samples in the applied pressure range, up to

o

1.4

1.2

1.0

0.80

0.60

0.40

' [ . . . . ~ . . . . ] . . . . I . . . . L * ' ' '

', CePd2(AIo~Ga001) a ,

• =~Z 0 G P a

• = 0.6 , * °= 1 . 0

[_ .':..':,..;:. • , ,

• ÷ ' : 4 ~

1 2 3 4 5 T(K)

Fig. 1. Results of the specific heat measurement at various pressures for CePdz(Alo.99Gao.01)3.

3.20

3.00

2.80 I -

2.60

2.40

T o o

o D ° . o [] [3 A a

o o ~ D oa [ 3 ° ° o

x x x

x × D CePd2(A Io~Gao.o t )a

x x x x CePd2(A lo .~Gaoo6) a

o CePd2AI s (R IH .4 ) I i h L i I I ~ t r I L i = ~ I I I ]

0 0.5 1 1.5 P(GPa)

Fig. 2. Pressure dependence of the magnetic transition temper- ature TN for CePd2(Al~_xGax)a, The arrow marks show the position of pressure Pmax giving the maximum TN.

1.6 GPa. The pressure Pmax giving rise to the max- imum TN with arrow marks increased with an increase of Ga content. TN for x = 0.01 reached 3.02 K at 1.0 GPa from 2.94 K at 0 GPa. On the other hand, T N of 2.52 K at 0 GPa for x = 0.05 became 2.70 K at 1.2 GPa. Since TN showed a pos- itive pressure derivative, the samples with Ga sub- stitutions are considered to be on the left side of the maximum TN in the Doniach phase diagram. Com- paring with the results from samples with Ga sub- stitutions, in which TN decreased with increasing Ga content [5], the present results suggest that the interplanar coupling, Jcf(f p), played a dominant

210 J. Tang et al. / Physica B 230-232 (1997) 208-210

i.

1.2 I T I I

+ Ref .6

0 . 9

0 . 8 + +

+

0 . 6 + + +

0 . 5 + + +

0.31 I I I 0.8

i I i p

+

+ + + +

+ +

+

J I I i 1 .0

J / JrN,rnax

Fig. 3. The magnetic transition temperature TN versus the coupling strength Jef for CePd2(All- xGax)3, together with data from Ref. [6]. Both quantities are normalized to their values where TN exhibits a maximum value.

role in the antiferromagnetic ordering in the CePd2(All-xGax)3 system.

We estimated the change of Jef with f -p hybrid- ization for both samples by using the results of X-ray diffraction experiments under high pressure and at ambient pressure, as well as that for CePd2Ala [4]. Fig. 3 shows the relationship of TN/TN,max versus Jcf/JTN . . . . for the samples of CePd2(All-xGax)3, together with that for some other Ce compounds [6], where TN . . . . and JTN . . . . denote the maximum transition temper- ature and the value of coupling constant giving TN . . . . . respectively. It can be seen that the value of J a increased with pressure for CePd2(All_xGax)3 and all data points can be mapped to a universal curve for the one-dimensional Kondo necklace model predicted by Doniach [7]. This behavior of TN under pressure can be understood with the competit ion between the R K K Y interaction and the Kondo effect by means of the Doniach phase diagram, as shown in previous studies. This J~f

dependence of TN suggests that CePd2(All-xGax)3 be located on the left-side region and is close to the maximum TN in the Doniach phase diagram. In other words, in this system, the substitution of A1 by Ga resulted in a decrease of J a and the external pressure caused an increase of TN in the region with small Jcf, where the R K K Y interaction dominates the Kondo effect. This result indicates that the decrease of TN with increasing Ga content in the CePdE(All_xGax)3 system was due to the expan- sion of c-axis, which resulted in a decrease of Ja.

F rom the present work, it is concluded that the magnetic ordering transition temperature rises with pressure increase and the 5% Ga substituted com- pound is located on the left side of the 1% Ga substituted in the Doniach phase diagram. This suggests that the reason for TN decreasing with increasing Ga content is the expansion of the c-axis. These results also indicate that the hybridization between the f- and p-electrons plays an important pa r t . in the antiferromagnetic order in the CePd2A13 compound.

R e f e r e n c e s

[1] H. Kitazawa, C. Schank, S. Thies, B. Seidel, C. Geibel and F. Steglich, J. Phys. Soc. Japan 61 (1992) 1461.

[2] A. D6nni, P. Fischer, B. Roessli and H. Kitazawa, Z. Phys. B 93 (1994) 449.

I-3] S.A.M. Mentink, G. J. Nieuwenhuys, A.A. Menovsky, J.A. Mydosh, H. Tou and Y. Kitaoka, Phys. Rev. B 49 (1994) 15 759.

[4] J. Tang, A. Matsushita, H. Kitazawa and T. Matsumoto, Physica B 217 (1996) 97.

I-5] B. Ludoph, S. Siillow, B. Becker, G.J. Nieuwenhuys, A.A. Menovsky and J.A. Mydosh, Proc. SCES'95.

[6] A.L. Cornelius and J.S. Schilling, Phys. Rev. B 49 (1994) 3955.

1-7] S. Doniach, in: Valence Instability and Related Narrow Band Phenomena, ed. R.D. Parks (Plenum, New York, 1977).