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Spin glass behaviour in URh2Ge2
ELSEVIER
Physica B 230-232 (1997) 105-107
Spin glass behaviour in URh2Ge2 S. Sfillow a'*, S . A . M . M e n t i n k b, T.E. M a s o n h, W . J . L . B u y e r s c, G.J. N i e u w e n h u y s a, A . A . M e n o v s k y a'd, J.A. M y d o s h a a Kamerlinyh Onnes Laboratory, Leiden University, P.O. Box 9506, 2300 RA Leiden, The Netherlands b Department of Physics, University o f Toronto, Canada CAECL, Chalk River, Ont., Canada d Van der Waals Zeeman Laboratorium, University of Amsterdam, The Netherlands
Abstract URh2Ge2 occupies an extraordinary position among the 122 heavy-electron compounds. Its physical properties strongly deviate from those of the other members of this materials class by exhibiting some previously unidentified form of magnetic correlations at low temperatures, instead of the usual antiferromagnetic ground state. Here, we present new results of ACsusceptibility and neutron diffraction measurements on single-crystalline as-grown URhzGe2. These data clearly indicate that crystallographic disorder on a local scale produces spin-glass behaviour in the sample. We therefore conclude that URh2Ge2 is a random-bond, heavy-fermion spin glass.
Keywords: URh2Ge2; Spin glass
The intermetallic compound URhzGe2 remains the subject of a long-standing debate [1-5]. Its properties deviate from those of the other 122 compounds; at low temperatures some unusual form of magnetic correlations are present instead of a conventional antiferromagnetic ground state. In particular, Ptasiewiez-Bak et al. [1,2] investigated the structural and magnetic properties of URhzGe2 by neutron diffraction and concluded that the compound crystallizes in a tetragonal structure, though the details could not be determined conclusively. Two possibilities were proposed: A structure closely related to the ThCr2Si2-1attice with Rh and Ge randomly distributed on the Cr and Si positions (symmetry class P4/mmm), or a CaBezGe2-unit cell (P4/nmm). No magnetic ordering could be observed by neutron diffraction down to 4.2 K, although it was inferred * Corresponding author.
from bulk techniques that an antiferromagnetic transition takes place at about 8 K. These results were corroborated by a study on single crystalline URh2Ge2 [5]. Here, it was coneluded that no magnetic ordering occurs down to 35inK. Still, the single crystal exhibited a maximum in the susceptibility at 11 K and a change in slope of the specific heat at 12 K was found, with an unusual low-temperature specific heat behaviour Cp = 7T + fiT 2. These features were then basically interpreted as arising from crystalline electric fields. In order to unambiguously determine the magnetic nature of the ground state of the system we started a new investigation of single crystalline URhzGe2. We now present the results of neutron-diffraction and AC-susceptibility studies on URhzGe2. A single crystal URh2Ge2 was grown at FOMALMOS, employing the Czochralski technique in a tri-arc furnace, similar to the procedure used for the
0921-4526/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved PH S092 1 - 4 5 2 6 ( 9 6 ) 0 0 5 6 0 - 1
106
S. Sallow et al. / Physica B 230-232 (1997) 105-107
100
3
50
~"
0 Fig. 1. Selected nuclear intensities of URh2Ge2 from Ref. [1] (grey), for our crystal (black) and calculated on basis ofa P4/nmm symmetry (white).
