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Influence of composition on the form factor measured in CeSn3
Journal
of Magnetism
and Magnetic
INFLUENCE
Materials
54-57
OF COMPOSITION
J.X. BOUCHERLE J. SCHWEIZER
+, G. FILLION
421
(1986) 421-422
ON THE FORM FACTOR *, J. FLOUQUET
MEASURED
IN CeSn,
**, F. GIVORD *, P. LEJAY
** and
+
+ C. E. N. -G., DRF/SPh - MDN. 85 X, 38041 Grenoble Cedex, France * Lab. Louis N&l, CNRS, 166X, 38042 Grenoble Cedex, France ** CRTBT, CNRS, 166 X, 38042 Grenoble Cedex, France
with different stoichiometries: CeSn z 9 and Ce form factor measurements have been performed on CeSn, compounds CeSn,,. The 4f-type localized moment is found to be the same on both samples, but the amplitude of the 5d contribution previously reported by Stassis et al. depends strongly on the composition: it is present on the stoichiometric sample but it has data of the two samples and its correlation with totally disappeared in CeSn, 9. An analysis of the magnetization crystallographic studies of the Ce-Sn phase diagram has allowed us to evaluate the contribution of CeSn, in each sample. A larger CeSn, susceptibility is found in the stoichiometric sample, in agreement with the form factor results.
The compound CeSn, (AuCu, cubic type structure) is in a mixed valence state as shown by its physical properties such as high electronic specific heat constant [2] or large thermal expansion [3]. Its bulk magnetic susceptibility [2.4,5] has a main contribution of the enhanced
Pauli-type
and
exhibits
a broad
maximum
as temperature is decreased below 40 K. The amplitude of this “tail” depends strongly on the sample measured and has commonly been attributed to the presence of paramagnetic impurities. It is therefore very difficult to correlate the results obtained by different techniques or/and on different samples. Among all the techniques used to understand the magnetic behaviour of such a compound, polarised neutron diffraction on single crystals are particularly relevant as they measure directly the Fourier components of the magnetic distribution around the atoms of the crystal structure. An experiment performed by Stassis et al. [I], has shown that, at low temperature, the magnetization induced by an applied field contains a large component of 5d electronic character, superimposed on the 4f type magnetization. On their two samples, the 5d contribution represents 40 and 50% of the total magnetization and can account partly for the increase in the bulk magnetic susceptibility of CeSn, at low temperatures. This large 5d contribution, which appears below 40 K, has been correlated with the mixed valence properties of the CeSn, compound and explained in terms of a simple model in which the 4f level is positioned slightly above the Fermi energy and is hybridized with the conduction band. It has been considered as a characteristic of Ce compounds in intermediate valence state. However, other Ce samples which crystallize in the same AuCu,-type structure do not exhibit such a large 5d contribution (only 17% of the total magnetization in CePd, [6], 10% in CeInSn, [7]) and nor does it depend on the temperature between 75 and 5.5 K. In order to check a possible correlation between the around
150
K. However
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it increases
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suddenly
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Publishers
intermediate valence state, the addition of a strong 5d contribution to the induced magnetization and the increase of the bulk susceptibility at low temperatures, two samples of different stoichiometries CeSn,,, and the single crystals were CeSn 3.0 have been studied: prepared by the Bridgman method, using 99.99% pure Ce and 99.999% pure Sn. Form factor measurements were performed on the D3 spectrometer at I.L.L. at 4.2 K in a 4.6 T magnetic field applied parallel to a [ITO] axis of the crystal. The measured magnetic amplitudes are presented in figs. 1 and 2. The solid line curve is the theoretical magnetic form factor of the Ce’+ free ion, evaluated in the dipolar approximation, using relativistic electronic wave functions [8]. The values of the 4f susceptibilities thus obtained are almost the same for the two samples (see table 1) and are also close to the values previously published [l] (1.7 x 10m3 emu/ma/). but the 5d contribution observed on our samples is much weaker: about 25% of the total magnetization in CeSn,,, and zero in CeSn,,. Accurate magnetization measurements [9] of the same samples, performed on a SQUID magnetometer have allowed to identify 2 kinds of impurities present in our samples: paramagnetic impurities (x proportional to T-‘) and impurities which order antiferromagnetically below 3 K. On the other hand, a crystallographic study [lo] of the neighbouring phases of CeSn, has shown the
Table 1 Amount of impurities Deduced susceptibility susceptibility Sample
% Ce,Sn,
CeSn 2,9 4.7 (2) CeSn,, 1.50(5)
B.V.
