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Influence of external pressure on thermal expansion and electrical resistivity of UNi2Si2 single crystals
Journal of Magnetism and Magnetic Materials 177 181 (1998) 49-50
,~
ELSEVIER
Journal of magnetism and magnetic materials
Influence of external pressure on thermal expansion and electrical resistivity of UNizSi2 single crystals F. Honda a, G. Oomi b, T. Kagayama b, A.V. Andreev c, V. Sechovsk~ d'*, L. Havela d, M.I. Bartashevich e, T. Goto e, A.A. Menovsky f "Department of Physics, Kumamoto University, Kumamoto 860, Japan b Department of Mechanical Engineering and Materials Science, Kumamoto University, Kumamoto 860, Japan Clnstitute of Physics, Academy of Sciences, Na Slovance 2, 180 40 Prague, Czech Republic aDepartment of Metal Physics, Charles' University, Ke Karlovu 5, 121 16 Prague, Czech Republic elSSP University of Tokyo, Roppongi 7-22-1, Minato-ku, Tokyo 106, Japan f University of Amsterdam, Valckenierstraat 65, 1018 XE Amsterdam, Netherlands'
Abstract Magnetization under ambient pressure, thermal expansion and electrical resistivity under external pressure up to 2.2 GPa were measured on single crystals of UNi2Si2. A p - T magnetic phase diagrams is proposed. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Actinide compounds; Magnetic phase diagram; Magnetoelasticity; Electrical resistivity
UNi2Si 2 with the tetragonal ThCr2Si2-type structure orders antiferromagnetically below TN = 124K and exhibits two additional magnetic phase transitions at lower temperatures. All magnetic structures consist of ferromagnetic basal planes of U moments oriented along the c-axis and sinusoidally modulated with a propagation vector (0,0, q). The high-temperature (T = 103 124 K) phase is an incommensurate LSDW with q ~ 0.74. Between 53 and 103 K, a simple body-centered antiferromagnetic structure with/~tJ = 1.6 ~tBis stabilized. The q = 2 of the ground-state phase yields a non-zero spontaneous magnetization [1]. All the transitions (those at 53 and 103 K are of the first order) are accompanied by anomalies in magnetic, transport, thermal, and magnetoelastic properties. We report on the magnetization, thermal expansion and electrical resistivity study performed on single crystals grown by the Czochralski method in a tri-arc furnace. At ambient and external pressure up to 2.2 GPa we have measured the thermal expansion along the a- and c-axis (77-300 K) and the electrical resistivity along the c-axis (4.2-300 K). The a-, (1 1 0) and c-axis magneti-
*Corresponding author. Tel.: 420-2-2191-1367; fax: 420-221911351; e-mail: [email protected].
zation curves at ambient pressure were measured at 4.2 K in pulsed fields up to 40 T. The magnetization as a function of temperature (5 300 K) and magnetic field (0.01-5 T) has been also studied with a SQUID magnetometer. High-field magnetization curves (Fig. 1) document a huge uniaxial magnetic anisotropy in UNizSi2. The linear extrapolation of the easy- and hard-magnetization direction curves to high fields yields an estimate of the anisotropy field Ba ~ 200 T. This value falls in the range typical for uranium compourds with uniaxial anisotropy. No anisotropy was found within the basal plane: the curves along the a- and the (1 1 0) axis, respectively, coincide. The observed spontaneous moment of 0.53/tB at 4.2 K is in a good agreement with Ref. [2]. The magnetic hysteresis of UNi2Si2 (the inset in Fig. 1) is characteristic for a strongly anisotropic ferromagnet, which has narrow domain walls with a high intrinsic coercivity. The coercive field observed in steady fields is twice as low as in a pulsed field with ~B/~t ~ 1 T/ms, which indicates a considerable magnetic viscosity. The three magnetic phase transitions and their evolution with pressure are clearly seen in the temperature dependence of the c-axis electrical resistivity (in Fig. 2, the results at ambient and 2.2 GPa pressure are shown). The hysteresis gradually decreases with increasing pressure.
0304-8853/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved PII S 0 3 0 4 - 8 8 5 3 ( 9 7 ) 0 0 2 8 7-4
50
F. Honda et al. / Journal of Magnetism and Magnetic Materials 177-181 (1998) 49 50 1200 0.6
1000
' ~i~axi<001> s
O .J
0.6
0.4 " 0.4
800 600
-a
<~ 400
0.2
=E
0.2 ' 0.0
axis <100> 200
0.0
80
10
0.0 0
20
30
100
40
B (T)
Fig. 1. High-field magnetization curves along the principal axes at 4.2 K. For clarity, the (1 1 0) curve is shifted down. Inset: the virgin magnetization curve and a part of the hysteresis loop along the c-axis.
2.2 GPa
160
2.5
AF
iLSDW !
Para
~ " 1.5 ft.
