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Electrical resistivity of UTGe (T: Ni, Pd, Pt) single crystals
Journal of Magnetism and Magnetic Materials 104-107 (1992) 53-54 North-Holland
Electrical resistivity of UTGe (T: Ni, Pd, Pt) single crystals S. K a w a m a t a ~'~, H . I w a s a k i a n d T. K o m a t s u b a r a b
a, N.
Kobayashi
a
K. I s h i m o t o a, y . Y a m a g u c h i ~
u Institute for Materials Research, Tohoku University, Aoba-ku, Sendai 980, Japan 1,Department of Physics, Faculty of Science, Tohoku University, Aoba-ku, Sendal 980, Japan
The electrical resistivity of three single crystals of UTGe (T: Ni, Pd, Pt) have been measured in a magnetic field up to 60 kOe. In all samples, Kondo-like behavior was observed between 100 and 300 K in zero field. A large reduction of the resistivity accompanied by a metamagnetic transition in UPdGe was observed for H lib- or c-axis. It is suggested that a magnetic superzone is formed in the antiferromagnetic state with long-period structure in UPdGe and UPtGe. Various magnetic and transport properties were reported on the equiatomic ternary compounds UTX, where T and X represent the transition metal and the group III or IV elements, respectively [1]. As for UTGe (T: Ni, Pd, Pt) with CeCu2(TiNiSi)-type crystal structure, we have reported on the magnetic phase transitions in single crystalline samples [2]. Details of the magnetic structures determined by neutron diffraction measurements on single crystals are to be reported elsewhere [3]. UNiGe is a simple collinear antiferromagnet below 42 K. UPdGe shows simple ferromagnetism below 28 K, while it exhibits antiferromagnetism with a longitudinal sinusoidal structure between 28 and 50 K. UPtGe becomes an antiferromagnet with a cycloidal structure below 50 K. We have observed a large reduction of the electrical resistivity accompanied with ferromagnetic ordering in poly-crystalline UPdGe [4]. In this paper, the behaviors of the electrical resistivity of UTGe (Ti: Ni, Pd, Pt) single crystals are reported and discussed based on the magnetic phase transitions. Single crystals of UTGe (T: Ni, Pd, Pt) were prepared by the Czochralski method using a tri-arc furnace under an argon atmosphere. By the neutron diffraction measurements on the single crystals, it was confirmed that the transition metal and germanium atoms occupy ordered sites; i.e. UTGe compounds have a TiNiSi-type crystal structure. The electrical resistivity at the c-axis was measured by a dc four-probe method in zero field or an ac four-probe method in a magnetic field up to 60 kOe. Fig. 1 shows the temperature dependence of the electrical resistivity of UTGe (T: Ni, Pd, Pt) single crystals in zero field. In all three samples Kondo-like behaviors are observed between 100 and 300 K, whereas the electronic specific heat coefficients are not so en-
hanced as the heavy-fermion compounds (between 20 and 30 m J / m o l K 2) [5]. The behaviors of the electrical resistivity due to the magnetic phase transitions are different among these compounds. The resistivity of UNiGe decreases as the temperature decreases below TN(= 42 K). In UPdGe, the resistivity begins to decrease at TN(= 50 K) and is reduced abruptly near To(= 28 K) with decreasing temperature. On the other hand, the resistivity of UPtGe begins to increase near TN(= 50 K), has a peak around 30 K and then decreases when the temperature decreases. The decrease of the resistivity due to the metamagnetic transition is a common property in the intermetalic compounds including cerium or uranium. However in UNiGe, little magnetic field dependence in the electrical resistivity was observed below 60 kOe, although metamagnetic transitions were observed in the magnetization measurements. Figs. 2 and 3 show the magnetic field dependence of the magnetization and the resistivity of UPdGe for a magnetic field H parallel to the c-axis, respectively. The large reduction of
I
,oo
I
I
I
UPdGe
E
U
Q2, 0 0 ~ I
O0
i
I
100
I
I
I
100
200
r 1 Present address: Department of Electronics, College of Engineering, University of Osaka Prefecture, Sakai, Osaka 591, Japan.
I
I//c-oxis
(K)
Fig. 1. Temperature dependence of the electrical resistivity of UTGe (T: Ni, Pd, Pt) single crystals with the current, I, parallel to the c-axis.