crystal examined in Ref. [5]. The crystal was checked by EPMA and found to be single phase with composition URh2.00~0.06Gel.96±0.06. It was then sectioned and each piece was checked for single crystallinity and alignment. The crystallographic structure of the as-grown crystal URh2Ge2 was investigated with neutron diffraction at Chalk River Laboratories, Canada. Qualitatively, the list of nuclear Bragg peaks observed for our crystal agrees well with the data of Ptasiewicz-Bak et al. [1 ], and indicates that the symmetry of the crystal is tetragonal and related to either P4/nmm or P4/mmm. Unfortunately, we cannot unambiguously distinguish between the two. Quantitatively, however, there are differences between our data and those of Ref. [1], as well as with spectra calculated on the basis of P4/mnm or P4/mmm symmetries. This is illustrated in Fig. 1, where we plot selected nuclear intensities as measured for our crystal in comparison to the data of Ref. [1] and those calculated assuming a P4/nmm symmetry (similar discrepancies are found assuming P4/mmm symmetry). In the calculation we used (notation as in Ref. [1]) zu = 0.74, ZRh = 0.13 and zGe = 0.37 with lattice parameters a - - 4 . 1 4 6 4 / ~ and c = 9.7510A. The most important differences are the much larger intensities of the (0 0 3) and (0 04) reflections observed in our experiment. Presently, we are not able to resolve the structural details denoted by those differences. Nevertheless, the conclusion can be drawn that the crystallographic structure of URh2Ge2 is not fully described by a unit cell with symmetry P4/nmm or P4/mmm, but that
(a) there is additional superstructural ordering (according to the large intensities of (0 0 3) and (0 0 4)), and (b) this superstructural ordering is strongly sample dependent (according to the differences between our data and those of Ref. [1]). The major future challenge will be to resolve the structural details of URh2Ge2, since, as we show below, the physical properties of the compound are critically dependent on the structural disorder. Physically, the crystal was characterized by several bulk techniques (resistivity, specific heat, DCsusceptibility Zoc, and magnetization). The results for our crystal are in good agreement with earlier reports. As in Refs. [2, 5], we found a maximum in ZDC (measured in 0.1 T) at 10 K and the uncommon lowtemperature specific heat Cp • 7 T + fiT s (not shown; a full account of our experiments will be given elsewhere [6]). In addition, we investigated the susceptibility anomaly of URhzGe2 by means of frequencydependent AC-susceptibility ZAC in zero magnetic field. The results of our measurements are shown in Fig. 2. It is clear from this plot that the anomaly in the susceptibility arises from spin-glass behaviour. As for canonical spin glasses the cusp, which is broadened in the high fields used to measure Xoc in Refs. [2, 5], becomes a sharp feature in zero field ZAC. Further, we find a frequency dependence of the temperature of the maximum in ZAC, TF, and absorption is visible at TF in ZAC" The frequency shift of TF ( = 9.75 K at 1.157Hz) is characterized by the normalized value ATF(~) = ATv/TFA log(co). For both crystallographic directions we find ATF(~O) -- 0.02-0.03, which is a typical order of magnitude for metallic spin glasses (CuMn: 0.005; (La,Gd)AIz: 0.06 [7]). In order for spin-glass behaviour to appear, disorder or randomness is necessary to create the competing magnetic interactions and frustrations. At present we do not know whether the structural features denoted by the intensity difference between our measurement and the P4/nmm or P4/mmm symmetries create the necessary randomness. In addition, atomic disorder, e.g., a mixture of Rh and Ge site occupancies, might be present in our crystal of URh2Ge2, but it is difficult to resolve such disorder by diffraction techniques. Further structural investigations with local structurai or magnetic probes (like HREM or NMR) will be necessary to clarify this point.
S. Siillow et al. / Physica B 230-232 (1997) 105-107
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This work was partially supported by the Nederlandse Stichting FOM.
,
10
T (K)
Summarizing, we found that a new as-grown single crystal URh2Ge2 crystallizes in a structure related either to P4/nmm or P4/mmm. There are certain details about the structure that we could not resolve, but which might play an important role for the magnetic properties of the compound. The AC-susceptibility ZAC was measured for the first time revealing the system to be a spin glass with a freezing transition at 10 K. This implies that structural disorder is creating a random-bond type of spin glass in this heavy-fermion material.
,
" ) of URh2Ge2 for the two crystallographic
ZCA "
References
[1] H. Ptasiewicz-Bak et al., J. Phys. F: Metal Phys. 11 (1981) 1225. [2] H. Ptasiewicz-Bak et al., Solid State Commun. 55 (1985) 601. [3] J.D. Thompson eta[., Phys. Lett. A 110 (1985) 470. [4] B. Lloret et al., J. Magn. Magn. Mater. 67 (1987) 232. [5] A.J. Dirkmaat et al., Europhys. Lett. 11 (1990) 275. [6] S. Sfillow et al., Phys. Rev. Lett., to be published. [7] J.A. Mydosh, Spin Glasses (Taylor & Francis, London, 1993).
