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Effects of high magnetic field and pressure on the electrical resistivity of the heavy fermion compound URu2Si2
Physica B 177 (1992) North-Holland
147-150
Effects of high magnetic field and pressure on the electrical resistivity of the heavy fermion compound URu,Si, Y. Uwatoko,
K. Iki, G. Oomi, Y. 6nuki”
and T. Komatsubarab
Department of Physics, Faculty of General Education, Kumamoto 860, Japan “Institute of Materials Science, University of Tsukuba, lbaraki 305, Japan hDepartment of Physics, Tohoku University, Sendai 980, Japan
The effects of pressure and magnetic field on the electrical resistivity of the heavy fermion compound URu,Si, have been measured up to 20 kbar and 5 T. It is found that TN increases with pressure and the magnetoresistance shows a complicated behavior with pressure.
1. Introduction
The physical properties of Kondo compounds are dominated mainly by two characteristic temperatures, T, and TRKKY, which are related to the Kondo effect and to the RKKY interaction [l]. The electronic and magnetic states are determined by the competition between these two temperatures: if TRKKY is larger than T,, the system will be magnetic and vice versa. It is well known that T, and TRKKY are crucially dependent on external forces such as pressure or magnetic field, which may change the band width, the relative position of the 4f level, the hybridization between the 4f level and the conduction band, etc. We expect to obtain a lot of important information about the electronic structure by investigating the physical properties of Kondo compounds under external forces. URu,Si, crystallizes in the tetragonal bodycentered ThCr,Si, type structure and has a yvalue of 64 mJ/mol K* [2]. This compound has attracted a considerable amount of attention because of the coexistence of a magnetic phase transition at 17.5 K and a superconducting transition at 0.8 K [3]. The data of electrical resistivity and magnetic measurements indicate that a spin (or charge) density wave (SDW or CDW) transition may occur at TN = 17.5 K [3]. 0921-4526/92/$05.00
CQ 1992 - Elsevier
Science
Publishers
In order to examine the electronic properties of URu,Si, under two combined external forces, high pressure and high magnetic field, we measured the electrical resistivity and the magnetoresistance up to 20 kbar and up to 5 T.
2. Experiments The sample preparation has been reported previously [4]. Figure 1 shows the overall view of the high pressure apparatus in the present work, which was designed to measure the magnetic and electrical properties of condensed matter at high pressure, low temperature and high magnetic field. Hydrostatic pressure was generated by using a Cu-Be piston cylinder device up to 20 kbar. A 1: 1 mixture of transformer oil and kerosene was used as a pressure transmitting medium. The pressure inside the cylinder was kept constant throughout the experiment by controlling the oil pressure of the hydraulic press within +5%. Temperature was measured with a calibrated Cu(Fe)-Chrome1 thermocouple. The details about the construction and operation have been described previously [5]. The electrical resistivity was measured by a standard fourprobe method in the temperature range from 2.2 to 300 K under high pressure. The magneto-
B.V. All rights
reserved
148
Y. Uwatoko
et al.
I Magnetoresistance
of URu,Si,
r
T(K) Fig. 2. The electrical resistivity p of URuzSi, temperature at various pressures.
I
1
Fig. 1. Overview of the present high pressure apparatus. An OXFORD 5 T superconducting magnet and dewar (SMD-8) are used. The thermal insulation in the pressure transmitting columns 7,8 is supported by compression rod 4 using fiberglass reinforced plastic (FRP) and the thermal radiation shield 6. (1) Pressure intensifier up to 20 ton; (2) piston (SUS304); (3) O-ring; (4) compression rod (FRP); (5) tension rod (SUS 304); (6) thermal radiation shield (SUS304); (7) piston (WC); (8) pressure vessel (Cu-Be); (9) superconducting magnet (5 T).
resistance measurements were performed on a polycrystalline sample at T = 2.2 and 4.2 K in magnetic fields up to 5 T at high pressure.
