Superconductivity and magnetism in the Heusler alloys M Pd 2 Pb (cid:132) M (cid:53) rare earth, Th, and U (cid:133)

We report on an investigation of superconductivity and magnetism in the series of compounds M Pd 2 Pb (cid:126) M (cid:53) rare earth, Th, U (cid:33) through measurements of electrical resistivity, magnetic susceptibility, speciﬁc heat, and lattice parameters. Both single crystals and polycrystalline samples were studied. The compound UPd 2 Pb was examined for possible heavy-fermion behavior, and found to exhibit antiferromagnetic order below T (cid:53) 35 K.


INTRODUCTION
Several Heusler alloys ͑cubic L2 1 structure͒ with the general chemical formula RT 2 X have been found to exhibit superconductivity, including YPd 2 X ͑XϭSn, Pb, In, and Sb͒, 1 YAu 2 Sn, 2 and RPd 2 Sn ͑RϭSc, Y, Tm, Yb, and Lu͒, 1,3 with critical temperatures as high as 4.76 K. Moreover, superconductivity and antiferromagnetism coexist in the compound YbPd 2 Sn. Higher T c values were generally found for compounds with larger lattice parameters, with YPd 2 Pb yielding the highest reported value of 4.76 K. 1 For RPd 2 Pb samples, we also expected the effects of magnetic pair breaking by the magnetic R ions to be minimized because of the large lattice parameters, which could give rise to interesting effects due to the interaction and competition between superconductivity and magnetism. We report on an investigation of superconductivity and magnetism in the series of compounds M Pd 2 Pb ͑M ϭrare earth R, Th, and U͒, primarily through measurements of electrical resistivity and magnetic susceptibility. Both polycrystalline samples and single crystals were studied. We find that the magnetic rare-earth ions order antiferromagnetically at low temperatures with the lowest Néel temperature T N values among RPd 2 X compounds. Bulk superconductivity is found for RϭSc, Y, Tm, Yb, and Lu, but with lower T c values than expected, probably associated with sample quality. Motivated by heavy-fermion behavior previously observed in UPd 2 Sn, 4 the compound UPd 2 Pb was examined for possible heavy-fermion behavior, and found to exhibit antiferromagnetic order below Tϭ35 K, inferred from a sharp cusp observed in the magnetic susceptibility. The large value of the linear coefficient of the specific heat ␥Ϸ100 mJ/mol K 2 , determined from data measured below 35 K, along with the temperature dependence of the electrical resistivity, suggest that this may be an antiferromagnetic low effective mass heavy-fermion compound.

EXPERIMENTAL DETAILS
Single crystals and polycrystalline samples of M Pd 2 Pb were fabricated by flux growth and by melting in sealed Ta tubes, respectively. Powder x-ray-diffraction measurements were subsequently made using a Rigaku rotating-anode diffractometer utilizing Cu K␣ radiation. Electrical-resistivity measurements were made on the polycrystalline samples from 1 to 300 K in a 4 He cryostat using a self-balancing, four-wire ac resistance bridge operating at 16 Hz, with typical excitation current densities of ϳ100 A/cm 2 . Platinum electrical leads ͑50 m diameter͒ were attached with silver epoxy. Static magnetic-susceptibility measurements were performed on single crystals using a Quantum Design superconducting quantum interference device ͑SQUID͒ magnetometer from 1.8 to 300 K in magnetic fields up to 7 T. Lowtemperature ͑0.2рTр6 K͒ ac magnetic-susceptibility measurements were performed in a 3 He/ 4 He dilution refrigerator. Specific-heat measurements were made on a polycrystalline sample of UPd 2 Pb from 0.5 to 20 K using a semiadiabatic heat-pulse calorimeter utilizing a 3 He cryostat.

SAMPLE PREPARATION
Single crystals of M Pd 2 Pb were successfully grown in Pb flux for M ϭSc, Y, Gd, Tb, Dy, Ho, Er, Tm, and Lu. Attempts to grow crystals with M ϭYb and U were unsuccessful, even though polycrystalline samples could be prepared by arc melting. In addition, attempts to grow crystals consisting of the lighter rare-earth ions M ϭCe, Pr, Nd, Sm, and Eu were unsuccessful, consistent with our inability to make polycrystalline samples, most likely due to the large size of the trivalent ions. The crystals were prepared in the following way.
