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Magnetic properties of cerium ternary stannides and antimonides: frustration, correlations and short-range order
Journal of Magnetism and Magnetic Materials 140-144 (1994) 891-892
8m
Ja JR
journal of magnetism and magnetic materials
ELSEVIER
Magnetic properties of cerium ternary stannides and antimonides: frustration, correlations and short-range order J. Pierre a, K. Kaczmarska b, R.V. Skolozdra c, A. Slebarski b a C.N.R.S., Lab. Louis N~el, 166X, 38042 Grenoble, France b Uniwersytet Slaski, Instytut Fizyki, 40007 Katowice, Poland c L Franko University, Dept. oflnorg. Chemistry, 290005 Lviv, Ukraine
Abstract The frustration of magnetic interactions may lower the N6el temperature of Kondo lattices, with the onset of short-range correlations above TN. Examples are given for ternary Ce stannides and antimonides, using data obtained from neutron diffraction, susceptibility, heat capacity and resistivity measurements.
Numerous studies have been devoted to the study of ternary silicides and antimonides of cerium and transition metals (M), with a special interest in 1 : 2 : 2 compounds. Kondo-like to intermediate valence behaviour was observed. More recently, similar stannides and antimonides have been studied [1-4]. We describe in this paper the physical properties of new compounds, and compare them with previous results on other 1 : 2 : 2 compounds. Shortrange order above N6el temperature (T N) is sometimes associated with frustration of magnetic interactions. The studied compounds are listed in Table 1. Most 1 : 2 : 2 compounds crystallize with the CaBeaGe 2 structure, or a slightly distorted structure for CeNi2Sn 2 [5]. Some are not stoichiometric: a continuous solid solution exists between CeCu2Sb 2 and CeCuSb 2 (defect structure HfCuSi 2) [4]. All these compounds show a Kondo-like resistivity. Some magnetic properties were described previously [1-5]. Ordering phenomena occur at low temperature, the N6el temperature corresponds to small anomalies in susceptibility and resistivity; a low TN/TI,: ratio, short-range order a n d / o r a progressive onset of correlations give rise to broad specific heat anomalies. At present, only the magnetic structure for CeNi2Sn 2 is known [5]; it is derived from a propagation vector (½, 7, 0), the moments lying along the a-axis of the structure with a magnetic moment of 0.9/x B at 1.5 K. An important feature of this structure is frustration in the body-centered Ce tetragonal lattice: antiferromagnetism occurs in Ce planes perpendicular to c, due to interactions between nearest Ce neighbours (d = 4.4 A). No dipolar coupling occurs between two successive Ce planes perpendicular to c and couplin~ along c only occurs between planes distant by about 10 A, which are separated by 7 atomic layers.
Thus interactions along this axis are probably much smaller, giving the interactions a quasi-bidimensional character. Similar structures have been encountered in some other 1:2:2 compounds, like CePd2Si2, CeRh2Si2, or U(Ru 1 xRhx)2Si2 [6,7]. In this last case, several propaga1 qz) have been simultanetion vectors of the form (½, 7, ously observed in one sample for a composition near x = 0.3 [7], which was explained by frustration and chemical disorder. Frustration in the body-centered tetragonal lattice has been described much earlier. Its main consequences are the reduction of the long-range ordering temperature which