\
L0
E 200 ¢J
O. 1.0
150
0.5
Q_
140
Fig. 3. Thermal expansion along the a- and c-axis at ambient and 1.7 GPa pressure.
2.0 250
120
T (K)
\ 0 GPa 0.0
lOO
40
8O
I
!
120
T (K) 50
~
0
40
80
120
160
T (K) Fig. 2. Temperature dependence of the c-axis electrical resistivity of UNi2Siz under ambient and high (2.2 GPa) pressure. The magnetic phase transitions at ambient pressure are marked by arrows. Results of a thermal-expansion study are shown in Fig. 3. In the paramagnetic range, the thermal expansion is remarkably anisotropic, with a nearly zero coefficient ec. This correlates with the lower c-axis compressibility (measured at r o o m temperature): ~c, = 2.4 x 10-3 G P a - 1, ~cc = 1.6 x 10- 3 G P a 1. TN is marked by Al/l anomalies of opposite sign along the a- and c-axis, yielding a negligible volume effect. The transition at 103 K is accompanied by pronounced steps in A1/l both along the a- ( + 3.3 x 10 .5 ) and c-axis ( - 6 x 10-5), allowing a tiny volume effect of 5 x 10 -s. A p T magnetic phase diagram based on both resistivity and thermal-expansion results is presented in Fig. 4. TN is nearly pressure independent, whereas the '103 K' transition shifts to lower temperatures with a rate dTz/dp = - 2.5 K / G P a . On the other hand, the temperature T1 of the low-temperature transition rapidly grows
Fig. 4. p T magnetic phase diagram based on resistivity (circles) and thermal-expansion (squares) data. In the case of the low-temperature transition which exhibits a wide hysteresis, we have defined the critical temperature value as an average between those obtained upon heating and cooling.
with pressure (dT1/dp = + 10 K / G P a above 0.6 GPa). Thus, external pressure suppresses the intermediate magnetic phase as a consequence of the pressure-induced changes of the exchange interaction along the c-axis. This work has been supported by the Grant Agency of the Academy of Sciences of the Czech Republic (Grant A-1010614) and by the Ministry of Education of the Czech Republic (Grant ES 011).
References
[1] L. Rebelsky, H. Lin, M.W. McElfresh, M.F. Collins, J.D. Garrett, W.J.L Buyers, M.S. Torikachvili, Physica B 180&181 (1992) 43. [2] T. Takeuchi, K. Watanabe, T. Taniguchi, T. Kuwai, A. Yamagishi, Y. Miyako, Physica B 201 (1994) 243.
,~
ELSEVIER
Journal of magnetism and magnetic materials
Influence of external pressure on thermal expansion and electrical resistivity of UNizSi2 single crystals F. Honda a, G. Oomi b, T. Kagayama b, A.V. Andreev c, V. Sechovsk~ d'*, L. Havela d, M.I. Bartashevich e, T. Goto e, A.A. Menovsky f "Department of Physics, Kumamoto University, Kumamoto 860, Japan b Department of Mechanical Engineering and Materials Science, Kumamoto University, Kumamoto 860, Japan Clnstitute of Physics, Academy of Sciences, Na Slovance 2, 180 40 Prague, Czech Republic aDepartment of Metal Physics, Charles' University, Ke Karlovu 5, 121 16 Prague, Czech Republic elSSP University of Tokyo, Roppongi 7-22-1, Minato-ku, Tokyo 106, Japan f University of Amsterdam, Valckenierstraat 65, 1018 XE Amsterdam, Netherlands'
Abstract Magnetization under ambient pressure, thermal expansion and electrical resistivity under external pressure up to 2.2 GPa were measured on single crystals of UNi2Si2. A p - T magnetic phase diagrams is proposed. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Actinide compounds; Magnetic phase diagram; Magnetoelasticity; Electrical resistivity
UNi2Si 2 with the tetragonal ThCr2Si2-type structure orders antiferromagnetically below TN = 124K and exhibits two additional magnetic phase transitions at lower temperatures. All magnetic structures consist of ferromagnetic basal planes of U moments oriented along the c-axis and sinusoidally modulated with a propagation vector (0,0, q). The high-temperature (T = 103 124 K) phase is an incommensurate LSDW with q ~ 0.74. Between 53 and 103 K, a simple body-centered antiferromagnetic structure with/~tJ = 1.6 ~tBis stabilized. The q = 2 of the ground-state phase yields a non-zero spontaneous magnetization [1]. All the transitions (those at 53 and 103 K are of the first order) are accompanied by anomalies in magnetic, transport, thermal, and magnetoelastic properties. We report on the magnetization, thermal expansion and electrical resistivity study performed on single crystals grown by the Czochralski method in a tri-arc furnace. At ambient and external pressure up to 2.2 GPa we have measured the thermal expansion along the a- and c-axis (77-300 K) and the electrical resistivity along the c-axis (4.2-300 K). The a-, (1 1 0) and c-axis magneti-
*Corresponding author. Tel.: 420-2-2191-1367; fax: 420-221911351; e-mail: [email protected].
