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Magnetic and transport properties of UCuGa, U2CuGa3 and UAuGa
Journal of Alloys and Compounds, 199 (1993) 193-196 JALCOM 709
193
Magnetic and transport properties of UCuGa, U2CuGa3 and UAuGa V. H . T r a n , R. T r 0 6 a n d D . B a d u r s k i w. Trzebiatowski Institute of Low Temperature and Structure Research, Polish Academy of Sciences, 50-950 Wroctaw (Poland)
(Received February 25, 1993)
Abstract
The magnetic characteristics of UCuGa, U2CuGa 3 and UAuGa, have been studied from 4.2 to 300 K. From the magnetic susceptibility data we inferred the onset of an antiferromagnetic ordering in UCuGa and U2CuGa3 at 30 K and 15 K respectively. Their electrical resistivities at high temperatures are analysed in terms of a Kondolike behaviour. In the case of UAuGa, we found two phase transitions at 18 and 60 K being of antiferromagnetic character. Moreover, a strong anisotropy in the transport properties of this compound has been observed above TN=60+2 K. The obtained results for a new compound U2CuGa3 are compared with those of UCuGa and UGa2.
1. Introduction
The magnetic and transport properties of UCuGa were briefly mentioned several years ago [1], but its magnetic behaviour is still a puzzle. For example, the earlier magnetization and specific heat studies made on UCuGa by Andreev et al. (cited in ref. 1) showed that this compound undergoes two magnetic transitions at 29 and 15 K, while our recent susceptibility measurements [2] have only revealed an antiferromagnetic ordering below 30 K. On the contrary, our electrical resistivity results demonstrate the presence of a phase transition in UCuGa only at 15 K [2]. Therefore, the nature of the magnetic order in UCuGa is not still clear. In an attempt to clarify this situation, we have repeated the magnetic susceptibility, magnetization and electrical resistivity measurements on newly prepared UCuGa samples. In the case of UAuGa [3], both the magnetic susceptibility and the electrical resistivity data indicate that the phase transition from the paramagnetic to antiferromagnetic state in this compound takes place at TN = 60_+ 2 K. Above this temperature the values of p change as In T which is characteristic of a Kondo effect. However, the magnetic results presented here give evidence that its magnetic behaviour is more complex than those previously published [3]. Moreover, we synthesized a new compound U2CuGa3 and its magnetic properties we here report for the first time. We have also studied the formation of such a compound with Au. Its physical properties including the magnetic sus0925-8388/93/$6.00
ceptibility, electrical resistivity and neutron diffraction measurements will be reported elsewhere [4]. 2. Results and discussion
The polycrystalline specimens used in this study were prepared by arc melting, followed by vacuum annealing at 800 °C for 5 days. Powder X-ray diffraction analysis showed the investigated samples to be single phase. The crystal structure of U2CuGa 3 was found to be the hexagonal A1B2 type (space group P r / m m m ) with the lattice parameters a=4.348(1) /~ and c=3.605(1) /~. As reported previously [1, 2], UCuGa crystallizes in a structure of the hexagonal CaIn2 type. In agreement with a previous study [3], a newly synthesized sample of UAuGa has an orthorhombic CeCu2 (TiNiSi) type unit cell. The crystallographic parameters of all the compounds investigated in this work are given in Table 1. The magnetic susceptibility X was measured over the temperature range 4.2-300 K using a Cahn-RH electrobalance. Figure 1 displays the reciprocal susceptibility X-1 of UCuGa and U 2 C u G a 3 as a function of temperature. In good agreement with our previous results [2], a cusp in x(T) for UCuGa is well marked at TN =30 K, but rather a small change in its X(T) below TN is apparent. As shown in Fig. 1, the temperature dependence of the reciprocal susceptibility of UCuGa follows a Curie-Weiss (CW) law above about 80 K, leading to the effective magnetic moment/zeer = 2.58/zB and the paramagnetic Currie temperature 19 p = - 1 3 © 1993- Elsevier Sequoia. All rights reserved
