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Temperature and concentration dependence of the electrical resistivity in (RE, Y)Co2 (RE = rare earth element)
Journal of Magnetism and Magnetic Materials 70 (1987) 159-161 North-Holland, Amsterdam
159
T E M P E R A T U R E AND C O N C E N T R A T I O N D E P E N D E N C E OF T H E ELECTRICAL RESISTIVITY IN (RE, Y)C% (RE = RARE E A R T H E L E M E N T ) E. GRATZ, N. PILLMAYR, E. BAUER and G. H I L S C H E R Institute q[ Experimental Physics, Technical Uniuersity Vienna, Austria
We study the influence of magnetic scattering processes on the temperature dependence of the electrical resistivity. This influence is caused by localized M-moments and spin-fluctuations. The pronounced minima in p vs. T curves observed in the diluted RE-region are discussed in terms of enhanced spin-fluctuations and of Kondo type scattering processes.
In previous papers transport properties, such as electrical resistivity and thermopower, have been studied in the REC%-compounds [1] and in some of the (RE, Y)Co2-pseudobinaries [2,3]. The temperature dependence of the resistivity and the thermopower shows pronounced discontinuities at T~ (T~.=Curie temperature) for DyCo 2 (T~= 135 K), HoCo 2 (T~=78 K) and ErC% (Tc = 3 2 K), which are of the first order type. Beyond these first order transitions the most interesting features are very deep minima in the p vs. T curves of ( H 0 ~ Y I _ ~ ) C o 2 around x = 0 . 5 , accompanied by large minima in the thermopower [3]. The aim of the present paper is to show that these resistivity minima first observed in (Ho, Y)Co 2 also exist in (Dy, Y)Co 2 and in (Er, Y)C%. This phenomenon will be discussed in view of spin-fluctuation, spinfreezing and Kondo type interaction. The o(T) curves of (Dy~Y1 ~.)Co2 and (Er~ YI_ ,)Co2 are presented in fig. 1 and fig. 2, respectively; ( H o r Y I _ , . ) C o 2 has been shown previously [3]. The concentration regions where the compounds show long range magnetic order are as follows 0.4 ~
uniform spin-fluctuations on the other hand give rise to spin dependent scattering processes of the conduction electrons in these systems. The o(T) behaviour of the exchange enhanced paramagnet YCo 2 shows a pronounced saturation tendency around 150 K (see fig. 1) and varies linearly in temperature up to 1000 K [6]. This behaviour at high temperature together with a T2-depen dence at low temperatures [6] indicate the dominant contribution of spin-fluctuations in YCo 2. Gschneidner and Ikeda [7] have established the existence of spin-fluctuations in YCo 2 by means of specific heat measurements in external fields.
5 16G
(Dy~ YT-x )C°2
lO0 8O 60
"4Y//"°7//"" Oi
' 2'0 ' Z.'O ' 6'0 ' dO ' 100 16~
.,o. 120
; l~O
160 T[KJ
Fig. 1. The temperature dependence of the electrical resistivity of the (Dy,.Y1 ,.)Co 2 series for (0 ~< x ~< 0.7). The inset shows the resistivity at 4.2 K.
0304-8853/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
160
E. Gratz et al.
11o
[i
j
/
/
Electrtcal resixtit~i(v in (RE. Y ) O 0
x:07 200
90
/ j
160 ~ ~ _ -
70
12G
200
~--~
~~6(ErxYf-x)C°2 ~ ~
\\
/
Er---~', , ~.)~ ~;}/
']160 120
50 30
80"'*"--..!. .....
10
~0 (Gd'Y)AI2/¢~~'-
so !
x=03
30~
40
\//
::t .". 50
20
\/
20 0
0
Ydo:02 & o6"~ 20
40
60
80
100
120
IZO T[K]
Fig. 2. The temperature dependence of the electrical resistivity of the (Er,Y I • )Co2 series for (0.3 ~
06
x
04
80 .
" = .40 02
0 YC°2 YAI2
Fig. 3. The temperature dependent part of the electrical resistivity at 273 K, given by Ap = 0(273 K ) - ,0(4.2 K) versus the concentration for (RE, Y)Co2 (RE = Dy, Ho. I-{r)and for (Gd, Y)AI 2.
