Study of magnetism in Ce rich Ce1−xUxPd2Al3

I/Im ELSEVIER

Physica B 205 (1995) 231-233

Study of magnetism in Ce rich Cel-xUxPd2A13 K. Ghosh, S. Ramakrishnan*, S.K. Dhar, Girish Chandra Low Temperature Physics Group, Tata Institute of Fundamental Research, Homi Bhabha Road, Bombay 400 005, India

Received 18 August 1994

Abstract

In this paper, we report our resistivity and magnetic susceptibility studies from 1.5 to 300 K and heat capacity studies from 2 to 20 K of Ce rich Ce I _xU~Pd2A13 alloys. Our studies show that the antiferromagnetic (AFM) ordering temperature (TN) of Ce l_xUxPdzA13 decreases with the increase of U substitution and is suppressed below 1.2 K for x = 0.3. The estimated Sommerfeld coefficient (y) also decreases with the decrease of TN unlike the increase of ? observed in many disordered heavy-electron antiferromagnets.

1. Introduction It is well known that the physical properties of the heavy-electron systems are extremely sensitive to the interatomic distance which has a direct bearing on the hybridization of the f orbitals with neighbouring ligands. High pressure studies have been used successfully to vary the relevant physical parameters. On the other hand, chemical pressure also helps one to understand the physical properties of the heavy-electron systems (in particular about the closeness of a heavy-electron state to an antiferromagnetic instability). Our studies [1] show that Ce replaces U in the Ul-xCexPd2Al3 system for 0 < x ~< 1. Thus, this system provides an opportunity to study the evolution of a 4f heavy-electron system (namely, CePd2AI3) starting from a 5f heavy-electron system (namely, UPd2AI3 [2-4]). Although similar attempts [5, 6] were made on U~-~CexRu2Si2, one has a miscibility gap in this system. In this note, we report our electrical resistivity, magnetic susceptibility and heat *Corresponding author.

capacity studies on the Ce rich (0 < x ~< 0.5) C e l - xUxPd2A13 system.

2. Experimental details All of the samples were made by melting the individual constituents (99.9% purity or better) in an arc furnace under a high purity Ar atmosphere. They were annealed at 950°C for two weeks. All the samples were found to have the reported PrNi2A13 [2] structure. The DC magnetic susceptibility was measured using a Faraday method and a SQUID magnetometer (Quantum Design, USA) whereas electrical resistivity and heat capacity measurements are made using home made set-ups.

3. Magnetic susceptibility studies The temperature dependence of the magnetic susceptibility (X) for Cel -xUxPd2A13 (x = 0, 0.1, 0.3 and 0.5) alloys from 2.0 to 50 K is shown in Fig. 1. The ~ data for heavy-electron antiferromagnet

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CePd2A13 (x = 0) are in agreement with previously published reports [6, 7]. At high temperatures (100K < T < 3OOK), Z data show Curie-Weiss dependence with increasing #~ff and 0 with the increase o f x upto 0.2 (for instance, ]2eff 2.4 #B and 0 = - 35 K for x = 0 which have increased to 2.@B and -- 46 K, respectively for x = 0.1). Further increase of x results in a decrease of 0 while/~ff continues to show a small increase. The antiferromagnetic transition temperature (TN) decreases with the increase of x from 2.8 K for x = 0 to 1.1 K for x = 0.2. Further increase of x resuits in the suppression of TN below 1.0 K. The value of g for x = 0.3 is large and it does not order down to 1.2 K. This could be due to the presence of large disorder in the K o n d o lattice at x = 0.3 I-8]. However, for x = 0.4, we have magnetic ordering at 2.9 K which increases to 4.5 K for x = 0.5. The value of #eft is 2.9#B and that of 0 -- - 9.8 K for x -- 0.5. Similar behaviour has been observed in the C e l - x U x R u E S i 2 system [6].

