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Conduction electron spin polarization in some intermetallic europium compounds
89 CONDUCTION ELECTRON SPIN POLARIZATION IN SOME INTERMETALLIC EUROPIUM COMPOUNDS F.J. V A N STEENWIJK, W.J. H U I S K A M P , H.Th. L E F E V E R , R.C. THIEL Kamerlingh Onnes Laboratory, Leiden, The Netherlands
and K.H.J. B U S C H O W Philips Research Laboratories, Eindhoven, The Netherlands F o r the C a C u s - t y p e c o m p o u n d s E u X , (X = Cu, Zn, Ag, Au) the m a g n e t i c p r o p e r t i e s , the l a t t i c e c o n s t a n t s , the ISlEu h y p e r f i n e fields a n d the i s o m e r s h i f t s w e r e d e t e r m i n e d . T h e t r a n s f e r r e d h y p e r f i n e fields w e r e e s t i m a t e d b y m e a n s of d a t a o b t a i n e d o n s o m e p s e u d o b i n a r i e s EuI_,R~X~ (R = Ca, L a , Gd). T h e a p p l i c a b i l i t y of the R K K Y m o d e l is d i s c u s s e d .
The compounds EuCus, EuZns, EuAgs, EuAu5 and related pseudobinaries were prepared by heating the constituent metals inside a molybdenum container as has been described before [1]. X-ray diffraction showed that all compounds exhibit the CaCu5 structure and were of single phase or contained only minor amounts of impurity phases. Only in the case of the Ag corn-
pound we had to use an excess Ag to obtain the CaCu5 phase without contamination of EuAg4. The lattice constants are given in table I. The magnetic measurements were performed on polycrystalline samples using an adaptation of the Faraday method. M6ssbauer data were obtained by means of the 21.7 keV resonance of ISlEu (cf. refs 1 and 2).
Table I Lattice constants
Ordering temperature*
(A)
(K) Tc = 5 7 7"o=57 Tc = 42
Euo.zLao.gZns
a =5.138 c =4.114 a = 5.126 c = 4.096 a =5.101 c = 4.099 a=5.143 c =4.103 a = 5.177 4.088 a = 5.100 c = 4.099 a = 5.437 c = 4.312 a = 5.44 c = 4.267 a = 5.468 c = 4.267
EuAg5
a = 5.629 c = 4.638
Tc = 19 To = 18
19
7.43
a c a c
Tc = 13 To = 14
14
Tc = 25
20
EuCu~ Euo.sCao.2Cus Euo.2CaosCus Euo ,Lao :Cu5 • " Euo 2Lao 8Cu~ •
"
Euo 2Gdo,Cu~ EuZn5 E t h 2Cao sZns
EuAu5 GdCus
c
0p** (K)
/~cn (/z J E u )
t~(18 k O e ) (/~B/Eu)
[Hcnl*** (kOe)
58
7.6
7.2
269(3)
7.5(6)
- 8.2
39
8.04
4.8
270(3)
7.5(6)
- 8.1
265(3)
6.5(6)
-8.3
274(3)
7.2(6)
- 8.1
To < 6
274(5)
7.9(6)
- 8.1
18 < To < 40
298(3)
6.2(6)
-7.9
292(3)
0.4(6)
- 10.0
To ~- 5
298(4)
0.6(6)
- 10.0
To < 6
284(4)
- 1.2(6)
-9.9
6.3
245(3)
6.2(6)
- 10.1
7.78
6.6
274(3)
9.4(6)
- 10.4
7.89
5.4
--
To = 15 L=47 To---45
43
7.87
5.0
Vzz (1017 V c m -2)
I.S.**** (mmls)
=
= 5.545 = 4.587 = 5.039 =4.111
T, = 10 To = 10.6
- 24
7.99
--
*To, TN f r o m m a g n e t i c m e a s u r e m e n t s , To f r o m M 6 s s b a u e r m e a s u r e m e n t s . ** P a r a m a g n e ~ i c C u r i e t e m p e r a t u r e . *** E x t r a p o l a t e d to 0 K. **** R e l a t i v e to '5'Eu : SmF~.
Physica 86--88B (1977) 8 9 - 9 0 © North-Holland
90 As a representative example, the temperature dependence of the magnetization and the reciprocal susceptibility of the compound EuAu5 are shown in fig. I. The data obtained from these measurements are listed in table 1, together with the results of other compounds. The effective moments are close to 7.94/~a and thus show that the Eu compounds contain Eu in its divalent state. The Mfssbauer spectra for EuAu5 are shown in fig. 2. They were analysed in terms of 18 lines as described in ref. 1. Spectra were also collected for a number of pseudobinary compounds, based on EuCu5 and EuZns, but with part of the Eu atoms replaced by Ca, La, or Gd. Also these spectra could be analysed in terms of one hyperfine field. For all compounds the results for hyperfine field, electric field gradient (V=) and isomer shift are listed in table I.