V. H. Tran et aL I Properties of UCuGa, U2CuGas and UAuGa
194
T A B L E 1. Crystal structure and magnetic and transport characteristics of the UGa2, U2CuGa3, U C u G a and U A u G a compounds Compound
Lattice parameters (/~)
Spin order TON
Op (K)
10 -3 (e.m.u. mol -z)
A'ox
/.~cfr (/ZB)
#(T) at high temperature (/~fl crn)
References
(K) UGa2
a=4.213 c =4.017
F; 126
134
1.32
2.80
Kondo behaviour
5-7
U2CuGa3
a =4.348 c = 3.605
AF; 15
7
0.78
2.79
1 2 3 9 - 5 6 In T
This work
UCuGa
a =4.341 c = 6.994
AF; 29, 15
0
0.02
2.36
Not reported
1
UCuGa
a=4.345 c = 7.002
AF; 30
-7
2.54
2 5 7 - 9 . 5 In T
2
UCuGa
a =4.366 c = 7.006
AF; 30
-13
0.01
2.58
2 7 8 - 1 6 In T
This work
UAuGa sample 1
a=3.881 b = 7.996 c=7.641
AF; 60
-8
0.6
2.70
1 5 0 0 - 0 . 1 In T
3
UAuGa sample 2
a =3.881 b=7.990 c =7.644
AF, AF; 18, 60
-16
0.5
2.66
A broad maximum at about 130 K
This work
=0
AF, antiferromagnetic; F, ferromagnetic.
400
350
"'.
300
~"
~
50 o
t"
250
t
200
o
""
~3o E
....-'-.. "'"'..... 2~
,-"""-
50
."
,,,,""
20 ?o
t °
75 ~oo
.
""
.,t"
150
......
0
.
50
......
..
100
150
} /YD<'t 200
250
300
T (K)
Fig. 1. T e m p e r a t u r e dependence of the reciprocal susceptibility of U C u G a and U2CuGa3. Plot A shows their low temperature magnetic susceptibility. Plot B shows the magnetization of these compounds as a function of magnetic field at 4.2 K.
K. The magnetic and transport data of UCuGa are collected in Table 1. As also seen in Fig. 1, the magnetic susceptibility of U a C u G a 3 exhibits a different thermal behaviour compared with that of UCuGa. The curvature of the X-I(T) function suggests its description in terms of a modified CW law. For the X data above 20 K the fit to this law yields Xo = 0.78 x 10 - 3 e . m . u , mol- 1, (gp = 7 K and/zen = 2.79/zB. A pronounced maximum in the X vs. T curve of U 2 C u G a 3 , but slightly broadened, occurs at about 15 K (see curves A of Fig. 1), suggesting an antiferromagnetic ordering also in this compound. However, in this case the magnetic susceptibility at, for
example, Tm~, seems to be very large compared with that for a common U-based antiferromagnet: at this temperature X reaches a value as large as 55× 10 - 3 e.m.u, mol-1. Hence we ascribe this fact as well as the appearance of the positive Op value to the formation of a complex antiferromagnetic phase. The presence in this sample of some ferromagnetic impurities can be excluded, because the magnetization at 4.2 K (see curves B of Fig. 1) exhibits an almost linear increase to the limit of our measurements, i.e. to a field of 4 T, but it does display a small hysteresis when the magnetic field is varied down. For a comparison, plot B of Fig. 1 also shows the results of magnetization measurements performed at 4.2 K for UCuGa. In contrast, its magnetization increases linearly with the magnetic field, but without any hysteresis. In Fig. 2 the reciprocal susceptibility of two UAuGa samples is displayed as a function of temperature. Although these two specimens were obtained in a similar thermal condition, their magnetic properties are different. For sample 1 we found the same behaviour as reported previously [3] but, for sample 2, two maxima are seen in the x(T) curve: that located at 60 K presents a cusp in agreement with a previous observation [3], and the second maximum at 18 K, which probably signals a magnetic phase transition into another antiferromagnetic state. Despite these differences both sets of lattice parameters a, b and c of samples 1 and 2 appear to be close to each other (Table 1). Also, the values of effective magnetic moments/zeer are comparable for these two samples. Nevertheless, the difference observed in x(T) at low temperatures between
V. H. Tran et al. / Properties of UCuGa, U2CuGa 3 and UAuGa
1.35
'
250
I
:;f
o
UAuGa
o~°[
150
i
12
%
i
°//./ . . . . . .1
m
..,......-
L
o :
/
/
1.30 ~ 2
J C
-~
// /
°
2oo-
k
J
~ ~
195
25
50
75
T(K)
"
i
i00
I
1.25
F
1.20
-
o
//
1oo
5O
/ 1.15
i ~i
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!