0312-8853/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
S. Kawarnata et al. / Electrical resistiuity of UTGe single crystals
54
8000
UPdGe '
' H//c-axi's ~
"~ 6000 .
E4000
~ ~ 3 ~ ~ ~
o
20/K
•
2000
I
2 / 3 ~ # ~ / f - 4oo -1
502
•
00
20
40 H
60 (kOe)
Fig. 2. Magnetization curves of UPdOe at various temperatures when H IIc-axis.
the resistivity accompanied by the metamagnetic transitions can be seen as reported on the poly-crystalline samples previously [4]. Similar results were also obtained for H ]] b-axis. On the contrary, when the magnetic field was applied parallel to the a-axis, there was no reduction in the resistivity, although metamagnetic transitions were observed in the magnetization for H ][ a-axis. The resistivity of U P t G e did not depend on the magnetic field at all as expected from the fact that no metamagnetic transition was observed below 60 kOe. The relationship between the resistivity and the magnetic structure is considered as follows. The reduction of the resistivity caused by the magnetic transitions is usually attributed to the reduction of the magnetic scattering. In addition to this effect of the magnetic scattering, the enhancement of the resistivity caused by the magnetic superzone due to magnetic ordering with long-period structure should be taken into account [6]. Both effects, the magnetic scattering
and the magnetic superzone, are responsible for the resistivity of U P d G c in the longitudinal sinusoidal state with long-period structure. The fact that the resistivity does not change with the metamagnetic transition for H II a-axis seems to indicate that the reduction of the magnetic scattering caused by the metamagnetic transition is negligible in U P d G e . Therefore the sudden reduction of the resistivity, both with the transition from the sinusoidal state to the ferromagnetic state in zero field and the metamagnetic transition in finite field, is considered to bc caused by the disappearance of the magnetic superzone. In the case of U P t G c , the formation of a magnetic superzone with a cycloidal structure with long period seems to cause the increase of the resistivity near T N with decreasing temperature in zero field. On the other hand, in the case of UNiGe, a magnetic superzone is not formed because the magnetic unit cell is the same as the chemical unit cell [3]. No decrease of the resistivity with metamagnetic transitions seems to indicate that the reduction of the magnetic scattering accompanied by the metamagnctic transition is very. small in U N i G e . In summary, the electrical resistivities have been measured on single crystals of U T G e (T: Ni, Pd, Pt). A variety in the behaviors of the resistivity accompanied by magnetic phase transitions was observed, It is suggested that a magnetic superzone is formed in the antiferromagnetic states with long-period structure in U P d G e and U P t G e . We are very grateful to Proffesors Y. Nakagawa, J.A. Mydosh, Y. Muto, T. Suzuki, O. Sakai, G. Kido, M. |kebe, Doctors V. Sechovsky, M. Ohashi for helpful discussions. We also thank Prof. T. Mitsugashira and Dr. A. Ochiai for the support in the sample preparation. This work was supported by the Grant-in-Aid (01790222) for Scientific Research from the Ministry of Education, Science and Culture, Japan. References
I
I
I
I
I
I
H II I II c - a x i s
UPdGe
400 u
60.2K
200
30.
Q.. 4.2 I
0
0
I
I
20
I
I
40 H
25.5 20.0 l
60 (kOe)
Fig. 3. Magnetic field dependence of the resistivity of UPdGe when H IIc-axis.
[l] V. Sechovsk~ and L. Havela, in: Ferromagnetic Materials, vol. 4, eds. E.P. Wohlfarth and K.H.J. Bushow (NorthHolland, Amsterdam, 1988) p. 309. [2] S. Kawamata, K. Ishimoto, H. Iwasaki, N. Kobayashi, Y. Yamaguchi, T. Komatsubara, G. Kido, T. Mitsugashira and Y. Muto, J. Magn. Magn. Mater. 90&91 (1990) 513. [3] S. Kawamata, K. Ishimoto, Y. Yamaguchi and T. Komatsubara, J. Magn. Magn. Mater. 104-107 (1992) 5l. [4] S. Kawamata, H. Iwasaki, N. Kobayashi, T. Mitsugashira and Y. Muto, J. Phys. Sac. Jpn. 58 (1989) 2654. [5] S. Kawamata, H. Iwasaki and N. Kobayashi, J. Magn. Magn. Mater. 104-107 (1992) 55. [6] S. Legvold, in: Magnetic Properties of Rare Earth Metals, ed. RJ. Elliot (Plenum Press, New York, 1972) p. 335.