Stoichiometric amounts of 99.9% pure M and 99.99% pure Pd were placed in an alumina crucible with 99.999% pure Pb, which comprised 90 mol % of the mixture. Quartz wool was then placed inside the crucible, which was then sealed in a quartz tube and backfilled with 150 torr ultrahighpurity ͑UHP͒ Ar. The sample was heated to 1200°C, then slowly cooled ͑10°C/h͒, allowing single crystals to nucleate. In our case, Pb from the flux was incorporated into the crystal structure. The sample was removed from the furnace at 800°C and centrifuged immediately so that the molten flux spun through the quartz wool, leaving the crystals in the crucible. To etch away any remaining free Pb, the crystals were placed in a 1:1 solution of H 2 O 2 ͑30%͒: glacial acetic acid for no more than 5 min. The crystals formed as cubes or clusters of cubes which appeared to grow in the ͑111͒ direction. Of these, crystals with M ϭGd and Sc formed the largest-͑ϳ1 mm on a side͒ and smallest-sized cubes, respectively. Subsequent x-ray powder diffraction of powdered crystals verified the single-phase L2 1 cubic Heusler structure for RϭLu, Tm, Er, Ho, Dy, Tb, Gd, Y, and Sc. No impurity peaks, including those of free Pb, were evident in the x-ray patterns. The intensity of the background noise level was only ϳ0.1% of the most intense peak corresponding to the ͑220͒ direction. The flux growths with Gd and Eu contained long, needle-shaped crystals ͑ϳ5 mm length͒, but they exhibited multiple x-ray peaks which did not belong to the Heusler structure. The Gd flux growths also contained the cube-shaped crystals with the Heusler structure. ThPd 2 Pb formed, though it contained several impurity phases, one of which was identified as ThPb. Attempts to grow single crystals of RϭYb, Eu, Sm, Nd, Pr, Ce, and U were unsuccessful.
Polycrystalline samples were fabricated by melting in Ta tubes as follows. Stoichiometric amounts of each element were sealed in a 1-cm-diameter Ta tube, with 1 atm UHP Ar. The tube was then sealed in a quartz tube backfilled to 150 torr UHP Ar, heated to 1300°C for 1 h, and subsequently quenched by placing the 1300°C tube in water at room temperature. Samples with the Heusler structure were made for RϭSc, Y, Tb, Dy, Ho, Tm, Yb, and U. X-ray powderdiffraction data revealed trace amounts of elemental Pb ͑a superconductor with T c ϭ7.2 K͒ in all samples, Pb 2 Pd ͑a superconductor with T c ϭ3.0 K͒ for RϭY, Dy, Ho, and Tm, and RPd 3 ͑not superconducting͒ for RϭYb. This method was not successful for RϭCe, Eu, Gd, Lu, or Th, although a minority Heusler phase was still discernible for Lu. The RϭEr sample was made by arc melting with excess Pb to compensate for loss due to its high vapor pressure. None of the samples were subsequently annealed.
It is interesting to note that the related compounds RPd 2 Sn, with RϭTb and Dy, exhibit structural transformations at ϳ250 and 50 K, respectively, which lowers the crystal symmetries. 5 The transformations were not evident from magnetic susceptibility, and we cannot rule out similar transformations occurring in our RPd 2 Pb samples.

RESULTS AND DISCUSSION
RPd 2 Pb "R‫؍‬rare earth: Sc,Y,Gd,Tb,Dy,Ho,Er,Tm,Yb,Lu… Fig. 1 is the temperature dependence of the normal-state electrical resistivity (T), normalized to its room-temperature value, of RPd 2 Pb compounds in which the 4 f electron shell of the R ions is ͑a͒ empty or filled and ͑b͒ partially filled. Except for RϭGd, for which a single crystal was used, polycrystalline samples were measured because of the difficulty of attaching electrical leads to the extremely small crystals. All samples exhibit metallic behavior with positive temperature coefficient of resistance at all temperatures below 295 K. The data display downward curvature except at the lowest temperatures where phonon scattering freezes out and the data eventually saturate towards the zerotemperature value due to scattering by defects and impurities. Although the degree of curvature varies among the RPd 2 Pb samples, it does not appear to depend in a systematic way on the R ion. In particular, temperature-dependent magnetic scattering due to splitting of the ground-state multiplet by the crystalline electric field ͑CEF͒ is not obvious. The resistance ratio RRϭ͑295 K͒/͑0 K͒ also varies a lot among samples, attesting to varying amounts of impurity scattering.

