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Magnetic ordering of the heavy-fermion alloy CeCu5.1Ag0.9
ELSEVIER
Physica B 230-232 (1997) 253-255
Magnetic behaviour of the new intermetallic compound Ce2Pd3Ge5 B. Becker, S. Ramakrishnan 1, D. Groten, S. Sfillow, C.C. Mattheus, G.J. Nieuwenhuys*, J.A. Mydosh Kamerlingh Onnes Laboratory, Leiden University, P.O. Box 9506, 2300 RA Leiden, The Netherlands
Abstract
We have prepared a new Ce-intermetallic compound Ce2Pd3Ge5 crystallizing in the orthorhombic U2Co3Sis-structure, which is related to the CaBe2Ge2-1attice. Specific heat cp, resistivity p and magnetic susceptibility X show antiferromagnetic ordering below TN = 3.8 K. The specific heat cp, corrected for the lattice contributions, is described in terms of mean field theory. Also we derive the low-lying crystalline electric field levels. Our results are compared to those of CePd2Ge2. Keywords: Ce2Pd3Ges; Crystal electric field
During the last decade many investigations on cerium intermetallic compounds with the formula unit CeT2X2 (T = transition metal and X = Si or Ge) have been performed (for a review see Ref. [1]). In this class of compounds the competition between Kondo effect and magnetic ordering has been studied and the ordering temperatures have been described within the framework of a semi-quantitative band-structure approach [2]. Here the Ce-T atomic distance and the transition metal d-electron count determine the effective f~l electron hybridization which enabled Endstra et al. [2] to map the series of compounds onto the Kondo-lattice phase diagram. Compositional studies are a useful tool to further test this type of description. Starting from the CeTzGe2 class of compounds we can proceed to the related class of Ce2T3Ges-compounds by replacing every fourth T atom by a Ge atom. CezT3Ge5 (previously reported for T = Rh; Ir [3]) crystallizes in the orthorhombic U2C03Si5- structure which can be * Corresponding author. l Permanent address: Tata Institute of Fundamental Research, Bombay, India.
viewed at as a combination of the CaBe2Ge2 and the BaNiSn3-structures. Here we report for the first time the magnetic properties of Ce2Pd3Ges. Polycrystalline samples of nominal composition Ce2Pd3Ge5 and La2Pd3Ge5 have been fabricated by arc-melting the constituents in stoichiometric ratios on a water-cooled copper crucible (Ce: 4N, Pd: 3N, Ge: 5N and La: 3N purity). Both compounds have been annealed in high vacuum at 900°C for seven days. Analysis by X-ray and electron-probe microanalysis (EPMA) proved the samples to be of the correct composition and crystal structure and to contain less than 1% second phase. The lattice parameters for Ce2Pd3Ge5 are a--10.164 (9)A, b = 12.13 (2)/~ and c = 6.167 (3) A. La2Pd3 Ge5 cystallizes in the same structure with slightly larger lattice parameters, a = 10.19 (2)/~, b = 12.22 ( 3 ) A and c = 6.19 (3)A. The inverse susceptibility versus temperature data are shown in Fig. 1. Above 180 K the susceptibility can be described via a Curie-Weiss law with an effective moment of 2.43#B/Ce-atom (solid line) which is close to the value of the free Ce 3+ ion. The deviations from the Curie-Weiss law at low temperature
0921-4526/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved PH S0921-4526(96)00669-2
254
B. Becker et al. / Physica B 230-232 (1997.) 253-255
12
0.08 0.06
"8 6 U .~ X
A "
o.o,
400 x
300
t
"\ ,
0
,
,
5
~-
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e
6
~
4
?:... '
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1
'
0
~
200 100 0 0
100
200
300
T [K]
0
,
0
2
4
I
I
i
I
6
8
10
12
14
T [K]
Fig. 1. Inverse DC-susceptibility XD -1c versus temperature of Ce2Pd3Ge5 (o) with Curie-Weiss fit (solid line). (Inset) Low-temperature susceptibility Zoc with antiferromagnetic ordering. 20
u
Fig. 3. Magnetic contributions to the specific heat versus temperature of Ce2Pd3Ge5 (0T (o), 2 T (O), 4 T (+), 6 T (A)). (Solid line) Molecular field prediction for a S = 1 antiferromagnet in zero field.