Physica B 230-232 (1997) 105-107
Spin glass behaviour in URh2Ge2 S. Sfillow a'*, S . A . M . M e n t i n k b, T.E. M a s o n h, W . J . L . B u y e r s c, G.J. N i e u w e n h u y s a, A . A . M e n o v s k y a'd, J.A. M y d o s h a a Kamerlinyh Onnes Laboratory, Leiden University, P.O. Box 9506, 2300 RA Leiden, The Netherlands b Department of Physics, University o f Toronto, Canada CAECL, Chalk River, Ont., Canada d Van der Waals Zeeman Laboratorium, University of Amsterdam, The Netherlands
Abstract URh2Ge2 occupies an extraordinary position among the 122 heavy-electron compounds. Its physical properties strongly deviate from those of the other members of this materials class by exhibiting some previously unidentified form of magnetic correlations at low temperatures, instead of the usual antiferromagnetic ground state. Here, we present new results of ACsusceptibility and neutron diffraction measurements on single-crystalline as-grown URhzGe2. These data clearly indicate that crystallographic disorder on a local scale produces spin-glass behaviour in the sample. We therefore conclude that URh2Ge2 is a random-bond, heavy-fermion spin glass.
Keywords: URh2Ge2; Spin glass
The intermetallic compound URhzGe2 remains the subject of a long-standing debate [1-5]. Its properties deviate from those of the other 122 compounds; at low temperatures some unusual form of magnetic correlations are present instead of a conventional antiferromagnetic ground state. In particular, Ptasiewiez-Bak et al. [1,2] investigated the structural and magnetic properties of URhzGe2 by neutron diffraction and concluded that the compound crystallizes in a tetragonal structure, though the details could not be determined conclusively. Two possibilities were proposed: A structure closely related to the ThCr2Si2-1attice with Rh and Ge randomly distributed on the Cr and Si positions (symmetry class P4/mmm), or a CaBezGe2-unit cell (P4/nmm). No magnetic ordering could be observed by neutron diffraction down to 4.2 K, although it was inferred * Corresponding author.
from bulk techniques that an antiferromagnetic transition takes place at about 8 K. These results were corroborated by a study on single crystalline URh2Ge2 [5]. Here, it was coneluded that no magnetic ordering occurs down to 35inK. Still, the single crystal exhibited a maximum in the susceptibility at 11 K and a change in slope of the specific heat at 12 K was found, with an unusual low-temperature specific heat behaviour Cp = 7T + fiT 2. These features were then basically interpreted as arising from crystalline electric fields. In order to unambiguously determine the magnetic nature of the ground state of the system we started a new investigation of single crystalline URhzGe2. We now present the results of neutron-diffraction and AC-susceptibility studies on URhzGe2. A single crystal URh2Ge2 was grown at FOMALMOS, employing the Czochralski technique in a tri-arc furnace, similar to the procedure used for the
0921-4526/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved PH S092 1 - 4 5 2 6 ( 9 6 ) 0 0 5 6 0 - 1
106
S. Sallow et al. / Physica B 230-232 (1997) 105-107
100
3
50
~"
0 Fig. 1. Selected nuclear intensities of URh2Ge2 from Ref. [1] (grey), for our crystal (black) and calculated on basis ofa P4/nmm symmetry (white).
