Magnetism and heavy fermion-like behavior in the RBiPt series

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ETH. Zurich, Switzerland
Members of the RBiPt (R =Ce-Lu with the exceptions of Pm and Eu) series have been grown as single crystals.Magnetic susceptibility and electrical resistance have been measured on all members of the series, and specific heat measurements have been performed on rcprcscntatives.Tue high temperature resistance uniformly changes from that of a small-gap semiconductor or semimetal seen in NdBiPt to that of a heavy-fermion meta! seen in YbBiPt, which shows a linear coefficient of specific heat at low temperatures of 8 J/K. 2

mole.
Further, the lighter rare earth members s how an unusually sharp increase in their resistance associated with antiferromagnetic ordering at low temperatures.
Recently, during attempts to grow single crystals of the R 3 Bi 4 Pt 3 series past the La, Ce, and Pr members that have bcen reported elsewhere, 1 • 2 we discovered that the synthesis technique being used favored the growth of RBiPt (R = rare earth) for R members beyond Pr.X-ray ditrraction of powdered single crystals show that the RBiPt series forms in tbe cubic, AgAsMg structure, which can be viewed as three face-centered-cubic sublattices placed at (0,0,0), (l/4,1/4,1/4), and (3/4,3/4,3/4) along the body diagonal.Until now, all that has been reported about these compounds is an incomplete set of lattice parameters. 3-s However, two other systems, UNiSn and MnNiSb (MnPtSb), in the same structure have been studied 6 • 7 in some detail because of their interesting "half-metallic" ferromagnetic properties.
Single crystals of RBiPt (R = Nd-Lu and Y with the exceptions of Pm and Eu) were grown out of Bi flux 1 and characterized by powder x-ray diffraction, four-probc electrical resistance and de magnetic susceptibility measure~ ments.In somc cascs thc low temperature (T <20 K) spe-cific heat was also determined.
Figure 1 shows the temperature variation of the electrical resistance for representative members of the series normalized at room temperature to the resistance of Nd-BiPt.Above approximately 150 K, all members, except Yb and Lu, have a negative dR/ dT, suggesting semiconductor or semimetal-like behavior.Indeed, a plot of In R versus 1/T for NdBiPt is linear in the range of 130 < T < 300 with a slope of ä = l 7S K .Interestingly, the magnitude of dR/ dT as seen in the high temperature resistance of these compounds (Fig. l inset) decreascs monotonically as the rare earth series is traversed.This trend is reftected as weil in the absolute magnitude of p(290 K) which ranges from 0.95±0.05mn cm for Nd, and 0.9±0.2mfi cm for Od to 0.35 ±0.05 mfi cm for Yb.These systematics suggest that the resistance may be dominated by variations in the unit cell volumc.Lattice parameters a 0 for members of the series are shown in Fig. 2 and agree reasonably weil with previously reported values. 3• 4 As expected from the wellknown lanthanide contraction, we see a monotonic decrease in a 0 in going from Ce to Lu.However, we note an apparently discontinuous drop in ao between Gd and Th for which we have no explanation. 9To test the assumption that a 0 dominates changes in the high temperature resistance, alloys Ndo.sRo.sBiPt and YBiPt were studied as well Their lattice parameters and temperature dependent resistances follow the systematics observed in the RBiPt compounds, as shown in Figs. 1 and 2.
At low temperatures anomalies appear in R (T) for R =Nd, Sm (not shown in Fig. 1), Gd, Th, and D y.In all cases, distinct features in the magnetic susceptibility and specific heat coincide with the temperature at which these anomalies occur.As an example, we show in Fig. 3, the resistance, susceptibility and specific heat divided by temperature for GdBiPt.Tue temperature variation of x and C/T near the anomaly at 9 K is characteristic of antiferromagnetic order.to The Neei temperatures, defined by these measurements, for the RBiPt compounds are given in Table 1.Assuming a linear extrapolation of C/T from the lowest measured temperature ( 1.5 K) to T = 0 allows an estimate of tht: t:ntropy S associated with the magnetic transition.In the case of Gd, we find S -0.8R ln 8 between T = 0 and the peak in C/T, which is close to the cxpected R ln 8 entropy for ordering in the J = 7/2 multiplet.Entropy below T N for Nd, Th, and Dy (Table I ) is considerably less than the R ln(2J + 1) expected for full Hund's rule J multiplet, implicating partial lifting of the /-ground state degeneracy.Above approximately 100 K, the inverse magnetic susceptibility l /x is linear in temperature for all members of the series except Sm which probably has a nontrivial Van Vleck contribution to its suset:ptibility.From these data, values of the effective moment µetr and paramagnetic 6 may bc determined.Results are summarized in Table I where we see that µerr is very close to that expected from Hund's rules for the trivalent rare-earth ions.In alloyed crystals Ndo.sRo.sBiPt, the effective moment per mole of rare earth falls bctween that of Nd and tbe R ion.Tue negative paramagnetic 6's argue for antiferromagnetic coupling between the rare-earth ions and tend to decrease across the series.At a temperature below 100 K, which depends on the R ion, llx deviates below the high-temperature cxtrapola- 1" \ ,_:- -' --. 1 .---' ! anomalous region in R( T) and C( T).A possible interpretation of this behavior is that tbis is a manifestation of strong electronic correlatioos that produce a low-temperature heavy-electron state out of which, for example, small momcnt ordering might occur.Clearly, additional measuremeots, e.g., nuclear magnetic resonance or muoo spin resonance, are called for to test for the existence of long range, small moment ordering.If our interpretation withstands additional investigation, YbBiPt represents by far the most strongly renormalized effective mass state yet reported.
Our Observations on RBiPt compounds raise a number of unanswered questions.First, the semiconductor-like resistance of NdBiPt seems quite unusual for an intermetallic compound.Whatever mechanism is responsible for this be- havior appears to be dominated by volumetric consideratious, given the systematics of Figs. 1 and 2. Further, the 4/ electrons play no rote in this, except for their Janthanide   contraction, since YBiPt falls within the progression.Second, there is the question of the mechanism responsible for the antiferromagnetic order.With the exception of Gd-BiPt, in which crystal-field etfects are absent because L = 0 for a 4/1 configuration, T N is relatively constant for R = Ce through Dy.A Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction may be responsible for tbe ordering but it would have to be mediated by a possibly small number of conduction electrons, give~ the high resistivity of the lighter rare-earth compounds, particularly NdBiPt.(This would require a process in which the conduction band is populated by electrons excited across tbe small gap; i.e., virtual excitations.)Obviously, T N does not follow de-Oennes scaling TN = (gJ -1) 2 J(J + 1), ifHund'srules J values are used; although, crystal-field effccts may explain this discrepancy.Further, the resistive anomalies seen in several of the lighter RBiPt members at T N suggest the development of a superzone gap, implying the existence of an s-f interaction necessary for the RKKY mechanism.Finally, there is the possibility of a very beavy-electron state in YbBiPt that coexists with small-moment ordering.If this is indeed true., the systematics displayed by the RBiPt series should provide important new insight into the origin of heavy-electron behavior in this material.
Magnetism and heavy fermion-like behavior in the RBiPt series P. C. Canfield, J. D. Thompson, W. P. Beyermann, A. Lacerda, M. F. Hundley, E. Peterson, and Z. Fisk Los Alamos National Laboratory, Los Alamos, New Mexico 87545 H. R. Ott

TABU~ 1 ."
High temperature cffective moment µ,.. paramagnetic 9, Neel temperature T NI and entropy up to T N for the RBiPt series.Orow out of Pb fiux.