,,
,.
o o o
,
o°
15 o
•
o
•
o
O
," s"
oS
10
i I / , S
~
o° o
,.
,,
A
5 0 0
10
20
30
T IK] Fig. 2. Specific heat versus temperature of Ce2Pd3Ges(o), La2Pd3Ge5 (dashed line) and the specific heat of Ce2Pd3Ge5 corrected for lattice contributions (A). (Solid line) Fit to the electronic and CEF contributions as described in the text.
are due to crystalline electric field effects (CEF). The inset of Fig. 1 shows a sharp kink in the susceptibility at 3.8 K revealing the antiferromagnetic character of the transition. An extrapolation of the susceptibility to 0 K gives z ( O ) / ) f ( T N ) ,,~ ] as expected for a polycrystal with conventional antiferromagnetic ordering. The specific heat data are presented in Fig. 2. An anomaly at T = 3.8 K is clearly visible. The specific heat of La2Pd3Ge5 was used to estimate the lattice contributions in Ce2Pd3Ges. By subtracting the specific heat of La2Pd3Ge5 from Ce2Pd3Ge5 we obtain
the 4f-specific heat Cu as indicated in the plot. The influence of an externally applied magnetic field on the specific heat is shown in Fig. 3. With increasing field the transition broadens, which is typical for a polycrystal with an anisotropic Zeeman splitting. The magnetic entropy has the same value at 10 K for all fields up to 6 T. The jump in zero field ACM ~ 10.8 J/CemolK is close to the theoretical value 23-Rfor a magnetic S = ½ system in a mean field approach [4]. The solid line in Fig. 3 represents the molecular field predictions. The fit describes the measurements in zero field very well. The entropy gain at 3.8 K is 0.85 R ln(2) and reaches Rln(2) at 7 K, indicating that a doublet is responsible for the magnetic ordering. The specific heat above the transition temperature can be described by using a CEF level scheme of three doublets at 0, 65 and 160 K, respectively, and this includes an electronic contribution of ~HT '~ 50 mJ/Ce mol K 2 (solid line in Fig. 2). Finally, we plot the resistivity of Ce2Pd3Ge5 and La2Pd3Ge5 in Fig. 4. The monotonic decrease of the resistivity with decreasing temperature is mainly governed by the electron-phonon scattering. The magnetic contribution to the resistivity PM of Ce2Pd3Ges, obtained by subtracting the resistivity of La2Pd3Ges, is shown in the inset of Fig. 4. The maximum around 100 K in PM is due to CEF field effects. The antiferromagnetic ordering is reflected in a sharp kink at the
255
B. Becket et al./ Physica B 230-232 (1997) 253-255
3.19A, two at dce--Pd = 3.30A, one at 3.49 A and two more at d c e - P d = 3.52 A. In CePd2Ge2 the distance of the eight nearest-neighbor Pd atoms is d c e - P d = 3.33 A. Hence, slightly less hybridization is expected and, combined with the fact that CePd2Ge2 is weakly hybridized, this agrees with the lower Trq. Systematic studies of Ce2T3Ge5 compounds are required to understand the influence of the lower crystal symmetry on the hybridization and the magnetic exchange. To conclude, we have found that the new intermetallic compound CeEPdaGe5 crystallizes in the U2Co3Sis-structure and undergoes antiferromagnetic ordering below 3.8 K. The high-temperature properties can be understood in terms of a CEF model three doublets at 0, 65 and 160 K. The ordering temperature and the CEF splitting are lower than in the related CePd2Ge2 compound. dce--Pd :
$0
dce-Pd =
E
200 o
* * * * * * * * 1o
150
T[K]
100 .
