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Evidence for antiferromagnetism in Ce1−xLaxCu2.2Si2 below 10 K
Journal of M a g n e t i s m and M a g n e t i c M a t e r i a l s 76 & 77 (1988) 5 2 3 - 5 2 4 North-Holland, Amsterdam
EVIDENCE FOR ANTIFERROMAGNETISM
523
I N Ce 1_ xLaxCu2.2Si 2 B E L O W 10 K
P. van AKEN, U. A H L H E I M , C.D. B R E D L F.R. de BOER §, D. E N K L E R , G. SPARN, H. SPILLE and F. STEGLICH lnstitut ]'£tr Festk&perphvsik, Technische Hoehschule Darmstadt and SFB 252, D-6100 Darmstadt. Fed. Rep. Germw~v § Natuurkundig Laboratorium, Unmersiteit t,an Amsterdam, NL-IOI8 XE Amsterdam, The Netherlands Preliminary results of the resistivity, d c - s u s c e p t i b i l i t y a n d specific heat for C e 1 _ ~La~Cu2.2Si 2 (0.015 < x _< 0.05) reveal evidence for a n t i f e r r o m a g n e t i c o r d e r b e l o w T N = 3 7 K. T N ( x ) is m a x i m u m n e a r x = 0.03. The low e n t r o p y associated with the transition p o i n t s to small ordered Ce moments.
The magnetic phase diagram of CeCu2Si 2, though being complex and far from understood, appears to be dominated by V~, the unit-cell volume [1]: Stoichiometric CeCuzSi 2 single crystals are not superconducting at ambient pressure, but seem to be antiferromagnets below TN = 3.5 K. For CeCu2.2Si 2 polycrystalline samples l<,~ is reduced by ( 2 - 3 ) × 1 0 -3 compared to the stoichiometric single crystals. Upon doping CeCu22Si 2 with a few at % of La, the average value of V,~ will increase and become comparable to V,~ of single-crystalline CeCu2Si 2. If V~c is the key parameter in determining the magnetic properties of this material, antiferromagnetism should develop for C% _,. La ,Cu 2.2Si ~ with a few at% La. La-doping will at the same time lead to a Tcdepression. But since one can afford up to 11 at% of La impurities are required before T~ vanishes [2], for dilute Ce~ xLa~Cu2.2Si2 heavy-fermion superconductivity should coexist below T~(x) with antiferromagnetic order, which eventually develops already above 1 K. In this contribution we present preliminary results of the resistivity (fig. la), of the dc, low-field susceptibility (fig. lb) [3] and of the specific heat (fig. 2) for polycrystalline C%_~LaxCu2.2Si 2 s a m p l e s (0 _< x _< 0.13). These results are consistent with an antiferromagnetic transition to occur at T N = 3-10 K. All samples studied were single phase according to X-ray powder diffractometry and microprobe. In fig. l a the electrical resistivity p ( T ) is shown for CeCu22Si 2 and a number of its La-alloys. To avoid problems with the determination of the actual shape factor arising from microcracks in the polycITstalline samples, all p ( T ) curves are matched at 300 K. The surprising result of fig. l a is a depression and upward-shift of the low-T peak of the resistivity upon increasing La con-
centration from x = 0 up to 5 at%. At higher x-values, the low-T resistivity increases and the peak position is pushed downwards. The systematic variation in the o(T) dependencies of Cel_.~LaxCu22Si 2 is underlined by the fact that both height (0m~) and position (T~,) of the low-T peak are connected by a linear relationship in the whole concentration range 0 < x _< 0.13 [3]. DC magnetization experiments carried out on these alloys at T = 4.2 K up to B = 35 T, reveal the low-T value of the intrinsic susceptibility Xi and an extrinsic ("impurity") contribution X e C J ( T + 0~). If the Curie constant Ce is expressed in terms of "non-transformed", stable Ce 3+ moments ( J = 5 / 2 ) , their concentration Xc.e)+ is found to decrease steadily from about 2 at% for CeCu2.2Si2 to 0.3 at% for the 13 at% La sample [3]. This may be ascribed to the overall lattice expansion, which allows Ce 3+_,,impuri ties" to "relax", i.e. to transform into CF-split Kondo ions. As expected, the characteristic single-ion Kondo temperature T * that determines the essential E
\E
Ce. × LoxCu2.2 Si~
I
~
I
~
t
~
190
\
/
.. o_ •
--
0
--
I
100
I
200 V {KI
H
b
I
0
×
o
I0
~
=0
= 0.015 = 0.03
•
:
• u ×
= 0.09 : 0.11 = 0.13
I
2O
~
0.05
]
30 V (KI
......