2OO
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T (K)
UAuGa
-
i
I
0
0
50
100
150
200
250
300
0
T (K)
980
240 i
960
O
-
UoCuGa 3
' /
I :<, /
'
"'\ ~
~- 200-
\~-
\-.
"~
UCuGa
940
~ Q
~-.~ 920
I
180 0
50
100
150
100 T
Fig. 2. Temperature dependence of the reciprocal susceptibility for two samples of UAuGa. Their low temperature susceptibility is also shown.
220
50
150
200
250
900 300
T (K)
Fig. 3. Electrical resistivity of UCuGa and UzCuGa 3 as a function of temperature.
these two samples and also in their absolute values of h' indicates a high sensitivity of magnetic properties to any small variation in stoichiometry, even if it is not reflected in the lattice parameters. Electrical resistivity measurements for UCuGa, U2CuGa3 and UAuGa were carried out with a fourprobe technique between 4.2 and 300 K. The results are presented in Figs. 3 and 4. As Fig. 3 illustrates, the p(T) curves of both UCuGa and U2CuGa3 (above 100 K) have a negative slope, which is indicative of a Kondo effect. Thus the repeated electrical resistivity measurements on UCuGa have confirmed the previous results [2]. It appears that the absolute p value of UECuGa3 at room temperature is about four times higher than that of UCuGa (see the different scales of Fig. 3). Also, the residual resistivity of the former compound is relatively large; a value of po as high as 9 3 0 / ~ cm at the liquid helium temperature was found. This probably results from the fact that U2CuGa3 is crystallographically not ordered, which brings about a
200
250
300
(K~
Fig. 4. Temperature dependence of the electrical resistivity of two U A u G a samples. The derivative dp(T)/dT as a function of temperature for these two samples is also shown.
strong scattering of electrons. The p(T) dependence for these compounds at low temperatures is also different, e.g. p of UCuGa exhibits a sudden increase at 15 K, whereas p of U2CuGa3 at temperatures between 4.2 and 30 K follows a quadratic temperature dependence: p(T) = 936 + 0.03T 1. At higher temperatures p(T) of U2CuGa3 has a broad maximum centred around 70 K. It is worth noticing that the shape of the p(T) curve of UECuGa3 is rather similar to that observed in C e A l 2 [8] or in URu2Si2 [9], for which, among others, the influence of the electric crystalline field on the Kondo effect has been considered. Surprisingly, the magnetic transition detected in the magnetic susceptibility measurements of U2CuGa3 at 15 K does not affect its p(T). Such a situation is not so rare among intermetallic U compounds and may indicate a large disorder on the various crystallographic sites. Only measurements on a single-crystal sample may solve this problem. A striking feature of the UAuGa case is the drastic change in the transport properties with respect to several samples measured. As shown in Fig. 4, the electrical resistivity of sample 1 decreases rapidly with increasing temperature according to the Kondo expression p(T) = 1500-0.1 In T and in conformity with our previous results [3]. In contrast, the resistivity values of sample 2 are larger and, above TN, p first increases and then goes through a very broad maximum centred at about 120 K. From these two different plots, we can stress that the electrical resistivity data confirm the antiferromagnetic ordering in this compound around 60 + 2 K for each sample (see the dp(T)/dT curves in Fig. 4); however, no clear anomaly can be seen in the p(T) curve around 18 K for either of these samples. One important feature of the resistivity behaviour of UAuGa, which should be emphasized here, is a large value of the residual resistivity for both the samples investigated, being even higher than 1 mf~ cm. We feel
196
V. H. Tran et al. / Properties of UCuGa, U2CuGa3 and UAuGa