Electrical resistivity of UTGe (T: Ni, Pd, Pt) single crystals S. K a w a m a t a ~'~, H . I w a s a k i a n d T. K o m a t s u b a r a b
a, N.
Kobayashi
a
K. I s h i m o t o a, y . Y a m a g u c h i ~
u Institute for Materials Research, Tohoku University, Aoba-ku, Sendai 980, Japan 1,Department of Physics, Faculty of Science, Tohoku University, Aoba-ku, Sendal 980, Japan
The electrical resistivity of three single crystals of UTGe (T: Ni, Pd, Pt) have been measured in a magnetic field up to 60 kOe. In all samples, Kondo-like behavior was observed between 100 and 300 K in zero field. A large reduction of the resistivity accompanied by a metamagnetic transition in UPdGe was observed for H lib- or c-axis. It is suggested that a magnetic superzone is formed in the antiferromagnetic state with long-period structure in UPdGe and UPtGe. Various magnetic and transport properties were reported on the equiatomic ternary compounds UTX, where T and X represent the transition metal and the group III or IV elements, respectively [1]. As for UTGe (T: Ni, Pd, Pt) with CeCu2(TiNiSi)-type crystal structure, we have reported on the magnetic phase transitions in single crystalline samples [2]. Details of the magnetic structures determined by neutron diffraction measurements on single crystals are to be reported elsewhere [3]. UNiGe is a simple collinear antiferromagnet below 42 K. UPdGe shows simple ferromagnetism below 28 K, while it exhibits antiferromagnetism with a longitudinal sinusoidal structure between 28 and 50 K. UPtGe becomes an antiferromagnet with a cycloidal structure below 50 K. We have observed a large reduction of the electrical resistivity accompanied with ferromagnetic ordering in poly-crystalline UPdGe [4]. In this paper, the behaviors of the electrical resistivity of UTGe (Ti: Ni, Pd, Pt) single crystals are reported and discussed based on the magnetic phase transitions. Single crystals of UTGe (T: Ni, Pd, Pt) were prepared by the Czochralski method using a tri-arc furnace under an argon atmosphere. By the neutron diffraction measurements on the single crystals, it was confirmed that the transition metal and germanium atoms occupy ordered sites; i.e. UTGe compounds have a TiNiSi-type crystal structure. The electrical resistivity at the c-axis was measured by a dc four-probe method in zero field or an ac four-probe method in a magnetic field up to 60 kOe. Fig. 1 shows the temperature dependence of the electrical resistivity of UTGe (T: Ni, Pd, Pt) single crystals in zero field. In all three samples Kondo-like behaviors are observed between 100 and 300 K, whereas the electronic specific heat coefficients are not so en-
hanced as the heavy-fermion compounds (between 20 and 30 m J / m o l K 2) [5]. The behaviors of the electrical resistivity due to the magnetic phase transitions are different among these compounds. The resistivity of UNiGe decreases as the temperature decreases below TN(= 42 K). In UPdGe, the resistivity begins to decrease at TN(= 50 K) and is reduced abruptly near To(= 28 K) with decreasing temperature. On the other hand, the resistivity of UPtGe begins to increase near TN(= 50 K), has a peak around 30 K and then decreases when the temperature decreases. The decrease of the resistivity due to the metamagnetic transition is a common property in the intermetalic compounds including cerium or uranium. However in UNiGe, little magnetic field dependence in the electrical resistivity was observed below 60 kOe, although metamagnetic transitions were observed in the magnetization measurements. Figs. 2 and 3 show the magnetic field dependence of the magnetization and the resistivity of UPdGe for a magnetic field H parallel to the c-axis, respectively. The large reduction of
I
,oo
I
I
I
UPdGe
E
U
Q2, 0 0 ~ I
O0
i
I
100
I
I
I
100
200
r 1 Present address: Department of Electronics, College of Engineering, University of Osaka Prefecture, Sakai, Osaka 591, Japan.
I
I//c-oxis
(K)
Fig. 1. Temperature dependence of the electrical resistivity of UTGe (T: Ni, Pd, Pt) single crystals with the current, I, parallel to the c-axis.
0312-8853/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
S. Kawarnata et al. / Electrical resistiuity of UTGe single crystals
54
8000
UPdGe '
' H//c-axi's ~
"~ 6000 .
