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Heavy-fermion behavior in amorphous and crystallized CeCu6 alloys produced by sputter deposition
Journal of Magnetism and Magnetic Materials 108 (1992) 161-162 North-Holland
/H= i
iii
Heavy-fermion behavior in amorphous and crystallized CeCu. alloys produced by sputter deposition K. S u z u k i a, K. S u m i y a m a
~, H . A m a n o
", H . Y a m a u c h i
" and T. Suzuki b
a Institute for Materials Research, t, Department of Physics, Tohoku Unieersity, Senc'.ai 980, Japan Amorphous CeCu(, alloys have been produced by dc triode sputtering. Its remarkable enhancement of the electronic specific heat coefficient indicates heavy-fermion characteristics at low temperatures, however, the temperature dependence of the magnetic susceptibility and electrical resistivity indicate no coherent Fermi condensation down to 2 K, in contrast to the typical heavy-fermion characteristics in the crystallized CeCum, alloys. In "heavy-fermion" systems, mixing and strong correlation effects of the 4f and conduction electrons give rise to exceptionally large masses at low temperatures [1,2]. An amorphous alloy has advantages for the extensive study of heavy-fermion characteristics over crystalline compounds whose structures are restricted to nearly stoichiometric compositions. Although amorphous Ce alloys have been produced to study heavyfermion characteristics, no systematic study has been done due to the limited formation range by liquid quenching [3] and to the small amount of specimen due to thermal evaporation [4]. The sputter-deposition method is a powerful vapor deposition method to produce random alloys. We have successfully produced amorphous CeCe 6 thin plates of about 300 ~ m thickness using triode sputtering equipment [5]. Although the chemical composition of 84.7 at% Cu determined by induction coupled plasma analysis (ICP) was slightly different from the original target composition of 85.7 at% Cu, wc adopt the nominal composition, C c C u , , for the present alloy. The X-ray diffraction measurement at 290 K displayed a halo pattern for the as-sputtered alloy, indicating this alloy to be amorphous. A portion of the plates was annealed to 500 ° C for 48 h and orthorhombic peaks were detected in the X-ray diffraction pattern. Fig. 1 shows the results ,of the low-temperature 010
0.08 1
..-
oooi-"
......-Y"
0,04 "
"o
600
k
400-
crystallized
~ . ~ . . / ' ~.- " ~ - - . . ~ . . . . _ _
... "-'''"
"
"
..-.'" "
...........
"x,,. .... SI:::.:::":"" 200 -
~,,
amorphous 0
I 50
0
I 100
I 150
I 200
250
T 2 (K 2)
Fig. 1. Low-temperature specific heat of amorphc Js and crystallized CeCum, alloys produced by triode sputtering in the form of a C~ T versus T 2 curve.
specific heat observed between 1.5 and 10 K by a c(mventional heat pulse method in the form of a C / T versus T 2 curve, where C / T is normalized to a mole of Ce atoms. The electronic specific heat coefficient, y, estimated from the results at ?; > 10 K, is about 110 and 250 mJ tool- ~ K - 2 for the amorphous and crystallized alloys. C / T increases rapidly with decreasing temperature below T = 5.5 K. The y-value extrapo-
.°"'~" ""..
........"'"
t
50
I 100
400
0.04
........ ......
300 -'
oE
200 ~
2~ 0.02 '~-
=
- 100 I 150
0.05
400
// 0 00 I..~ 0
800f~
amorphous 81
I
0.021 -
......
,500
-.
×
1000,
I. . . . . . . . . . . r . . . . . . . . . . 0 200 250 I00
300 ×
0.03
200 ~ 100
0.01 0.00
°•""'"''"'"'°
/ 0
. . . . . . . . . . ,...... . . . . . . . . . . . . . . . . . 50
T (K)
100
150 T (K)
Fig. 2. Magnetic susceptibility, X, of CeCum, alloys produced by triode sputtering.
