Magnetic and transport properties of UPd2Sb

Solid State Communications 133 (2005) 625–628 www.elsevier.com/locate/ssc

Magnetic and transport properties of UPd2Sb Krzysztof Gofryk, Dariusz Kaczorowski*, Andrzej Czopnik Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 1410, 50-950 Wrocław, Poland Received 1 October 2004; accepted 6 January 2005 by H. Akai Available online 12 January 2005

Abstract A new compound UPd2Sb was prepared and studied by means of X-ray diffraction, magnetization, electrical resistivity, magnetoresistivity, thermoelectric power and specific heat measurements. The phase crystallizes with a cubic structure of the  MnCu2Al-type (s.g. Fm3m). It orders antiferromagnetically at TNZ55 K and exhibits a modified Curie–Weiss behaviour with reduced effective magnetic moment at higher temperatures. The electrical resistivity behaves in a manner characteristic of systems with strong electronic correlations, showing Kondo effect in the paramagnetic region and Kondo-like response to the applied magnetic field. The Seebeck coefficient exhibits a behaviour expected for scattering of conduction electrons on a narrow quasiparticle band near the Fermi energy. The low-temperature electronic specific heat in UPd2Sb is moderately enhanced being about 81 mJ/mol K2. q 2005 Elsevier Ltd. All rights reserved. PACS: 72.15.Qm; 72.15.Eb; 75.30.Mb; 75.50.Ee Keywords: A. Magnetically ordered materials; D. Electronic transport; D. Kondo effect; D. Heavy femions

1. Introduction In the course of our systematic study on the magnetic, electrical and thermal behaviour in rare-earth-based Heusler phases REPd2Sb and REPd2Bi we reported recently on intriguing semimetallic-like properties of the compounds with REZY, Gd–Er [1,2]. All these ternaries, except for diamagnetic Y-based phases, exhibit localized magnetism of RE3C ions, and a few of them order antiferromagnetically at low temperatures (TNZ2–7 K). Their electrical behaviour indicates fairly complex character of the underlying electronic structure with multiple electron and hole bands with different temperature and magnetic field variations of carrier concentrations and mobilities. The uranium-based alloys with the composition UT2M, where T stands for a d-electron transition metal and M is a p* Corresponding author. Tel.: C4871 34 350 21; fax: C4871 34 410 29. E-mail address: [email protected] (D. Kaczorowski). 0038-1098/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2005.01.007

electron element, attracted during the last decade much attention due to large variety of their physical behaviour originating from sensitive nature of uranium 5f-electrons to degree of their hybridisation with s, p, and d-electrons of neighbouring atoms. These phases crystallize with various crystal structures like hexagonal ZrPt2Al-, orthorhombic Fe3C- or cubic MnCu2Al-types [3–5]. Among them UPd2M intermetallics usually form with the latter structure type, characteristic of Heusler phases. UPd2Pb was reported to be antiferromagnetic below TNZ35 K and found to display characteristics of heavy-fermion state [6]. UPd2In is an antiferromagnet with TNZ20 K and has also been considered as a heavy-fermion system [7]. These findings motivated us to extend our studies on Heusler phases containing antimony and bismuth to 5f-based materials, and at the outset we focused on a hypothetical compound UPd2Sb. In this paper we report on the formation of this compound and give preliminary account on our study of its physical properties. To the best of our knowledge such data are communicated here for the first time.

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2. Experimental details Polycrystalline samples of UPd2Sb were synthesized by arc-melting the stoichiometric amounts of the constituents (U: 99.8 wt%, Pd: 99.999 wt%, Sb: 99.999 wt%) under titanium-gettered argon atmosphere. The buttons were remelted several times to ensure good homogeneity. No further heat treatment was applied. The obtained alloys were checked at room temperature using a Siemens D5000 X-ray powder diffractometer with Cu Ka radiation. The magnetic properties were studied between 1.7 K and room temperature and in magnetic fields up to 5 T employing a Quantum Design MPMS-5 SQUID magnetometer. The electrical resistivity was measured in the temperature range 4.2– 300 K using a conventional four-point dc technique. Magnetoresistivity measurements were made at temperatures 4, 20 and 40 K and in fields up to 8 T generated by a commercial AMI superconducting magnet. The Seebeck coefficient was measured from 6 to 300 K employing a differential method with copper as a reference material. Specific heat measurements were carried out over the interval 3–70 K by adiabatic method.

