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Magnetization and specific heat of CeCu6−xMx (MNi, Pd, Pt)
Physica B 223&224 (1996) 325-328
ELSEVIER
Magnetization and specific heat of CeCu6-xMx (M = Ni, Pd, Pt) M. Sieck, C. Speck, M. Waffenschmidt, S. Mock, H.v. Lrhneysen* Physikalisches Institut, Universitiit Karlsruhe, D-76128 Karlsruhe, Germany
Abstract Polycrystalline alloys of CeCu6 with Ni, Pd, and Pt have been studied with magnetization (M) and specific-heat (C) measurements in the concentration range where they still retain the orthorhombic structure. Sharp maxima in M ( T ) and C(T) for Pd alloys with x > 0.07 indicate antiferromagnetic order. At the critical concentration where magnetic order sets in, C varies as C / T ~ - ln(T/To) and M shows a cusp for T --* 0, indicative of non-Fermi-liquid behavior. Similar features are found for Pt alloys. Ni alloys, on the other hand, evolve towards intermediate valence behavior with increasing x. These differences are discussed in terms of changes of volume and band structure upon alloying.
1. Introduction The nonmagnetic heavy-fermion system Cefu6 is at the onset of long-range antiferromagnetic order, as evidenced by the existence of short-range antiferromagnetic correlations [1] and also by the observation of longrange magnetic order upon alloying with Au [2-4] or Ag [2, 5]. Other alloying experiments with CeCu6 have also been reported [6]. The onset of magnetic order in Cefu6-xAux which up to now is the best-studied system based on CeCu6, has been interpreted in terms of a weakening of the conduction-electron-4f-electron exchange interaction J because of the lattice expansion upon alloying [7], thereby weakening the Kondo effect and stabilizing the 4f magnetic moments. Here we report on a systematic study of CeCu6-xMx with M = Ni, Pd, Pt to investigate further the interplay between on-site Kondo compensation and intersite magnetic interactions.
2. Experimental Polycrystalline samples were arc-melted under highpurity argon from appropriate amounts of ingots (Ce 4N, * Corresponding author.
Cu 5N, Ni 4N, Pd 4N, Pt 4N). The samples were annealed at 700 or 750°C for 14 days. X-ray diffraction showed the single-phase orthorhombic Pnma structure of C e f u 6 up to the concentrations xs = 0.2, 0.4, and 0.2 for M = Ni, Pd, and Pt, respectively. Beyond these concentrations, a hexagonal phase develops whose volume fraction quickly grows with x > xs. While the alloys with Pd and Pt expand upon alloying, alloys with Ni have a smaller molar volume than CeCu6 does. For M = Au and Pd, a and c increase and b decreases with increasing x (for Au alloys this could be observed up to x ~ 1 [4]). For the Pt alloys the same trend was found but the number of samples prepared were too few to arrive at a definite conclusion for the b-axis behavior. The data are compatible with a linear volume change A V / V o ~ x for Pd and Pt alloys where Vo is the molar volume of C e f u 6 , although the scatter of the data precludes a definite statement. We obtain AV/Vo = 0.016x for Pd and 0.021x for Pt, compared to 0.035x for Au [4]. The DC magnetization and specific heat have been measured as described elsewhere I-4]. F o r the magnetization measurements, the samples were powdered to obtain a correct average over crystal orientations. This is important because of the strong magnetic anisotropy of, e.g., Cefu6-xAux [4].
0921-4526/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved PII S 0 9 2 1 - 4 5 2 6 ( 9 6 ) 0 0 1 1 3-5
M. Sieck et al. / Physica B 223&224 (1996) 325 328
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T (K) Fig. 1. Magnetization M divided by applied magnetic field B = 0.1 T for polycrystalline CeCu6-xPdx alloys as a function of temperature T, from bottom to top x = 0, 0.05, 0.1, 0.15, 0.2, 0.3, 0.4. Inset shows the N6el temperature TN(x) as determined from the maximum of d(M. T)/dT (circles and solid line). Dashed line shows TN(X) for CeCu6 ~Aux,
Fig. 2. Specific heat C plotted as C/T versus log. T for different CeCu6 ~Pdx alloys.