0312-8853/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
200
250
300
0
162
K. Suzuki et al. / Heat3,-fermion beharior in CeCu 6
lated to 0 K, y(0), is about 1300 and 900 mJ mol-t K -2 for the amorphous and crystallized alloys. These alloys, have heavy-fermion characteristic. In bulk CeCut, alloy.,, y(0) is about 1000 and 1500 mJ mol-~ K -2 for the poly-crystai and single crystal [1,6]. The magnetic susceptibility, X, was observed between 2 and 300 K using a torsion balance magnetometer. The values of A' are plotted as a function of temperature in fig. 2 for amorphous and crystallized CeCu 6 alloys. Above i00 K, X of these alloys roughly obeys the Curie-Weis~ law: The paramagnetic Curie temperature is - 10 and - 4 0 K and the effective Bohr magneton number, ~,:tt, is 2.49 and 2.78tx m respectively, for tt-c amorphous and crystallized alloys. For the bulk CeCu:, alloy, tzen. is near 2.69~B [1]. The diviafion from the Curie-Weiss law below 100 K is probably due to the crystal field effect [1,2]. The upward deviation from a straight line in the l / X - T plot and the Pauli paramagnetic behavior at low temperatures ha,~e been ascribed to degeneracy of the 4f electrons in the Fermi liquid in the bulk crystalline CeCum, alloy [1,2]. The above effects are only slightly detectable in the present crystallized alloy, however, they are much faded in the amorphous alloy, indicating that the magnetic moment of the Ce atoms is retained down to 2 K. In the amorphous CeCu 6 alloys, the magnetic moment of the 4f electrons may be stabilized by a localization effect of the 4f electrons originating from the disorder. Fig. 3 shows the electrical resistivity, p, of amorphous and crystallized CeCu 6 alloys observed between 2.7 and 300 K by the four-probe method. With decreasing temperature, T, p of the amorphous alloy is independent of T down to 250 K, slightly increases below 250 K and markedly increases below 25 K. On the other hand, in the crystallized CeCu 6 alloy, p gradually decreases with decreasing temperature down to T = 250 K and shows a shallow minimum at about 250 K. It logarithmically increases down to 40 K, shows a maximum at 10 K and decreases below 10 K. The Iogarith120
110
amorphous E
100
CL
90
80
5'o
,;o
2;0
1300
T (K)
Fig. 3. Electrical resistivity, p, of amorphous and crystallized CeCum, alloys produced by triode sputtering.
mic increase for both alloys is ascribed to the Kondo effect, while the AT2-dependence below 10 K for the crystallized alloys is ascribed to formation of the coherent Kondo state (Fermi condensation of the 4f electrons). However, the A-value of 4.5 ~ l l cm K -2 is much smaller than 111 ~['l cm K -2 for the bulk poly-crystalline alloy [7]. These results indicate that a coherent Kondo lattice is not formed in the present amorphous alloys down to 2 K and the coherency is not perfect even in the present crystallized alloy, whose residual resistivity is extrapolated to be about 90 p, fl cm. In conclusion, the comparative study of amorphous and crystallized CeCu 6 alloys clearly shows that the amorphous CeCu 6 is a heavy-fermion system characterized by an enhanced low-temperature specific heat and not a false heavy-fermion system [8]. However, the amorphous structure masks the electron correlations for forming a singlet state a n d / o r magnetic ordering. Since the amorphous structure also suppresses the itinerancy of the 4f electrons, the contribution of the Fermi liquid degeneracy of the 4f electrons to the electrical resistivity is smeared out at low temperatures. The authors wish to thank Dr. K. Takada for doing the ICP analysis. They are indebted to Santoku Co. Ltd. for supplying Ce metal ingots. This work was supported partially by a Grant-in-Aid for Scientific Research on Priority Areas (Grant No. 02216102) and a Grant-in-Aid for General Scientific Research on Priority Areas (Grant No. 03452029) given by the Ministry of Education, Science and Culture and also by the New Energy and Industrial Technology Dcvclopmcnt Organization (NEDO). References
[1] G.R. Stewart, Rev. Mod. Phys. 56 (1984) 755. [2] N.B. Brandt and V.V. Moshchaikov, Adv. Phys. 33 (1984) 373. [3] H. v. L6hneysen, H.J. Schink, W. Felsch, K. Samwer and H. Schr6der, Physica B 107 (1981) 631; R. Greening, H. Schr6der and W. Felsch, J. Magn. Magn. Mater. 62 (1986) 188. [4] D. Malterre, J. Durand and G. Marchal, J. Non-Cryst. Solids 61 & 62 (1984) 1137; D. Maiterre, G. Krill, J. Durand and G. Marchal, Phys. Rev. B 34 (1986) 2176. [5] H. Amano, K. S,miyama, T. Suzuki and K. Suzuki, J. Phys. Soc. Jpn. 60 (1991) 397. [6] T. Fujita, K. Satoh, Y. Onuki and T. Komatsubara, J. Magn. Magn. Mater. 47 & 48 (1985) 66. [7] A. Sumiyama, Y. Oda, H. Nagano, Y. Onoki, K. Shibutani and T. Komatsubara, .I. Phys. Soc. Jpn. 55 (1986) 1294. [8] K.A. Gschneider Jr., J. T~,~, S.K. Dhar and A. Goldman, Physica B 163 (1990) 507.