in the CeSn,, and CeSn,,, samples, of CeSn, and comparison with the 4f S Ce3+
XC&n, X‘tf T = 412 K and H = 4.65 T (X 10e3 emu/mol-‘)
2.70 (5) 0.38 (1)
1.7 (3) 2.0 (1)
1.65 (3) 1.55 (6)
I
I
I
I
CeSn,, T=4.ZK H=4.6T u
tJ.UU
sinO/h Fig. 1. Magnetic
form factor measured
O.ZU
(A-‘)
on CeSn, 4 at 4.2 K.
occurrence of some crystallographic defects: Sn atoms can be replaced by Ce atoms. Zones of higher density in Ce atoms are thus created. in which Ce should be trivalent. These defects can be either disordered (corresponding to the paramagnetic impurities), or ordered: especially. one defect each 7 CeSn, cells leads to the orthorhombic structure of Ce,Sn,. The magnetic properties of Ce,Sn, have been studied [ll]. It is antiferromagnetic, with TN = 3 K. The second kind of impurity is then the neighbouring phase Ce,Sn,. The amounts of the 2 kinds of impurities are given in table 1 and the contribution of pure CeSn, to the bulk magnetization measured in our 2 samples at 4.2 K and in 4.6 T. could then be evaluated. Values are given in table 1 and they correspond on figs. 1 and 2 with the values of the magnetic amplitudes pf at sin B/h = 0. In spite of the large error bar on the value obtained for CeSn,,,, due to the large amount of impurities in that compound, the values compare well with the extrapolation of the directly measured magnetic amplitudes. Especially m CeSnlO, this bulk magnetization corresponds with an additional contribution to the 4f moment of 24%. in very good agreement with the amount of 5d contribution. The absence of a 5d contribution in with the assumption of CeSn 2.y is compatible Gschneidner [12]: the bulk susceptibility tail could be due to spin fluctuations which are suppressed by the presence of impurities. In conclusion. as the Sn concentration is decreased from CeSn,,, to C&n,,. Ce>Sn, ordered regions and
I
I
CeSn,., T=4.2K H=4.67
0.40
0.60
sine/h
Fig. 2. Magnetic
Ce”
form factor
measured
0.80
(A-')
on CeSn? ,, at 4.2 K
i
disordered paramagnetic impurities develop simultaneously and the 5d contribution to the induced magnetization tends to disappear. Though both type of impurities are very difficult to show by usual technics it is of the highest importance, to select samples in which these impurities are absent in order to reach the intrinsic properties of CeSn,. This result suggests the importance of the coherence between all Ce atoms for inducing a large 5d contribution. [I] C. Stassis, C.K. Loong, B.N. Harmon. S.H. Liu and R.M. Moon, J. Appl. Phys. 50 (1979) 7567. [2] J.R. Cooper, C. Rizzuto and G. Olcese. J. de Phys. 32 (1971) Ci-1136. Metals. Y [31 I.R. Harris and G.V. Raynor. J. Less-Common (1965) 7. 141T. Tsuchida and W.E. Wallace, J. Chem. Phyh. 43 (1965) 3811. S.K. Garg and R.J. [51 SK. Malik. R. Vijayaraguavan, Rlpmeester. Phys. Stat. Sol. (b) 68 (1975) 399. [61 C. Stassis, C.K. Loong. J. Zarestky and O.D. McMasterb. J. Appl. Phys. 53 (1982) 7890. [71 A. Benoit, J.X. Boucherle, J. Flouquet. J. Sakurai and J. Schweizer. J. Magn. Magn. Mat. 47&48 (1985) 149. PI A.J. Freeman and J.P. Desclaux. J. Magn. Magn. Mat. 12 (1978) 11. PI J.X. Boucherle. G. Fillion, J. Flouquqt. F. Givord, P. Le~ay and J. Schweizer, to be published. (101 F. Givord. P. Lejay. J. Schweizer and A. Stunault. to he published. [111 F. Givord, P. LeJay. J. Schweizer and A. Stunault. to he published. PI K.A. Gschneidner. Jr.. J. Magn. Magn. Mat. 47&48 (19X5) 57.