3
CeCu6 xPtx
3. Results and discussion
We first focus on the data for CeCu6-xPd~ which in the low-x range has been investigated most intensively. Fig. 1 shows M/B measured in a magnetic field B = 0.1 T versus temperature T. The "susceptibility" M/B rises towards low T with an increasing slope as x is increased from x = 0. For x >/0.10 maxima in M/B are clearly visible. As the inset of Fig. 1 reveals, the temperature TN of the maximum varies non-linearly with x. Included in the inset are two data points for the AC susceptibility which showed maxima in XAc(T) at about the same temperature. Fig. 2 shows the specific heat C for several CeCu6-xPdx alloys plotted as C/T versus log T. The agreement of the specific-heat maximum (visible in C/T as the sharp kink) with the maximum in M ( T ) suggests the onset of long-range antiferromagnetic order as previously found for other CeCu6 alloys, notably with Au where also neutron-scattering results are available [8]. Fig. 2 further shows that samples at the onset of antiferromagnetic order, around x ~ 0.05, show nonFermi-liquid behavior, i.e. C/T ~ ln(T/To). This was
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Fig. 3. Specific heat C plotted as C/T versus log T for different CeCu6 xPt~ alloys.
M. Sieck et al. / Physica B 223&224 (1996) 325-328
previously shown to occur for C e C u 6 - x A u x for xc -~ 0.1 [9]. However, even the sample with x = 0.05 for which no magnetic transition was found in XACor M/B down to 40 mK shows definite positive deviations from C/T ~ - ln(T/To) below ~ 0.1 K. This might be due to magnetic ordering at still lower T as inferred by comparison with the C/T data for x = 0.1. Comparing all these data with CeCur_xAu:,, we find overall similarity between the two systems. Details, of course, would have to be investigated in a study of single crystals as has been done for CeCu6_xAux. We note, however, that a simple volume expansion which is the primary driving force for the occurrence of magnetic order in C e C u 6 - x A u x alloys [7] is not sufficient to explain magnetic ordering in the Pd alloys: while the nonmagnetic Kondo singlet ground state of C e f u 6 is actually destabilized faster upon alloying with Pd than with Au (cf. inset in Fig. 1) the volume expands more slowly. By a simple electron valency argument, one might expect that the Ce 5d electrons hybridize strongly with the Pd 4d states, thus effectively filling the Pd 4d orbitals which would lead to a reduction of the density of states at the Fermi level N(EF) and hence to a decrease of J and a concomitant stabilization of magnetic moments. The Pt alloy with x = 0.15 shows a shoulder around 0.25 K which indicates some sort of magnetic order (see
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T (K) Fig. 4. M/B measured in B = 0.1 T for several CeCu6_xM x alloys in the vicinity of magnetic ordering. Data are offset vertically by 0.I/~B/T each for clarity. Inset shows M/B versus T °5. Symbols are the same as in Fig. 5.
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T (K) Fig. 5. C/T versus log T for several CeCu6_xM~, alloys in the vicinity of magnetic ordering. Data are offset vertically by 0.5 J/mol K z each for clarity.