for Ce compounds will lead to a further reduction of the ordered moment, and the occurrence of short-range order above T N. A much longer correlation length is expected within the basal plane than along the c-axis. Such an anisotropy of the correlation length was found in URu0.94Rho.06Si2, which does not exhibit any long-range order [6]. It is more difficult to give evidence for the reduction of the N6el temperature, but for isomorphous Gd compounds GdNi2Sb 2 and GdCu2Sb2, the T N / I Tp I ratio of N6el to Curie-Weiss temperature is very low, which may be in-
Table 1 Properties of studied compounds Compound
Structure
Tp (K)
TN(K)
CeNi2 Sn 2 CeNi2 Sb 2 CeCuzSn 2 CeCuzSb 2 CeCu S b 2
LaPt2Ge 2 CaBe2Ge 2 CaBezGe 2 CaBe2Ge 2
- 9 - 28 - 12 - 7 - 11
2.2 1.1 2.1 5.0 6.5
0304-8853/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 3 0 4 - 8 8 5 3 ( 9 4 ) 0 0 9 8 7 - 2
HfCu Si 2
892
J. Pierre et al. /Journal of Magnetism and Magnetic Materials 140-144 (1995) 891-892
dicative of this phenomenon. We shall see in the following that short-range order occurs in some of these systems. Fig. 1 represents the reciprocal susceptibility of CeCuSb 2 and CeCu2Sb 2. Tp is obtained from a CurieWeiss fit, TN from the behaviour of magnetic isotherms and resistivity. For CeCuSb2, the magnitudes of T N and Tp are larger than for the second compound, which may be explained by larger interactions. The main feature in the first compound is that the reciprocal susceptibility deviates from a Curie-Weiss law below 25 K, contrary to that of dilute solid solutions where Ce is diluted by La. Thus short-range order arises between T N = 7 and 25 K, with influence on the resistivity (see below). The specific heat Cp was measured for some stannides [1,2], more recently for CeNizSb 2 [8]. Magnetic ordering is often accompanied by broad specific heat bumps instead of sharp lambda-type anomalies. In the case of CeNi2Sn 2, cp [1,2] has a peak value reduced to about 6 J / m o l - K , instead of 12.5 for a doublet in the ionic model. This can be explained by a value of TN/TI,: close to 1, according to Besnus et al. [9]. Above T N, Cp may be accounted for by a Kondo term with T K = 3.4 K, only a small contribution due to short-range correlations is obtained. For CeNieSb 2, Cava et al. [8] observed a broad bump with c / T reaching 2.3 J / m o l . K 2 at 1.1 K. Between 2 and 6 K, the specific heat may be explained by a Kondo anomaly on the ground state doublet with TI< = 1.6 K. Below 2 K, the peak value and the shape of the anomaly can be explained by a progressive onset of AF correlations, giving a magnetic splitting of about 0.8 K at 0 K which vanishes above 2.5 K. A N6el temperature of 2.5 K was indeed attributed to this compound in Ref. [3], on the basis of susceptibility and resistivity anomalies: this temperature is only that where magnetic correlations begin. The resistivity of some Ce stannides and antimonides was previously studied, together with that of La compounds [1,3,4]. The temperature dependence cannot be gO
~~ e
80
+ *
BO 50 70
o~ CeCu2Sb2
%
CeCu Sb2 L a . 4 C e . SCuS~2
/ /
-
2o 1o o
h
i
I0
20
i
30
I
40 Temperature
E
50
60
(K)
Fig. 1. Reciprocal susceptibility for CeCuSb z, La0.4Ce0.6CuSb2 and CeCuzSb 2.
50
m u
!
~ /o
45
-+- CeCu Sb2 +++ CeCu Sb2 -o- CeCu2Sb2 2
0T 5 T 0 T 5 T
<--.
40 0
35
/
30
2
5
~0°
1
~
1
5
1
1
tO1
1
1
1
5
tO2
Temperature
5 (K)
103
80
Fig. 2. Resistivity under 0 and 5 T for CeCu2Sb 2 and CeCuSb 2.