of Magnetism
and Magnetic
INFLUENCE
Materials
54-57
OF COMPOSITION
J.X. BOUCHERLE J. SCHWEIZER
+, G. FILLION
421
(1986) 421-422
ON THE FORM FACTOR *, J. FLOUQUET
MEASURED
IN CeSn,
**, F. GIVORD *, P. LEJAY
** and
+
+ C. E. N. -G., DRF/SPh - MDN. 85 X, 38041 Grenoble Cedex, France * Lab. Louis N&l, CNRS, 166X, 38042 Grenoble Cedex, France ** CRTBT, CNRS, 166 X, 38042 Grenoble Cedex, France
with different stoichiometries: CeSn z 9 and Ce form factor measurements have been performed on CeSn, compounds CeSn,,. The 4f-type localized moment is found to be the same on both samples, but the amplitude of the 5d contribution previously reported by Stassis et al. depends strongly on the composition: it is present on the stoichiometric sample but it has data of the two samples and its correlation with totally disappeared in CeSn, 9. An analysis of the magnetization crystallographic studies of the Ce-Sn phase diagram has allowed us to evaluate the contribution of CeSn, in each sample. A larger CeSn, susceptibility is found in the stoichiometric sample, in agreement with the form factor results.
The compound CeSn, (AuCu, cubic type structure) is in a mixed valence state as shown by its physical properties such as high electronic specific heat constant [2] or large thermal expansion [3]. Its bulk magnetic susceptibility [2.4,5] has a main contribution of the enhanced
Pauli-type
and
exhibits
a broad
maximum
as temperature is decreased below 40 K. The amplitude of this “tail” depends strongly on the sample measured and has commonly been attributed to the presence of paramagnetic impurities. It is therefore very difficult to correlate the results obtained by different techniques or/and on different samples. Among all the techniques used to understand the magnetic behaviour of such a compound, polarised neutron diffraction on single crystals are particularly relevant as they measure directly the Fourier components of the magnetic distribution around the atoms of the crystal structure. An experiment performed by Stassis et al. [I], has shown that, at low temperature, the magnetization induced by an applied field contains a large component of 5d electronic character, superimposed on the 4f type magnetization. On their two samples, the 5d contribution represents 40 and 50% of the total magnetization and can account partly for the increase in the bulk magnetic susceptibility of CeSn, at low temperatures. This large 5d contribution, which appears below 40 K, has been correlated with the mixed valence properties of the CeSn, compound and explained in terms of a simple model in which the 4f level is positioned slightly above the Fermi energy and is hybridized with the conduction band. It has been considered as a characteristic of Ce compounds in intermediate valence state. However, other Ce samples which crystallize in the same AuCu,-type structure do not exhibit such a large 5d contribution (only 17% of the total magnetization in CePd, [6], 10% in CeInSn, [7]) and nor does it depend on the temperature between 75 and 5.5 K. In order to check a possible correlation between the around
150
K. However
0304-8853/86/$03.50
it increases
0 Elsevier
suddenly
Science
Publishers
intermediate valence state, the addition of a strong 5d contribution to the induced magnetization and the increase of the bulk susceptibility at low temperatures, two samples of different stoichiometries CeSn,,, and the single crystals were CeSn 3.0 have been studied: prepared by the Bridgman method, using 99.99% pure Ce and 99.999% pure Sn. Form factor measurements were performed on the D3 spectrometer at I.L.L. at 4.2 K in a 4.6 T magnetic field applied parallel to a [ITO] axis of the crystal. The measured magnetic amplitudes are presented in figs. 1 and 2. The solid line curve is the theoretical magnetic form factor of the Ce’+ free ion, evaluated in the dipolar approximation, using relativistic electronic wave functions [8]. The values of the 4f susceptibilities thus obtained are almost the same for the two samples (see table 1) and are also close to the values previously published [l] (1.7 x 10m3 emu/ma/). but the 5d contribution observed on our samples is much weaker: about 25% of the total magnetization in CeSn,,, and zero in CeSn,,. Accurate magnetization measurements [9] of the same samples, performed on a SQUID magnetometer have allowed to identify 2 kinds of impurities present in our samples: paramagnetic impurities (x proportional to T-‘) and impurities which order antiferromagnetically below 3 K. On the other hand, a crystallographic study [lo] of the neighbouring phases of CeSn, has shown the
Table 1 Amount of impurities Deduced susceptibility susceptibility Sample
% Ce,Sn,
CeSn 2,9 4.7 (2) CeSn,, 1.50(5)
B.V.