that the preferred orientation, crystallographic disorder and the presence of microcracks in the UAuGa samples are the most probable origin for their large residual value. With respect to the position of U 2 C u G a 3 , lying between the UCuGa and UGa2, it is interesting to compare its magnetic behaviour with these two latter compounds. According to Hill's idea, these compounds should be local-moment systems (du_tj = 4/~). In fact, the localmoment behaviour of UGa2 was confirmed by many experiments, such as pressure studies [10], magnetization measurements [5] and so on. In this case, the ferromagnetic structure of UGa2 is probably simple with the U magnetic moments lying in the basal plane [5]. A ferromagnetic ordering of UGa2 [5, 6] in a hexagonal type of unit cell is a result of competition of the magnetic exchange interactions Jo acting within the U atom plane and the interactions J1 which couple magnetically the adjacent U atom layers. For UGa2, both Jo and J1 are positive. The magnetic interactions between interlayered U atoms are not direct because of the separation by Ga atoms. Thus the integral J: behaves as a Ruderman-Kittel-Kasuya-Yoshida type interaction, in which the number of sp electrons on the outermost shell of the Ga atoms will have an important role. The partial replacement of Ga atoms by the Cu atoms in UGa2 decreases the number of conduction electrons. This change naturally modifies the interaction characteristics, especially those described by J1. Also, the introduction of the d electrons by Cu in U 2 C u G a 3 may cause to a certain degree the delocalization of 5f electrons as a result of the U 5f-Cu 3d hybridization effect. As mentioned above, U2CuGa3 possesses the AlB2-type structure with the sequence of the layers being U-(Cu, Ga)-U along the c axis. Hence, we notice that antiferromagnetic ordering in UzCuGa3 is possible when the value of Ja becomes negative. A similar situation probably takes place in UCuGa, since this compound adopts a CaIn2-type unit cell, with the sequence of layers analogous to that of UGa2, i.e. U-(Cu, Ga)-U-(Cu, Ga)-U, but with distances between the U atoms (du_tj = (1/4)c) being slightly shortened. In both U2CuGa3 and UCuGa the exchange integral J0 within the U atom plane probably has a positive value, as is the ease of UGa2. 3. Conclusion
We have found that the new compound U 2 C u G a 3 , crystallizing in the hexagonal A I B 2 - t y p e structure, undergoes an antiferromagnetic phase transition at around 15 IC We suggest that the number of sp-electrons on the outermost shell of Ga atoms and the degree of U
5f-Cu 3d hybridization are crucial parameters influencing the magnetic behaviour in the series UGa2, U 2 C u G a 3 and UCuGa. Therefore, the transition from the ferromagnetic in UGa2 to antiferromagnetic properties in U2CuGa3 and UCuGa is a consequence of the change in the sign of-/1, but the magnetic ordering within the U atom plane probably remains always ferromagnetic. Two polycrystalline samples of UAuGa were magnetically measured: one of them has revealed the existence of two magnetic transitions at 18 and 60 K, while the other has one transition only, at 60 K. The electrical resistivities of both samples exhibit the transition only at TN = 60 K and different behaviours above their antiferromagnetic transition temperature. Thus, a definite conclusion as to the observed large anisotropy in the transport properties of UAuGa in the paramagnetic state has to wait until the results of transport measurements on a single-crystal specimen become available.
Acknowledgment
The work was supported by the Polish Sciences Foundation "PONT" number 624392.
Note added in proof
Neutron diffraction experiment does not confirm an antiferromagnetic order in U2CuGa3.