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with the D C magnetic susceptibility data. The electrical resistivity of the x = 0.1 sample is similar to that of the x = 0.0 sample except for the change of slope which occurs at 2.0 K for x = 0.1 which is in agreement with TN = 2.0 K observed from the z(T) data. However, the temperature at which p shows a m a x i m u m value (Tp) decreases monotonically as x increases upto x = 0.5 (for instance, T , = 33.4 K for x = 0 which decreases to 6.7 K for x = 0.5). We also know from a previous study [7] that the crystal field splitting in CePd2A13 is small (A, = 33 K), and we believe that the observed variation in the p m a x i m u m could be due to crystal field effects.

4. Electrical resistivity studies 5. Heat capacity studies The temperature dependence of the electrical resistivity (p) of Cel -xUxPd2Al3 (x = 0, 0.1, 0.2, 0.3 and 0.5) from 1.5 to 300 K is shown in Fig. 2. The p data for x = 0 are in agreement with that of earlier reports [4, 7]. The p data for different values of x show interesting features which do not have any correspondence

The heat capacity (C) data from 2 to 20 K show the decrease of TN with x which can be inferred from the plot of C/T versus T as shown for x = 0.0, 0.1, 0.3 and x = 0.5 in Fig. 3. This decrease of TN with the increase of x is also seen in g data. The value of TN of pure CePd2A13 (x = 0) decreases

K. Ghosh et al./Physica B 205 (1995) 231-233

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electron compound. The possible existence of short-range ordering (along with the observed antiferromagnetic ordering) could lead to the observed behaviour in the intermediate concentration and subsequent suppression of TN below 1.0 K for x - - 0 . 3 . Similar behaviour is seen in the Cel-xUxRuESi2system [6]. However, this speculation could only be confirmed by detailed neutron diffraction studies of the Cel-xUxPd2A13 system.

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Fig. 3. Plot of Cp/T versus T for various values of x in the Ce~-:,UxPd2A13 system.

from 2.8 to 2.0 K as x increases from 0.0 to 0.1. On a further increase of x, TN decreases below 1.5 K for x >i 0.2. However, for x = 0.5, TN has a value of 4.5 K. The broad anomaly in Cp at this temperature indicates an inhomogeneous magnetically ordered state. After subtracting the Schottky term for substituted samples (considering an appropriate mole fraction of Ce and using A I = 33 K for pure CePd2A13 [7] ), the C / T at 0 K (7) values obtained by extrapolating the high temperature heat capacity data monotonically decreases from 380 m J / m o l K 2 (for x = 0) to 100 m J / m o l K 2 (for x = 0.5). Usually a non-magnetic rare earth substitution results in an increase of 7 while decreasing the magnetic transition temperature of a heavy-

We have shown that the value of TN decreases monotonically with increase of x in C e l - x U x PdEA13 till x = 0.3 where TN has a value of 1.0 K. Further increase of x results in the increase of TN to 4.5 K at x = 0.5. A possible presence of short-range ordering along with long-range antiferromagnetic ordering is suspected for intermediate concentrations in analogy with the Cel_xUxRu2Si2 system. The value of Tp decreases with the increase of x which could be due to the crystal field effects in this system. The 7 value (extrapolated from the paramagnetic region after subtracting the Schottky contribution) monotonically decreases with x which could be due to the presence of short range-magnetic order which can only be confirmed by detailed neutron scattering studies.

References [-ll K. Ghosh et al., J. Magn. Magn. Mater., to be published. 1-2] C. Geibel et al., Z. Phys. B 84 (1991) 1. [3] A. Krimmel et al., Z. Phys. B 86 (1992) 161. [4] H. Kitazawa et al., J. Phys. Soc. Japan 61 (1992) 1461. [5] M. Mihalik et al., Physica B 186-188 (1993) 507. 1-6] J. G. Park and B. R. Coles, Physica B 186-188 (1993) 795. [7] S. A. M. Mentink et al., Physica B 186-188 (1993) 460. [8] V. Dobrosavljevicet al., Phys. Rev. Lett. 69 (1992) 1113.