- Email: [email protected]
Transport and magnetic properties of the heavy-fermion compounds UCu3+xGa2−x(0.1 ≤ x ≤ 0.8)
ELSEVIER
Physica B 199&200 (1994)31-33
Transport and magnetic properties of the heavy-fermion compounds UCu3+xGa2_ x (0.1 < x < 0.8) Y. Maeda a, T. Takabatake ~'*, H. Fujii a, I. Oguro b, K. Sugiyama ¢, K. Oda ~, M. Date ¢'d aFaculty of Integrated Arts and Sciences, Hiroshima University, Higashi-Hiroshima 724, Japan blnstitute.for Solid State Physics, University of Tokyo, Tokyo f06, Japan eFaculty r~f Science and d The Research Centre.]br Extreme Materials, Osaka University, Tovonaka Osaka 560, Japan.
Abstract
We report on the measurements of electrical resistivity, magnetic susceptibility, high-field magnetization and magnetoresistance on UCu3 + xGaz-x (0.1 _< x _< 0.8). Both the N6el temperature and the metamagnetic-transition field have maxima at x = 0.3. F o r x _> 0.6, nonmagnetic heavy-fermion behavior appears, whereas the high-temperature properties are hardly changed.
In a previous paper [1], we have reported that UCu3+xGa2-x with the CaCus-type structure for 0.1 < x < 0.8 exhibits a crossover from an antiferromagnetic (AF) heavy-fermion state to a nonmagnetic heavyfermion state with increasing x. The Nrel temperature TN initially increases from 8.2 K for x = 0.1 up to 17 K for x = 0.3, and then decreases with further increase of x. For x > 0.6, no indication of magnetic order above 1.3 K was observed in either resistivity [1] or specific heat C(7") [2]. At x = 0.6, the ratio C/T attains a large value of 430mJ/K 2 mol at 1.2 K. The behavior of TN(x) with a maximum is similar to what is expected from the competition between Kondo and RKKY interactions in the Doniach's Kondo-necklace model [3]. Therefore, the evolution of the nonmagnetic heavy-fermion state was attributed to the effect of the increase in the 5f-ligand hybridization with increasing x [I]. In order to obtain ins',ght into the mechanism of the crossover in this system, we have measured the resistivity, magnetic * Corresponding author.
susceptibility, high-field magnetization and magnetoresistance in the whole concentration range 0.1 < x < 0.8. The preparation and characterization of the samples used in this study were described in Ref. [1]. We note here that the samples with x = 0 and 1.0 were found to be -nultiphase by electron-probe microanalysis. F r o m the ~.ampositions of the main phases in these samples, the phase limit was determined as 0.1 < x < 0.8. The variations of the lattice parameters are presented in Fig. l With increasing x from 0.1 to 0.8, the a-parameter decreases almost linearly by 1.5%, whereas the c-parameter increases by 0.5%. For all the samples, the high-temperature susceptibility 7.(T) above "~'K~w "-",w~,~,~ . . . . . . ,,,~k~ Curie-Weiss law. Both the paramagnetic Curie temperature 0 and effective magnetic moment It¢rr are changed slightly (see Fig. 1). The g(T) curve for x < 0.5 passes through a rounded peak as temperature is decreased. The temperature where d x I T ) / d T is at maximum agrees with the value of TN determined from the specific-heat anomaly. TN(x) in Fig. 1 has the maximum near x = 0.3, as mentioned above.
0921-4526/94/$07.00 (~ 1994 Elsevier Science B.V. All rights reserved SSDI 0921-4526(93)E0298-U
Y, Maeda et al, t Physica B 199&200 (1994) 31-33
32 ,
5.2
,
,
+
i
4.2
o<
1.5 I "i 0
o I
i
0 ='01
tree powder T
o:5
o.8
3,0
0.5
C
:
:
oTn •
-r
He
10
O
5 .,
t
f, + +
I
i
.-I
E
I
0
• p~
15
0
i
3,2
,-¢ e~ II
,
¢
-30
"" -40
!
UCu3+xGa2"x
=. '~
5.1
i
I
I
o
I /0
',
' i
(b)
0.4 o
t::: 02 0.2
0.4 X
0.6
0,5
• 1-..... O. "- -0,4 <:1 " -0.6
d 0
-0.2
0.8
Fig. 1. Variations of physical properties of UCu3+~Ga2-~ versus x (see text).