References 1 V. Sechovsky and L. Havela, in E. P. Wohlfarth and H. K. J. Buschow (eds.), Ferromagnetic Materials, Vol. 4, NorthHolland, Amsterdam, 1988, p. 436. 2 V. H. Tran and R. Tror, Proc. Int. Conf. on the Physics of Transition Metals, Darmstadt, 1992, to be published. 3 V. H. Tran and R. Tror, Physica B, (1993), to be published. 4 V. H. Tran, F. Boure6 and R. Tror, tO be published. 5 A. V. Andreev, K. P. Belov, A. V. Deryagin. R. Z. Levitin and A. Menovsky, J. Phys. (Paris), Colloq. C4, 40 (1979) 82. 6 V. Ansorge and A. Menovsky, Phys. Status Solidi, 30 (1968) K31. 7 R. Z. Levitin, A. S. Dmitrievskii, Z. Henkie and A. Misiuk, Phys. Star. Sol., 27 (9) (1975) K109. 8 A. Percheron, J. C. Achard, O. Gorochov, B. Cornut, D. Jerome and B. Coqblin, Solid State Commun., 12 (1973) 1289. 9 A. Le R. Dawson, W. R. Datars, J. D. Garrett and F. S. Razavi, J. Phys.: Condens. Matter, 1 (1989) 6817. 10 J . J . M . Franse, P. H. Frings, F. R. de Boer and A. Menovsky, in J. S. Schilling and R. N. Shelton (eds.), Physics of Solids Under High Pressure, North-Holland, Amsterdam, 1981, p. 181.
193
Magnetic and transport properties of UCuGa, U2CuGa3 and UAuGa V. H . T r a n , R. T r 0 6 a n d D . B a d u r s k i w. Trzebiatowski Institute of Low Temperature and Structure Research, Polish Academy of Sciences, 50-950 Wroctaw (Poland)
(Received February 25, 1993)
Abstract
The magnetic characteristics of UCuGa, U2CuGa 3 and UAuGa, have been studied from 4.2 to 300 K. From the magnetic susceptibility data we inferred the onset of an antiferromagnetic ordering in UCuGa and U2CuGa3 at 30 K and 15 K respectively. Their electrical resistivities at high temperatures are analysed in terms of a Kondolike behaviour. In the case of UAuGa, we found two phase transitions at 18 and 60 K being of antiferromagnetic character. Moreover, a strong anisotropy in the transport properties of this compound has been observed above TN=60+2 K. The obtained results for a new compound U2CuGa3 are compared with those of UCuGa and UGa2.
1. Introduction
The magnetic and transport properties of UCuGa were briefly mentioned several years ago [1], but its magnetic behaviour is still a puzzle. For example, the earlier magnetization and specific heat studies made on UCuGa by Andreev et al. (cited in ref. 1) showed that this compound undergoes two magnetic transitions at 29 and 15 K, while our recent susceptibility measurements [2] have only revealed an antiferromagnetic ordering below 30 K. On the contrary, our electrical resistivity results demonstrate the presence of a phase transition in UCuGa only at 15 K [2]. Therefore, the nature of the magnetic order in UCuGa is not still clear. In an attempt to clarify this situation, we have repeated the magnetic susceptibility, magnetization and electrical resistivity measurements on newly prepared UCuGa samples. In the case of UAuGa [3], both the magnetic susceptibility and the electrical resistivity data indicate that the phase transition from the paramagnetic to antiferromagnetic state in this compound takes place at TN = 60_+ 2 K. Above this temperature the values of p change as In T which is characteristic of a Kondo effect. However, the magnetic results presented here give evidence that its magnetic behaviour is more complex than those previously published [3]. Moreover, we synthesized a new compound U2CuGa3 and its magnetic properties we here report for the first time. We have also studied the formation of such a compound with Au. Its physical properties including the magnetic sus0925-8388/93/$6.00
ceptibility, electrical resistivity and neutron diffraction measurements will be reported elsewhere [4]. 2. Results and discussion
The polycrystalline specimens used in this study were prepared by arc melting, followed by vacuum annealing at 800 °C for 5 days. Powder X-ray diffraction analysis showed the investigated samples to be single phase. The crystal structure of U2CuGa 3 was found to be the hexagonal A1B2 type (space group P r / m m m ) with the lattice parameters a=4.348(1) /~ and c=3.605(1) /~. As reported previously [1, 2], UCuGa crystallizes in a structure of the hexagonal CaIn2 type. In agreement with a previous study [3], a newly synthesized sample of UAuGa has an orthorhombic CeCu2 (TiNiSi) type unit cell. The crystallographic parameters of all the compounds investigated in this work are given in Table 1. The magnetic susceptibility X was measured over the temperature range 4.2-300 K using a Cahn-RH electrobalance. Figure 1 displays the reciprocal susceptibility X-1 of UCuGa and U 2 C u G a 3 as a function of temperature. In good agreement with our previous results [2], a cusp in x(T) for UCuGa is well marked at TN =30 K, but rather a small change in its X(T) below TN is apparent. As shown in Fig. 1, the temperature dependence of the reciprocal susceptibility of UCuGa follows a Curie-Weiss (CW) law above about 80 K, leading to the effective magnetic moment/zeer = 2.58/zB and the paramagnetic Currie temperature 19 p = - 1 3 © 1993- Elsevier Sequoia. All rights reserved
