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Thermal expansion and magnetostriction of UPd3 and U(Pd1−xPtx)3 single crystals
PHYSICA Physica B 199&200 11994) 416-418
ELSEVIER
Thermal expansion and magnetostriction of UPd3 and U(Pdl-xPtx)3 single crystals S.W. Zochowski*, K.A. McEwen Department of Physics, Birkbeck College, University of London, Malet Street, London WCIE 7HX, UK
Abstract The localised-moment system UPd3 exhibits quadrupolar and magnetic phase transitions, at T~ ~ 7 K and Tz ~ 4.5 K, respectively. We have followed the magnetic field dependence of the transitions in thermal expansion and magnetostriction measurements. The thermal expansion is large and highly anisotropic, as is the magnetostriction below 7"2. The transitions are extremely sensitive to Pt doping.
1. Introduction UPd3 is a Iocalised-moment uranium intermetallic compound with the dhcp crystal structure and a U U distance very similar to that of HCP UPt a. Measurements of the specific heat [1, 2], thermal expansion [3] and elastic constants [4] indicate the existence of two transitions in zero applied magnetic field, at Tt -~ 7 K and 7"2 ~ 4.5 K. Recent polarised neutron scattering experiments [5] have shown that the transition at Tt is structural, whilst that at T2 has both structural and magnetic components, with the magnitude of the ordered magnetic moment being extremely small, ,~ 10-2 Fa/Uatom. These results also confirmed earlier suggestions [I ~.,6] that the phase transition at Tt arises from a strong quadrupolar interaction between the 5f2 U-ions. The crystal field states at both sites are singlets, with excited doublets at 15 meV (hexagonal sites) and 1.5 meV (quasi-cubic sites). It has been proposed [7] that the transition is to an antiferroquadrupolar phase characterised by P=.~ and Pr: distortions, which correspond to
* Corresponding author.
a tilting of the charge distribution away from the c-axis. This paper summarises our thermal expansion and magnetostriction studies of UPd3. A more detailed description of the results of the experiments on the complete U(Pdt-xPt~)3 system will be published elsewhere.
2. Experimental details and results Samples of U(Pd~ _ ~Pt,)3 were cut from single crystals prepared at the University of Birmingham using the Czochralski technique. Typical sizes were 4 x 4 x 4 mm 3, with faces cut perpendicular to the a-, b- and c-directions. Strain measurements were made relative to the body of a capacitance dilatometer as a function of temperature and applied magnetic field (H _< 7T, with the field applied parallel to the measurement direction). Length measurements along tile a- b- and c-axes for UPd3, made as a function of increasing temperature, are shown in Fig. 1 for applied magnetic fields of zero and 7.0 T. The transitions at Tt and 7"2 are clearly evident in the a- and b-axis data: the response is qualitatively similar, except for the discontinuity at the lower transition, for which AI/Ib(T 2 ) ~ 2Al/l~( T 2 I. Also, the b-axis shows
0921-4526/94/$07.00 c) 1994 Elsevier Science B.V. All rights reserved SSDI 0921-4526(93)E0399-2
S.W. Zochowski. K.A. McEwen/ Physica B 199&200 (1994)416-418 an additional anomaly at T~, about 1 K above T~. In contrast to the basal plane directions, the c-axis shows a steady expansion with decreasing temperature, until it reaches a maximum around 8 K (i.e. at T~). Measurements over an extended temperature range show that the c-axis expansion begins at ~ 250 K. The c-axis length changes at Tt and T2 are in the opposite direction to those of the basal plane. Thermal expansion measurements in various applied magnetic fields indicate significant anisotropy between the symmetry axes: in 7 T the a- and b-axis results now differ qualitatively. The results at zero and 1 T are ,,cry similar, as reported previously [3]. Above 1 T the ob-
i
m
I I I I ~ I I I I I I I I I I
- H=O.OT UPd3
m
--
. j /
417
served phase transitions, at Tz, Tt and T't, all exhibit some field dependence. As can be seen in Fig. !, the weak anomaly in the b-axis at T'I becomes enhanced in 7 T, and is now seen also in the a-axis results. The a-axis discontinuity at T t has become more pronounced. Tz, and to a lesser extent 7'1, have a positive field dependence for the a- and b-axes and a negative dependence for the c-axis. The field dependence of 7"2 for all directions shows significant changes in slope near 1 T and between 4 and 5 T. We note also that the sign of the discontinuity in Al/l, at Te changes between 4 and 5 T. Magnetostriction results along the three symmetry axes for UPd~ at T = 2.8 K (i,e. < T2) are shown in Fig. 2. The highly anisotropic and hysteretic nature of the phase below 7"2 is compared to the high-temperature (9 K) data. The latter show a straightforward, quadratic, negative magnetostriction with only a small anisotropy between the c-axis and the basal plane directions. At
aax,,
Ii
I I I III }1 < .o
.°-
I I I I lit c-axis
I -
""" ............. ~
w
o°.°°
I ~,
'~-1
~
~
f
--
b-axis -~
Lr'L/ _ j -
.."
