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Magnetic anomalies in Gd2PdSi3
Physica B 259—261 (1999) 892 — 893
Magnetic anomalies in Gd PdSi R. Mallik , E.V. Sampathkumaran *, P.L. Paulose , H. Sugawara, H. Sato Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400 005, India Department of Physics, Tokyo Metropoliton University, Tokyo 192-03, Japan
Abstract The results of AC and DC magnetic susceptibility, thermopower and Hall effect measurements of a compound, Gd PdSi , establish that this compound orders magnetically below (¹ ") 20 K. Though the ordering appears to be of , an antiferromagnetic-type, the paramagnetic Curie temperature is positive suggestive of the existence of ferromagnetic correlations. The thermopower at 300 K is large, apparently due to Pd 4d electrons, decreasing monotonically with temperature. There is a change in the sign of Hall constant well below ¹ . The overall thermal, transport and magnetic , behavior of this compound appears to be interesting. 1999 Elsevier Science B.V. All rights reserved. Keywords: Gd PdSi ; Magnetic anomalies; Hall effect; Thermopower
Earlier, we noticed [1] Kondo-lattice-like characteristics in the heat-capacity and electrical resistivity behavior in polycrystalline Gd PdSi . We carried out AC and DC magnetic susceptibility (s), Hall effect and thermopower (S) measurements on the same sample, the results of which are reported here. The plot of inverse s versus temperature (Fig. 1a) is found to be practically linear above 30 K and the value of the paramagnetic Curie temperature (h ) is found to be positive with a magnitude of 19 K, indicating the existence of ferromagnetic correlations. However, there is a distinct peak in s at the same temperature (19 K) which implies the onset of long range antiferromagnetic order at this temperature. The ZFC and FC data (Fig. 1a, left inset) are practically the same above 15 K, while the difference can be distinctly seen to set in below 15 K, but not at the peak. This may be related to the fact there are two magnetic transitions, one close to 21 K (2b site) and the other at 15 K (6h site), from two different Gd sites characteristic of this structure, as shown by Mossbauer effect measurements [1]. In order to address the question whether the ZFC—FC behavior arises from spin—glass
* Corresponding author. Fax: (091)-22-2152110/2181; e-mail: [email protected].
behavior, we also performed AC s measurements at various frequencies (Fig. 1a, right inset) and we do not find any upward shift of the peak with increasing frequency expected for the spin glass phenomena. The shoulder around 15 K in AC s data presumably arises from the magnetic ordering from 6h site. The results of S measurements are shown in Fig. 1b both for Gd PdSi and Lu PdSi . It is to be noted that S of the Gd sample is large at 300 K, which is true even for Lu PdSi , and is comparable to the magnitude known for the Pd metal [2], thereby indicating large Pd 4d density of states at the Fermi level. The values are larger (about !10 lV/K) for the Gd sample, compared to the Lu sample. S varies (decreases) linearly down to about 20 K (as expected). There is a tendency to a constant value for a narrow temperature range below 20 K. Since this shoulder appears in the non-magnetic compound as well, the feature due to magnetic ordering is not clearly separable from the phonon drag effect. Thus the magnetic ordering near 21 K does not have any noticeable effect on S in the Gd alloy. In contrast to this, the Hall coefficient (R ) of the Gd sample is completely & different from that of the Lu sample (Fig. 1c). While the R is a small positive number for Lu sample at all & temperatures, it is negative above 8 K for the Gd PdSi with a distinct peak in the vicinity of magnetic ordering.
0921-4526/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 8 ) 0 1 1 2 9 - 6
R. Mallik et al. / Physica B 259—261 (1999) 892 — 893
893
We, however, find that the peak occurs only near 15 K (Fig. 1c, inset), without any noticeable feature at 20 K and thus the 20 K transition is not tracked by Hall measurements. A noteworthy finding is that there is a sign reversal below 8 K (see inset of Fig. 1c); possibly band structure effects due to magnetic superzone-boundary effects are responsible for this anomaly. In the paramagnetic region (above 21 K), the R data were fitted to an & expression R "R #A/(¹!h ) (where A is a constant) & and this fit clearly showed that the anomalous Hall term dominates over the entire temperature range above 21 K. In conclusion, considering also the observation of Kondo-like characteristics above 21 K earlier by us [1], this Gd compound exhibits interesting thermal, transport and magnetic behaviors. We plan single crystal investigations to characterize this compound better.
References
Fig. 1. (a) Inverse susceptibility (s) (with a line through the linear region), with the low temperature DC s data (field cooled and zero-field-cooled, H"100 Oe) and AC s behavior at various frequencies shown in the insets, as a function of temperature for Gd PdSi . The thermopower and the Hall coefficient (R ) of & Gd PdSi and Lu PdSi as a function of temperature are also shown in (b) and (c), respectively. The low temperature data of R is shown in an expanded form for the Gd sample in the inset & of (c).
[1] R. Mallik, E.V. Sampathkumaran, M. Strecker, G. Wortmann, Europhys. Lett. 41 (1998) 315. [2] B. Coqblin, J.R. Iglesias-sicard, R. Julian, Contemp. Phys. 19 (1978) 327.