30 -
Eu Au 5
These comparisons show that in all c a s e s H N is rather small. For ferromagnets H N and 0p are proportional in the RKKY model. The small values obtained for EuCu5 and GdCu5 contrast with this view: e.g. HN(EuCus) is much too small when compared with the relative high value for 0p. Reasoning along similar lines as in ref. 4, this could perhaps indicate that the magnetic interactions in these compounds proceed to a large extent through d-type conduction electrons rather than s-type electrons. It was suggested by Nowik et al. [5], that correlation exists between the I.S. and H o p , the hyperfine field contribution due to the Eu spin's own 4f moment. Our value of H o p for EuCu5 ( - + 7 5 kOe) is relatively low in view of the rather large charge density at the nucleus, which may perhaps be regarded as a further indication that other than s-type electrons may play an active role in the magnetic exchange interactions in this compound.
5o
l
w
J
EuAus
L.@ - '~
H= 18 kOe
i
~.~,~¢~¢.~"r.
100 ,'~.~;~ ~~'~"~,~ ~
~
30 __ 98
*~ 2O 10 .
z o
205 K 96
~,
tn i./3 10
~I0C
oi
100
200
T(~,)
300
'"
"
" ""'~'""
,~
o
Fig. 1. Temperature dependence of the magnetization (tr) and reciprocal susceptibility (X -') of EuAu~.
The hyperfine fields can be decomposed as follows: Heft, = Hcp + Hop + HN-
The core polarization term H c p is approximately equal to - 3 4 0 kOe [3]. The transferred field H N due to conduction electron polarization by the surrounding Eu moments can be estimated for EuCu5 and EuZn5 by comparing the hyperfine fields observed for these compounds with those of Euo2CaosCu5 and Euo2Cao.sZns. An estimate of H~ in GdCu5 may be obtained by comparing the fields observed in Euo.2GdosCu5 and Euo.2Lao.sCus.
or"
~
4, ,
":
98
'
11
K
~, I
-40
I
I
1
t
I
10
I
-2 0 0 VELOCITY (MM/SEC)
Fig. 2. M6ssbauer spectrum of EuAu5 obtained at 20.5 and I.IK.
References [1] K.H.J. Buschow, W.J. Huiskamp, H.Th. LeFever and F.J. van Steenwijk, J. Phys. F: Metal Phys. 5 (1975) 1625. [2] F.J. van Steenwijk, H.Th. LeFever, R.C. Thiel and K.H.J. Buschow, Physica 79B (1975) 604. [3] J.M. Baker and F.I.B. Williams, Proc. Roy. Soc. A264 (1962) 283. [4] E. Dormann and K.H.J. Buschow, J. Appl. Phys. 47 (1976) 1662. [5] I. Nowik, B.D. Dunlap and J.H. Wernick, Phys. Rev. B8 (1973) 238.
and K.H.J. B U S C H O W Philips Research Laboratories, Eindhoven, The Netherlands F o r the C a C u s - t y p e c o m p o u n d s E u X , (X = Cu, Zn, Ag, Au) the m a g n e t i c p r o p e r t i e s , the l a t t i c e c o n s t a n t s , the ISlEu h y p e r f i n e fields a n d the i s o m e r s h i f t s w e r e d e t e r m i n e d . T h e t r a n s f e r r e d h y p e r f i n e fields w e r e e s t i m a t e d b y m e a n s of d a t a o b t a i n e d o n s o m e p s e u d o b i n a r i e s EuI_,R~X~ (R = Ca, L a , Gd). T h e a p p l i c a b i l i t y of the R K K Y m o d e l is d i s c u s s e d .
The compounds EuCus, EuZns, EuAgs, EuAu5 and related pseudobinaries were prepared by heating the constituent metals inside a molybdenum container as has been described before [1]. X-ray diffraction showed that all compounds exhibit the CaCu5 structure and were of single phase or contained only minor amounts of impurity phases. Only in the case of the Ag corn-
pound we had to use an excess Ag to obtain the CaCu5 phase without contamination of EuAg4. The lattice constants are given in table I. The magnetic measurements were performed on polycrystalline samples using an adaptation of the Faraday method. M6ssbauer data were obtained by means of the 21.7 keV resonance of ISlEu (cf. refs 1 and 2).
