- Email: [email protected]
Magnetic structures of Ce(AgxCu1 − x)1.8Sb2 Kondo compounds
~
,~ ELSEVIER
Journal of Magnetism and Magnetic Materials 196-197 (19991 891-892
Journalof magnetism
and
magnetic materials
Magnetic structures of Ce(AgxCul- x)l.8Sb2 Kondo compounds A. Guzik a'*, J. Pierre b, K. Kaczmarska a, B.
Ouladdiaf ~
aInstitute of Physics, Silesian University, 40-007 Katowice, Poland bLaboratory L. Reel, CNRS, 166X, 38042 Grenoble, France CInstitute Laue-Langevin, 156X,, 38042 Grenoble, France
Abstract
The magnetic structures of the Ce(AgxCu~-x)l.sSb2 compounds have been refined from neutron diffraction. The transition from the CeCu~.sSb2 commensurate frustrated antiferromagnetic structure to the CeAg~.8Sb2 incommensurate one occurs through ferromagnetic and spin-glass states. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Magnetic structure; Neutron diffraction; Konda systems; Cerium compounds
1. Introduction
The magnetic properties of cerium intermetallics strongly depend on electronic structure, because conduction electrons realize two fundamental interactions determining magnetic ground state: the magnetic RKKY interaction and the hybridization of localized 4f electrons with conduction electrons. Even for isoelectronic compounds, like CeCul.8Sbz and CeAgl.sSb2, the magnetic, transport and thermodynamics properties differ [1-31. In previous study 1-1] we reported the magnetic structure of border compounds CeAgl.sSb2 and CeCul.aSb2. In both compounds magnetic moment is aligned along the c-axis. The magnetic structure of CeCul.sSb2 is frustrated, described by the propagation vector q = (0, 0, 1/2), whereas the magnetic structure for CeAgl.sSb2 is an incommensurate one with q = (0.06, 0.06, 0). It is very close to a simple antiferromagnetic ( + - ) and such a structure is stable for low magnetic fields. An increase of magnetic field induces a metamagnetic transition to the ferromagnetic state. In the following we report the magnetic structures of the mixed compounds Ce(AgxCul_x)l.sSb2 x = 0.2, 0.6, 0.8. All compounds crystallize with the tetragonal
*Corresponding author. Fax: [email protected].
+ 48-32-588431; e-mail:
CaBe2Ge2 type of structure. The 'a' parameter remains constant and is equal to 4.4 A in the series, whereas the 'c' parameter increases linearly from 10.2 A for CeCul.sSb2 to 10.7 A for CeAgl.sSb2, at room temperature. In every compound a deficiency of transition metal (Ag, Cu) at position (2c) has been observed, hence the actual crystallographic structure inclines to the HfCuSi2 one. Both crystallographic and magnetic structures have been refined using the program F U L L P R O F [4].
2. Ce(Ago.sCuo.2)m.sSbz Fig. 1 presents the magnetic diffraction pattern made by subtraction of the spectra taken at 10 K from another taken at 1.5 K. Additional magnetic peaks arise at the place of crystallographic peaks (1 0 11 and (1 1 0). It indicates a ferromagnetic (FM) type of order, as in BCC tetragonal lattice the magnetic structural form-factor for F M state reads: F(hkl) =f(hkl)*[1 + ( - 1)h+k+'].
It means that the magnetic contribution is only at the position of the crystallographic peaks with an even sum of Miller indices. The lack of contribution at (0 0 2) peak indicates that magnetic moments are aligned along the c-axis. The magnetic moment in the ordered state of a cerium atom has a value 0.7#a.
0304-8853/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 8 ) 0 0 9 7 7 - 9
892
A. Guzik et al. /Journal of Magnetism and Magnetic Materials 196-197 (1999) 891-892 0.8
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Fig. 1. Magnetic contribution to the diffraction neutron spectra obtained by subtracting diffractograms taken at 1.5 and 10 K for Ce(Ago 8Cuo.2h.sSb~.
The magnetic isotherms taken at temperatures below 8 K show the spontaneous magnetization and clearly confirms the ferromagnetic state in that compound (Fig. 2).
3. Other compounds In the compound Ce(Ago.6Cu0.4)l.8Sb 2 we also found the same FM order with a value of magnetic moment equal to 0.5/~B at 1.5 K. In contrast, in the Ce(Ag0.zCuo.8h.sSb2 we did not find any magnetic order at 1.5 K, although magnetic measurements could suggest some very weak antiferromagnetism. Thus we suppose that in such a compound only short-range and frustrated magnetic correlations are present, making a spin-glass state.
Fig. 2. Arrot's plot for Ce(Ag0.sCuo.zh.sSb2. Lines are guides for the eyes.