50 30:
0 08 REC°2 GdA/2
"~,)><, Y " / i" I" Dy
273 K)
- o ( T = 4.2 K)) for the (RE, Y ) C % and the isostructural (Gd, Y)A12. In o r d e r to e l i m i n a t e the influence of the residual resistivity O0 we used At) as a measure of the t e m p e r a t u r e d e p e n d e n t scattering c o n t r i b u t i o n s (e.g. e l e c t r o n - p h o n o n , e l e c t r o n - s p i n w a v e s , e l e c t r o n - s p i n fluctuations, etc). At) of Y C o 2 is e n h a n c e d by a factor of 4 in c o m p a r i s o n to YA12. This e n h a n c e m e n t is caused by spin-fluctuations. A rather irregular variation of AO with increasing x for the (RE,Y~ , ) C o 2 c o m p o u n d s follows from fig. 3, while a fairly uniform x d e p e n d e n c e is o b s e r v e d for ( G d , Y)AI ~. The complex trend of A p ( x ) is i n t i m a t e l y connected with the a p p e a r a n c e of m i n i m a in the t e m p e r a t u r e d e p e n d e n c e of the electrical resistivity. The occurence of those m i n i m a in the O vs. T curves c a n n o t simply be a t t r i b u t e d to freezing
p h e n o m e n a , since for the classical (Gd,Y~ ,)AI 2 series in the spin-glass regime (0 < x < 0.3) pron o u n c e d m i n i m a neither in O vs. T nor in Ap vs. T are observable [8]. It is also rather unlikely that the onset of the induced C o - m o m e n t s can give rise to these minima, which are not observed for example in other (RE, Y ) C o 2 - c o m p o u n d s ( R E = Pr, N d , Gd), although in these c o m p o u n d s the onset of the i n d u c e d itinerant C o - m o m e n t s occurs below the respective critical c o n c e n t r a t i o n range [9,10]. Usually, m i n i m a in the t e m p e r a t u r e dependence of p ( T ) are a t t r i b u t e d to K o n d o type scattering processes. A closer inspection of the resistivity d a t a of (Ho, Y)Co z in the vicinity of m i n i m a shows a ( - I n T ) d e p e n d e n c e of the magnetic c o n t r i b u t i o n to p. This a p p e a r s to be a necessary c o n d i t i o n for the existence of K o n d o type interaction• In this connection it is notew o r t h y that we found by specific heat measurem e n t s [4,5] a drastic increase of the electronic specific heat coefficient and a significant loss of the magnetic e n t r o p y in the dilute R E concentration range, This was a t t r i b u t e d to an increasing extent of s p i n - f l u c t u a t i o n s and to a p r o b a b l e instability of the R E - m o m e n t s [4]. Both m e c h a n i s m s increase in these series when p r o c e e d i n g from Dy to Er. Such an instability of a R E - m o m e n t was recently p r o p o s e d by E d w a r d s [1 1] and may pre-
£2 Gratz et al. / Electrical resistiviO, in (RE, Y)Co 2
sumably be responsible for the pronounced tivity minima.
resis-
References [1] E. Gratz, H. Sassik and H. Nowotny, J. Phys. FI1 (1981) 429. [21 E. Gratz, H. Nowotny and W, Steiner, Proc. Intern. Conf. on Rare Earths and Actinides, eds. E. Burzo and M. Rogalski, Bucarest, 1983, p. 116. [3] E. Gratz, E. Bauer, V. Sechovsky and J. Chmist, J. Magn. Magn. Mat. 54-57 (1986) 517. [4] G. Hilscher, N. Pillmayr, C. Schmitzer and E. Gratz, to be published.
161
[5] N. Pillmayr, C. Schmitzer, E. Gratz, G. Hilscher and V. Sechovsky, J. Magn. Magn. Mat. 70 (1987) 162. [6] E. Gratz, E. Bauer, H. Nowotny, J. Magn. Magn. Mat. 70 (1987) 118, [7] K.A. Gschneidner Jr. and K. Ikeda, J. Magn. Magn. Mat. 31-34 (1983) 265. [8] E. Bauer, E. Gratz and H. Kirchmayr, Z. Phys. 68 (1987) 63. [9] N.H. Duc, T.H. Hien, P.E. Brommer and J.J.M. Franse, to appear in J. Phys. F (1987). [10] K. Yoshimura, S. Hirosawa and Y. Nakamura, J, Phys. Soc. Japan 53 (1984) 2120. [11] D.M. Edwards, Proc. 5th Intern. Conf. on Valence Fluctuations, Bangalore, India.