• .
* * *
* * "
100 50 0 0
100
200
300
T IK] Fig. 4. Resistivityversus temperaturefor Ce2Pd3Ge5 (.) and La2 Pd3Ge5 (solid line). (Inset) Resistivity PM = p(Ce2Pd3Ges)p (La2Pd3Ges) versus temperature. transition temperature TN = 3.8 K. In the antiferromagnetic state the resistance drops by about a factor of 4 down to 50 mK. Comparing the above results to CePd2Ge2 [5] we see a very similar behaviour for both compounds. For CePd2Ge2 antiferromagnetic ordering at 5.1 K and a CEF scheme with three doublets at 0, 110 and 220 K have been found. There are small differences between both compounds given by the lower splitting of the doublets in the CEF and the lower transition for the Ce2Pd3Ge5 compound. The unit-cell volume per Ce-atom is for Ce2PdaGe5 0.7% smaller than for CePd2Ge2. In CePd2Ge2 TN increase with increasing pressure [6]. Thus, the reduction of Trq cannot simply be explained by a pressure effect. Considering the nearest Ce-Pd distances, in Ce2Pd3Ges, each Ce is surrounded by a Pd at
This work was partially Nederlandse Stichting FOM.
supported
by
the
References
[1] G.J. Nieuwenhuys, Heavyfermionsand related compounds,in: Handbook of Magnetic Materials, Vol. 9, ed. K.H.J. Buschow (Elsevier, Amsterdam, 1995) p. 1. [2] T. Eodstra et al., Phys. Rev B 48 (1993) 9595. [3] C. Godart et al., Solid State Commun. 67 (1988) 677. [4] H.E. Stanley, Introduction to Phase Transitions and Critical Phenomena (Clarendon Press, Oxford, 1971). [5] M.J. Besnus et al., J. Magn. Magn. Mater. 104-107 (1992) 1387. [6] G. Oomi et al., these Proceedings, PhysicaB 230-232 (1997).
Physica B 230-232 (1997) 253-255
Magnetic behaviour of the new intermetallic compound Ce2Pd3Ge5 B. Becker, S. Ramakrishnan 1, D. Groten, S. Sfillow, C.C. Mattheus, G.J. Nieuwenhuys*, J.A. Mydosh Kamerlingh Onnes Laboratory, Leiden University, P.O. Box 9506, 2300 RA Leiden, The Netherlands
Abstract
We have prepared a new Ce-intermetallic compound Ce2Pd3Ge5 crystallizing in the orthorhombic U2Co3Sis-structure, which is related to the CaBe2Ge2-1attice. Specific heat cp, resistivity p and magnetic susceptibility X show antiferromagnetic ordering below TN = 3.8 K. The specific heat cp, corrected for the lattice contributions, is described in terms of mean field theory. Also we derive the low-lying crystalline electric field levels. Our results are compared to those of CePd2Ge2. Keywords: Ce2Pd3Ges; Crystal electric field
During the last decade many investigations on cerium intermetallic compounds with the formula unit CeT2X2 (T = transition metal and X = Si or Ge) have been performed (for a review see Ref. [1]). In this class of compounds the competition between Kondo effect and magnetic ordering has been studied and the ordering temperatures have been described within the framework of a semi-quantitative band-structure approach [2]. Here the Ce-T atomic distance and the transition metal d-electron count determine the effective f~l electron hybridization which enabled Endstra et al. [2] to map the series of compounds onto the Kondo-lattice phase diagram. Compositional studies are a useful tool to further test this type of description. Starting from the CeTzGe2 class of compounds we can proceed to the related class of Ce2T3Ges-compounds by replacing every fourth T atom by a Ge atom. CezT3Ge5 (previously reported for T = Rh; Ir [3]) crystallizes in the orthorhombic U2C03Si5- structure which can be * Corresponding author. l Permanent address: Tata Institute of Fundamental Research, Bombay, India.