Table I Lattice constants
Ordering temperature*
(A)
(K) Tc = 5 7 7"o=57 Tc = 42
Euo.zLao.gZns
a =5.138 c =4.114 a = 5.126 c = 4.096 a =5.101 c = 4.099 a=5.143 c =4.103 a = 5.177 4.088 a = 5.100 c = 4.099 a = 5.437 c = 4.312 a = 5.44 c = 4.267 a = 5.468 c = 4.267
EuAg5
a = 5.629 c = 4.638
Tc = 19 To = 18
19
7.43
a c a c
Tc = 13 To = 14
14
Tc = 25
20
EuCu~ Euo.sCao.2Cus Euo.2CaosCus Euo ,Lao :Cu5 • " Euo 2Lao 8Cu~ •
"
Euo 2Gdo,Cu~ EuZn5 E t h 2Cao sZns
EuAu5 GdCus
c
0p** (K)
/~cn (/z J E u )
t~(18 k O e ) (/~B/Eu)
[Hcnl*** (kOe)
58
7.6
7.2
269(3)
7.5(6)
- 8.2
39
8.04
4.8
270(3)
7.5(6)
- 8.1
265(3)
6.5(6)
-8.3
274(3)
7.2(6)
- 8.1
To < 6
274(5)
7.9(6)
- 8.1
18 < To < 40
298(3)
6.2(6)
-7.9
292(3)
0.4(6)
- 10.0
To ~- 5
298(4)
0.6(6)
- 10.0
To < 6
284(4)
- 1.2(6)
-9.9
6.3
245(3)
6.2(6)
- 10.1
7.78
6.6
274(3)
9.4(6)
- 10.4
7.89
5.4
--
To = 15 L=47 To---45
43
7.87
5.0
Vzz (1017 V c m -2)
I.S.**** (mmls)
=
= 5.545 = 4.587 = 5.039 =4.111
T, = 10 To = 10.6
- 24
7.99
--
*To, TN f r o m m a g n e t i c m e a s u r e m e n t s , To f r o m M 6 s s b a u e r m e a s u r e m e n t s . ** P a r a m a g n e ~ i c C u r i e t e m p e r a t u r e . *** E x t r a p o l a t e d to 0 K. **** R e l a t i v e to '5'Eu : SmF~.
Physica 86--88B (1977) 8 9 - 9 0 © North-Holland
90 As a representative example, the temperature dependence of the magnetization and the reciprocal susceptibility of the compound EuAu5 are shown in fig. I. The data obtained from these measurements are listed in table 1, together with the results of other compounds. The effective moments are close to 7.94/~a and thus show that the Eu compounds contain Eu in its divalent state. The Mfssbauer spectra for EuAu5 are shown in fig. 2. They were analysed in terms of 18 lines as described in ref. 1. Spectra were also collected for a number of pseudobinary compounds, based on EuCu5 and EuZns, but with part of the Eu atoms replaced by Ca, La, or Gd. Also these spectra could be analysed in terms of one hyperfine field. For all compounds the results for hyperfine field, electric field gradient (V=) and isomer shift are listed in table I.
30 -
Eu Au 5
These comparisons show that in all c a s e s H N is rather small. For ferromagnets H N and 0p are proportional in the RKKY model. The small values obtained for EuCu5 and GdCu5 contrast with this view: e.g. HN(EuCus) is much too small when compared with the relative high value for 0p. Reasoning along similar lines as in ref. 4, this could perhaps indicate that the magnetic interactions in these compounds proceed to a large extent through d-type conduction electrons rather than s-type electrons. It was suggested by Nowik et al. [5], that correlation exists between the I.S. and H o p , the hyperfine field contribution due to the Eu spin's own 4f moment. Our value of H o p for EuCu5 ( - + 7 5 kOe) is relatively low in view of the rather large charge density at the nucleus, which may perhaps be regarded as a further indication that other than s-type electrons may play an active role in the magnetic exchange interactions in this compound.
5o
l
w
J
EuAus
L.@ - '~
H= 18 kOe
i
~.~,~¢~¢.~"r.
100 ,'~.~;~ ~~'~"~,~ ~
~
30 __ 98
*~ 2O 10 .
z o
205 K 96
~,
tn i./3 10
~I0C
oi
100
200
T(~,)
300
'"
"
" ""'~'""
,~
o
Fig. 1. Temperature dependence of the magnetization (tr) and reciprocal susceptibility (X -') of EuAu~.
The hyperfine fields can be decomposed as follows: Heft, = Hcp + Hop + HN-
The core polarization term H c p is approximately equal to - 3 4 0 kOe [3]. The transferred field H N due to conduction electron polarization by the surrounding Eu moments can be estimated for EuCu5 and EuZn5 by comparing the hyperfine fields observed for these compounds with those of Euo2CaosCu5 and Euo2Cao.sZns. An estimate of H~ in GdCu5 may be obtained by comparing the fields observed in Euo.2GdosCu5 and Euo.2Lao.sCus.
or"
~
4, ,
":
98
'
11
K
~, I
-40
I
I
1
t
I
10
I
-2 0 0 VELOCITY (MM/SEC)
Fig. 2. M6ssbauer spectrum of EuAu5 obtained at 20.5 and I.IK.
References [1] K.H.J. Buschow, W.J. Huiskamp, H.Th. LeFever and F.J. van Steenwijk, J. Phys. F: Metal Phys. 5 (1975) 1625. [2] F.J. van Steenwijk, H.Th. LeFever, R.C. Thiel and K.H.J. Buschow, Physica 79B (1975) 604. [3] J.M. Baker and F.I.B. Williams, Proc. Roy. Soc. A264 (1962) 283. [4] E. Dormann and K.H.J. Buschow, J. Appl. Phys. 47 (1976) 1662. [5] I. Nowik, B.D. Dunlap and J.H. Wernick, Phys. Rev. B8 (1973) 238.