Fig. 3). For x = 0.2 maxima in ~AC and C around 1 K were observed (not shown). Here the influence of the minority phase has yet to be elucidated. Nevertheless, an alloy with x = 0.1 shows a nice - I n ( T / T o ) behavior in C/T. C/T for x = 0.05 approaches the Fermi-liquid behavior of pure C e f u 6. In Fig. 4 we show M/B measured in B = 0.1 T for all (polycrystalline) C e C u 6 - x Mx alloys we have investigated so far in the vicinity of the critical concentration of the onset of magnetic order. For comparison, we include Ni data with x = 0.12 and data for pure CeCu6. Fig. 5 shows C/T for the same alloys. Clearly, both C and M data for the different alloys fall in two categories which will be discussed in turn. (i) The Ni data are close to those for pure CeCu6 indicating that the nonmagnetic ground state is stabilized by the addition of Ni. This is in keeping with the volume dependence where the volume actually shrinks upon alloying with Ni, thereby increasing the Kondo compensation via an increase of J. For completeness, we mention that the hexagonal structure found upon further alloying is likely to be the CeCusderived Ce2(Cu, Ni)12 structure with some excess of Cu or Ni with (Cu, Ni) pairs on a Ce site. For these alloys, nominally CeCu6-xNix with x/> 0.18, we found indications of antiferromagnetic or spin-glass like ordering as evidenced by maxima in ZAc(T) and broad specific heat
328
M. Sieck et al. / Physica B 223&224 (1996) 325-328
maxima in the same temperature region (not shown). This is not surprising since CeCu5 is known to order antiferromagnetically [10]. Further increase of x towards x = 5 renders the crystals intermediate valent, again similar to the related compound CeNi~ [10]. (ii) The data for Au (x = 0.1), Pd (x = 0.05), and Pt (x = 0.1) alloys all show non-Fermi-liquid behavior. Note that the specificheat measurements for the Au alloy have been extended down to 60 m K and show a perfect C / T ~ - l n ( T / T o ) behavior over nearly two decades in T as will be discussed elsewhere [11]. Of course, this suggests that the characteristic temperature scale To is equal. We obtain To = 6.16, 6.39, and 5.5 K for the Au, Pd, and Pt alloy, respectively. While the microscopic origin of To is not clear, the values are close to the single-ion Kondo temperatures in these alloys, as inferred for CeCu6 and CeCu6-xAux from the specific heat [11]. It is interesting to note that at the critical concentration for the onset of antiferromagnetism the C / T ~ - l n ( T / T o ) behavior as observed down to ~ 60 m K is nearly identical for the three different alloys. Again, the slight positive deviations below that temperature may be due to an onset of magnetic ordering below ~ 40 mK. The low-field magnetization for the Pt x = 0.1 alloy shows a (1 - ~ x / T ) dependence as found for CeCus.9Auo. 1 [9], see inset of Fig. 4. (Note that the present data are for polycrystals. Apparently, the large easy-axis contribution dominates the weak and almost T-independent a- and b-axis contributions.) For the Pd x = 0.05 alloy, a simple algebraic T dependence is not observed. In conclusion, CeCu6-xMx alloys with M = Pd and Pt showing magnetic order above a concentration threshold xc also exhibit non-Fermi-liquid behavior for
x ~ xc, as previously found for M = Au. O n the other hand, alloys with M = Ni do not exhibit magnetic order in the orthorhombic phase. These differences can only partly be understood in terms of simple volume effects. Hybridization of the M d states with the Ce 3d states might play an equally or even more important role.
Acknowledgements We thank W. Kuhs for his help with the structure analysis of the CeCu6-xNix alloys. This work was supported by the Deutsche Forschungsgemeinschaft.
References [1] G. Aeppli et al., Phys. Rev. Lett. 57 (1986) 122; J. RossatMignod et al., J. Magn. Magn. Mater. 76-77 (1988) 376. I-2] A. Germann et al., J. de Phys. 49 (1988) C8-755. [3] M.R. Lees et al., J. Phys.: Condens. Matter. 2 (1990) 6403. 1-4] H.G. Schlager et al., J. Low Temp. Phys. 90 (1993) 181. [5] A.K. Ganghopadhyay et al., Phys. Rev. B. 38 (1988) 2603; G. Fraunberger et al., Phys. Rev. B 40 (1989) 4735. 1-6] M.R. Lees and R.B. Coles, J. Magn. Magn. Matter. 76 & 77 (1988) 173; M.R. Lees et al., J. Phys.: Condens. Matter. 2 (1990) 4773. 1-7] A. Germann and H.v. L6hneysen, Europhys. Lett. 9 (1989) 367. 1-8] A. Schr6der et al., Physica B 199 & 200 (1994) 47. 1-9] H.v. L6hneysen et al., Phys. Rev. Lett. 72 (1994) 3262. [10] N.B. Brandt et al., Soy. Phys. Solid State 26 (1984) 1279. [11] H.v. L6hneysen et al., Physica B 223&224 (1996) 471.