viewed at as a combination of the CaBe2Ge2 and the BaNiSn3-structures. Here we report for the first time the magnetic properties of Ce2Pd3Ges. Polycrystalline samples of nominal composition Ce2Pd3Ge5 and La2Pd3Ge5 have been fabricated by arc-melting the constituents in stoichiometric ratios on a water-cooled copper crucible (Ce: 4N, Pd: 3N, Ge: 5N and La: 3N purity). Both compounds have been annealed in high vacuum at 900°C for seven days. Analysis by X-ray and electron-probe microanalysis (EPMA) proved the samples to be of the correct composition and crystal structure and to contain less than 1% second phase. The lattice parameters for Ce2Pd3Ge5 are a--10.164 (9)A, b = 12.13 (2)/~ and c = 6.167 (3) A. La2Pd3 Ge5 cystallizes in the same structure with slightly larger lattice parameters, a = 10.19 (2)/~, b = 12.22 ( 3 ) A and c = 6.19 (3)A. The inverse susceptibility versus temperature data are shown in Fig. 1. Above 180 K the susceptibility can be described via a Curie-Weiss law with an effective moment of 2.43#B/Ce-atom (solid line) which is close to the value of the free Ce 3+ ion. The deviations from the Curie-Weiss law at low temperature
0921-4526/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved PH S0921-4526(96)00669-2
254
B. Becker et al. / Physica B 230-232 (1997.) 253-255
12
0.08 0.06
"8 6 U .~ X
A "
o.o,
400 x
300
t
"\ ,
0
,
,
5
~-
lO
e
6
~
4
?:... '
'
1
'
0
~
200 100 0 0
100
200
300
T [K]
0
,
0
2
4
I
I
i
I
6
8
10
12
14
T [K]
Fig. 1. Inverse DC-susceptibility XD -1c versus temperature of Ce2Pd3Ge5 (o) with Curie-Weiss fit (solid line). (Inset) Low-temperature susceptibility Zoc with antiferromagnetic ordering. 20
u
Fig. 3. Magnetic contributions to the specific heat versus temperature of Ce2Pd3Ge5 (0T (o), 2 T (O), 4 T (+), 6 T (A)). (Solid line) Molecular field prediction for a S = 1 antiferromagnet in zero field.
,,
,.
o o o
,
o°
15 o
•
o
•
o
O
," s"
oS
10
i I / , S
~
o° o
,.
,,
A
5 0 0
10
20
30
T IK] Fig. 2. Specific heat versus temperature of Ce2Pd3Ges(o), La2Pd3Ge5 (dashed line) and the specific heat of Ce2Pd3Ge5 corrected for lattice contributions (A). (Solid line) Fit to the electronic and CEF contributions as described in the text.
are due to crystalline electric field effects (CEF). The inset of Fig. 1 shows a sharp kink in the susceptibility at 3.8 K revealing the antiferromagnetic character of the transition. An extrapolation of the susceptibility to 0 K gives z ( O ) / ) f ( T N ) ,,~ ] as expected for a polycrystal with conventional antiferromagnetic ordering. The specific heat data are presented in Fig. 2. An anomaly at T = 3.8 K is clearly visible. The specific heat of La2Pd3Ge5 was used to estimate the lattice contributions in Ce2Pd3Ges. By subtracting the specific heat of La2Pd3Ge5 from Ce2Pd3Ge5 we obtain
the 4f-specific heat Cu as indicated in the plot. The influence of an externally applied magnetic field on the specific heat is shown in Fig. 3. With increasing field the transition broadens, which is typical for a polycrystal with an anisotropic Zeeman splitting. The magnetic entropy has the same value at 10 K for all fields up to 6 T. The jump in zero field ACM ~ 10.8 J/CemolK is close to the theoretical value 23-Rfor a magnetic S = ½ system in a mean field approach [4]. The solid line in Fig. 3 represents the molecular field predictions. The fit describes the measurements in zero field very well. The