properly described by the classical Cornut-Coqblin theory, as the ratio of T K (HT) for 6-fold degeneracy to the overall crystal field splitting is not small. Coherence between 4f wave functions contributes to the decrease of resistivity at temperatures lower than the lowest crystal field splitting. We compare in Fig. 2 the resistivity for CeCu2Sb 2 and CeCuSb 2 under 0 and 5 T: the crystal field splittings are smaller for CeCuSb2, the magnetoresistance is much larger for CeCuSb 2, becomes significant below 25 K, where magnetic correlations begin to set it and the large resistivity anomaly is washed out by a field of 5 T. In conclusion, magnetic frustation may lower appreciably the ordering temperature in these compounds, leaving short-range order above T N. All physical data show the influence of short-range correlations. References
[1] T. Takabatake, F. Teshima, H. Fujii, S. Nishigori, T. Suzuki T. Fujita, Y. Yamaguchi and J. Sakurai, J. Magn. Magn. Mater. 90-91 (1990) 474. [2] W.P. Beyermann, M.F. Hundley, P. Canfield, J.D. Thompson, M. Latroche, C. Godart, M. Selsane, Z. Fisk and J.L. Smith, Phys. Rev. B 43 (1991) 13130. [3] K. Kaczmarska, J. Pierre, A. Slebarski and R.V. Skolozdra, J. Alloys Compounds 196 (1993) 165. [4] RN. Skolozdra, Ja. F. Mikhalski, K. Kaczmarska and J. Pierre, J. Alloys Compounds 206 (1994) 141. [5] J. Pierre, B. Lambert-Andron, R.V. Skolozdra, J. RodriguezCarvajal and K. Kaczmarska, Physica B 202 (1994) 143. [6] P. Buffet, F. Bourdarot, S. Quezel, J. Rossat-Mignot, P. Lejay, B. Chevalier and H. Hickey, J. Magn. Magn. Mater. 108 (1992) 202. [7] S. Kawarazaki, Y. Kobashi, T. Taniguchi, Y. Miyako and H. Amitsuka, J. Phys. Soc. Japan 63 (1994) 716. [8] R.J. Cava, A.P. Ramirez, H. Takagi, J. Krajewski and W.F. Peck, J. Magn. Magn. Mater. 128 (1993) 124. [9] M.J. Besnus, A. Braghta, N. Hamdaoui and A. Meyer, J. Magn. Magn. Mater. 104-107 (1992) 1385.
8m
Ja JR
journal of magnetism and magnetic materials
ELSEVIER
Magnetic properties of cerium ternary stannides and antimonides: frustration, correlations and short-range order J. Pierre a, K. Kaczmarska b, R.V. Skolozdra c, A. Slebarski b a C.N.R.S., Lab. Louis N~el, 166X, 38042 Grenoble, France b Uniwersytet Slaski, Instytut Fizyki, 40007 Katowice, Poland c L Franko University, Dept. oflnorg. Chemistry, 290005 Lviv, Ukraine
Abstract The frustration of magnetic interactions may lower the N6el temperature of Kondo lattices, with the onset of short-range correlations above TN. Examples are given for ternary Ce stannides and antimonides, using data obtained from neutron diffraction, susceptibility, heat capacity and resistivity measurements.
Numerous studies have been devoted to the study of ternary silicides and antimonides of cerium and transition metals (M), with a special interest in 1 : 2 : 2 compounds. Kondo-like to intermediate valence behaviour was observed. More recently, similar stannides and antimonides have been studied [1-4]. We describe in this paper the physical properties of new compounds, and compare them with previous results on other 1 : 2 : 2 compounds. Shortrange order above N6el temperature (T N) is sometimes associated with frustration of magnetic interactions. The studied compounds are listed in Table 1. Most 1 : 2 : 2 compounds crystallize with the CaBeaGe 2 structure, or a slightly distorted structure for CeNi2Sn 2 [5]. Some are not stoichiometric: a continuous solid solution exists between CeCu2Sb 2 and CeCuSb 2 (defect structure HfCuSi 2) [4]. All these compounds show a Kondo-like resistivity. Some magnetic properties were described previously [1-5]. Ordering phenomena occur at low temperature, the N6el temperature corresponds to small anomalies in susceptibility and resistivity; a low TN/TI,: ratio, short-range order a n d / o r a progressive onset of correlations give rise to broad specific heat anomalies. At present, only the magnetic structure for CeNi2Sn 2 is known [5]; it is derived from a propagation vector (½, 7, 0), the moments lying along the a-axis of the structure with a magnetic moment of 0.9/x B at 1.5 K. An important feature of this structure is frustration in the body-centered Ce tetragonal lattice: antiferromagnetism occurs in Ce planes perpendicular to c, due to interactions between nearest Ce neighbours (d = 4.4 A). No dipolar coupling occurs between two successive Ce planes perpendicular to c and couplin~ along c only occurs between planes distant by about 10 A, which are separated by 7 atomic layers.