as a function
However, the p(T) values around 4.2 and 300 K are not much affected by pressure. p(T) around the broad maximum decreases significantly with pressure. Such an anomalous pressure effect on the p(T) curve has not been observed in heavy fermion compounds containing Ce, such as CeCu, [7], CeAl, [a], etc. This kind of behavior may thus be a characteristic of U compounds. The magnitude of the hump at T, decreases with increasing pressure. This magnitude is inversely proportional to the gap energy A [6]. Therefore, the energy gap associated with the antiferromagnetic order may increase with pressure. This conclusion is supported by the results of previous resistivity measurements at high pressure [9]. Figure 3 shows T, as a function of pressure up to 20 kbar. The value of T, increases with pressure, the rate of increase becoming larger above approximately 10 kbar. The pressure dependence of T, is approximately linear below 10 kbar, having a rate of dT,/dP = 0.16 K/kbar. This is
3. Results and discussion Figure 2 shows the temperature dependent resistivity p(T) of URu,Si, at various pressures up to 20 kbar. All the p(T) curves show humplike anomalies at T,, which are reminiscent of the resistance anomaly due to a spin density wave in the antiferromagnet Cr. TN is determined as the temperature of the inflection point in the p(T) curves. At 1 bar, p(T) shows almost the same behavior as obtained previously [6].
of
251 0
0
10 20 P (kbar)
Fig. 3. T, of URuzSiz
as a function
of pressure.
149
Y. Uwatoko et al. I Magnetoresistanceof URuJi,
similar to the value of 0.13 K/kbar obtained by McElfresh et al. [6]. In fig. 4, the variation of the magnetoresistante Aplp = {p(H) - p(O)} /p(O) .with pressure is shown at (a) 4.2 K and (b) 2.2 K. The magnetoresistance is positive up to 5 T. Aplp is as much as 5.5% at 4T at 1 bar. The behavior of the positive magnetoresistance is almost the same as the results obtained by &uki et al. [4]
0.1 7,
and by Palstra et al. [lo]. There is no significant difference in the overall behavior of Aplp versus H curves for T = 2.2 K and for T = 4.2 K. The value of Aplp at H = 4 T is shown in the inset of fig. 2. The magnetoresistance increases slightly with increasing pressure up to 15 kbar. But, Aplp at 20 kbar is smaller than that at 15 kbar. The origin of this complicated behavior of Aplp with pressure is not clear at present. The field dependence of the transverse magnetoresistance of metals and alloys may be given as follows [ll]: AP -= P
0
1
2
4
5
H(6 (a)
(b) Fig. 4. Magnetoresistance versus pressure for two temperatures, (a) T = 4.2 K and (b) T = 2.2 K. The solid lines are a fit to Aplp = AH’. The inset shows the pressure dependence of the magnetoresistance at H = 4 T.
p(H)
- p(O) = AH2
(1)
P(0)
A least-square fit to the observed Aplp was carried out at various pressures. The coefficients A which are obtained are shown in fig. 5. A shows a maximum around P = 15 kbar at both 4.2 and 2.2 K. It has been reported that T, increases with pressure, but that the rate of increase becomes larger above approximately 15 kbar [9]. The pressure at which the maxima in the A versus P curves occurs agrees well with this value. We have reported previously that the effect of crystal electric field (CEF) splitting disappears at high pressure around 10 kbar [12]. The maximum in the A versus P curves may be partly due to the disappearance of the CEF spitting. In any case, the maximum suggests a change in the magnetic state of URu,Si, induced by high pressure. Further experimental work
zo-
P
Fig. 5. The coefficient p = AH2 as a function
30
(kbar)
A of the of pressure.
HZ contribution
to Apl
150
Y. Uwatoko et al. I Magnetoresistance
including crystal structure and magnetic measurements are needed to clarify this point.