/H= i
iii
Heavy-fermion behavior in amorphous and crystallized CeCu. alloys produced by sputter deposition K. S u z u k i a, K. S u m i y a m a
~, H . A m a n o
", H . Y a m a u c h i
" and T. Suzuki b
a Institute for Materials Research, t, Department of Physics, Tohoku Unieersity, Senc'.ai 980, Japan Amorphous CeCu(, alloys have been produced by dc triode sputtering. Its remarkable enhancement of the electronic specific heat coefficient indicates heavy-fermion characteristics at low temperatures, however, the temperature dependence of the magnetic susceptibility and electrical resistivity indicate no coherent Fermi condensation down to 2 K, in contrast to the typical heavy-fermion characteristics in the crystallized CeCum, alloys. In "heavy-fermion" systems, mixing and strong correlation effects of the 4f and conduction electrons give rise to exceptionally large masses at low temperatures [1,2]. An amorphous alloy has advantages for the extensive study of heavy-fermion characteristics over crystalline compounds whose structures are restricted to nearly stoichiometric compositions. Although amorphous Ce alloys have been produced to study heavyfermion characteristics, no systematic study has been done due to the limited formation range by liquid quenching [3] and to the small amount of specimen due to thermal evaporation [4]. The sputter-deposition method is a powerful vapor deposition method to produce random alloys. We have successfully produced amorphous CeCe 6 thin plates of about 300 ~ m thickness using triode sputtering equipment [5]. Although the chemical composition of 84.7 at% Cu determined by induction coupled plasma analysis (ICP) was slightly different from the original target composition of 85.7 at% Cu, wc adopt the nominal composition, C c C u , , for the present alloy. The X-ray diffraction measurement at 290 K displayed a halo pattern for the as-sputtered alloy, indicating this alloy to be amorphous. A portion of the plates was annealed to 500 ° C for 48 h and orthorhombic peaks were detected in the X-ray diffraction pattern. Fig. 1 shows the results ,of the low-temperature 010
0.08 1
..-
oooi-"
......-Y"
0,04 "
"o
600
k
400-
crystallized
~ . ~ . . / ' ~.- " ~ - - . . ~ . . . . _ _
... "-'''"
"
"
..-.'" "
...........
"x,,. .... SI:::.:::":"" 200 -
~,,
amorphous 0
I 50
0
I 100
I 150
I 200
250
T 2 (K 2)
Fig. 1. Low-temperature specific heat of amorphc Js and crystallized CeCum, alloys produced by triode sputtering in the form of a C~ T versus T 2 curve.
specific heat observed between 1.5 and 10 K by a c(mventional heat pulse method in the form of a C / T versus T 2 curve, where C / T is normalized to a mole of Ce atoms. The electronic specific heat coefficient, y, estimated from the results at ?; > 10 K, is about 110 and 250 mJ tool- ~ K - 2 for the amorphous and crystallized alloys. C / T increases rapidly with decreasing temperature below T = 5.5 K. The y-value extrapo-
.°"'~" ""..
........"'"
t
50
I 100
400
0.04
........ ......
300 -'
oE
200 ~
2~ 0.02 '~-
=
- 100 I 150
0.05
400
// 0 00 I..~ 0
800f~
amorphous 81
I
0.021 -
......
,500
-.
×
1000,
I. . . . . . . . . . . r . . . . . . . . . . 0 200 250 I00
300 ×
0.03
200 ~ 100
0.01 0.00
°•""'"''"'"'°
/ 0
. . . . . . . . . . ,...... . . . . . . . . . . . . . . . . . 50
T (K)
100
150 T (K)
Fig. 2. Magnetic susceptibility, X, of CeCum, alloys produced by triode sputtering.