V. H. Tran et aL I Properties of UCuGa, U2CuGas and UAuGa
194
T A B L E 1. Crystal structure and magnetic and transport characteristics of the UGa2, U2CuGa3, U C u G a and U A u G a compounds Compound
Lattice parameters (/~)
Spin order TON
Op (K)
10 -3 (e.m.u. mol -z)
A'ox
/.~cfr (/ZB)
#(T) at high temperature (/~fl crn)
References
(K) UGa2
a=4.213 c =4.017
F; 126
134
1.32
2.80
Kondo behaviour
5-7
U2CuGa3
a =4.348 c = 3.605
AF; 15
7
0.78
2.79
1 2 3 9 - 5 6 In T
This work
UCuGa
a =4.341 c = 6.994
AF; 29, 15
0
0.02
2.36
Not reported
1
UCuGa
a=4.345 c = 7.002
AF; 30
-7
2.54
2 5 7 - 9 . 5 In T
2
UCuGa
a =4.366 c = 7.006
AF; 30
-13
0.01
2.58
2 7 8 - 1 6 In T
This work
UAuGa sample 1
a=3.881 b = 7.996 c=7.641
AF; 60
-8
0.6
2.70
1 5 0 0 - 0 . 1 In T
3
UAuGa sample 2
a =3.881 b=7.990 c =7.644
AF, AF; 18, 60
-16
0.5
2.66
A broad maximum at about 130 K
This work
=0
AF, antiferromagnetic; F, ferromagnetic.
400
350
"'.
300
~"
~
50 o
t"
250
t
200
o
""
~3o E
....-'-.. "'"'..... 2~
,-"""-
50
."
,,,,""
20 ?o
t °
75 ~oo
.
""
.,t"
150
......
0
.
50
......
..
100
150
} /YD<'t 200
250
300
T (K)
Fig. 1. T e m p e r a t u r e dependence of the reciprocal susceptibility of U C u G a and U2CuGa3. Plot A shows their low temperature magnetic susceptibility. Plot B shows the magnetization of these compounds as a function of magnetic field at 4.2 K.
K. The magnetic and transport data of UCuGa are collected in Table 1. As also seen in Fig. 1, the magnetic susceptibility of U a C u G a 3 exhibits a different thermal behaviour compared with that of UCuGa. The curvature of the X-I(T) function suggests its description in terms of a modified CW law. For the X data above 20 K the fit to this law yields Xo = 0.78 x 10 - 3 e . m . u , mol- 1, (gp = 7 K and/zen = 2.79/zB. A pronounced maximum in the X vs. T curve of U 2 C u G a 3 , but slightly broadened, occurs at about 15 K (see curves A of Fig. 1), suggesting an antiferromagnetic ordering also in this compound. However, in this case the magnetic susceptibility at, for
example, Tm~, seems to be very large compared with that for a common U-based antiferromagnet: at this temperature X reaches a value as large as 55× 10 - 3 e.m.u, mol-1. Hence we ascribe this fact as well as the appearance of the positive Op value to the formation of a complex antiferromagnetic phase. The presence in this sample of some ferromagnetic impurities can be excluded, because the magnetization at 4.2 K (see curves B of Fig. 1) exhibits an almost linear increase to the limit of our measurements, i.e. to a field of 4 T, but it does display a small hysteresis when the magnetic field is varied down. For a comparison, plot B of Fig. 1 also shows the results of magnetization measurements performed at 4.2 K for UCuGa. In contrast, its magnetization increases linearly with the magnetic field, but without any hysteresis. In Fig. 2 the reciprocal susceptibility of two UAuGa samples is displayed as a function of temperature. Although these two specimens were obtained in a similar thermal condition, their magnetic properties are different. For sample 1 we found the same behaviour as reported previously [3] but, for sample 2, two maxima are seen in the x(T) curve: that located at 60 K presents a cusp in agreement with a previous observation [3], and the second maximum at 18 K, which probably signals a magnetic phase transition into another antiferromagnetic state. Despite these differences both sets of lattice parameters a, b and c of samples 1 and 2 appear to be close to each other (Table 1). Also, the values of effective magnetic moments/zeer are comparable for these two samples. Nevertheless, the difference observed in x(T) at low temperatures between
V. H. Tran et al. / Properties of UCuGa, U2CuGa 3 and UAuGa
1.35
'
250
I
:;f
o
UAuGa
o~°[
150
i
12
%
i
°//./ . . . . . .1
m
..,......-
L
o :
/
/
1.30 ~ 2
J C
-~
// /
°
2oo-
k
J
~ ~
195
25
50
75
T(K)
"
i
i00
I
1.25
F
1.20
-
o
//
1oo
5O
/ 1.15
i ~i
150
!