T=9.0K all axes b-axis
A
b
UPd 3
m
T=2.8K
g
i
ht
a-axis a
,I,Illll=lJl,IR 0 2 4 6 8 10
"%
12
14
6
T E M P E R A T U R E tr~) .... Fig. I. Thermal expansion of UPd3, measu.~ed along the symmetry axes at zero field (open circles) and at H = 7.0 T (solid squares). The arrows indicate the transition temperatures for the zero-field data. Zero-field results for the c-axis of U(Pdo.95Pto.o5)3 are also shown. The data for each axis have been displaced vertically for ease of presentation; the differences between the zero and 7 T measurements indicate the magnetostriction.
I ~ I~la 0
1
2
I J I till
3
4
5
6
7
A P P L I E D M A G N E T I C FIELD (T) Fig. 2. Magnetostriction of UPd~ measured in increasing field along the symmetry axes at T = 2.8 and 9.0 K. The dashed lines indicate results in decreasing field for the a- and b-axes.
418
S. IV. Zochowski, K.A. McF,wen/ Physica B 199&200 (1994) 416-418
2,8 K, the c-axis response is positive and much weaker than for the a- and b-axes. Results for T2 < T < Tt (~ot shown) are similar to those taken at 9 K, although the magnitude of the quadratic term is greater at lower temperatures. The expansivitics of U(Pdl-.,Ptx)3, with x = 0.05 and 0.10, were measured in the c-direction in zero applied field. The results for x = 0.05 are shown in Fig. 1. Although the expansion coefficient is negative, there was no evidence (for T > 1.6 K) of the series of transitions observed in UPd3. The expansivity of both Pt-doped compounds decreases with increasing temperature, up to 15 K. Furthermore, for T > T'~ we find that the magnitude of the expansion coefficient of the three concentrations is greatest for x = 0 and least for x = 0.05.
T'~ remains unknown. Although it has been seen in zerofield heat capacity measurements I-2], no corresponding superlattice peaks have been observed by neutron diffraction. The highly anisotropic and hysteretic nature of the thermal expansion and magnetostriction results for measurements along the symmetry directions of a single crystal of UPda strongly indicate that the state below T2 is indeed magnetic. The transitions in UPd3 have been seen to be extremely sensitive to Pt doping. The observed rapid destruction of both the magnetic and quadrupolar phases of UPd3 may be due to the general expansion of the lattice by the addition of Pt.
Acknowledgements 3. Discussion The results of the thermal expansion measurements in zero field are qualitatively consistent with the proposed phases and crystal field states, With decreasing temperature, as the excited crystal field states depopulate and the axial quadrupolar moment develops, the charge distribution becomes increasingly more prelate in nature and the c-axis expands while the a- and b-axes contract. This depopulation begins near 250 K, in conjunction with the c-axis expansion, At TI the stress within the crystal becomes too great and it undergoes the antiferroquadrupolar transition with the charge distribution tilting away from the c-axis, leading to a relaxation along the c-axis and almost equivalent expansions along both the a- and h-axes. Further reduction of temperature leads to an induced dipolar transition at T2 with the antiferromagnet moments aligned out of the basal plane, and thus the expansion along the c-axis. The changes observed in the lower transition temperature 7"2 and the nature of the transition at 7"2 as a function of applied field suggest the presence of more than one magnetic phase in applied fields below T2. The origin of the transition at
We are grateful to the Argonne National Laboratory for the uranium used in these experiments. We thank D. Fort and Y.J. Bi for growing the crystals. This work was supported by the SERC and The Royal Society.
References [1] K. Andres, D. Davidov, P. Dernier, F, Hsu, W.A. Reed and G.J. Nieuwenhuys, Solid State Commun. 28 (1978) 405, [2] N. Patrikios, M. de Podesta and K.A. McEwen, to be published, [3] H.R. Ott, K. Andres and P.H. Schmidt, Physica B 102 (1980) 148. [4] M. Yoshizawa, B. Liithi, T. Goto, T. Suzuki, B. Renker, A. de Visser, P. Frings and J.J.M. Franse, J. Magn. Magn. Mater. 52 (1985) 413. [5] U. Steigenberg ~ K.A. McEwen, J.L. Martinez and D. Fort, J. Magn. Magn. Mater. 108 (1992} 163. [6] W.J.L. Buyers and TM. Holden, in: Handbook of the Physics and Chemistry of Actinides, Vol. 2 (North-Holland, Amsterdam, 1985) p. 239. [7] K.A McEwen, U. Steigenberger and J.L. Martinez, Physica B 1~6-188 (1993) 670.