Thus interactions along this axis are probably much smaller, giving the interactions a quasi-bidimensional character. Similar structures have been encountered in some other 1:2:2 compounds, like CePd2Si2, CeRh2Si2, or U(Ru 1 xRhx)2Si2 [6,7]. In this last case, several propaga1 qz) have been simultanetion vectors of the form (½, 7, ously observed in one sample for a composition near x = 0.3 [7], which was explained by frustration and chemical disorder. Frustration in the body-centered tetragonal lattice has been described much earlier. Its main consequences are the reduction of the long-range ordering temperature which for Ce compounds will lead to a further reduction of the ordered moment, and the occurrence of short-range order above T N. A much longer correlation length is expected within the basal plane than along the c-axis. Such an anisotropy of the correlation length was found in URu0.94Rho.06Si2, which does not exhibit any long-range order [6]. It is more difficult to give evidence for the reduction of the N6el temperature, but for isomorphous Gd compounds GdNi2Sb 2 and GdCu2Sb2, the T N / I Tp I ratio of N6el to Curie-Weiss temperature is very low, which may be in-
Table 1 Properties of studied compounds Compound
Structure
Tp (K)
TN(K)
CeNi2 Sn 2 CeNi2 Sb 2 CeCuzSn 2 CeCuzSb 2 CeCu S b 2
LaPt2Ge 2 CaBe2Ge 2 CaBezGe 2 CaBe2Ge 2
- 9 - 28 - 12 - 7 - 11
2.2 1.1 2.1 5.0 6.5
0304-8853/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 3 0 4 - 8 8 5 3 ( 9 4 ) 0 0 9 8 7 - 2
HfCu Si 2
892
J. Pierre et al. /Journal of Magnetism and Magnetic Materials 140-144 (1995) 891-892
dicative of this phenomenon. We shall see in the following that short-range order occurs in some of these systems. Fig. 1 represents the reciprocal susceptibility of CeCuSb 2 and CeCu2Sb 2. Tp is obtained from a CurieWeiss fit, TN from the behaviour of magnetic isotherms and resistivity. For CeCuSb2, the magnitudes of T N and Tp are larger than for the second compound, which may be explained by larger interactions. The main feature in the first compound is that the reciprocal susceptibility deviates from a Curie-Weiss law below 25 K, contrary to that of dilute solid solutions where Ce is diluted by La. Thus short-range order arises between T N = 7 and 25 K, with influence on the resistivity (see below). The specific heat Cp was measured for some stannides [1,2], more recently for CeNizSb 2 [8]. Magnetic ordering is often accompanied by broad specific heat bumps instead of sharp lambda-type anomalies. In the case of CeNi2Sn 2, cp [1,2] has a peak value reduced to about 6 J / m o l - K , instead of 12.5 for a doublet in the ionic model. This can be explained by a value of TN/TI,: close to 1, according to Besnus et al. [9]. Above T N, Cp may be accounted for by a Kondo term with T K = 3.4 K, only a small contribution due to short-range correlations is obtained. For CeNieSb 2, Cava et al. [8] observed a broad bump with c / T reaching 2.3 J / m o l . K 2 at 1.1 K. Between 2 and 6 K, the specific heat may be explained by a Kondo anomaly on the ground state doublet with TI< = 1.6 K. Below 2 K, the peak value and the shape of the anomaly can be explained by a progressive onset of AF correlations, giving a magnetic splitting of about 0.8 K at 0 K which vanishes above 2.5 K. A N6el temperature of 2.5 K was indeed attributed to this compound in Ref. [3], on the basis of susceptibility and resistivity anomalies: this temperature is only that where magnetic correlations begin. The resistivity of some Ce stannides and antimonides was previously studied, together with that of La compounds [1,3,4]. The temperature dependence cannot be gO
~~ e
80
+ *
BO 50 70
o~ CeCu2Sb2
%
CeCu Sb2 L a . 4 C e . SCuS~2
/ /
-
2o 1o o
h
i
I0
20
i
30
I
40 Temperature
E
50
60
(K)
Fig. 1. Reciprocal susceptibility for CeCuSb z, La0.4Ce0.6CuSb2 and CeCuzSb 2.