E4000
~ ~ 3 ~ ~ ~
o
20/K
•
2000
I
2 / 3 ~ # ~ / f - 4oo -1
502
•
00
20
40 H
60 (kOe)
Fig. 2. Magnetization curves of UPdOe at various temperatures when H IIc-axis.
the resistivity accompanied by the metamagnetic transitions can be seen as reported on the poly-crystalline samples previously [4]. Similar results were also obtained for H ]] b-axis. On the contrary, when the magnetic field was applied parallel to the a-axis, there was no reduction in the resistivity, although metamagnetic transitions were observed in the magnetization for H ][ a-axis. The resistivity of U P t G e did not depend on the magnetic field at all as expected from the fact that no metamagnetic transition was observed below 60 kOe. The relationship between the resistivity and the magnetic structure is considered as follows. The reduction of the resistivity caused by the magnetic transitions is usually attributed to the reduction of the magnetic scattering. In addition to this effect of the magnetic scattering, the enhancement of the resistivity caused by the magnetic superzone due to magnetic ordering with long-period structure should be taken into account [6]. Both effects, the magnetic scattering
and the magnetic superzone, are responsible for the resistivity of U P d G c in the longitudinal sinusoidal state with long-period structure. The fact that the resistivity does not change with the metamagnetic transition for H II a-axis seems to indicate that the reduction of the magnetic scattering caused by the metamagnetic transition is negligible in U P d G e . Therefore the sudden reduction of the resistivity, both with the transition from the sinusoidal state to the ferromagnetic state in zero field and the metamagnetic transition in finite field, is considered to bc caused by the disappearance of the magnetic superzone. In the case of U P t G c , the formation of a magnetic superzone with a cycloidal structure with long period seems to cause the increase of the resistivity near T N with decreasing temperature in zero field. On the other hand, in the case of UNiGe, a magnetic superzone is not formed because the magnetic unit cell is the same as the chemical unit cell [3]. No decrease of the resistivity with metamagnetic transitions seems to indicate that the reduction of the magnetic scattering accompanied by the metamagnctic transition is very. small in U N i G e . In summary, the electrical resistivities have been measured on single crystals of U T G e (T: Ni, Pd, Pt). A variety in the behaviors of the resistivity accompanied by magnetic phase transitions was observed, It is suggested that a magnetic superzone is formed in the antiferromagnetic states with long-period structure in U P d G e and U P t G e . We are very grateful to Proffesors Y. Nakagawa, J.A. Mydosh, Y. Muto, T. Suzuki, O. Sakai, G. Kido, M. |kebe, Doctors V. Sechovsky, M. Ohashi for helpful discussions. We also thank Prof. T. Mitsugashira and Dr. A. Ochiai for the support in the sample preparation. This work was supported by the Grant-in-Aid (01790222) for Scientific Research from the Ministry of Education, Science and Culture, Japan. References
I
I
I
I
I
I
H II I II c - a x i s
UPdGe
400 u
60.2K
200
30.
Q.. 4.2 I
0
0
I
I
20
I
I
40 H
25.5 20.0 l
60 (kOe)
Fig. 3. Magnetic field dependence of the resistivity of UPdGe when H IIc-axis.
[l] V. Sechovsk~ and L. Havela, in: Ferromagnetic Materials, vol. 4, eds. E.P. Wohlfarth and K.H.J. Bushow (NorthHolland, Amsterdam, 1988) p. 309. [2] S. Kawamata, K. Ishimoto, H. Iwasaki, N. Kobayashi, Y. Yamaguchi, T. Komatsubara, G. Kido, T. Mitsugashira and Y. Muto, J. Magn. Magn. Mater. 90&91 (1990) 513. [3] S. Kawamata, K. Ishimoto, Y. Yamaguchi and T. Komatsubara, J. Magn. Magn. Mater. 104-107 (1992) 5l. [4] S. Kawamata, H. Iwasaki, N. Kobayashi, T. Mitsugashira and Y. Muto, J. Phys. Sac. Jpn. 58 (1989) 2654. [5] S. Kawamata, H. Iwasaki and N. Kobayashi, J. Magn. Magn. Mater. 104-107 (1992) 55. [6] S. Legvold, in: Magnetic Properties of Rare Earth Metals, ed. RJ. Elliot (Plenum Press, New York, 1972) p. 335.