2OO
J
T (K)
UAuGa
-
i
I
0
0
50
100
150
200
250
300
0
T (K)
980
240 i
960
O
-
UoCuGa 3
' /
I :<, /
'
"'\ ~
~- 200-
\~-
\-.
"~
UCuGa
940
~ Q
~-.~ 920
I
180 0
50
100
150
100 T
Fig. 2. Temperature dependence of the reciprocal susceptibility for two samples of UAuGa. Their low temperature susceptibility is also shown.
220
50
150
200
250
900 300
T (K)
Fig. 3. Electrical resistivity of UCuGa and UzCuGa 3 as a function of temperature.
these two samples and also in their absolute values of h' indicates a high sensitivity of magnetic properties to any small variation in stoichiometry, even if it is not reflected in the lattice parameters. Electrical resistivity measurements for UCuGa, U2CuGa3 and UAuGa were carried out with a fourprobe technique between 4.2 and 300 K. The results are presented in Figs. 3 and 4. As Fig. 3 illustrates, the p(T) curves of both UCuGa and U2CuGa3 (above 100 K) have a negative slope, which is indicative of a Kondo effect. Thus the repeated electrical resistivity measurements on UCuGa have confirmed the previous results [2]. It appears that the absolute p value of UECuGa3 at room temperature is about four times higher than that of UCuGa (see the different scales of Fig. 3). Also, the residual resistivity of the former compound is relatively large; a value of po as high as 9 3 0 / ~ cm at the liquid helium temperature was found. This probably results from the fact that U2CuGa3 is crystallographically not ordered, which brings about a
200
250
300
(K~
Fig. 4. Temperature dependence of the electrical resistivity of two U A u G a samples. The derivative dp(T)/dT as a function of temperature for these two samples is also shown.