entropy gain at 3.8 K is 0.85 R ln(2) and reaches Rln(2) at 7 K, indicating that a doublet is responsible for the magnetic ordering. The specific heat above the transition temperature can be described by using a CEF level scheme of three doublets at 0, 65 and 160 K, respectively, and this includes an electronic contribution of ~HT '~ 50 mJ/Ce mol K 2 (solid line in Fig. 2). Finally, we plot the resistivity of Ce2Pd3Ge5 and La2Pd3Ge5 in Fig. 4. The monotonic decrease of the resistivity with decreasing temperature is mainly governed by the electron-phonon scattering. The magnetic contribution to the resistivity PM of Ce2Pd3Ges, obtained by subtracting the resistivity of La2Pd3Ges, is shown in the inset of Fig. 4. The maximum around 100 K in PM is due to CEF field effects. The antiferromagnetic ordering is reflected in a sharp kink at the
255
B. Becket et al./ Physica B 230-232 (1997) 253-255
3.19A, two at dce--Pd = 3.30A, one at 3.49 A and two more at d c e - P d = 3.52 A. In CePd2Ge2 the distance of the eight nearest-neighbor Pd atoms is d c e - P d = 3.33 A. Hence, slightly less hybridization is expected and, combined with the fact that CePd2Ge2 is weakly hybridized, this agrees with the lower Trq. Systematic studies of Ce2T3Ge5 compounds are required to understand the influence of the lower crystal symmetry on the hybridization and the magnetic exchange. To conclude, we have found that the new intermetallic compound CeEPdaGe5 crystallizes in the U2Co3Sis-structure and undergoes antiferromagnetic ordering below 3.8 K. The high-temperature properties can be understood in terms of a CEF model three doublets at 0, 65 and 160 K. The ordering temperature and the CEF splitting are lower than in the related CePd2Ge2 compound. dce--Pd :
$0
dce-Pd =
E
200 o
* * * * * * * * 1o
150
T[K]
100 .
• .
* * *
* * "
100 50 0 0
100
200
300
T IK] Fig. 4. Resistivityversus temperaturefor Ce2Pd3Ge5 (.) and La2 Pd3Ge5 (solid line). (Inset) Resistivity PM = p(Ce2Pd3Ges)p (La2Pd3Ges) versus temperature. transition temperature TN = 3.8 K. In the antiferromagnetic state the resistance drops by about a factor of 4 down to 50 mK. Comparing the above results to CePd2Ge2 [5] we see a very similar behaviour for both compounds. For CePd2Ge2 antiferromagnetic ordering at 5.1 K and a CEF scheme with three doublets at 0, 110 and 220 K have been found. There are small differences between both compounds given by the lower splitting of the doublets in the CEF and the lower transition for the Ce2Pd3Ge5 compound. The unit-cell volume per Ce-atom is for Ce2PdaGe5 0.7% smaller than for CePd2Ge2. In CePd2Ge2 TN increase with increasing pressure [6]. Thus, the reduction of Trq cannot simply be explained by a pressure effect. Considering the nearest Ce-Pd distances, in Ce2Pd3Ges, each Ce is surrounded by a Pd at
This work was partially Nederlandse Stichting FOM.
supported
by
the
References
[1] G.J. Nieuwenhuys, Heavyfermionsand related compounds,in: Handbook of Magnetic Materials, Vol. 9, ed. K.H.J. Buschow (Elsevier, Amsterdam, 1995) p. 1. [2] T. Eodstra et al., Phys. Rev B 48 (1993) 9595. [3] C. Godart et al., Solid State Commun. 67 (1988) 677. [4] H.E. Stanley, Introduction to Phase Transitions and Critical Phenomena (Clarendon Press, Oxford, 1971). [5] M.J. Besnus et al., J. Magn. Magn. Mater. 104-107 (1992) 1387. [6] G. Oomi et al., these Proceedings, PhysicaB 230-232 (1997).