50
m u
!
~ /o
45
-+- CeCu Sb2 +++ CeCu Sb2 -o- CeCu2Sb2 2
0T 5 T 0 T 5 T
<--.
40 0
35
/
30
2
5
~0°
1
~
1
5
1
1
tO1
1
1
1
5
tO2
Temperature
5 (K)
103
80
Fig. 2. Resistivity under 0 and 5 T for CeCu2Sb 2 and CeCuSb 2.
properly described by the classical Cornut-Coqblin theory, as the ratio of T K (HT) for 6-fold degeneracy to the overall crystal field splitting is not small. Coherence between 4f wave functions contributes to the decrease of resistivity at temperatures lower than the lowest crystal field splitting. We compare in Fig. 2 the resistivity for CeCu2Sb 2 and CeCuSb 2 under 0 and 5 T: the crystal field splittings are smaller for CeCuSb2, the magnetoresistance is much larger for CeCuSb 2, becomes significant below 25 K, where magnetic correlations begin to set it and the large resistivity anomaly is washed out by a field of 5 T. In conclusion, magnetic frustation may lower appreciably the ordering temperature in these compounds, leaving short-range order above T N. All physical data show the influence of short-range correlations. References
[1] T. Takabatake, F. Teshima, H. Fujii, S. Nishigori, T. Suzuki T. Fujita, Y. Yamaguchi and J. Sakurai, J. Magn. Magn. Mater. 90-91 (1990) 474. [2] W.P. Beyermann, M.F. Hundley, P. Canfield, J.D. Thompson, M. Latroche, C. Godart, M. Selsane, Z. Fisk and J.L. Smith, Phys. Rev. B 43 (1991) 13130. [3] K. Kaczmarska, J. Pierre, A. Slebarski and R.V. Skolozdra, J. Alloys Compounds 196 (1993) 165. [4] RN. Skolozdra, Ja. F. Mikhalski, K. Kaczmarska and J. Pierre, J. Alloys Compounds 206 (1994) 141. [5] J. Pierre, B. Lambert-Andron, R.V. Skolozdra, J. RodriguezCarvajal and K. Kaczmarska, Physica B 202 (1994) 143. [6] P. Buffet, F. Bourdarot, S. Quezel, J. Rossat-Mignot, P. Lejay, B. Chevalier and H. Hickey, J. Magn. Magn. Mater. 108 (1992) 202. [7] S. Kawarazaki, Y. Kobashi, T. Taniguchi, Y. Miyako and H. Amitsuka, J. Phys. Soc. Japan 63 (1994) 716. [8] R.J. Cava, A.P. Ramirez, H. Takagi, J. Krajewski and W.F. Peck, J. Magn. Magn. Mater. 128 (1993) 124. [9] M.J. Besnus, A. Braghta, N. Hamdaoui and A. Meyer, J. Magn. Magn. Mater. 104-107 (1992) 1385.