ELSEVIER
Thermal expansion and magnetostriction of UPd3 and U(Pdl-xPtx)3 single crystals S.W. Zochowski*, K.A. McEwen Department of Physics, Birkbeck College, University of London, Malet Street, London WCIE 7HX, UK
Abstract The localised-moment system UPd3 exhibits quadrupolar and magnetic phase transitions, at T~ ~ 7 K and Tz ~ 4.5 K, respectively. We have followed the magnetic field dependence of the transitions in thermal expansion and magnetostriction measurements. The thermal expansion is large and highly anisotropic, as is the magnetostriction below 7"2. The transitions are extremely sensitive to Pt doping.
1. Introduction UPd3 is a Iocalised-moment uranium intermetallic compound with the dhcp crystal structure and a U U distance very similar to that of HCP UPt a. Measurements of the specific heat [1, 2], thermal expansion [3] and elastic constants [4] indicate the existence of two transitions in zero applied magnetic field, at Tt -~ 7 K and 7"2 ~ 4.5 K. Recent polarised neutron scattering experiments [5] have shown that the transition at Tt is structural, whilst that at T2 has both structural and magnetic components, with the magnitude of the ordered magnetic moment being extremely small, ,~ 10-2 Fa/Uatom. These results also confirmed earlier suggestions [I ~.,6] that the phase transition at Tt arises from a strong quadrupolar interaction between the 5f2 U-ions. The crystal field states at both sites are singlets, with excited doublets at 15 meV (hexagonal sites) and 1.5 meV (quasi-cubic sites). It has been proposed [7] that the transition is to an antiferroquadrupolar phase characterised by P=.~ and Pr: distortions, which correspond to
* Corresponding author.
a tilting of the charge distribution away from the c-axis. This paper summarises our thermal expansion and magnetostriction studies of UPd3. A more detailed description of the results of the experiments on the complete U(Pdt-xPt~)3 system will be published elsewhere.
2. Experimental details and results Samples of U(Pd~ _ ~Pt,)3 were cut from single crystals prepared at the University of Birmingham using the Czochralski technique. Typical sizes were 4 x 4 x 4 mm 3, with faces cut perpendicular to the a-, b- and c-directions. Strain measurements were made relative to the body of a capacitance dilatometer as a function of temperature and applied magnetic field (H _< 7T, with the field applied parallel to the measurement direction). Length measurements along tile a- b- and c-axes for UPd3, made as a function of increasing temperature, are shown in Fig. 1 for applied magnetic fields of zero and 7.0 T. The transitions at Tt and 7"2 are clearly evident in the a- and b-axis data: the response is qualitatively similar, except for the discontinuity at the lower transition, for which AI/Ib(T 2 ) ~ 2Al/l~( T 2 I. Also, the b-axis shows
0921-4526/94/$07.00 c) 1994 Elsevier Science B.V. All rights reserved SSDI 0921-4526(93)E0399-2
S.W. Zochowski. K.A. McEwen/ Physica B 199&200 (1994)416-418 an additional anomaly at T~, about 1 K above T~. In contrast to the basal plane directions, the c-axis shows a steady expansion with decreasing temperature, until it reaches a maximum around 8 K (i.e. at T~). Measurements over an extended temperature range show that the c-axis expansion begins at ~ 250 K. The c-axis length changes at Tt and T2 are in the opposite direction to those of the basal plane. Thermal expansion measurements in various applied magnetic fields indicate significant anisotropy between the symmetry axes: in 7 T the a- and b-axis results now differ qualitatively. The results at zero and 1 T are ,,cry similar, as reported previously [3]. Above 1 T the ob-
i
m
I I I I ~ I I I I I I I I I I
- H=O.OT UPd3
m
--
. j /
417
served phase transitions, at Tz, Tt and T't, all exhibit some field dependence. As can be seen in Fig. !, the weak anomaly in the b-axis at T'I becomes enhanced in 7 T, and is now seen also in the a-axis results. The a-axis discontinuity at T t has become more pronounced. Tz, and to a lesser extent 7'1, have a positive field dependence for the a- and b-axes and a negative dependence for the c-axis. The field dependence of 7"2 for all directions shows significant changes in slope near 1 T and between 4 and 5 T. We note also that the sign of the discontinuity in Al/l, at Te changes between 4 and 5 T. Magnetostriction results along the three symmetry axes for UPd~ at T = 2.8 K (i,e. < T2) are shown in Fig. 2. The highly anisotropic and hysteretic nature of the phase below 7"2 is compared to the high-temperature (9 K) data. The latter show a straightforward, quadratic, negative magnetostriction with only a small anisotropy between the c-axis and the basal plane directions. At
aax,,
Ii
I I I III }1 < .o
.°-
I I I I lit c-axis
I -
""" ............. ~
w
o°.°°
I ~,
'~-1
~
~
f
--
b-axis -~
Lr'L/ _ j -
.."