Shown in
Qualitatively speaking, those samples with the largestmoment R ions, namely, RϭGd, Ho, and Dy, exhibit the smallest RR's.
All of the polycrystalline samples have elemental Pb impurities, which causes (T) to drop below TϷ7 K owing to the superconductivity of Pb. For those samples in which (T) does not drop to zero by then, an additional drop is observed below TϷ3 K due to the presence of a Pb 2 Pd impurity phase, which is superconducting with T c ϭ3.0 K. 6 No such resistive drops are observed for the relatively impurityfree single-crystal specimen of GdPd 2 Pb, allowing a small anomaly in (T), shown in the inset of Fig. 1͑b͒, to be observed near Tϭ4.5 K. This anomaly is due to antiferromagnetic ordering of the Gd moments, as inferred from the magnetic susceptibility discussed below. Fig. 2 is the inverse magnetic susceptibility Ϫ1 as a function of temperature T for the RPd 2 Pb compounds with magnetic rare-earth ions R. The (T) data were acquired for many clusters of single crystals with random orientations with respect to the magnetic-field direction, except for RϭEr for which a polycrystalline sample was measured. Over a wide temperature range ͑10рTр300 K͒, the data for all of these samples are well described by the Curie-Weiss law (T)ϭN eff 2 /3k B ͑TϪ CW ͒, where N is the number of rare-earth ions, eff is the effective magnetic moment, and CW is the Curie-Weiss temperature. The solid lines in Fig. 2 represent least-squares linear fits of the data, which yield values for eff and CW listed in Table I. The values of eff are close to the theoretical Hund's rules ground-state values for free trivalent rare-earth ions as illustrated in Fig. 3 and do not deviate much from the Curie-Weiss fits above the antiferromagnetic ordering temperatures, suggesting that the CEF energy level splittings ⌬ CEF of the Hund's rules groundstate multiplets are small ͑⌬ CEF Ͻ10 KϷ1 meV͒ in these compounds.

Shown in
The Curie-Weiss temperatures CW are all negative, indicative of antiferromagnetic ͑AFM͒ exchange interactions. These interactions give rise to AFM order at low temperatures, revealed as a peak in the magnetic susceptibility for some of these samples at the Néel temperature T N , as shown in Fig. 4 and listed in Table I. Data for RϭGd, Tb, and Dy represent dc from static ͑dc͒ magnetization measurements taken in a field of 10 Oe ͑100 Oe for RϭGd͒ after zero-field cooling ͑ZFC͒ and during field cooling ͑FC͒. Data for RϭHo and Er are from low-temperature ac magnetic-susceptibility ac measurements. Two interesting aspects of the magnetic-susceptibility data are ͑1͒ there is only a small decrease in (T) below T N as T→0 ͓the ratio ͑0͒/(T N ) is much larger than the theoretical value of 2/3 for an isotropic, uniaxial AFM powder specimen 7 ͔ and ͑2͒ irreversibility in dc appears below an onset temperature T irr , reminiscent of spin-glass freezing. The large ͑0͒/(T N ) ratio is characteristic of spin glasses, but can also occur for an antiferromagnetically ordered fcc lattice due to low anisotropy energy. 8 In the latter case, the measured susceptibility below T N approaches Ќ ͑in which the antiparallel moments are perpendicular to the applied magnetic field H͒, which is more energetically favorable than ʈ ͑moments parallel to H͒. Paramagnetic impurities could also increase the measured value of ͑0͒/(T N ). The irreversible dc (T) for GdPd 2 Pb was measured with Hϭ1, 10, 100, and 200 Oe, with little change in (T) or T N . This observation, as well as the observed scaling of T N with the de Gennes factor ͑described below͒, and the electricalresistivity results ͑described above͒ eliminate the possibility that these anomalies are due to superconductivity. The irreversibility in the magnetization might be due to structural disorder ͑defects͒ in the samples, which could give rise to FIG. 4. Low-temperature magnetic susceptibility vs temperature T of RPd 2 Pb single crystals, revealing antiferromagnetic order below the Néel temperature T N , determined from the cusp in the data. The data for RϭGd, Tb, and Dy are from dc measurements taken in fields of 100, 10, and 10 Oe, respectively, upon zero-field cooling ͑ZFC͒ and field cooling ͑FC͒. The data for RϭHo and Er are from ac susceptibility measurements as described in the text. spin-glass-like behavior, or possibly geometrical frustration of the ordered moments on the fcc lattice. The existence of bulk superconductivity was confirmed for some of the RPd 2 Pb samples from dc magneticsusceptibility measurements, taken upon both ZFC and FC in a magnetic field Hϭ10 Oe. ZFC data are shown in Fig. 5. The (T) data were irreversible with the FC data being typically 50% of the ZFC values for each sample, which we attribute to flux pinning. A significant diamagnetic response, indicative of bulk superconductivity, is observed for RPd 2 Pb compounds in which R is a nonmagnetic trivalent ion, including RϭSc, Y, and Lu. Evidently, superconductivity is the ground state of this system in the absence of magnetic pair breaking by magnetic R ions. The d electrons of the Pd ions are believed to be mainly responsible for superconductivity in these compounds. 