3. Results and discussion The X-ray powder diffraction measurements have shown that the prepared samples of UPd2Sb are single phase with cubic symmetry. The lattice parameter refined from the X˚ . Close similarity in the diffraction ray data is aZ6.766(2) A patterns and the values of a between UPd2Sb and the rareearth ternaries studied in Refs. [1,2] suggest that the uranium compound, alike the other materials, crystallizes with the cubic MnCu2Al-type of structure (space group  characteristic of Heusler phases. Fm3m) The temperature dependence of the inverse magnetic susceptibility of UPd2Sb measured in applied magnetic field of 0.1 T is shown in Fig. 1. A distinct minimum in cK1(T) manifests an antiferromagnetic behaviour with the Ne´el temperature TNZ55 K. In the paramagnetic region the inverse susceptibility is slightly curvilinear and may be described by a modified Curie–Weiss law in the form cðTÞ ZðNm2eff =3kB ðT K QÞÞC c0 with the parameters: m effZ 2.7 mB, qpZK86 K and c0Z3!10K4 emu/mol (note the solid line in Fig. 1). The experimental effective magnetic moment meff is considerably smaller than the free-ion value expected for trivalent or tetravalent uranium ions (g[J(JC 1)]1/2Z3.62 and 3.58, respectively). The negative paramagnetic Curie temperature qp is consistent with the antiferromagnetic ordering, however, its absolute value is much larger than the Ne´el temperature TN. Both these findings might hint at the presence in UPd2Sb moderate Kondo interactions. The magnetization measured at TZ 1.7 K is proportional to the applied magnetic field with no hysteresis effect nor metamagnetic-like anomaly up to 5 T

Fig. 1. Temperature dependence of the magnetic susceptibility of UPd2Sb measured in magnetic field BZ0.1 T. The solid line represents a modified Curie–Weiss fit with the parameters given in the text. Inset: magnetisation versus magnetic field measured at TZ 1.7 K with increasing (full circles) and decreasing (open circles) magnetic field.

(see the inset to Fig. 1). Such a behaviour is characteristic of systems with strong magnetocrystalline anisotropy. Fig. 2 displays the temperature variation of the electrical resistivity of UPd2Sb. At room temperature the resistivity is about 210 mU cm that is a magnitude typical for uranium intermetallics. With decreasing temperature r(T) shows a negative slope down to TN, below which it forms a hump. The resistivity behaviour in the paramagnetic region may be well approximated by the formula rðTÞ Z r0 C rN 0 C cK ln T

(1)

appropriate for describing scattering of conduction electrons on defects (first term), disordered spins (second term) and Kondo impurities (third term). A least-squares fit of this expression to the experimental data above 60 K yielded the parameters: r0CrN 0 Z287 mU cm and cKZK14 mU cm. As

Fig. 2. Temperature dependence of the electrical resistivity of UPd2Sb. The solid line is a least-squares fit to the experimental data of the function defined in Eq. (1) (see the text). Inset: temperature derivative of the resistivity in the vicinity of TN. The arrows mark the antiferromagnetic phase transition.

K. Gofryk et al. / Solid State Communications 133 (2005) 625–628

it is apparent from the inset to Fig. 2, the onset of the antiferromagnetic ordering in UPd2Sb results in a distinct minimum in the temperature derivative of the resistivity dr/ dT(T). The rapid rise of the resistivity below TN may likely be attributed to enhanced scattering of conduction electrons due to formation of new Brillouin zone boundaries in the ordered state. Upon applying external magnetic field in the direction perpendicular to that of the electrical current the resistivity measured at low temperatures hardly changes. The transverse (Bti) magnetoresistivity Dr=rZ rðBÞK rð0Þ=rð0Þ is slightly negative in the antiferromagnetic region and reaches only K0.13% in a field of 8 T being nearly temperature independent in the range 4–40 K (Fig. 3). Interestingly, the overall shape of Dr=rðTÞ in UPd2Sb is similar to that expected for systems with Kondo interactions. The temperature variation of the thermoelectric power of UPd2Sb is shown in Fig. 4. At room temperature the Seebeck coefficient is about 9 mV/K. Then, the thermopower smoothly decreases with decreasing temperature down to about 100 K, below which S(T) forms a broad hump. In the main this S(T) curve resembles closely that measured for a heavy-fermion compound UPt2In [8]. Accordingly, the data for UPd2Sb were analysed in the framework of a model that takes into account scattering conduction electrons by 5f quasiparticle band of a Lorentzian form [9]. Within this socalled two-band model the thermoelectric power is given by a modified Mott expression SðTÞ Z

aT b2 C T 2

(2)

in which the coefficients a and b are defined by aZ

2ð3 K 3F Þ ; jej

b2 Z

3ðð3 K 3F Þ2 C G2 Þ p2 kB2

The symbol 3 denotes the energy position of the 5f band and G stands for its bandwidth. As displayed in Fig. 4, the model provides pretty good approximation of the experi-

Fig. 3. Field variation of the transverse magnetoresistivity of UPd2Sb measured at 4, 20 and 40 K.