T=9.0K all axes b-axis
A
b
UPd 3
m
T=2.8K
g
i
ht
a-axis a
,I,Illll=lJl,IR 0 2 4 6 8 10
"%
12
14
6
T E M P E R A T U R E tr~) .... Fig. I. Thermal expansion of UPd3, measu.~ed along the symmetry axes at zero field (open circles) and at H = 7.0 T (solid squares). The arrows indicate the transition temperatures for the zero-field data. Zero-field results for the c-axis of U(Pdo.95Pto.o5)3 are also shown. The data for each axis have been displaced vertically for ease of presentation; the differences between the zero and 7 T measurements indicate the magnetostriction.
I ~ I~la 0
1
2
I J I till
3
4
5
6
7
A P P L I E D M A G N E T I C FIELD (T) Fig. 2. Magnetostriction of UPd~ measured in increasing field along the symmetry axes at T = 2.8 and 9.0 K. The dashed lines indicate results in decreasing field for the a- and b-axes.
418
S. IV. Zochowski, K.A. McF,wen/ Physica B 199&200 (1994) 416-418
2,8 K, the c-axis response is positive and much weaker than for the a- and b-axes. Results for T2 < T < Tt (~ot shown) are similar to those taken at 9 K, although the magnitude of the quadratic term is greater at lower temperatures. The expansivitics of U(Pdl-.,Ptx)3, with x = 0.05 and 0.10, were measured in the c-direction in zero applied field. The results for x = 0.05 are shown in Fig. 1. Although the expansion coefficient is negative, there was no evidence (for T > 1.6 K) of the series of transitions observed in UPd3. The expansivity of both Pt-doped compounds decreases with increasing temperature, up to 15 K. Furthermore, for T > T'~ we find that the magnitude of the expansion coefficient of the three concentrations is greatest for x = 0 and least for x = 0.05.
T'~ remains unknown. Although it has been seen in zerofield heat capacity measurements I-2], no corresponding superlattice peaks have been observed by neutron diffraction. The highly anisotropic and hysteretic nature of the thermal expansion and magnetostriction results for measurements along the symmetry directions of a single crystal of UPda strongly indicate that the state below T2 is indeed magnetic. The transitions in UPd3 have been seen to be extremely sensitive to Pt doping. The observed rapid destruction of both the magnetic and quadrupolar phases of UPd3 may be due to the general expansion of the lattice by the addition of Pt.
Acknowledgements 3. Discussion The results of the thermal expansion measurements in zero field are qualitatively consistent with the proposed phases and crystal field states, With decreasing temperature, as the excited crystal field states depopulate and the axial quadrupolar moment develops, the charge distribution becomes increasingly more prelate in nature and the c-axis expands while the a- and b-axes contract. This depopulation begins near 250 K, in conjunction with the c-axis expansion, At TI the stress within the crystal becomes too great and it undergoes the antiferroquadrupolar transition with the charge distribution tilting away from the c-axis, leading to a relaxation along the c-axis and almost equivalent expansions along both the a- and h-axes. Further reduction of temperature leads to an induced dipolar transition at T2 with the antiferromagnet moments aligned out of the basal plane, and thus the expansion along the c-axis. The changes observed in the lower transition temperature 7"2 and the nature of the transition at 7"2 as a function of applied field suggest the presence of more than one magnetic phase in applied fields below T2. The origin of the transition at
We are grateful to the Argonne National Laboratory for the uranium used in these experiments. We thank D. Fort and Y.J. Bi for growing the crystals. This work was supported by the SERC and The Royal Society.
References [1] K. Andres, D. Davidov, P. Dernier, F, Hsu, W.A. Reed and G.J. Nieuwenhuys, Solid State Commun. 28 (1978) 405, [2] N. Patrikios, M. de Podesta and K.A. McEwen, to be published, [3] H.R. Ott, K. Andres and P.H. Schmidt, Physica B 102 (1980) 148. [4] M. Yoshizawa, B. Liithi, T. Goto, T. Suzuki, B. Renker, A. de Visser, P. Frings and J.J.M. Franse, J. Magn. Magn. Mater. 52 (1985) 413. [5] U. Steigenberg ~ K.A. McEwen, J.L. Martinez and D. Fort, J. Magn. Magn. Mater. 108 (1992} 163. [6] W.J.L. Buyers and TM. Holden, in: Handbook of the Physics and Chemistry of Actinides, Vol. 2 (North-Holland, Amsterdam, 1985) p. 239. [7] K.A McEwen, U. Steigenberger and J.L. Martinez, Physica B 1~6-188 (1993) 670.