0312-8853/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
200
250
300
0
162
K. Suzuki et al. / Heat3,-fermion beharior in CeCu 6
lated to 0 K, y(0), is about 1300 and 900 mJ mol-t K -2 for the amorphous and crystallized alloys. These alloys, have heavy-fermion characteristic. In bulk CeCut, alloy.,, y(0) is about 1000 and 1500 mJ mol-~ K -2 for the poly-crystai and single crystal [1,6]. The magnetic susceptibility, X, was observed between 2 and 300 K using a torsion balance magnetometer. The values of A' are plotted as a function of temperature in fig. 2 for amorphous and crystallized CeCu 6 alloys. Above i00 K, X of these alloys roughly obeys the Curie-Weis~ law: The paramagnetic Curie temperature is - 10 and - 4 0 K and the effective Bohr magneton number, ~,:tt, is 2.49 and 2.78tx m respectively, for tt-c amorphous and crystallized alloys. For the bulk CeCu:, alloy, tzen. is near 2.69~B [1]. The diviafion from the Curie-Weiss law below 100 K is probably due to the crystal field effect [1,2]. The upward deviation from a straight line in the l / X - T plot and the Pauli paramagnetic behavior at low temperatures ha,~e been ascribed to degeneracy of the 4f electrons in the Fermi liquid in the bulk crystalline CeCum, alloy [1,2]. The above effects are only slightly detectable in the present crystallized alloy, however, they are much faded in the amorphous alloy, indicating that the magnetic moment of the Ce atoms is retained down to 2 K. In the amorphous CeCu 6 alloys, the magnetic moment of the 4f electrons may be stabilized by a localization effect of the 4f electrons originating from the disorder. Fig. 3 shows the electrical resistivity, p, of amorphous and crystallized CeCu 6 alloys observed between 2.7 and 300 K by the four-probe method. With decreasing temperature, T, p of the amorphous alloy is independent of T down to 250 K, slightly increases below 250 K and markedly increases below 25 K. On the other hand, in the crystallized CeCu 6 alloy, p gradually decreases with decreasing temperature down to T = 250 K and shows a shallow minimum at about 250 K. It logarithmically increases down to 40 K, shows a maximum at 10 K and decreases below 10 K. The Iogarith120
110
amorphous E
100
CL
90
80
5'o
,;o
2;0
1300
T (K)
Fig. 3. Electrical resistivity, p, of amorphous and crystallized CeCum, alloys produced by triode sputtering.
mic increase for both alloys is ascribed to the Kondo effect, while the AT2-dependence below 10 K for the crystallized alloys is ascribed to formation of the coherent Kondo state (Fermi condensation of the 4f electrons). However, the A-value of 4.5 ~ l l cm K -2 is much smaller than 111 ~['l cm K -2 for the bulk poly-crystalline alloy [7]. These results indicate that a coherent Kondo lattice is not formed in the present amorphous alloys down to 2 K and the coherency is not perfect even in the present crystallized alloy, whose residual resistivity is extrapolated to be about 90 p, fl cm. In conclusion, the comparative study of amorphous and crystallized CeCu 6 alloys clearly shows that the amorphous CeCu 6 is a heavy-fermion system characterized by an enhanced low-temperature specific heat and not a false heavy-fermion system [8]. However, the amorphous structure masks the electron correlations for forming a singlet state a n d / o r magnetic ordering. Since the amorphous structure also suppresses the itinerancy of the 4f electrons, the contribution of the Fermi liquid degeneracy of the 4f electrons to the electrical resistivity is smeared out at low temperatures. The authors wish to thank Dr. K. Takada for doing the ICP analysis. They are indebted to Santoku Co. Ltd. for supplying Ce metal ingots. This work was supported partially by a Grant-in-Aid for Scientific Research on Priority Areas (Grant No. 02216102) and a Grant-in-Aid for General Scientific Research on Priority Areas (Grant No. 03452029) given by the Ministry of Education, Science and Culture and also by the New Energy and Industrial Technology Dcvclopmcnt Organization (NEDO). References
[1] G.R. Stewart, Rev. Mod. Phys. 56 (1984) 755. [2] N.B. Brandt and V.V. Moshchaikov, Adv. Phys. 33 (1984) 373. [3] H. v. L6hneysen, H.J. Schink, W. Felsch, K. Samwer and H. Schr6der, Physica B 107 (1981) 631; R. Greening, H. Schr6der and W. Felsch, J. Magn. Magn. Mater. 62 (1986) 188. [4] D. Malterre, J. Durand and G. Marchal, J. Non-Cryst. Solids 61 & 62 (1984) 1137; D. Maiterre, G. Krill, J. Durand and G. Marchal, Phys. Rev. B 34 (1986) 2176. [5] H. Amano, K. S,miyama, T. Suzuki and K. Suzuki, J. Phys. Soc. Jpn. 60 (1991) 397. [6] T. Fujita, K. Satoh, Y. Onuki and T. Komatsubara, J. Magn. Magn. Mater. 47 & 48 (1985) 66. [7] A. Sumiyama, Y. Oda, H. Nagano, Y. Onoki, K. Shibutani and T. Komatsubara, .I. Phys. Soc. Jpn. 55 (1986) 1294. [8] K.A. Gschneider Jr., J. T~,~, S.K. Dhar and A. Goldman, Physica B 163 (1990) 507.