ELSEVIER
Magnetization and specific heat of CeCu6-xMx (M = Ni, Pd, Pt) M. Sieck, C. Speck, M. Waffenschmidt, S. Mock, H.v. Lrhneysen* Physikalisches Institut, Universitiit Karlsruhe, D-76128 Karlsruhe, Germany
Abstract Polycrystalline alloys of CeCu6 with Ni, Pd, and Pt have been studied with magnetization (M) and specific-heat (C) measurements in the concentration range where they still retain the orthorhombic structure. Sharp maxima in M ( T ) and C(T) for Pd alloys with x > 0.07 indicate antiferromagnetic order. At the critical concentration where magnetic order sets in, C varies as C / T ~ - ln(T/To) and M shows a cusp for T --* 0, indicative of non-Fermi-liquid behavior. Similar features are found for Pt alloys. Ni alloys, on the other hand, evolve towards intermediate valence behavior with increasing x. These differences are discussed in terms of changes of volume and band structure upon alloying.
1. Introduction The nonmagnetic heavy-fermion system Cefu6 is at the onset of long-range antiferromagnetic order, as evidenced by the existence of short-range antiferromagnetic correlations [1] and also by the observation of longrange magnetic order upon alloying with Au [2-4] or Ag [2, 5]. Other alloying experiments with CeCu6 have also been reported [6]. The onset of magnetic order in Cefu6-xAux which up to now is the best-studied system based on CeCu6, has been interpreted in terms of a weakening of the conduction-electron-4f-electron exchange interaction J because of the lattice expansion upon alloying [7], thereby weakening the Kondo effect and stabilizing the 4f magnetic moments. Here we report on a systematic study of CeCu6-xMx with M = Ni, Pd, Pt to investigate further the interplay between on-site Kondo compensation and intersite magnetic interactions.
2. Experimental Polycrystalline samples were arc-melted under highpurity argon from appropriate amounts of ingots (Ce 4N, * Corresponding author.
Cu 5N, Ni 4N, Pd 4N, Pt 4N). The samples were annealed at 700 or 750°C for 14 days. X-ray diffraction showed the single-phase orthorhombic Pnma structure of C e f u 6 up to the concentrations xs = 0.2, 0.4, and 0.2 for M = Ni, Pd, and Pt, respectively. Beyond these concentrations, a hexagonal phase develops whose volume fraction quickly grows with x > xs. While the alloys with Pd and Pt expand upon alloying, alloys with Ni have a smaller molar volume than CeCu6 does. For M = Au and Pd, a and c increase and b decreases with increasing x (for Au alloys this could be observed up to x ~ 1 [4]). For the Pt alloys the same trend was found but the number of samples prepared were too few to arrive at a definite conclusion for the b-axis behavior. The data are compatible with a linear volume change A V / V o ~ x for Pd and Pt alloys where Vo is the molar volume of C e f u 6 , although the scatter of the data precludes a definite statement. We obtain AV/Vo = 0.016x for Pd and 0.021x for Pt, compared to 0.035x for Au [4]. The DC magnetization and specific heat have been measured as described elsewhere I-4]. F o r the magnetization measurements, the samples were powdered to obtain a correct average over crystal orientations. This is important because of the strong magnetic anisotropy of, e.g., Cefu6-xAux [4].
0921-4526/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved PII S 0 9 2 1 - 4 5 2 6 ( 9 6 ) 0 0 1 1 3-5
M. Sieck et al. / Physica B 223&224 (1996) 325 328
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T (K) Fig. 1. Magnetization M divided by applied magnetic field B = 0.1 T for polycrystalline CeCu6-xPdx alloys as a function of temperature T, from bottom to top x = 0, 0.05, 0.1, 0.15, 0.2, 0.3, 0.4. Inset shows the N6el temperature TN(x) as determined from the maximum of d(M. T)/dT (circles and solid line). Dashed line shows TN(X) for CeCu6 ~Aux,
Fig. 2. Specific heat C plotted as C/T versus log. T for different CeCu6 ~Pdx alloys.