0.3
T
=
4
~ i
0
10
20 30 Magneticfield (T)
40
Fig. 3. Field dependenceof (a) magnetization and (b) magnetoresistance of UCu3+.Ga2-. at 4.2 K 4oo[..~, o , , .,,'
2,0
,=ol
.,t.
~ o.
'
,0
K
..o,,
)
1.4 0.7
1.0
.......................
10 T(K)
100
Fig. 2. Temperature dependence of normalized resistivity of UCu~+.,Ga~_,. The inset shows temperature dependence of resistivity for U C u 3 , t G a t . 9 in fields of 0, 7 and 14 T.
Figure 2 shows the temperature dependence of the resistivity p which is normalized to ~.he room-temperature value. As the samples have microcracks, the absolute values of p were roughly estimated to be 200-400 pQ cm at 300 K. We note that p/p(300 K) is almost proportional to - In T above 30 K and the slope is independent of x. This observation suggests that the whole system at high temperatures can be characterized by one temperature scale of T* ~ 100 K. The abrupt
increase in p(T) below TN, which is most prominent for x = O. 1, indicates that the AF transition induces a superzone gap in the Fermi surface. This gap is suppressed by the application of magnetic field of 14 T as inferred from the inset of Fig. 2. The o(T) curve at 14 T is similar to that of x = 0.8 in zero field which shows metallic behavior below 20 K. However, the development of coherence is interfered by the intrinsic disorder in the sublattice of the Cu and Ga atoms. Figures 3(a) and 3(b) represent, respectively, the magnetization M(H) and magnetoresistance Ap/p(H = O) measured at 4.2 K using pulsed fields. The M(H) curve of the powdered sample for x = 0.1 exhibits a pronounced metamagnetic transition at H~ = 8.8 T. The transition becomes less pronounced for x = 0.3 and 0.5. T h e dependence of He on x tracks that of TN on x (see Fig. 1). The metamagnetic transition disappears in the nonma'~netic sample with x = 0.8. Interestingly, all the M(H) curves seem to be saturated to a constant value of 1.5pa/f.u. In Fig. 3(b). a strong reduction in the resistivity occurs at /-/~ for x = 0.1, 0.3 and 0.5. The reduction is therefore attributed to the loss of the gap in the Fermi surface. In fields higher than FIo both M(H) and Ap/p still change significantly. Similar behavior observed in URu2Si2 was interpreted as a result of the field-induced magnetic moment out of the heavy-fermion state [4].
Y. Maeda et al./ Physica B 199&200 (1994) 31-33
In summary, we find that the high-temperature magnetic and transport properties of UCus+~Gaa-~ are hardly changed in the whole concentration range 0.1 _< x < 0.8. Nevertheless, the ground state changes from the AF state to the nonmagnetic heavy-fermion state near x - 0.6. This fact is difficult to be understood in the framework of the Doniach's Kondo-lattice model. The observed heavy-fermion behavior might originate from the short-range order, as was found in U ( I n t - , S n , h 1"5]. However, in UPd,AI3 with the same structure type with the present system, the heavy-fermion behavior coexists with a large magnetic moment of 0.85#B/U [6]. In order to study the origin of the heavyfermion behavior in UCu3 +~Ga2-~, microscopic studies using neutron scattering and NMR techniques are highly anticipated.
33
References [!] T. Takabatak¢, Y. Maeda, H. Fujii, S. lkeda, S. Nishigori,T. Fujita, A. Minami,I. Oguro, K. Sugiyama,K. Oda and M. Date, Physica B 186-188 (1993) 734. ['2] S. lkeda, S. Nishigori, T. Suzuki, T. Fujita, Y. Maeda, T. Takabatake and H. Fujii, Physica B 194-196 {1994) 465. [3"] S. Doniach, in: Valence Instabilities and Related ~arrowBand Phenomena, ed. R.D. Parks (Plenum, New York. 1977) p, 169. [4"] K. Sugiyamaand M. Date, J. Magn. Magn. Mater. 90&91 (1990) 194. [5"] T. Yuen, N. Bykovetz,G.Y. Jiang, C.L. Lin, P.P. Wise and J.E. Crow, Physica B 171 (1991) 367. [6] A. Krimmel,P. Fischer,B. Roessli,H. Maletta,C. Geibel,C. Schank, A. Grau¢l, A. Loidl and F. Steglich,Z. Phys. B 86 0992) 161.