References
[ll
N.B. Brandt and V.V. Moschalkov, Adv. Phys. 33 (1984) 373. PI A. de Visser, F.E. Kayzel, A.A. Menovsky, J.J. M. Frame, J. van den Berg and G.J. Nieuwenhuys, Phys. Rev. B 34 (1986) 8168. J. van den Berg, A.J. [31 T.T.M. Palstra, A.A. Menovsky, Dirkmaat. P.H. Kes, G.J. Nieuwenhuys and J.A. Mydosh. Phys. Rev. Lett. 55 (1986) 2727. I. Ukon, T. Omi, K. Shibutani, [41 Y. Gnuki, T. Yamazaki, K. Yonemitsu, A. Umezawa, W.K. Kwok, G.W. Crabtree and D.G. Himks, Physica B 148 (1987) 29.
of lJRu2Si2
G. Oomi, J. High Press. Ins. Jpn., to be [51 Y. Uwatoko, published (in Japanese). [61 M.W. McElfresh, J.D. Thompson, J.O. Willis, M.B. Maple, T. Kohara and M.S. Torikachvili, Phys. Rev. B 35 (1987) 43. [71 S. Yomo, L. Gao, R.L. Meng, P.H. Hor, C.W. Chu and J. Susaki, J. Magn. Magn. Mater. 76 & 77 (1988) 257. PI A. Peroheron, J.C. Achard, 0. Gorochov, B. Cornut, J. Jerome and B. Coqblin, Solid State Commun. 12 (1973) 1289. H. Takahashi, N. Mori, [91 K. Iki, G. Oomi, Y. Uwatoko, Y. Gnuki and T. Komatsubara, J. Less-Common Met. to be published. Palstra, A.A. Menovsky and J.A. Mydosh, [lOI T.T.M. Phys. Rev. B 33 (1986) 6527. [ill J.P. Jan, in: Solid State Physics, Vol. 5, eds. F. Seitz and D. Turubull (Academic Press, New York, 1957) p. 1. H. Okita, Y. Uwatoko, G. Oomi, [121 K. Iki, H. Takakura, Y. Gnuki and T. Komatsubara, J. Magn. Magn. Mater. 90 & 91 (1990) 526.
147-150
Effects of high magnetic field and pressure on the electrical resistivity of the heavy fermion compound URu,Si, Y. Uwatoko,
K. Iki, G. Oomi, Y. 6nuki”
and T. Komatsubarab
Department of Physics, Faculty of General Education, Kumamoto 860, Japan “Institute of Materials Science, University of Tsukuba, lbaraki 305, Japan hDepartment of Physics, Tohoku University, Sendai 980, Japan
The effects of pressure and magnetic field on the electrical resistivity of the heavy fermion compound URu,Si, have been measured up to 20 kbar and 5 T. It is found that TN increases with pressure and the magnetoresistance shows a complicated behavior with pressure.
1. Introduction
The physical properties of Kondo compounds are dominated mainly by two characteristic temperatures, T, and TRKKY, which are related to the Kondo effect and to the RKKY interaction [l]. The electronic and magnetic states are determined by the competition between these two temperatures: if TRKKY is larger than T,, the system will be magnetic and vice versa. It is well known that T, and TRKKY are crucially dependent on external forces such as pressure or magnetic field, which may change the band width, the relative position of the 4f level, the hybridization between the 4f level and the conduction band, etc. We expect to obtain a lot of important information about the electronic structure by investigating the physical properties of Kondo compounds under external forces. URu,Si, crystallizes in the tetragonal bodycentered ThCr,Si, type structure and has a yvalue of 64 mJ/mol K* [2]. This compound has attracted a considerable amount of attention because of the coexistence of a magnetic phase transition at 17.5 K and a superconducting transition at 0.8 K [3]. The data of electrical resistivity and magnetic measurements indicate that a spin (or charge) density wave (SDW or CDW) transition may occur at TN = 17.5 K [3]. 0921-4526/92/$05.00
CQ 1992 - Elsevier
Science
Publishers
In order to examine the electronic properties of URu,Si, under two combined external forces, high pressure and high magnetic field, we measured the electrical resistivity and the magnetoresistance up to 20 kbar and up to 5 T.