/-,0
--
30 20
-
---10
~--0
~0
Fig. 1. N o r m a l i z e d resistivity p ( T ) / o ( 3 0 0 K) (a) a n d intrinsic d c - s u s c e p t i b i l i t y x i ( T ) (b) for C % _ , L a ~ L a , C u 2 2 S i 2 (0 __
0304-8853/88/$03.50 © Elsevier Science Publishers B.V.
__
524
P. van Aken et al. / Antiferromagnetism in CeI
t h e r m o d y n a m i c p r o p e r t i e s of the alloys decreases with increasing L a c o n c e n t r a t i o n , as c o n c l u d e d from an increase in Xi(4.2 K) from 7.3 × 10 S m 3 / m o l - C e for x = 0 to 8.0 x 10 -~ m 3 / m o l Ce for x = 13 at% [3]. T h e t e m p e r a t u r e d e p e n d e n c e of the intrinsic susceptibility x i ( T ) as o b t a i n e d from the raw d a t a x ( T ) (taken at B = I T) via x ~ ( T ) = x ( T ) C e / ( T + 0~) is shown in fig. ] b for CeCu2.2Si 2 a n d the samples with 1.5, 3, 5 a n d 13 at% La concentration. Clearly, three qualitatively different type of x i ( T ) curves are o b t a i n e d : (i) The pure c o m p o u n d a n d the 1.5 at% La s a m p l e exhibit a c o n s t a n t ( " P a u l i - l i k e " ) value b e l o w 3 K which c o n t i n u o u s l y decreases u p o n w a r m i n g [1]. (ii) F o r x > 7 at%, x ~ ( T ) shows a negative t e m p e r a t u r e coefficient d o w n to 2 K, the lowest accessible t e m p e r a t u r e . A c c o r d i n g to earlier w o r k on stoichiometric C e l _ x L a x C u 2 S i 2 alloys [4] we might expect some " s p i n - g l a s s " type o r d e r i n g at lower temperatures. In the same c o n c e n t r a t i o n range ( x > 7 at%), a d d i t i o n of La causes an increased incoherent scattering a b o v e 4.2 K (fig. ]a). (iii) A flat m a x i m u m in x i ( T ) n e a r 5 - 1 0 K is o b s e r v e d for x = 3 at% a n d at 3 - 4 K for x = 5 at%. In this c o n c e n t r a t i o n range increasing Lac o n c e n t r a t i o n causes a r e d u c t i o n of i n c o h e r e n t scattering (fig. l a ) . It has been suggested [3] to a t t r i b u t e these findings to some long range (inhom o g e n e o u s l y b r o a d e n e d or m o d u l a t e d ) antiferromagnetic o r d e r to develop b e l o w T N of the o r d e r of several K. In o r d e r to s u b s t a n t i a t e this p r o p o s a l we show in figs. 2a a n d 2b results of the specific heat for CeCu2.2Si 2 and the 1.5 and 3 at% La alloys for T > T~(x). In fig. 2a the 4f-derived specific heat is shown vs. T. It was o b t a i n e d b y s u b t r a c t i n g the m o l a r specific heat of LaCu2. 2 Si 2 (also shown) f r o m that of the a f o r e - m e n t i o n e d s a m p l e s a n d expressing it p e r m o l e of Ce. A c c o r d i n g to the C4r ( T ) results, there is no difference b e t w e e n x = 0 a n d x = 1.5 at%, whereas slight differences are resolved in the p ( T ) a n d x i ( T ) results. C o m p a r e d with the results for these two samples, the 3 at% alloy exhibits a c o n s i d e r a b l y larger specific heat. As is shown in fig. 2b, this difference AC4f = C4f(x =0.03)-C4r(X=0.015), in fact, suggests the presence of a p h a s e transition n e a r 6 7 K, close to the location of the b r o a d x i ( T ) p e a k for the s a m e sample. The e n t r o p y associated with this a n o m a l y