Physica B 199&200 (1994)31-33
Transport and magnetic properties of the heavy-fermion compounds UCu3+xGa2_ x (0.1 < x < 0.8) Y. Maeda a, T. Takabatake ~'*, H. Fujii a, I. Oguro b, K. Sugiyama ¢, K. Oda ~, M. Date ¢'d aFaculty of Integrated Arts and Sciences, Hiroshima University, Higashi-Hiroshima 724, Japan blnstitute.for Solid State Physics, University of Tokyo, Tokyo f06, Japan eFaculty r~f Science and d The Research Centre.]br Extreme Materials, Osaka University, Tovonaka Osaka 560, Japan.
Abstract
We report on the measurements of electrical resistivity, magnetic susceptibility, high-field magnetization and magnetoresistance on UCu3 + xGaz-x (0.1 _< x _< 0.8). Both the N6el temperature and the metamagnetic-transition field have maxima at x = 0.3. F o r x _> 0.6, nonmagnetic heavy-fermion behavior appears, whereas the high-temperature properties are hardly changed.
In a previous paper [1], we have reported that UCu3+xGa2-x with the CaCus-type structure for 0.1 < x < 0.8 exhibits a crossover from an antiferromagnetic (AF) heavy-fermion state to a nonmagnetic heavyfermion state with increasing x. The Nrel temperature TN initially increases from 8.2 K for x = 0.1 up to 17 K for x = 0.3, and then decreases with further increase of x. For x > 0.6, no indication of magnetic order above 1.3 K was observed in either resistivity [1] or specific heat C(7") [2]. At x = 0.6, the ratio C/T attains a large value of 430mJ/K 2 mol at 1.2 K. The behavior of TN(x) with a maximum is similar to what is expected from the competition between Kondo and RKKY interactions in the Doniach's Kondo-necklace model [3]. Therefore, the evolution of the nonmagnetic heavy-fermion state was attributed to the effect of the increase in the 5f-ligand hybridization with increasing x [I]. In order to obtain ins',ght into the mechanism of the crossover in this system, we have measured the resistivity, magnetic * Corresponding author.
susceptibility, high-field magnetization and magnetoresistance in the whole concentration range 0.1 < x < 0.8. The preparation and characterization of the samples used in this study were described in Ref. [1]. We note here that the samples with x = 0 and 1.0 were found to be -nultiphase by electron-probe microanalysis. F r o m the ~.ampositions of the main phases in these samples, the phase limit was determined as 0.1 < x < 0.8. The variations of the lattice parameters are presented in Fig. l With increasing x from 0.1 to 0.8, the a-parameter decreases almost linearly by 1.5%, whereas the c-parameter increases by 0.5%. For all the samples, the high-temperature susceptibility 7.(T) above "~'K~w "-",w~,~,~ . . . . . . ,,,~k~ Curie-Weiss law. Both the paramagnetic Curie temperature 0 and effective magnetic moment It¢rr are changed slightly (see Fig. 1). The g(T) curve for x < 0.5 passes through a rounded peak as temperature is decreased. The temperature where d x I T ) / d T is at maximum agrees with the value of TN determined from the specific-heat anomaly. TN(x) in Fig. 1 has the maximum near x = 0.3, as mentioned above.
0921-4526/94/$07.00 (~ 1994 Elsevier Science B.V. All rights reserved SSDI 0921-4526(93)E0298-U
Y, Maeda et al, t Physica B 199&200 (1994) 31-33
32 ,
5.2
,
,
+
i
4.2
o<
1.5 I "i 0
o I
i
0 ='01
tree powder T
o:5
o.8
3,0
0.5
C
:
:
oTn •
-r
He
10
O
5 .,
t
f, + +
I
i
.-I
E
I
0
• p~
15
0
i
3,2
,-¢ e~ II
,
¢
-30
"" -40
!
UCu3+xGa2"x
=. '~
5.1
i
I
I
o
I /0
',
' i
(b)
0.4 o
t::: 02 0.2
0.4 X
0.6
0,5
• 1-..... O. "- -0,4 <:1 " -0.6
d 0
-0.2
0.8
Fig. 1. Variations of physical properties of UCu3+~Ga2-~ versus x (see text).