1 Curiously, superconductivity was not observed down to 1.8 K for ThPd 2 Pb, Th being a nonmagnetic tetravalent ion. Superconductivity was also observed for RϭTm and Yb, which are trivalent magnetic ions with the smallest values of the de Gennes factor (gϪ1) 2 J(Jϩ1) among the heavy magnetic rare earths. The ⌬ data shown in Fig. 5 for RϭTm and Yb were obtained by subtracting the magnetic contribution of the Tm and Yb ions, respectively. As shown in Fig. 5, the diamagnetic shielding fractions, estimated from the ZFC measurements, vary from ϳ10% to ϳ60% for single-crystal specimens, and are as high as 140% for polycrystalline YbPd 2 Pb. These values should be multiplied by a factor of ϳ2/3 to account for the demagnetization correction if one approximates the cube-shaped crystals as spheres.
The superconducting transitions are rather broad, with onset temperatures ranging from 2.1 to 2.8 K ͑see Table I͒. Previous measurements of T c for arc-melted polycrystalline samples of YPd 2 Pb yielded higher values of T c ϭ4.76 ͑Ref. 1͒ and 4.05 K. 9 The precise value of T c is known to be highly sensitive to the stoichiometry in these Heusler systems, with a possible correlation with lattice parameter, which could be due to a sharp feature in the electronic density of states. 1 It is not clear what value of T c represents that of the stoichiometric compound YPd 2 Pb. We note, however, that one must be very careful to account for the effects of any superconducting impurities including Pb ͑T c ϭ7.2 K͒ and Pb 2 Pd ͑T c ϭ3.0 K͒.
Magnetic interactions in a metal with rare-earth ions are governed by the s f exchange interaction between the conduction electron spins s and the total angular momentum J of the local 4 f states, given by the Hamiltonian where J s f is the exchange coupling parameter and g J is the Landé g factor of the rare-earth ion. The factor ͑g J Ϫ1͒ results from replacing the total spin S with its projection onto the total J for rare earths with finite orbital angular momentum L. For superconducting metals, this interaction leads to magnetic pair breaking as described, for example, by the Abrikosov-Gorkov theory. For a lattice of rare-earth ions, this interaction leads to the RKKY indirect exchange interaction between rare-earth ions, with Hamiltonian H RKKY ϭϪ͚J eff J i •J j , where J eff ϳJ s f 2 (g J Ϫ1) 2 . The RKKY interactions are responsible for magnetic order at low temperatures and magnetic correlations at high temperatures which result in finite Curie-Weiss temperatures. If the magnitude of J s f is constant, the characteristic temperatures T N and CW are predicted to scale among the rare earths with the de Gennes factor (g J Ϫ1) 2 J(Jϩ1), which represents the square of the component of the total spin S in the total J direction, ͉͑S•J͒J͉ 2 . Figure 6 summarizes the results of the magneticsusceptibility measurements on the RPd 2 Pb compounds, showing CW , T N , and T c for the heavy rare-earth R ions. Both CW and T N scale approximately with the de Gennes factor (g J Ϫ1) 2 J(Jϩ1), represented by the solid line in Fig.  6 ͑normalized to the values for RϭGd͒. Crystalline electric field effects could be responsible for deviations from this scaling. It is interesting to note that the values of T N and CW are very small compared to other rare-earth intermetallic compounds, implying small exchange interactions. Of particular interest is the existence of superconductivity for RϭTm and Yb. Apparently, because of the small de Gennes factors, the magnetic pair breaking due to the magnetic Tm and Yb ions is not sufficiently strong to destroy superconductivity. In addition, since Tm is a non-Kramer's ion, it is possible for it to have a nonmagnetic CEF ground state, which might also reduce magnetic pair breaking. Supercon- FIG. 5. Superconducting contribution to the magnetic susceptibility ⌬, after zero-field cooling ͑ZFC͒ and then applying a 10 Oe field, for single-crystal specimens of the RPd 2 Pb compounds which were found to be superconducting. The data for RϭTm and Yb were corrected for the magnetic background contribution of the R ions.