zation curves at ambient pressure were measured at 4.2 K in pulsed fields up to 40 T. The magnetization as a function of temperature (5 300 K) and magnetic field (0.01-5 T) has been also studied with a SQUID magnetometer. High-field magnetization curves (Fig. 1) document a huge uniaxial magnetic anisotropy in UNizSi2. The linear extrapolation of the easy- and hard-magnetization direction curves to high fields yields an estimate of the anisotropy field Ba ~ 200 T. This value falls in the range typical for uranium compourds with uniaxial anisotropy. No anisotropy was found within the basal plane: the curves along the a- and the (1 1 0) axis, respectively, coincide. The observed spontaneous moment of 0.53/tB at 4.2 K is in a good agreement with Ref. [2]. The magnetic hysteresis of UNi2Si2 (the inset in Fig. 1) is characteristic for a strongly anisotropic ferromagnet, which has narrow domain walls with a high intrinsic coercivity. The coercive field observed in steady fields is twice as low as in a pulsed field with ~B/~t ~ 1 T/ms, which indicates a considerable magnetic viscosity. The three magnetic phase transitions and their evolution with pressure are clearly seen in the temperature dependence of the c-axis electrical resistivity (in Fig. 2, the results at ambient and 2.2 GPa pressure are shown). The hysteresis gradually decreases with increasing pressure.
0304-8853/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved PII S 0 3 0 4 - 8 8 5 3 ( 9 7 ) 0 0 2 8 7-4
50
F. Honda et al. / Journal of Magnetism and Magnetic Materials 177-181 (1998) 49 50 1200 0.6
1000
' ~i~axi<001> s
O .J
0.6
0.4 " 0.4
800 600
-a
<~ 400
0.2
=E
0.2 ' 0.0
axis <100> 200
0.0
80
10
0.0 0
20
30
100
40
B (T)
Fig. 1. High-field magnetization curves along the principal axes at 4.2 K. For clarity, the (1 1 0) curve is shifted down. Inset: the virgin magnetization curve and a part of the hysteresis loop along the c-axis.
2.2 GPa
160
2.5
AF
iLSDW !
Para
~ " 1.5 ft.
\
L0
E 200 ¢J
O. 1.0
150
0.5
Q_
140
Fig. 3. Thermal expansion along the a- and c-axis at ambient and 1.7 GPa pressure.
2.0 250
120
T (K)
\ 0 GPa 0.0
lOO
40
8O
I
!
120
T (K) 50
~
0
40
80
120
160
T (K) Fig. 2. Temperature dependence of the c-axis electrical resistivity of UNi2Siz under ambient and high (2.2 GPa) pressure. The magnetic phase transitions at ambient pressure are marked by arrows. Results of a thermal-expansion study are shown in Fig. 3. In the paramagnetic range, the thermal expansion is remarkably anisotropic, with a nearly zero coefficient ec. This correlates with the lower c-axis compressibility (measured at r o o m temperature): ~c, = 2.4 x 10-3 G P a - 1, ~cc = 1.6 x 10- 3 G P a 1. TN is marked by Al/l anomalies of opposite sign along the a- and c-axis, yielding a negligible volume effect. The transition at 103 K is accompanied by pronounced steps in A1/l both along the a- ( + 3.3 x 10 .5 ) and c-axis ( - 6 x 10-5), allowing a tiny volume effect of 5 x 10 -s. A p T magnetic phase diagram based on both resistivity and thermal-expansion results is presented in Fig. 4. TN is nearly pressure independent, whereas the '103 K' transition shifts to lower temperatures with a rate dTz/dp = - 2.5 K / G P a . On the other hand, the temperature T1 of the low-temperature transition rapidly grows
Fig. 4. p T magnetic phase diagram based on resistivity (circles) and thermal-expansion (squares) data. In the case of the low-temperature transition which exhibits a wide hysteresis, we have defined the critical temperature value as an average between those obtained upon heating and cooling.
with pressure (dT1/dp = + 10 K / G P a above 0.6 GPa). Thus, external pressure suppresses the intermediate magnetic phase as a consequence of the pressure-induced changes of the exchange interaction along the c-axis. This work has been supported by the Grant Agency of the Academy of Sciences of the Czech Republic (Grant A-1010614) and by the Ministry of Education of the Czech Republic (Grant ES 011).
References
[1] L. Rebelsky, H. Lin, M.W. McElfresh, M.F. Collins, J.D. Garrett, W.J.L Buyers, M.S. Torikachvili, Physica B 180&181 (1992) 43. [2] T. Takeuchi, K. Watanabe, T. Taniguchi, T. Kuwai, A. Yamagishi, Y. Miyako, Physica B 201 (1994) 243.