2. Experiments The sample preparation has been reported previously [4]. Figure 1 shows the overall view of the high pressure apparatus in the present work, which was designed to measure the magnetic and electrical properties of condensed matter at high pressure, low temperature and high magnetic field. Hydrostatic pressure was generated by using a Cu-Be piston cylinder device up to 20 kbar. A 1: 1 mixture of transformer oil and kerosene was used as a pressure transmitting medium. The pressure inside the cylinder was kept constant throughout the experiment by controlling the oil pressure of the hydraulic press within +5%. Temperature was measured with a calibrated Cu(Fe)-Chrome1 thermocouple. The details about the construction and operation have been described previously [5]. The electrical resistivity was measured by a standard fourprobe method in the temperature range from 2.2 to 300 K under high pressure. The magneto-
B.V. All rights
reserved
148
Y. Uwatoko
et al.
I Magnetoresistance
of URu,Si,
r
T(K) Fig. 2. The electrical resistivity p of URuzSi, temperature at various pressures.
I
1
Fig. 1. Overview of the present high pressure apparatus. An OXFORD 5 T superconducting magnet and dewar (SMD-8) are used. The thermal insulation in the pressure transmitting columns 7,8 is supported by compression rod 4 using fiberglass reinforced plastic (FRP) and the thermal radiation shield 6. (1) Pressure intensifier up to 20 ton; (2) piston (SUS304); (3) O-ring; (4) compression rod (FRP); (5) tension rod (SUS 304); (6) thermal radiation shield (SUS304); (7) piston (WC); (8) pressure vessel (Cu-Be); (9) superconducting magnet (5 T).
resistance measurements were performed on a polycrystalline sample at T = 2.2 and 4.2 K in magnetic fields up to 5 T at high pressure.
as a function
However, the p(T) values around 4.2 and 300 K are not much affected by pressure. p(T) around the broad maximum decreases significantly with pressure. Such an anomalous pressure effect on the p(T) curve has not been observed in heavy fermion compounds containing Ce, such as CeCu, [7], CeAl, [a], etc. This kind of behavior may thus be a characteristic of U compounds. The magnitude of the hump at T, decreases with increasing pressure. This magnitude is inversely proportional to the gap energy A [6]. Therefore, the energy gap associated with the antiferromagnetic order may increase with pressure. This conclusion is supported by the results of previous resistivity measurements at high pressure [9]. Figure 3 shows T, as a function of pressure up to 20 kbar. The value of T, increases with pressure, the rate of increase becoming larger above approximately 10 kbar. The pressure dependence of T, is approximately linear below 10 kbar, having a rate of dT,/dP = 0.16 K/kbar. This is
3. Results and discussion Figure 2 shows the temperature dependent resistivity p(T) of URu,Si, at various pressures up to 20 kbar. All the p(T) curves show humplike anomalies at T,, which are reminiscent of the resistance anomaly due to a spin density wave in the antiferromagnet Cr. TN is determined as the temperature of the inflection point in the p(T) curves. At 1 bar, p(T) shows almost the same behavior as obtained previously [6].
of
251 0
0
10 20 P (kbar)
Fig. 3. T, of URuzSiz
as a function
of pressure.