FIG. 6. Curie-Weiss temperatures CW ͑solid circles͒, Néel temperatures T N ͑open circles͒, and onset superconducting critical temperatures T c ͑open squares͒ of the RPd 2 Pb compounds, derived from magnetic-susceptibility measurements. The values CW and T N are compared to the de Gennes factor (g J Ϫ1) 2 J(Jϩ1), represented by the solid line, and normalized to the values for RϭGd. ductivity was previously observed in TmPd 2 Sn and YbPd 2 Sn with T c values of 2.8 and 2 K, respectively, 1,3 and YbPd 2 Sn was discovered to exhibit coexistence of superconductivity and antiferromagnetic order of the Yb ions below T N ϭ0.23 K. 3 This coexistence has been attributed to the small de Gennes factor of Yb. The TmPd 2 Sn compound was postulated to have a nonmagnetic CEF ground state. There does not appear to be any sign of magnetic order for TmPd 2 Pb or YbPd 2 Pb down to 1.8 K from our magnetic-susceptibility measurements.
From magnetic-susceptibility and 119 Sn Mössbauer measurements on TmPd 2 Sn, Malik, Umarji, and Shenoy 10 deduced that the Tm 3ϩ ions have a ⌫ 3 nonmagnetic ground state with a first excited triplet state ϳ1 meV ͑ϳ10 K͒ above the ground state. Shown in Fig. 7 are magnetization M (H) data for TmPd 2 Pb crystals at 1.8 K, measured in fields H up to 7 T. The M (H) data are nonlinear and consist of a contribution that appears to saturate like a Brillouin function in high fields and a linear term with a slope 0 ϭ0.07 emu/mol, represented by the dashed line. The linear term could be a second-order van Vleck susceptibility term. Extrapolation of the linear term to zero yields the saturation magnetization of ϳ19ϫ10 3 emu/mol, corresponding to g J Jϭ3.4 B , which is less than the saturation moment of 7 B for the full Tm 3ϩ multiplet. This reduced value could be due to the cubic CEF which splits the multiplet such that the ⌫ 5 ͑2͒ triplet, which has the magnetic moment g J ͗⌫ 5 (2) ͉J z ͉⌫ 5 (2) ͘ϭ3.13 B , lies lowest. 11 A nonmagnetic ground state appears to be ruled out unless the CEF splittings are very small compared to 1.8 K. Shown in the inset is a plot of the temperature dependence of the inverse magnetic susceptibility from 1.8 to 10 K as measured in a low field of 1 kOe where M (H) is approximately linear down to 1.8 K. The dashed line in the inset represents a fit to a Curie-Weiss law in this temperature range, which has nearly the same eff and CW as the hightemperature fits, again suggesting that the CEF splitting is very small, or that the ⌫ 5 ͑2͒ triplet lies lowest in energy. In either case, it is surprising that superconductivity exists in TmPd 2 Pb despite the highly magnetic character of the Tm ions even at low temperatures TϳT c .

UPd 2 Pb
In a previous study of uranium-based ternary compounds with the chemical formula UT 2 X with TϭPd, Au and XϭIn, Si, Ge, Sn, Sb, it was found that only those with M ϭIn formed the cubic Heusler structure. 4 The others revealed a more complicated structure in the x-ray-diffraction patterns, believed to be orthorhombic. None of those materials studied were superconducting down to 80 mK. The compound UPd 2 Sn, however, displayed characteristics of valence fluctuation or Kondo lattice phenomena below a characteristic temperature ϳ10 K, and is considered 4 to be a nonmagnetic, nonsuperconducting heavy-electron material. In particular, the electronic specific-heat coefficient ␥ϭC el /T was found to be strongly temperature dependent with a maximum value of ϳ270 mJ/mol K 2 at 9.7 K and an extrapolated value of ϳ70 mJ/mol K 2 at Tϭ0 K.