crystal examined in Ref. [5]. The crystal was checked by EPMA and found to be single phase with composition URh2.00~0.06Gel.96±0.06. It was then sectioned and each piece was checked for single crystallinity and alignment. The crystallographic structure of the as-grown crystal URh2Ge2 was investigated with neutron diffraction at Chalk River Laboratories, Canada. Qualitatively, the list of nuclear Bragg peaks observed for our crystal agrees well with the data of Ptasiewicz-Bak et al. [1 ], and indicates that the symmetry of the crystal is tetragonal and related to either P4/nmm or P4/mmm. Unfortunately, we cannot unambiguously distinguish between the two. Quantitatively, however, there are differences between our data and those of Ref. [1], as well as with spectra calculated on the basis of P4/mnm or P4/mmm symmetries. This is illustrated in Fig. 1, where we plot selected nuclear intensities as measured for our crystal in comparison to the data of Ref. [1] and those calculated assuming a P4/nmm symmetry (similar discrepancies are found assuming P4/mmm symmetry). In the calculation we used (notation as in Ref. [1]) zu = 0.74, ZRh = 0.13 and zGe = 0.37 with lattice parameters a - - 4 . 1 4 6 4 / ~ and c = 9.7510A. The most important differences are the much larger intensities of the (0 0 3) and (0 04) reflections observed in our experiment. Presently, we are not able to resolve the structural details denoted by those differences. Nevertheless, the conclusion can be drawn that the crystallographic structure of URh2Ge2 is not fully described by a unit cell with symmetry P4/nmm or P4/mmm, but that
(a) there is additional superstructural ordering (according to the large intensities of (0 0 3) and (0 0 4)), and (b) this superstructural ordering is strongly sample dependent (according to the differences between our data and those of Ref. [1]). The major future challenge will be to resolve the structural details of URh2Ge2, since, as we show below, the physical properties of the compound are critically dependent on the structural disorder. Physically, the crystal was characterized by several bulk techniques (resistivity, specific heat, DCsusceptibility Zoc, and magnetization). The results for our crystal are in good agreement with earlier reports. As in Refs. [2, 5], we found a maximum in ZDC (measured in 0.1 T) at 10 K and the uncommon lowtemperature specific heat Cp • 7 T + fiT s (not shown; a full account of our experiments will be given elsewhere [6]). In addition, we investigated the susceptibility anomaly of URhzGe2 by means of frequencydependent AC-susceptibility ZAC in zero magnetic field. The results of our measurements are shown in Fig. 2. It is clear from this plot that the anomaly in the susceptibility arises from spin-glass behaviour. As for canonical spin glasses the cusp, which is broadened in the high fields used to measure Xoc in Refs. [2, 5], becomes a sharp feature in zero field ZAC. Further, we find a frequency dependence of the temperature of the maximum in ZAC, TF, and absorption is visible at TF in ZAC" The frequency shift of TF ( = 9.75 K at 1.157Hz) is characterized by the normalized value ATF(~) = ATv/TFA log(co). For both crystallographic directions we find ATF(~O) -- 0.02-0.03, which is a typical order of magnitude for metallic spin glasses (CuMn: 0.005; (La,Gd)AIz: 0.06 [7]). In order for spin-glass behaviour to appear, disorder or randomness is necessary to create the competing magnetic interactions and frustrations. At present we do not know whether the structural features denoted by the intensity difference between our measurement and the P4/nmm or P4/mmm symmetries create the necessary randomness. In addition, atomic disorder, e.g., a mixture of Rh and Ge site occupancies, might be present in our crystal of URh2Ge2, but it is difficult to resolve such disorder by diffraction techniques. Further structural investigations with local structurai or magnetic probes (like HREM or NMR) will be necessary to clarify this point.
S. Siillow et al. / Physica B 230-232 (1997) 105-107
14.0 0
-
X=
13.0
,~,
12.0
55 ;,<
11.0
"~
0.3
50
.
45 ©
0
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,
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10
,
15
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,,
,
,
15
T (K)
Fig. 2. The frequency dependence of the AC-susceptibility (in phase ZAC° ~ and out phase directions. (-) 1.157 Hz; ( A ) 11.57 Hz; (o) 115.7 Hz; ( + ) 1157 Hz.
This work was partially supported by the Nederlandse Stichting FOM.
,
10
T (K)
Summarizing, we found that a new as-grown single crystal URh2Ge2 crystallizes in a structure related either to P4/nmm or P4/mmm. There are certain details about the structure that we could not resolve, but which might play an important role for the magnetic properties of the compound. The AC-susceptibility ZAC was measured for the first time revealing the system to be a spin glass with a freezing transition at 10 K. This implies that structural disorder is creating a random-bond type of spin glass in this heavy-fermion material.
,
" ) of URh2Ge2 for the two crystallographic
ZCA "
References
[1] H. Ptasiewicz-Bak et al., J. Phys. F: Metal Phys. 11 (1981) 1225. [2] H. Ptasiewicz-Bak et al., Solid State Commun. 55 (1985) 601. [3] J.D. Thompson eta[., Phys. Lett. A 110 (1985) 470. [4] B. Lloret et al., J. Magn. Magn. Mater. 67 (1987) 232. [5] A.J. Dirkmaat et al., Europhys. Lett. 11 (1990) 275. [6] S. Sfillow et al., Phys. Rev. Lett., to be published. [7] J.A. Mydosh, Spin Glasses (Taylor & Francis, London, 1993).