,
La ~Cu: 2Si 2 below 10 K
1.5
]1[I
,~
r
~ ) :0.03 o : o.ols
,~
•
0.1 ~
:o
2d
Ce ×Lax Cu225i 2 d
0.05
0.5
~d
a ----..~
2
b
LaCumSi2 ~
"~
&
6 T{K}
"*"
"°: "°*
i
8
0
2
~
6 T (K)
8
10
Fig. 2. Specific heat, C(T), of C% xLaxCu22Si2. (a) Ci,a(T) per mole of LaCu2.2Si2, and 4f-derived part per mole of Ce, C4f(T)=(1 x)-I[C(T)-CLa(T)], for T>T~ (b) AC4r= C4r(x = 0.03)- C4f(X = 0.015) VS. T. (between T = 0 a n d 9 K) is A S a f = 0 . 0 3 R I n 2 / mol-Ce. In conclusion, a weak a n t i f e r r o m a g n e t i c transition, ( p r e s u m a b l y ) associated with very small o r d e r e d Ce m o m e n t s , seems to exist in dilute Ce 1 ~La,Cu2.2Si 2 systems in the range of several K. F o r x ~ 0, the N6el t e m p e r a t u r e a p p e a r s to decrease. O u r e x p e r i m e n t s allow no decision as to whether in pure CeCu2.2Si 2 a similar type of " i t i n e r a n t " a n t i f e r r o m a g n e t i s m below a lower T N eventually coexists with superconductivity, as was recently c o n c l u d e d from high-field ( B > Be2 ) N M R [5] and B = 0 ~tSR [6] experiments. This coexistence could, however, easily be investigated for the L a - d o p e d alloys, with T~ > 0.35 K for 1.5 < x < 5 at%. Investigations to c o m p l e t e the m a g n e t i c p h a s e d i a g r a m of Cel_xLaxCu2.2Si2 as well as ~ S R studies on these systems are in p r e p e r a t i o n .
References [1] F. Steglich, Springer Series in Solid-State Sciences 62 (1985) 23, and references therein. [2] U. Ahlheim, P. van Aken, H. Spille and F. Steglich, Helv. Phys. Acta 61 (1988) 518. [3] P. van Aken, F.R. de Boer, H. Spille, M. Winkelmann, U. Ahlheim, C.D. Bredl and F. Steglich, Physica C 153 155 (1988) 449. [4] F.G. Aliev, N.B. Brandt, V.V. Moshchalkov and S.M. Chudinov, J. Low Temp. Phys. 57 (1984) 61. [5] Y. Kitaoka, H. Yamada and K. Asayama, J. Magn. Magn. Mat., to be published. [6] Y.J. Uemura, W.J. Kossler, X.H. Yu, H.E. Schone, J.R. Kempton, C.E. Stronach, S. Barth, F.N. Gygax, B. Hini, A. Schenck, C. Baines, W.F. Lankford, Y. Onuki and T. Komatsubara (preprint, 1988).
EVIDENCE FOR ANTIFERROMAGNETISM
523
I N Ce 1_ xLaxCu2.2Si 2 B E L O W 10 K
P. van AKEN, U. A H L H E I M , C.D. B R E D L F.R. de BOER §, D. E N K L E R , G. SPARN, H. SPILLE and F. STEGLICH lnstitut ]'£tr Festk&perphvsik, Technische Hoehschule Darmstadt and SFB 252, D-6100 Darmstadt. Fed. Rep. Germw~v § Natuurkundig Laboratorium, Unmersiteit t,an Amsterdam, NL-IOI8 XE Amsterdam, The Netherlands Preliminary results of the resistivity, d c - s u s c e p t i b i l i t y a n d specific heat for C e 1 _ ~La~Cu2.2Si 2 (0.015 < x _< 0.05) reveal evidence for a n t i f e r r o m a g n e t i c o r d e r b e l o w T N = 3 7 K. T N ( x ) is m a x i m u m n e a r x = 0.03. The low e n t r o p y associated with the transition p o i n t s to small ordered Ce moments.