149
Y. Uwatoko et al. I Magnetoresistanceof URuJi,
similar to the value of 0.13 K/kbar obtained by McElfresh et al. [6]. In fig. 4, the variation of the magnetoresistante Aplp = {p(H) - p(O)} /p(O) .with pressure is shown at (a) 4.2 K and (b) 2.2 K. The magnetoresistance is positive up to 5 T. Aplp is as much as 5.5% at 4T at 1 bar. The behavior of the positive magnetoresistance is almost the same as the results obtained by &uki et al. [4]
0.1 7,
and by Palstra et al. [lo]. There is no significant difference in the overall behavior of Aplp versus H curves for T = 2.2 K and for T = 4.2 K. The value of Aplp at H = 4 T is shown in the inset of fig. 2. The magnetoresistance increases slightly with increasing pressure up to 15 kbar. But, Aplp at 20 kbar is smaller than that at 15 kbar. The origin of this complicated behavior of Aplp with pressure is not clear at present. The field dependence of the transverse magnetoresistance of metals and alloys may be given as follows [ll]: AP -= P
0
1
2
4
5
H(6 (a)
(b) Fig. 4. Magnetoresistance versus pressure for two temperatures, (a) T = 4.2 K and (b) T = 2.2 K. The solid lines are a fit to Aplp = AH’. The inset shows the pressure dependence of the magnetoresistance at H = 4 T.
p(H)
- p(O) = AH2
(1)
P(0)
A least-square fit to the observed Aplp was carried out at various pressures. The coefficients A which are obtained are shown in fig. 5. A shows a maximum around P = 15 kbar at both 4.2 and 2.2 K. It has been reported that T, increases with pressure, but that the rate of increase becomes larger above approximately 15 kbar [9]. The pressure at which the maxima in the A versus P curves occurs agrees well with this value. We have reported previously that the effect of crystal electric field (CEF) splitting disappears at high pressure around 10 kbar [12]. The maximum in the A versus P curves may be partly due to the disappearance of the CEF spitting. In any case, the maximum suggests a change in the magnetic state of URu,Si, induced by high pressure. Further experimental work
zo-
P
Fig. 5. The coefficient p = AH2 as a function
30
(kbar)
A of the of pressure.
HZ contribution
to Apl
150
Y. Uwatoko et al. I Magnetoresistance
including crystal structure and magnetic measurements are needed to clarify this point.
References
[ll
N.B. Brandt and V.V. Moschalkov, Adv. Phys. 33 (1984) 373. PI A. de Visser, F.E. Kayzel, A.A. Menovsky, J.J. M. Frame, J. van den Berg and G.J. Nieuwenhuys, Phys. Rev. B 34 (1986) 8168. J. van den Berg, A.J. [31 T.T.M. Palstra, A.A. Menovsky, Dirkmaat. P.H. Kes, G.J. Nieuwenhuys and J.A. Mydosh. Phys. Rev. Lett. 55 (1986) 2727. I. Ukon, T. Omi, K. Shibutani, [41 Y. Gnuki, T. Yamazaki, K. Yonemitsu, A. Umezawa, W.K. Kwok, G.W. Crabtree and D.G. Himks, Physica B 148 (1987) 29.
of lJRu2Si2
G. Oomi, J. High Press. Ins. Jpn., to be [51 Y. Uwatoko, published (in Japanese). [61 M.W. McElfresh, J.D. Thompson, J.O. Willis, M.B. Maple, T. Kohara and M.S. Torikachvili, Phys. Rev. B 35 (1987) 43. [71 S. Yomo, L. Gao, R.L. Meng, P.H. Hor, C.W. Chu and J. Susaki, J. Magn. Magn. Mater. 76 & 77 (1988) 257. PI A. Peroheron, J.C. Achard, 0. Gorochov, B. Cornut, J. Jerome and B. Coqblin, Solid State Commun. 12 (1973) 1289. H. Takahashi, N. Mori, [91 K. Iki, G. Oomi, Y. Uwatoko, Y. Gnuki and T. Komatsubara, J. Less-Common Met. to be published. Palstra, A.A. Menovsky and J.A. Mydosh, [lOI T.T.M. Phys. Rev. B 33 (1986) 6527. [ill J.P. Jan, in: Solid State Physics, Vol. 5, eds. F. Seitz and D. Turubull (Academic Press, New York, 1957) p. 1. H. Okita, Y. Uwatoko, G. Oomi, [121 K. Iki, H. Takakura, Y. Gnuki and T. Komatsubara, J. Magn. Magn. Mater. 90 & 91 (1990) 526.