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Fig. 4. Temperature dependence of the thermoelectric power of UPd2Sb. The solid line is a least-squares fit to the experimental data of the function defined in Eq. (2) (see the text). The arrow marks the Ne´el temperature TNZ55 K derived from the magnetic data.

mental S(T) above ca. 120 K. The so-obtained characteristics of the narrow band in UPd2Sb are 3Z4.04 meV and GZ48.2 meV, being similar to those derived for several uranium- and cerium-based intermetallics with strong electronic correlations [8]. Deviations from the two-band model observed at lower temperatures may be attributed to scattering processes that are beyond this simple approach, like phonon-drag effect, crystal-field interactions and/or long-range antiferromagnetic ordering. As regards the latter phenomenon, it should be noticed that no clear anomaly in S(T) of UPd2Sb is observed at the phase transition at TNZ 55 K. The measured specific heat of UPd2Sb is presented in Fig. 5. At the magnetic transition temperature no welldefined feature in Cp(T) is observed but only a small kink. This little anomaly is better discernible on the curve Cp/T(T) (note the upper inset to Fig. 5). Very similar behaviour was observed e.g. in GdCu5 and attributed to the formation of an incommensurate helimagnetic structure [10] or an amplitude-modulated structure [11]. The second alternative provides a particularly good description of both the shape and the position of the specific heat anomaly at the onset of magnetically ordered state [12]. It seems possible that similar type of magnetic ordering is present also in UPd2Sb, despite different crystal structure, but neutron diffraction experiments are required to conclude on this point. On the other hand, for magnetically ordered dense Kondo systems one does expect some reduction in the height of the jump in Cp(T) at TN that is the larger the higher is the characteristic temperature TK. Hence, also this latter phenomenon may contribute to the behaviour in the uranium compound studied, even though usually Kondo effect, unlike the present case, does not affect a mean-field l-shape of the Cp peak. In the lower inset to Fig. 5 there is shown the lowtemperature specific heat of UPd2Sb plotted as Cp/T versus

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heat. Also the temperature variation of the Seebeck coefficient and the field dependence of the magnetoresistivity measured for UPd2Sb are typical for systems with strong hybridization of 5f electrons with conduction band. Altogether the observed properties are consistent with a Kondo behavior and suggest classifying the novel uranium compound studied as a low effective-mass heavy-fermion material.

Acknowledgements

Fig. 5. Temperature dependence of the specific heat of UPd2Sb. Upper inset: temperature variation of the ratio Cp/T. Lower inset: low-temperature data plotted as Cp/T versus T2. The solid line represents a least-squares fit to the experimental data of the function defined in Eq. (3) (see the text). The arrows mark the antiferromagnetic phase transition.

T2. The solid line represents a fit of the experimental data to the standard formula Cp ðTÞ Z gT C bT 3

(3) 2

with the fitting parameters: gZ81 mJ/mol K and bZ 4.1 mJ/mol K4. The Debye temperature calculated from the ffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi equation QD Z 3 12Rp4 =5b (where R is gas constant) is equal to 124 K. Such a low value may be compared to those reported for related Heusler phases: QDZ127 K for UPd2Pb [6] and 180 K for UPd2In [7]. In turn, the Sommerfeld coefficient g is an order of magnitude larger than those expected for simple metals, thus clearly manifesting the presence in UPd2Sb of strong electronic correlations.

4. Summary UPd2Sb is an antiferromagnet with TNZ55 K that shows some features characteristic of strongly correlated electron systems, like reduced effective magnetic moment, large negative paramagnetic Curie temperature, logarithmic decrease in the electrical resistivity, negative magnetoresistivity, enhanced low-temperature electronic specific

This work was supported by the Polish State Committee for Scientific Research KBN under Grant No. 1 P03B 090 27.

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