0.3
T
=
4
~ i
0
10
20 30 Magneticfield (T)
40
Fig. 3. Field dependenceof (a) magnetization and (b) magnetoresistance of UCu3+.Ga2-. at 4.2 K 4oo[..~, o , , .,,'
2,0
,=ol
.,t.
~ o.
'
,0
K
..o,,
)
1.4 0.7
1.0
.......................
10 T(K)
100
Fig. 2. Temperature dependence of normalized resistivity of UCu~+.,Ga~_,. The inset shows temperature dependence of resistivity for U C u 3 , t G a t . 9 in fields of 0, 7 and 14 T.
Figure 2 shows the temperature dependence of the resistivity p which is normalized to ~.he room-temperature value. As the samples have microcracks, the absolute values of p were roughly estimated to be 200-400 pQ cm at 300 K. We note that p/p(300 K) is almost proportional to - In T above 30 K and the slope is independent of x. This observation suggests that the whole system at high temperatures can be characterized by one temperature scale of T* ~ 100 K. The abrupt
increase in p(T) below TN, which is most prominent for x = O. 1, indicates that the AF transition induces a superzone gap in the Fermi surface. This gap is suppressed by the application of magnetic field of 14 T as inferred from the inset of Fig. 2. The o(T) curve at 14 T is similar to that of x = 0.8 in zero field which shows metallic behavior below 20 K. However, the development of coherence is interfered by the intrinsic disorder in the sublattice of the Cu and Ga atoms. Figures 3(a) and 3(b) represent, respectively, the magnetization M(H) and magnetoresistance Ap/p(H = O) measured at 4.2 K using pulsed fields. The M(H) curve of the powdered sample for x = 0.1 exhibits a pronounced metamagnetic transition at H~ = 8.8 T. The transition becomes less pronounced for x = 0.3 and 0.5. T h e dependence of He on x tracks that of TN on x (see Fig. 1). The metamagnetic transition disappears in the nonma'~netic sample with x = 0.8. Interestingly, all the M(H) curves seem to be saturated to a constant value of 1.5pa/f.u. In Fig. 3(b). a strong reduction in the resistivity occurs at /-/~ for x = 0.1, 0.3 and 0.5. The reduction is therefore attributed to the loss of the gap in the Fermi surface. In fields higher than FIo both M(H) and Ap/p still change significantly. Similar behavior observed in URu2Si2 was interpreted as a result of the field-induced magnetic moment out of the heavy-fermion state [4].
Y. Maeda et al./ Physica B 199&200 (1994) 31-33
In summary, we find that the high-temperature magnetic and transport properties of UCus+~Gaa-~ are hardly changed in the whole concentration range 0.1 _< x < 0.8. Nevertheless, the ground state changes from the AF state to the nonmagnetic heavy-fermion state near x - 0.6. This fact is difficult to be understood in the framework of the Doniach's Kondo-lattice model. The observed heavy-fermion behavior might originate from the short-range order, as was found in U ( I n t - , S n , h 1"5]. However, in UPd,AI3 with the same structure type with the present system, the heavy-fermion behavior coexists with a large magnetic moment of 0.85#B/U [6]. In order to study the origin of the heavyfermion behavior in UCu3 +~Ga2-~, microscopic studies using neutron scattering and NMR techniques are highly anticipated.
33
References [!] T. Takabatak¢, Y. Maeda, H. Fujii, S. lkeda, S. Nishigori,T. Fujita, A. Minami,I. Oguro, K. Sugiyama,K. Oda and M. Date, Physica B 186-188 (1993) 734. ['2] S. lkeda, S. Nishigori, T. Suzuki, T. Fujita, Y. Maeda, T. Takabatake and H. Fujii, Physica B 194-196 {1994) 465. [3"] S. Doniach, in: Valence Instabilities and Related ~arrowBand Phenomena, ed. R.D. Parks (Plenum, New York. 1977) p, 169. [4"] K. Sugiyamaand M. Date, J. Magn. Magn. Mater. 90&91 (1990) 194. [5"] T. Yuen, N. Bykovetz,G.Y. Jiang, C.L. Lin, P.P. Wise and J.E. Crow, Physica B 171 (1991) 367. [6] A. Krimmel,P. Fischer,B. Roessli,H. Maletta,C. Geibel,C. Schank, A. Grau¢l, A. Loidl and F. Steglich,Z. Phys. B 86 0992) 161.