The magnetic phase diagram of CeCu2Si 2, though being complex and far from understood, appears to be dominated by V~, the unit-cell volume [1]: Stoichiometric CeCuzSi 2 single crystals are not superconducting at ambient pressure, but seem to be antiferromagnets below TN = 3.5 K. For CeCu2.2Si 2 polycrystalline samples l<,~ is reduced by ( 2 - 3 ) × 1 0 -3 compared to the stoichiometric single crystals. Upon doping CeCu22Si 2 with a few at % of La, the average value of V,~ will increase and become comparable to V,~ of single-crystalline CeCu2Si 2. If V~c is the key parameter in determining the magnetic properties of this material, antiferromagnetism should develop for C% _,. La ,Cu 2.2Si ~ with a few at% La. La-doping will at the same time lead to a Tcdepression. But since one can afford up to 11 at% of La impurities are required before T~ vanishes [2], for dilute Ce~ xLa~Cu2.2Si2 heavy-fermion superconductivity should coexist below T~(x) with antiferromagnetic order, which eventually develops already above 1 K. In this contribution we present preliminary results of the resistivity (fig. la), of the dc, low-field susceptibility (fig. lb) [3] and of the specific heat (fig. 2) for polycrystalline C%_~LaxCu2.2Si 2 s a m p l e s (0 _< x _< 0.13). These results are consistent with an antiferromagnetic transition to occur at T N = 3-10 K. All samples studied were single phase according to X-ray powder diffractometry and microprobe. In fig. l a the electrical resistivity p ( T ) is shown for CeCu22Si 2 and a number of its La-alloys. To avoid problems with the determination of the actual shape factor arising from microcracks in the polycITstalline samples, all p ( T ) curves are matched at 300 K. The surprising result of fig. l a is a depression and upward-shift of the low-T peak of the resistivity upon increasing La con-
centration from x = 0 up to 5 at%. At higher x-values, the low-T resistivity increases and the peak position is pushed downwards. The systematic variation in the o(T) dependencies of Cel_.~LaxCu22Si 2 is underlined by the fact that both height (0m~) and position (T~,) of the low-T peak are connected by a linear relationship in the whole concentration range 0 < x _< 0.13 [3]. DC magnetization experiments carried out on these alloys at T = 4.2 K up to B = 35 T, reveal the low-T value of the intrinsic susceptibility Xi and an extrinsic ("impurity") contribution X e C J ( T + 0~). If the Curie constant Ce is expressed in terms of "non-transformed", stable Ce 3+ moments ( J = 5 / 2 ) , their concentration Xc.e)+ is found to decrease steadily from about 2 at% for CeCu2.2Si2 to 0.3 at% for the 13 at% La sample [3]. This may be ascribed to the overall lattice expansion, which allows Ce 3+_,,impuri ties" to "relax", i.e. to transform into CF-split Kondo ions. As expected, the characteristic single-ion Kondo temperature T * that determines the essential E
\E
Ce. × LoxCu2.2 Si~
I
~
I
~
t
~
190
\
/
.. o_ •
--
0
--
I
100
I
200 V {KI
H
b
I
0
×
o
I0
~
=0
= 0.015 = 0.03
•
:
• u ×
= 0.09 : 0.11 = 0.13
I
2O
~
0.05
]
30 V (KI
......
/-,0
--
30 20
-
---10
~--0
~0
Fig. 1. N o r m a l i z e d resistivity p ( T ) / o ( 3 0 0 K) (a) a n d intrinsic d c - s u s c e p t i b i l i t y x i ( T ) (b) for C % _ , L a ~ L a , C u 2 2 S i 2 (0 __
0304-8853/88/$03.50 © Elsevier Science Publishers B.V.
__
524
P. van Aken et al. / Antiferromagnetism in CeI
t h e r m o d y n a m i c p r o p e r t i e s of the alloys decreases with increasing L a c o n c e n t r a t i o n , as c o n c l u d e d from an increase in Xi(4.2 K) from 7.3 × 10 S m 3 / m o l - C e for x = 0 to 8.0 x 10 -~ m 3 / m o l Ce for x = 13 at% [3]. T h e t e m p e r a t u r e d e p e n d e n c e of the intrinsic susceptibility x i ( T ) as o b t a i n e d from the raw d a t a x ( T ) (taken at B = I T) via x ~ ( T ) = x ( T ) C e / ( T + 0~) is shown in fig. ] b for CeCu2.2Si 2 a n d the samples with 1.5, 3, 5 a n d 13 at% La concentration. Clearly, three qualitatively different type of x i ( T ) curves are o b t a i n e d : (i) The pure c o m p o u n d a n d the 1.5 at% La s a m p l e exhibit a c o n s t a n t ( " P a u l i - l i k e " ) value b e l o