3
CeCu6 xPtx
3. Results and discussion
We first focus on the data for CeCu6-xPd~ which in the low-x range has been investigated most intensively. Fig. 1 shows M/B measured in a magnetic field B = 0.1 T versus temperature T. The "susceptibility" M/B rises towards low T with an increasing slope as x is increased from x = 0. For x >/0.10 maxima in M/B are clearly visible. As the inset of Fig. 1 reveals, the temperature TN of the maximum varies non-linearly with x. Included in the inset are two data points for the AC susceptibility which showed maxima in XAc(T) at about the same temperature. Fig. 2 shows the specific heat C for several CeCu6-xPdx alloys plotted as C/T versus log T. The agreement of the specific-heat maximum (visible in C/T as the sharp kink) with the maximum in M ( T ) suggests the onset of long-range antiferromagnetic order as previously found for other CeCu6 alloys, notably with Au where also neutron-scattering results are available [8]. Fig. 2 further shows that samples at the onset of antiferromagnetic order, around x ~ 0.05, show nonFermi-liquid behavior, i.e. C/T ~ ln(T/To). This was
• x = 0.15 o x=0.1 • x = 0.05 ox=O
---2 Y o E
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1 T
4
(K)
Fig. 3. Specific heat C plotted as C/T versus log T for different CeCu6 xPt~ alloys.
M. Sieck et al. / Physica B 223&224 (1996) 325-328
previously shown to occur for C e C u 6 - x A u x for xc -~ 0.1 [9]. However, even the sample with x = 0.05 for which no magnetic transition was found in XACor M/B down to 40 mK shows definite positive deviations from C/T ~ - ln(T/To) below ~ 0.1 K. This might be due to magnetic ordering at still lower T as inferred by comparison with the C/T data for x = 0.1. Comparing all these data with CeCur_xAu:,, we find overall similarity between the two systems. Details, of course, would have to be investigated in a study of single crystals as has been done for CeCu6_xAux. We note, however, that a simple volume expansion which is the primary driving force for the occurrence of magnetic order in C e C u 6 - x A u x alloys [7] is not sufficient to explain magnetic ordering in the Pd alloys: while the nonmagnetic Kondo singlet ground state of C e f u 6 is actually destabilized faster upon alloying with Pd than with Au (cf. inset in Fig. 1) the volume expands more slowly. By a simple electron valency argument, one might expect that the Ce 5d electrons hybridize strongly with the Pd 4d states, thus effectively filling the Pd 4d orbitals which would lead to a reduction of the density of states at the Fermi level N(EF) and hence to a decrease of J and a concomitant stabilization of magnetic moments. The Pt alloy with x = 0.15 shows a shoulder around 0.25 K which indicates some sort of magnetic order (see
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T (K) Fig. 4. M/B measured in B = 0.1 T for several CeCu6_xM x alloys in the vicinity of magnetic ordering. Data are offset vertically by 0.I/~B/T each for clarity. Inset shows M/B versus T °5. Symbols are the same as in Fig. 5.
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T (K) Fig. 5. C/T versus log T for several CeCu6_xM~, alloys in the vicinity of magnetic ordering. Data are offset vertically by 0.5 J/mol K z each for clarity.