159
T E M P E R A T U R E AND C O N C E N T R A T I O N D E P E N D E N C E OF T H E ELECTRICAL RESISTIVITY IN (RE, Y)C% (RE = RARE E A R T H E L E M E N T ) E. GRATZ, N. PILLMAYR, E. BAUER and G. H I L S C H E R Institute q[ Experimental Physics, Technical Uniuersity Vienna, Austria
We study the influence of magnetic scattering processes on the temperature dependence of the electrical resistivity. This influence is caused by localized M-moments and spin-fluctuations. The pronounced minima in p vs. T curves observed in the diluted RE-region are discussed in terms of enhanced spin-fluctuations and of Kondo type scattering processes.
In previous papers transport properties, such as electrical resistivity and thermopower, have been studied in the REC%-compounds [1] and in some of the (RE, Y)Co2-pseudobinaries [2,3]. The temperature dependence of the resistivity and the thermopower shows pronounced discontinuities at T~ (T~.=Curie temperature) for DyCo 2 (T~= 135 K), HoCo 2 (T~=78 K) and ErC% (Tc = 3 2 K), which are of the first order type. Beyond these first order transitions the most interesting features are very deep minima in the p vs. T curves of ( H 0 ~ Y I _ ~ ) C o 2 around x = 0 . 5 , accompanied by large minima in the thermopower [3]. The aim of the present paper is to show that these resistivity minima first observed in (Ho, Y)Co 2 also exist in (Dy, Y)Co 2 and in (Er, Y)C%. This phenomenon will be discussed in view of spin-fluctuation, spinfreezing and Kondo type interaction. The o(T) curves of (Dy~Y1 ~.)Co2 and (Er~ YI_ ,)Co2 are presented in fig. 1 and fig. 2, respectively; ( H o r Y I _ , . ) C o 2 has been shown previously [3]. The concentration regions where the compounds show long range magnetic order are as follows 0.4 ~
uniform spin-fluctuations on the other hand give rise to spin dependent scattering processes of the conduction electrons in these systems. The o(T) behaviour of the exchange enhanced paramagnet YCo 2 shows a pronounced saturation tendency around 150 K (see fig. 1) and varies linearly in temperature up to 1000 K [6]. This behaviour at high temperature together with a T2-depen dence at low temperatures [6] indicate the dominant contribution of spin-fluctuations in YCo 2. Gschneidner and Ikeda [7] have established the existence of spin-fluctuations in YCo 2 by means of specific heat measurements in external fields.
5 16G
(Dy~ YT-x )C°2
lO0 8O 60
"4Y//"°7//"" Oi
' 2'0 ' Z.'O ' 6'0 ' dO ' 100 16~
.,o. 120
; l~O
160 T[KJ
Fig. 1. The temperature dependence of the electrical resistivity of the (Dy,.Y1 ,.)Co 2 series for (0 ~< x ~< 0.7). The inset shows the resistivity at 4.2 K.
0304-8853/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
160
E. Gratz et al.
11o
[i
j
/
/
Electrtcal resixtit~i(v in (RE. Y ) O 0
x:07 200
90
/ j
160 ~ ~ _ -
70
12G
200
~--~
~~6(ErxYf-x)C°2 ~ ~
\\
/
Er---~', , ~.)~ ~;}/
']160 120
50 30
80"'*"--..!. .....
10
~0 (Gd'Y)AI2/¢~~'-
so !
x=03
30~
40
\//
::t .". 50
20
\/
20 0
0
Ydo:02 & o6"~ 20
40
60
80
100
120
IZO T[K]
Fig. 2. The temperature dependence of the electrical resistivity of the (Er,Y I • )Co2 series for (0.3 ~
06
x
04
80 .
" = .40 02
0 YC°2 YAI2
Fig. 3. The temperature dependent part of the electrical resistivity at 273 K, given by Ap = 0(273 K ) - ,0(4.2 K) versus the concentration for (RE, Y)Co2 (RE = Dy, Ho. I-{r)and for (Gd, Y)AI 2.