We have synthesized polycrystalline samples of UPd 2 Pb and found that it forms the cubic L2 1 Heusler structure. The cubic lattice parameter aϭ6.85Ϯ0.01 Å is the largest of all the M Pd 2 X L2 1 Heusler compounds ͑see Table I͒,  crystal structure. We subsequently measured (T), (T), and C(T) in order to characterize it for possible heavy-fermion behavior.
The temperature dependences of the normalized electrical resistivity and magnetic susceptibility of UPd 2 Pb are shown in Figs. 8͑a͒ and 8͑b͒, respectively. The electrical resistivity (T) increases by only a few percent with decreasing temperature down to ϳ100 K, below which (T) decreases. The negative temperature coefficient of resistance above 100 K is indicative of Kondo-like magnetic scattering and is a typical feature of heavy-fermion materials, as is the subsequent drop below 100 K. A small kink in the (T) data is observed at Tϭ35 K, which we attribute to antiferromagnetic ordering of the U ions, inferred from the magnetic-susceptibility data in Fig. 8͑b͒. Like the other M Pd 2 Pb polycrystalline samples, (T) drops suddenly below 7.2 K due to Pb inclusions. The low-temperature resistivity of M Pd 2 Pb could be studied by applying a magnetic field sufficient to destroy the superconductivity of Pb and Pb 2 Pd impurity phases.
The magnetic susceptibility of UPd 2 Pb is shown in Fig.  8͑b͒. A sharp cusp is observed at Tϭ35 K, which we attribute to AFM order of the U ions. In contrast to the RPd 2 Pb compounds which exhibit AFM order, the ratio ͑0͒/(T N )Ϸ0.75 for UPd 2 Pb, which is only slightly larger than the theoretical value of 2/3 for an isotropic, uniaxial AFM powder specimen. 7 At higher temperatures, the (T) data can be described by a Curie-Weiss law plus a constant 0 , (T)ϭN eff 2 /3k B ͑TϪ CW ͒ϩ 0 , represented by the solid line, with eff ϭ2.77 B , CW ϭϪ50 K, and 0 ϭ6.2ϫ10 Ϫ4 emu/mol. The value of the effective moment is somewhat smaller than the free-ion values for trivalent ͑3.62 B ͒ and tetravalent ͑3.58 B ͒ U ions. The paramagnetic constant 0 could be due to an enhanced Pauli susceptibility for a heavyfermion compound. Significant CEF splitting ͑⌬ CEF ϳ100 K͒ might also give rise to the observed deviation from free-ion behavior. Not shown in Fig. 8͑b͒, the (T) data reveal slightly irreversible behavior at temperatures below T N . Shown in Fig. 9 are low-temperature specific-heat data for UPd 2 Pb, plotted as C/T vs T 2 , for 0.5рTр10 K. The solid line represents a least-squares fit of the data to the equation C(T)ϭ␥Tϩ␤T 3 , with ␥ϭ98 mJ/mol K 2 and ␤ϭ3.8 mJ/mol K 4 . The value of the Sommerfeld coefficient ␥ is at least an order of magnitude larger than that of a normal metal, although it is small compared to prototypical heavyfermion compounds for which 0.25Շ␥Շ1 J/mol K 2 . 12 However, the U ions are antiferromagnetically ordered at these temperatures, which might be responsible for the observed values of the coefficients ␥ and ␤. Measurements of C(T) to higher temperatures will give information about the ordering, as well as a measure of ␥ above T N .

CONCLUSION
Single crystals of RPd 2 Pb with very small amounts of impurity phases were fabricated. Measurements of magnetic susceptibility revealed bulk superconductivity for RϭSc, Y, Tm, Yb, and Lu, with T c Ϸ2-3 K. These values are lower than expected; a T c value as high as 4.76 K was previously reported for YPd 2 Pb. The discrepancy is most likely due to differences in sample composition and/or quality. In particular, the T c 's of these Heusler compounds are known to be sensitive to exact stoichiometry; off-stoichiometric polycrystalline samples might yield higher T c values. The magnetic rare-earth ions order antiferromagnetically with very low Néel temperatures T N р4.5 K. Both T N and CW scale approximately with the de Gennes factor. It is unlikely that the Tm ions in TmPd 2 Pb have a nonmagnetic ground state, making it more surprising that it is superconducting. ThPd 2 Pb was not superconducting above 1.8 K. The Heusler compound UPd 2 Pb orders antiferromagnetically below 35 K, and displays many characteristics of a low effective mass heavyfermion material.