Fig. 3). For x = 0.2 maxima in ~AC and C around 1 K were observed (not shown). Here the influence of the minority phase has yet to be elucidated. Nevertheless, an alloy with x = 0.1 shows a nice - I n ( T / T o ) behavior in C/T. C/T for x = 0.05 approaches the Fermi-liquid behavior of pure C e f u 6. In Fig. 4 we show M/B measured in B = 0.1 T for all (polycrystalline) C e C u 6 - x Mx alloys we have investigated so far in the vicinity of the critical concentration of the onset of magnetic order. For comparison, we include Ni data with x = 0.12 and data for pure CeCu6. Fig. 5 shows C/T for the same alloys. Clearly, both C and M data for the different alloys fall in two categories which will be discussed in turn. (i) The Ni data are close to those for pure CeCu6 indicating that the nonmagnetic ground state is stabilized by the addition of Ni. This is in keeping with the volume dependence where the volume actually shrinks upon alloying with Ni, thereby increasing the Kondo compensation via an increase of J. For completeness, we mention that the hexagonal structure found upon further alloying is likely to be the CeCusderived Ce2(Cu, Ni)12 structure with some excess of Cu or Ni with (Cu, Ni) pairs on a Ce site. For these alloys, nominally CeCu6-xNix with x/> 0.18, we found indications of antiferromagnetic or spin-glass like ordering as evidenced by maxima in ZAc(T) and broad specific heat
328
M. Sieck et al. / Physica B 223&224 (1996) 325-328
maxima in the same temperature region (not shown). This is not surprising since CeCu5 is known to order antiferromagnetically [10]. Further increase of x towards x = 5 renders the crystals intermediate valent, again similar to the related compound CeNi~ [10]. (ii) The data for Au (x = 0.1), Pd (x = 0.05), and Pt (x = 0.1) alloys all show non-Fermi-liquid behavior. Note that the specificheat measurements for the Au alloy have been extended down to 60 m K and show a perfect C / T ~ - l n ( T / T o ) behavior over nearly two decades in T as will be discussed elsewhere [11]. Of course, this suggests that the characteristic temperature scale To is equal. We obtain To = 6.16, 6.39, and 5.5 K for the Au, Pd, and Pt alloy, respectively. While the microscopic origin of To is not clear, the values are close to the single-ion Kondo temperatures in these alloys, as inferred for CeCu6 and CeCu6-xAux from the specific heat [11]. It is interesting to note that at the critical concentration for the onset of antiferromagnetism the C / T ~ - l n ( T / T o ) behavior as observed down to ~ 60 m K is nearly identical for the three different alloys. Again, the slight positive deviations below that temperature may be due to an onset of magnetic ordering below ~ 40 mK. The low-field magnetization for the Pt x = 0.1 alloy shows a (1 - ~ x / T ) dependence as found for CeCus.9Auo. 1 [9], see inset of Fig. 4. (Note that the present data are for polycrystals. Apparently, the large easy-axis contribution dominates the weak and almost T-independent a- and b-axis contributions.) For the Pd x = 0.05 alloy, a simple algebraic T dependence is not observed. In conclusion, CeCu6-xMx alloys with M = Pd and Pt showing magnetic order above a concentration threshold xc also exhibit non-Fermi-liquid behavior for
x ~ xc, as previously found for M = Au. O n the other hand, alloys with M = Ni do not exhibit magnetic order in the orthorhombic phase. These differences can only partly be understood in terms of simple volume effects. Hybridization of the M d states with the Ce 3d states might play an equally or even more important role.
Acknowledgements We thank W. Kuhs for his help with the structure analysis of the CeCu6-xNix alloys. This work was supported by the Deutsche Forschungsgemeinschaft.
References [1] G. Aeppli et al., Phys. Rev. Lett. 57 (1986) 122; J. RossatMignod et al., J. Magn. Magn. Mater. 76-77 (1988) 376. I-2] A. Germann et al., J. de Phys. 49 (1988) C8-755. [3] M.R. Lees et al., J. Phys.: Condens. Matter. 2 (1990) 6403. 1-4] H.G. Schlager et al., J. Low Temp. Phys. 90 (1993) 181. [5] A.K. Ganghopadhyay et al., Phys. Rev. B. 38 (1988) 2603; G. Fraunberger et al., Phys. Rev. B 40 (1989) 4735. 1-6] M.R. Lees and R.B. Coles, J. Magn. Magn. Matter. 76 & 77 (1988) 173; M.R. Lees et al., J. Phys.: Condens. Matter. 2 (1990) 4773. 1-7] A. Germann and H.v. L6hneysen, Europhys. Lett. 9 (1989) 367. 1-8] A. Schr6der et al., Physica B 199 & 200 (1994) 47. 1-9] H.v. L6hneysen et al., Phys. Rev. Lett. 72 (1994) 3262. [10] N.B. Brandt et al., Soy. Phys. Solid State 26 (1984) 1279. [11] H.v. L6hneysen et al., Physica B 223&224 (1996) 471.