w 3 K which c o n t i n u o u s l y decreases u p o n w a r m i n g [1]. (ii) F o r x > 7 at%, x ~ ( T ) shows a negative t e m p e r a t u r e coefficient d o w n to 2 K, the lowest accessible t e m p e r a t u r e . A c c o r d i n g to earlier w o r k on stoichiometric C e l _ x L a x C u 2 S i 2 alloys [4] we might expect some " s p i n - g l a s s " type o r d e r i n g at lower temperatures. In the same c o n c e n t r a t i o n range ( x > 7 at%), a d d i t i o n of La causes an increased incoherent scattering a b o v e 4.2 K (fig. ]a). (iii) A flat m a x i m u m in x i ( T ) n e a r 5 - 1 0 K is o b s e r v e d for x = 3 at% a n d at 3 - 4 K for x = 5 at%. In this c o n c e n t r a t i o n range increasing Lac o n c e n t r a t i o n causes a r e d u c t i o n of i n c o h e r e n t scattering (fig. l a ) . It has been suggested [3] to a t t r i b u t e these findings to some long range (inhom o g e n e o u s l y b r o a d e n e d or m o d u l a t e d ) antiferromagnetic o r d e r to develop b e l o w T N of the o r d e r of several K. In o r d e r to s u b s t a n t i a t e this p r o p o s a l we show in figs. 2a a n d 2b results of the specific heat for CeCu2.2Si 2 and the 1.5 and 3 at% La alloys for T > T~(x). In fig. 2a the 4f-derived specific heat is shown vs. T. It was o b t a i n e d b y s u b t r a c t i n g the m o l a r specific heat of LaCu2. 2 Si 2 (also shown) f r o m that of the a f o r e - m e n t i o n e d s a m p l e s a n d expressing it p e r m o l e of Ce. A c c o r d i n g to the C4r ( T ) results, there is no difference b e t w e e n x = 0 a n d x = 1.5 at%, whereas slight differences are resolved in the p ( T ) a n d x i ( T ) results. C o m p a r e d with the results for these two samples, the 3 at% alloy exhibits a c o n s i d e r a b l y larger specific heat. As is shown in fig. 2b, this difference AC4f = C4f(x =0.03)-C4r(X=0.015), in fact, suggests the presence of a p h a s e transition n e a r 6 7 K, close to the location of the b r o a d x i ( T ) p e a k for the s a m e sample. The e n t r o p y associated with this a n o m a l y
,
La ~Cu: 2Si 2 below 10 K
1.5
]1[I
,~
r
~ ) :0.03 o : o.ols
,~
•
0.1 ~
:o
2d
Ce ×Lax Cu225i 2 d
0.05
0.5
~d
a ----..~
2
b
LaCumSi2 ~
"~
&
6 T{K}
"*"
"°: "°*
i
8
0
2
~
6 T (K)
8
10
Fig. 2. Specific heat, C(T), of C% xLaxCu22Si2. (a) Ci,a(T) per mole of LaCu2.2Si2, and 4f-derived part per mole of Ce, C4f(T)=(1 x)-I[C(T)-CLa(T)], for T>T~ (b) AC4r= C4r(x = 0.03)- C4f(X = 0.015) VS. T. (between T = 0 a n d 9 K) is A S a f = 0 . 0 3 R I n 2 / mol-Ce. In conclusion, a weak a n t i f e r r o m a g n e t i c transition, ( p r e s u m a b l y ) associated with very small o r d e r e d Ce m o m e n t s , seems to exist in dilute Ce 1 ~La,Cu2.2Si 2 systems in the range of several K. F o r x ~ 0, the N6el t e m p e r a t u r e a p p e a r s to decrease. O u r e x p e r i m e n t s allow no decision as to whether in pure CeCu2.2Si 2 a similar type of " i t i n e r a n t " a n t i f e r r o m a g n e t i s m below a lower T N eventually coexists with superconductivity, as was recently c o n c l u d e d from high-field ( B > Be2 ) N M R [5] and B = 0 ~tSR [6] experiments. This coexistence could, however, easily be investigated for the L a - d o p e d alloys, with T~ > 0.35 K for 1.5 < x < 5 at%. Investigations to c o m p l e t e the m a g n e t i c p h a s e d i a g r a m of Cel_xLaxCu2.2Si2 as well as ~ S R studies on these systems are in p r e p e r a t i o n .
References [1] F. Steglich, Springer Series in Solid-State Sciences 62 (1985) 23, and references therein. [2] U. Ahlheim, P. van Aken, H. Spille and F. Steglich, Helv. Phys. Acta 61 (1988) 518. [3] P. van Aken, F.R. de Boer, H. Spille, M. Winkelmann, U. Ahlheim, C.D. Bredl and F. Steglich, Physica C 153 155 (1988) 449. [4] F.G. Aliev, N.B. Brandt, V.V. Moshchalkov and S.M. Chudinov, J. Low Temp. Phys. 57 (1984) 61. [5] Y. Kitaoka, H. Yamada and K. Asayama, J. Magn. Magn. Mat., to be published. [6] Y.J. Uemura, W.J. Kossler, X.H. Yu, H.E. Schone, J.R. Kempton, C.E. Stronach, S. Barth, F.N. Gygax, B. Hini, A. Schenck, C. Baines, W.F. Lankford, Y. Onuki and T. Komatsubara (preprint, 1988).