Magnetic anomalies in Gd PdSi R. Mallik , E.V. Sampathkumaran *, P.L. Paulose , H. Sugawara, H. Sato Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400 005, India Department of Physics, Tokyo Metropoliton University, Tokyo 192-03, Japan
Abstract The results of AC and DC magnetic susceptibility, thermopower and Hall effect measurements of a compound, Gd PdSi , establish that this compound orders magnetically below (¹ ") 20 K. Though the ordering appears to be of , an antiferromagnetic-type, the paramagnetic Curie temperature is positive suggestive of the existence of ferromagnetic correlations. The thermopower at 300 K is large, apparently due to Pd 4d electrons, decreasing monotonically with temperature. There is a change in the sign of Hall constant well below ¹ . The overall thermal, transport and magnetic , behavior of this compound appears to be interesting. 1999 Elsevier Science B.V. All rights reserved. Keywords: Gd PdSi ; Magnetic anomalies; Hall effect; Thermopower
Earlier, we noticed [1] Kondo-lattice-like characteristics in the heat-capacity and electrical resistivity behavior in polycrystalline Gd PdSi . We carried out AC and DC magnetic susceptibility (s), Hall effect and thermopower (S) measurements on the same sample, the results of which are reported here. The plot of inverse s versus temperature (Fig. 1a) is found to be practically linear above 30 K and the value of the paramagnetic Curie temperature (h ) is found to be positive with a magnitude of 19 K, indicating the existence of ferromagnetic correlations. However, there is a distinct peak in s at the same temperature (19 K) which implies the onset of long range antiferromagnetic order at this temperature. The ZFC and FC data (Fig. 1a, left inset) are practically the same above 15 K, while the difference can be distinctly seen to set in below 15 K, but not at the peak. This may be related to the fact there are two magnetic transitions, one close to 21 K (2b site) and the other at 15 K (6h site), from two different Gd sites characteristic of this structure, as shown by Mossbauer effect measurements [1]. In order to address the question whether the ZFC—FC behavior arises from spin—glass
* Corresponding author. Fax: (091)-22-2152110/2181; e-mail: [email protected].
behavior, we also performed AC s measurements at various frequencies (Fig. 1a, right inset) and we do not find any upward shift of the peak with increasing frequency expected for the spin glass phenomena. The shoulder around 15 K in AC s data presumably arises from the magnetic ordering from 6h site. The results of S measurements are shown in Fig. 1b both for Gd PdSi and Lu PdSi . It is to be noted that S of the Gd sample is large at 300 K, which is true even for Lu PdSi , and is comparable to the magnitude known for the Pd metal [2], thereby indicating large Pd 4d density of states at the Fermi level. The values are larger (about !10 lV/K) for the Gd sample, compared to the Lu sample. S varies (decreases) linearly down to about 20 K (as expected). There is a tendency to a constant value for a narrow temperature range below 20 K. Since this shoulder appears in the non-magnetic compound as well, the feature due to magnetic ordering is not clearly separable from the phonon drag effect. Thus the magnetic ordering near 21 K does not have any noticeable effect on S in the Gd alloy. In contrast to this, the Hall coefficient (R ) of the Gd sample is completely & different from that of the Lu sample (Fig. 1c). While the R is a small positive number for Lu sample at all & temperatures, it is negative above 8 K for the Gd PdSi with a distinct peak in the vicinity of magnetic ordering.
0921-4526/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 8 ) 0 1 1 2 9 - 6
R. Mallik et al. / Physica B 259—261 (1999) 892 — 893
893
We, however, find that the peak occurs only near 15 K (Fig. 1c, inset), without any noticeable feature at 20 K and thus the 20 K transition is not tracked by Hall measurements. A noteworthy finding is that there is a sign reversal below 8 K (see inset of Fig. 1c); possibly band structure effects due to magnetic superzone-boundary effects are responsible for this anomaly. In the paramagnetic region (above 21 K), the R data were fitted to an & expression R "R #A/(¹!h ) (where A is a constant) & and this fit clearly showed that the anomalous Hall term dominates over the entire temperature range above 21 K. In conclusion, considering also the observation of Kondo-like characteristics above 21 K earlier by us [1], this Gd compound exhibits interesting thermal, transport and magnetic behaviors. We plan single crystal investigations to characterize this compound better.
References
Fig. 1. (a) Inverse susceptibility (s) (with a line through the linear region), with the low temperature DC s data (field cooled and zero-field-cooled, H"100 Oe) and AC s behavior at various frequencies shown in the insets, as a function of temperature for Gd PdSi . The thermopower and the Hall coefficient (R ) of & Gd PdSi and Lu PdSi as a function of temperature are also shown in (b) and (c), respectively. The low temperature data of R is shown in an expanded form for the Gd sample in the inset & of (c).
[1] R. Mallik, E.V. Sampathkumaran, M. Strecker, G. Wortmann, Europhys. Lett. 41 (1998) 315. [2] B. Coqblin, J.R. Iglesias-sicard, R. Julian, Contemp. Phys. 19 (1978) 327.