50 30:
0 08 REC°2 GdA/2
"~,)><, Y " / i" I" Dy
273 K)
- o ( T = 4.2 K)) for the (RE, Y ) C % and the isostructural (Gd, Y)A12. In o r d e r to e l i m i n a t e the influence of the residual resistivity O0 we used At) as a measure of the t e m p e r a t u r e d e p e n d e n t scattering c o n t r i b u t i o n s (e.g. e l e c t r o n - p h o n o n , e l e c t r o n - s p i n w a v e s , e l e c t r o n - s p i n fluctuations, etc). At) of Y C o 2 is e n h a n c e d by a factor of 4 in c o m p a r i s o n to YA12. This e n h a n c e m e n t is caused by spin-fluctuations. A rather irregular variation of AO with increasing x for the (RE,Y~ , ) C o 2 c o m p o u n d s follows from fig. 3, while a fairly uniform x d e p e n d e n c e is o b s e r v e d for ( G d , Y)AI ~. The complex trend of A p ( x ) is i n t i m a t e l y connected with the a p p e a r a n c e of m i n i m a in the t e m p e r a t u r e d e p e n d e n c e of the electrical resistivity. The occurence of those m i n i m a in the O vs. T curves c a n n o t simply be a t t r i b u t e d to freezing
p h e n o m e n a , since for the classical (Gd,Y~ ,)AI 2 series in the spin-glass regime (0 < x < 0.3) pron o u n c e d m i n i m a neither in O vs. T nor in Ap vs. T are observable [8]. It is also rather unlikely that the onset of the induced C o - m o m e n t s can give rise to these minima, which are not observed for example in other (RE, Y ) C o 2 - c o m p o u n d s ( R E = Pr, N d , Gd), although in these c o m p o u n d s the onset of the i n d u c e d itinerant C o - m o m e n t s occurs below the respective critical c o n c e n t r a t i o n range [9,10]. Usually, m i n i m a in the t e m p e r a t u r e dependence of p ( T ) are a t t r i b u t e d to K o n d o type scattering processes. A closer inspection of the resistivity d a t a of (Ho, Y)Co z in the vicinity of m i n i m a shows a ( - I n T ) d e p e n d e n c e of the magnetic c o n t r i b u t i o n to p. This a p p e a r s to be a necessary c o n d i t i o n for the existence of K o n d o type interaction• In this connection it is notew o r t h y that we found by specific heat measurem e n t s [4,5] a drastic increase of the electronic specific heat coefficient and a significant loss of the magnetic e n t r o p y in the dilute R E concentration range, This was a t t r i b u t e d to an increasing extent of s p i n - f l u c t u a t i o n s and to a p r o b a b l e instability of the R E - m o m e n t s [4]. Both m e c h a n i s m s increase in these series when p r o c e e d i n g from Dy to Er. Such an instability of a R E - m o m e n t was recently p r o p o s e d by E d w a r d s [1 1] and may pre-
£2 Gratz et al. / Electrical resistiviO, in (RE, Y)Co 2
sumably be responsible for the pronounced tivity minima.
resis-
References [1] E. Gratz, H. Sassik and H. Nowotny, J. Phys. FI1 (1981) 429. [21 E. Gratz, H. Nowotny and W, Steiner, Proc. Intern. Conf. on Rare Earths and Actinides, eds. E. Burzo and M. Rogalski, Bucarest, 1983, p. 116. [3] E. Gratz, E. Bauer, V. Sechovsky and J. Chmist, J. Magn. Magn. Mat. 54-57 (1986) 517. [4] G. Hilscher, N. Pillmayr, C. Schmitzer and E. Gratz, to be published.
161
[5] N. Pillmayr, C. Schmitzer, E. Gratz, G. Hilscher and V. Sechovsky, J. Magn. Magn. Mat. 70 (1987) 162. [6] E. Gratz, E. Bauer, H. Nowotny, J. Magn. Magn. Mat. 70 (1987) 118, [7] K.A. Gschneidner Jr. and K. Ikeda, J. Magn. Magn. Mat. 31-34 (1983) 265. [8] E. Bauer, E. Gratz and H. Kirchmayr, Z. Phys. 68 (1987) 63. [9] N.H. Duc, T.H. Hien, P.E. Brommer and J.J.M. Franse, to appear in J. Phys. F (1987). [10] K. Yoshimura, S. Hirosawa and Y. Nakamura, J, Phys. Soc. Japan 53 (1984) 2120. [11] D.M. Edwards, Proc. 5th Intern. Conf. on Valence Fluctuations, Bangalore, India.