strong scattering of electrons. The p(T) dependence for these compounds at low temperatures is also different, e.g. p of UCuGa exhibits a sudden increase at 15 K, whereas p of U2CuGa3 at temperatures between 4.2 and 30 K follows a quadratic temperature dependence: p(T) = 936 + 0.03T 1. At higher temperatures p(T) of U2CuGa3 has a broad maximum centred around 70 K. It is worth noticing that the shape of the p(T) curve of UECuGa3 is rather similar to that observed in C e A l 2 [8] or in URu2Si2 [9], for which, among others, the influence of the electric crystalline field on the Kondo effect has been considered. Surprisingly, the magnetic transition detected in the magnetic susceptibility measurements of U2CuGa3 at 15 K does not affect its p(T). Such a situation is not so rare among intermetallic U compounds and may indicate a large disorder on the various crystallographic sites. Only measurements on a single-crystal sample may solve this problem. A striking feature of the UAuGa case is the drastic change in the transport properties with respect to several samples measured. As shown in Fig. 4, the electrical resistivity of sample 1 decreases rapidly with increasing temperature according to the Kondo expression p(T) = 1500-0.1 In T and in conformity with our previous results [3]. In contrast, the resistivity values of sample 2 are larger and, above TN, p first increases and then goes through a very broad maximum centred at about 120 K. From these two different plots, we can stress that the electrical resistivity data confirm the antiferromagnetic ordering in this compound around 60 + 2 K for each sample (see the dp(T)/dT curves in Fig. 4); however, no clear anomaly can be seen in the p(T) curve around 18 K for either of these samples. One important feature of the resistivity behaviour of UAuGa, which should be emphasized here, is a large value of the residual resistivity for both the samples investigated, being even higher than 1 mf~ cm. We feel
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that the preferred orientation, crystallographic disorder and the presence of microcracks in the UAuGa samples are the most probable origin for their large residual value. With respect to the position of U 2 C u G a 3 , lying between the UCuGa and UGa2, it is interesting to compare its magnetic behaviour with these two latter compounds. According to Hill's idea, these compounds should be local-moment systems (du_tj = 4/~). In fact, the localmoment behaviour of UGa2 was confirmed by many experiments, such as pressure studies [10], magnetization measurements [5] and so on. In this case, the ferromagnetic structure of UGa2 is probably simple with the U magnetic moments lying in the basal plane [5]. A ferromagnetic ordering of UGa2 [5, 6] in a hexagonal type of unit cell is a result of competition of the magnetic exchange interactions Jo acting within the U atom plane and the interactions J1 which couple magnetically the adjacent U atom layers. For UGa2, both Jo and J1 are positive. The magnetic interactions between interlayered U atoms are not direct because of the separation by Ga atoms. Thus the integral J: behaves as a Ruderman-Kittel-Kasuya-Yoshida type interaction, in which the number of sp electrons on the outermost shell of the Ga atoms will have an important role. The partial replacement of Ga atoms by the Cu atoms in UGa2 decreases the number of conduction electrons. This change naturally modifies the interaction characteristics, especially those described by J1. Also, the introduction of the d electrons by Cu in U 2 C u G a 3 may cause to a certain degree the delocalization of 5f electrons as a result of the U 5f-Cu 3d hybridization effect. As mentioned above, U2CuGa3 possesses the AlB2-type structure with the sequence of the layers being U-(Cu, Ga)-U along the c axis. Hence, we notice that antiferromagnetic ordering in UzCuGa3 is possible when the value of Ja becomes negative. A similar situation probably takes place in UCuGa, since this compound adopts a CaIn2-type unit cell, with the sequence of layers analogous to that of UGa2, i.e. U-(Cu, Ga)-U-(Cu, Ga)-U, but with distances between the U atoms (du_tj = (1/4)c) being slightly shortened. In both U2CuGa3 and UCuGa the exchange integral J0 within the U atom plane probably has a positive value, as is the ease of UGa2. 3. Conclusion
We have found that the new compound U 2 C u G a 3 , crystallizing in the hexagonal A I B 2 - t y p e structure, undergoes an antiferromagnetic phase transition at around 15 IC We suggest that the number of sp-electrons on the outermost shell of Ga atoms and the degree of U
5f-Cu 3d hybridization are crucial parameters influencing the magnetic behaviour in the series UGa2, U 2 C u G a 3 and UCuGa. Therefore, the transition from the ferromagnetic in UGa2 to antiferromagnetic properties in U2CuGa3 and UCuGa is a consequence of the change in the sign of-/1, but the magnetic ordering within the U atom plane probably remains always ferromagnetic. Two polycrystalline samples of UAuGa were magnetically measured: one of them has revealed the existence of two magnetic transitions at 18 and 60 K, while the other has one transition only, at 60 K. The electrical resistivities of both samples exhibit the transition only at TN = 60 K and different behaviours above their antiferromagnetic transition temperature. Thus, a definite conclusion as to the observed large anisotropy in the transport properties of UAuGa in the paramagnetic state has to wait until the results of transport measurements on a single-crystal specimen become available.
Acknowledgment
The work was supported by the Polish Sciences Foundation "PONT" number 624392.
Note added in proof
Neutron diffraction experiment does not confirm an antiferromagnetic order in U2CuGa3.
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