in the CeSn,, and CeSn,,, samples, of CeSn, and comparison with the 4f S Ce3+
XC&n, X‘tf T = 412 K and H = 4.65 T (X 10e3 emu/mol-‘)
2.70 (5) 0.38 (1)
1.7 (3) 2.0 (1)
1.65 (3) 1.55 (6)
I
I
I
I
CeSn,, T=4.ZK H=4.6T u
tJ.UU
sinO/h Fig. 1. Magnetic
form factor measured
O.ZU
(A-‘)
on CeSn, 4 at 4.2 K.
occurrence of some crystallographic defects: Sn atoms can be replaced by Ce atoms. Zones of higher density in Ce atoms are thus created. in which Ce should be trivalent. These defects can be either disordered (corresponding to the paramagnetic impurities), or ordered: especially. one defect each 7 CeSn, cells leads to the orthorhombic structure of Ce,Sn,. The magnetic properties of Ce,Sn, have been studied [ll]. It is antiferromagnetic, with TN = 3 K. The second kind of impurity is then the neighbouring phase Ce,Sn,. The amounts of the 2 kinds of impurities are given in table 1 and the contribution of pure CeSn, to the bulk magnetization measured in our 2 samples at 4.2 K and in 4.6 T. could then be evaluated. Values are given in table 1 and they correspond on figs. 1 and 2 with the values of the magnetic amplitudes pf at sin B/h = 0. In spite of the large error bar on the value obtained for CeSn,,,, due to the large amount of impurities in that compound, the values compare well with the extrapolation of the directly measured magnetic amplitudes. Especially m CeSnlO, this bulk magnetization corresponds with an additional contribution to the 4f moment of 24%. in very good agreement with the amount of 5d contribution. The absence of a 5d contribution in with the assumption of CeSn 2.y is compatible Gschneidner [12]: the bulk susceptibility tail could be due to spin fluctuations which are suppressed by the presence of impurities. In conclusion. as the Sn concentration is decreased from CeSn,,, to C&n,,. Ce>Sn, ordered regions and
I
I
CeSn,., T=4.2K H=4.67
0.40
0.60
sine/h
Fig. 2. Magnetic
Ce”
form factor
measured
0.80
(A-')
on CeSn? ,, at 4.2 K
i
disordered paramagnetic impurities develop simultaneously and the 5d contribution to the induced magnetization tends to disappear. Though both type of impurities are very difficult to show by usual technics it is of the highest importance, to select samples in which these impurities are absent in order to reach the intrinsic properties of CeSn,. This result suggests the importance of the coherence between all Ce atoms for inducing a large 5d contribution. [I] C. Stassis, C.K. Loong, B.N. Harmon. S.H. Liu and R.M. Moon, J. Appl. Phys. 50 (1979) 7567. [2] J.R. Cooper, C. Rizzuto and G. Olcese. J. de Phys. 32 (1971) Ci-1136. Metals. Y [31 I.R. Harris and G.V. Raynor. J. Less-Common (1965) 7. 141T. Tsuchida and W.E. Wallace, J. Chem. Phyh. 43 (1965) 3811. S.K. Garg and R.J. [51 SK. Malik. R. Vijayaraguavan, Rlpmeester. Phys. Stat. Sol. (b) 68 (1975) 399. [61 C. Stassis, C.K. Loong. J. Zarestky and O.D. McMasterb. J. Appl. Phys. 53 (1982) 7890. [71 A. Benoit, J.X. Boucherle, J. Flouquet. J. Sakurai and J. Schweizer. J. Magn. Magn. Mat. 47&48 (1985) 149. PI A.J. Freeman and J.P. Desclaux. J. Magn. Magn. Mat. 12 (1978) 11. PI J.X. Boucherle. G. Fillion, J. Flouquqt. F. Givord, P. Le~ay and J. Schweizer, to be published. (101 F. Givord. P. Lejay. J. Schweizer and A. Stunault. to he published. [111 F. Givord, P. LeJay. J. Schweizer and A. Stunault. to he published. PI K.A. Gschneidner. Jr.. J. Magn. Magn. Mat. 47&48 (19X5) 57.