Apart from the short wavevector modulation in CeAgl.8Sb2, the basal plane interactions are ferromagnetic in the whole composition range. Conversely, the interactions along the c axis change from ferro- to antiferromagnetic, giving a frustrated structure in the copper compound where there is no magnetic dipolar coupling between the atoms at the corner and at the center of the cell. Thus, rather weak and random interactions occur along the c axis, following the disorder and strains in the lattice due to the introduction of a few Cu atoms instead of Ag atoms. Analogous features have been observed in URhl xRuxSi2 [6]. Probably these effects lead to the vanishing of long range magnetic ordering in the Ce(Ago.2Cu0.sh.sSb2 compound.
Acknowledgements This work has been partly supported by Polish Research Committee (KBN) under grant no. 2P03B 08013.
4. Summary References Thus, when silver is replaced by copper, first the antiferromagnetic phase is washed out and the FM state, observed in CeAga.sSb2 above 1.5 T at 1.5 K, dominates. But hybridization of 4f electrons with the conduction electrons becomes larger and larger, leading to the reduction of the magnetic moment. Specific heat data collected by Muro et al. [4] suggests that the hybridization in CeAga.8Sb2 may be small, whereas width of the quasielastic line measured in inelastic neutron scattering [5] shows that in CeCua.sSb2 the hybridization effect plays an important role.
[1] A. Guzik, J. Pierre, K. Kaczmarska, B. Lambert-Andron, B. Ouladdiaf, J. Magn. Magn. Mater. 157/158 (1996) 683. [2] A. Guzik, J. Pierre, J. Alloys Compounds 264 (1998) 8. [3] Y. Muro, N. Takeda, M. Ishikawa, J. Alloys Compounds 264 (1997) 23. [4] J. Rodriguez-Carvajal, Physica B 192 (1993) 55. [5] A. Gruzik, K. Kaczmarska, J. Pierre, A. Murani. J. Magn. Magn. Mater. 161 (1996) 103. [6] P. Burlet, F. Bourdarot, S. Quezel, J. Rossat-Mignod, P. Lejay, B. Chevalier, J. Magn. Magn. Mater. 108 (1992) 202.
,~ ELSEVIER
Journal of Magnetism and Magnetic Materials 196-197 (19991 891-892
Journalof magnetism
and
magnetic materials
Magnetic structures of Ce(AgxCul- x)l.8Sb2 Kondo compounds A. Guzik a'*, J. Pierre b, K. Kaczmarska a, B.
Ouladdiaf ~
aInstitute of Physics, Silesian University, 40-007 Katowice, Poland bLaboratory L. Reel, CNRS, 166X, 38042 Grenoble, France CInstitute Laue-Langevin, 156X,, 38042 Grenoble, France
Abstract
The magnetic structures of the Ce(AgxCu~-x)l.sSb2 compounds have been refined from neutron diffraction. The transition from the CeCu~.sSb2 commensurate frustrated antiferromagnetic structure to the CeAg~.8Sb2 incommensurate one occurs through ferromagnetic and spin-glass states. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Magnetic structure; Neutron diffraction; Konda systems; Cerium compounds
1. Introduction
The magnetic properties of cerium intermetallics strongly depend on electronic structure, because conduction electrons realize two fundamental interactions determining magnetic ground state: the magnetic RKKY interaction and the hybridization of localized 4f electrons with conduction electrons. Even for isoelectronic compounds, like CeCul.8Sbz and CeAgl.sSb2, the magnetic, transport and thermodynamics properties differ [1-31. In previous study 1-1] we reported the magnetic structure of border compounds CeAgl.sSb2 and CeCul.aSb2. In both compounds magnetic moment is aligned along the c-axis. The magnetic structure of CeCul.sSb2 is frustrated, described by the propagation vector q = (0, 0, 1/2), whereas the magnetic structure for CeAgl.sSb2 is an incommensurate one with q = (0.06, 0.06, 0). It is very close to a simple antiferromagnetic ( + - ) and such a structure is stable for low magnetic fields. An increase of magnetic field induces a metamagnetic transition to the ferromagnetic state. In the following we report the magnetic structures of the mixed compounds Ce(AgxCul_x)l.sSb2 x = 0.2, 0.6, 0.8. All compounds crystallize with the tetragonal
*Corresponding author. Fax: [email protected].
+ 48-32-588431; e-mail:
CaBe2Ge2 type of structure. The 'a' parameter remains constant and is equal to 4.4 A in the series, whereas the 'c' parameter increases linearly from 10.2 A for CeCul.sSb2 to 10.7 A for CeAgl.sSb2, at room temperature. In every compound a deficiency of transition metal (Ag, Cu) at position (2c) has been observed, hence the actual crystallographic structure inclines to the HfCuSi2 one. Both crystallographic and magnetic structures have been refined using the program F U L L P R O F [4].
2. Ce(Ago.sCuo.2)m.sSbz Fig. 1 presents the magnetic diffraction pattern made by subtraction of the spectra taken at 10 K from another taken at 1.5 K. Additional magnetic peaks arise at the place of crystallographic peaks (1 0 11 and (1 1 0). It indicates a ferromagnetic (FM) type of order, as in BCC tetragonal lattice the magnetic structural form-factor for F M state reads: F(hkl) =f(hkl)*[1 + ( - 1)h+k+'].
It means that the magnetic contribution is only at the position of the crystallographic peaks with an even sum of Miller indices. The lack of contribution at (0 0 2) peak indicates that magnetic moments are aligned along the c-axis. The magnetic moment in the ordered state of a cerium atom has a value 0.7#a.
0304-8853/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 8 ) 0 0 9 7 7 - 9
892
A. Guzik et al. /Journal of Magnetism and Magnetic Materials 196-197 (1999) 891-892 0.8
I
[
I
•
(101) zzuu ~ ~"
1900
o Yobs -" YCaIC I Bragg position -- Yolos-Ycalc
0.6
=~ 1600
(110)
s % _=
(002)
700
I
I
I
!
I
400
i~ I
0.4
0.2
r
100
5
10
15
20
25
30
35
40
45
50
P,~'"
55
aa=" 2
I
,
J
4
6
8
2 Theta (°) HfM(psff.u.*T)
Fig. 1. Magnetic contribution to the diffraction neutron spectra obtained by subtracting diffractograms taken at 1.5 and 10 K for Ce(Ago 8Cuo.2h.sSb~.
The magnetic isotherms taken at temperatures below 8 K show the spontaneous magnetization and clearly confirms the ferromagnetic state in that compound (Fig. 2).
3. Other compounds In the compound Ce(Ago.6Cu0.4)l.8Sb 2 we also found the same FM order with a value of magnetic moment equal to 0.5/~B at 1.5 K. In contrast, in the Ce(Ag0.zCuo.8h.sSb2 we did not find any magnetic order at 1.5 K, although magnetic measurements could suggest some very weak antiferromagnetism. Thus we suppose that in such a compound only short-range and frustrated magnetic correlations are present, making a spin-glass state.
Fig. 2. Arrot's plot for Ce(Ag0.sCuo.zh.sSb2. Lines are guides for the eyes.
Apart from the short wavevector modulation in CeAgl.8Sb2, the basal plane interactions are ferromagnetic in the whole composition range. Conversely, the interactions along the c axis change from ferro- to antiferromagnetic, giving a frustrated structure in the copper compound where there is no magnetic dipolar coupling between the atoms at the corner and at the center of the cell. Thus, rather weak and random interactions occur along the c axis, following the disorder and strains in the lattice due to the introduction of a few Cu atoms instead of Ag atoms. Analogous features have been observed in URhl xRuxSi2 [6]. Probably these effects lead to the vanishing of long range magnetic ordering in the Ce(Ago.2Cu0.sh.sSb2 compound.
Acknowledgements This work has been partly supported by Polish Research Committee (KBN) under grant no. 2P03B 08013.
4. Summary References Thus, when silver is replaced by copper, first the antiferromagnetic phase is washed out and the FM state, observed in CeAga.sSb2 above 1.5 T at 1.5 K, dominates. But hybridization of 4f electrons with the conduction electrons becomes larger and larger, leading to the reduction of the magnetic moment. Specific heat data collected by Muro et al. [4] suggests that the hybridization in CeAga.8Sb2 may be small, whereas width of the quasielastic line measured in inelastic neutron scattering [5] shows that in CeCua.sSb2 the hybridization effect plays an important role.
[1] A. Guzik, J. Pierre, K. Kaczmarska, B. Lambert-Andron, B. Ouladdiaf, J. Magn. Magn. Mater. 157/158 (1996) 683. [2] A. Guzik, J. Pierre, J. Alloys Compounds 264 (1998) 8. [3] Y. Muro, N. Takeda, M. Ishikawa, J. Alloys Compounds 264 (1997) 23. [4] J. Rodriguez-Carvajal, Physica B 192 (1993) 55. [5] A. Gruzik, K. Kaczmarska, J. Pierre, A. Murani. J. Magn. Magn. Mater. 161 (1996) 103. [6] P. Burlet, F. Bourdarot, S. Quezel, J. Rossat-Mignod, P. Lejay, B. Chevalier, J. Magn. Magn. Mater. 108 (1992) 202.