- Email: [email protected]
Magnetoresistivity and Hall effect investigations under pressure on SmS
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 310 (2007) e54–e56 www.elsevier.com/locate/jmmm
Magnetoresistivity and Hall effect investigations under pressure on SmS K. Imuraa,, K. Matsubayashia, H.S. Suzukib, K. Deguchia, N.K. Satoa a
Department of Physics, Graduate School of Science, Nagoya University, Nagoya 464-8602, Japan b Nanomaterial Laboratory, National Institute for Materials Science, Tsukuba 305-0047, Japan Available online 10 November 2006
Abstract We report magnetoresistivity and Hall resistivity measurements on the black SmS. We observed positive magnetoresistance at low temperatures and negative magnetoresistance in a wide temperature range. The former seems to be ascribed to a magnetic field effect on impurity conduction, and the latter can be attributed to gap-energy reduction by magnetic fields. We also detected hysteresis in the Tdependence of the electrical resistivity under magnetic field. r 2006 Elsevier B.V. All rights reserved. PACS: 71.28.þd; 72.15.Gd Keywords: SmS; Magnetoresistivity; Hall effect
SmS is a well known mixed-valence compound showing a first-order phase transition at a critical pressure Pc (6 kbar at room temperature), accompanied by an isostructural large volume collapse of DV 8% [1]. In a low pressure phase below Pc , called the black phase, it is believed that SmS is a nonmagnetic semiconductor; temperature dependence of magnetic susceptibility can be explained by van Vleck paramagnetism of a stable Sm2þ ion (4f 6 , S ¼ 3, L ¼ 3, J ¼ 0) [2], and the electrical resistivity shows an activation-type temperature dependence with a narrow energy gap E g (0:1 eV at ambient pressure). On the other hand, a strong magnetic-field effect on the electrical resistivity was observed; at ambient pressure, the electrical resistivity decreases drastically with increasing magnetic field in a wide temperature range ð64 KpTp280 KÞ [3]. An increase of carrier density with magnetic field was also found at T ¼ 77 K. Furthermore, a positive magnetoresistance and a hysteresis phenomenon were detected in a low temperature region ðTp23 KÞ at 3.3 kbar [4]. An impurity effect, such as a bound magnetic polaron model, was proposed to explain these unusual transport phenomena in magnetic fields. However, it is not clear yet whether all of these observations are ascribed to Corresponding author. Tel.: +81 52 789 2889; fax: +81 52 789 2933.
E-mail address: [email protected] (K. Imura). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.547
an impurity effect. In this paper, we report systematic investigations of the magnetoresistance and Hall effect under pressure on the black SmS. The transverse magnetoresistance and Hall effect measurements under pressure were carried out using the DC van der Pauw method from 1.3 K to about 100 K. A singlecrystal sample of SmS with a typical dimension of 1:5 1:5 0:25 mm3 was cleaved from a bulk ingot. Pressure was applied at room temperature using a piston cylinder clamp cell, which consists of a BeCu outer cylinder and a NiCrAl inner cylinder. Daphne oil 7373 was used as a pressure-transmitting medium. Low temperature pressure was determined by a superconducting transition temperature of indium (6N-purity). Fig. 1(a) shows the temperature dependence of the electrical resistivity at 3.1 kbar under magnetic field. Below about 100 K, we find a negative magnetoresistance. Interestingly, a hysteretic behavior was observed below about T X 25 K in two curves measured at 6.8 T; FC and ZFC curves denote the electrical resistivity measured in a warming process under 6.8 T after cooled under 6.8 T and zero field, respectively. We observed the hysteresis at 2.4 and 3.7 kbar (not shown here), but failed to detect it at ambient pressure. Indeed, it is possible to ascribe the occurrence of the hysteresis to an impurity effect such as a spin glass. However, it is noteworthy that samples of
ARTICLE IN PRESS K. Imura et al. / Journal of Magnetism and Magnetic Materials 310 (2007) e54–e56
b
102
0T 1 3 6.8 6.8(FC)
(Ωcm)
101
2 e (cm2/Vs) ne(cm-3) × 10-17 (Ωcm)
a
e55
100
10-1 SmS 3.1kbar
10-2 1
10 T (K)
1 0 5
0
0
100
P = 3.1kbar T = 4.2K
50
0
2
4 H (T)
6
Fig. 1. (a) Temperature dependence of the electrical resistivity in constant magnetic fields at 3.1 kbar. Hysteresis was detected below about 25 K (marked by an arrow) in curves measured at 6.8 T (see text for detail). (b) Top panel: Magnetic field dependence of the electrical resistivity at P ¼ 3:1 kbar and T ¼ 4:2 K. Middle panel: Magnetic field dependence of carrier density at 3.1 kbar and 4.2 K. Bottom panel: Magnetic field dependence of charge mobility at 3.1 kbar and 4.2 K. The carrier density and charge mobility were calculated using a usual single band model. Note that a maximum in the magnetoresistance coincides with a minimum in the mobility.
b
a 1500
SmS ambient
0.7kbar Eg (K)
1000
2.4 500 3.1 H
3.7
0 0
2 P (kbar)
4
0
2
4 H (T)
6
Fig. 2. (a) Pressure dependence of the energy gap E g deduced from the temperature dependence of electrical resistivity and Hall resistivity. We note that the pressure was estimated at low temperature. (b) Magnetic field dependence of E g deduced from the temperature dependence of electrical resistivity in constant magnetic fields at selected pressures.
different origins yield almost the same value for T X . (Note that Konczykowski et al. gave T X 23 K, as mentioned above [4].) Thermal expansion experiments under magnetic field are in progress. As shown in Fig. 1(b), the electrical resistivity rðHÞ at 4.2 K shows a maximum at around H m 1:5 T. Similar positive magnetoresistance was detectable up to about
10 K. Note that the ratio rðH m Þ=rð0Þ exceeds 2. We find that whereas the carrier density ne ðHÞ shows a nearly monotonic increase through H m , the charge mobility me ðHÞ steeply decreases up to H m before increasing with magnetic field. This clearly indicates that the positive magnetoresistance is due to the decrease in the mobility. Since variable range hopping behavior, characterized by rðTÞ / T 0:25 ,
ARTICLE IN PRESS e56
K. Imura et al. / Journal of Magnetism and Magnetic Materials 310 (2007) e54–e56
was observed in a similar temperature range, it may be possible to attribute the positive magnetoresistance to a magnetic field effect on an impurity conduction. Fig. 2(a) shows the pressure dependence of the semiconducting energy gap E g deduced from the temperature dependence of electrical resistivity and Hall resistivity measured at 1 T. Both experiments yield almost the same pressure dependence of E g . We note that E g monotonically decreases as increasing pressure, but also above Pc , in golden phase, semiconducting energy gap E g was shown in the electrical resistivity [5]. It was very difficult to reveal the accurate behavior of E g in the vicinity of Pc . It deserves a further investigation. In Fig. 2(b) shown is the magnetic field dependence of E g at a constant pressure (below Pc ), which was deduced from the temperature dependence of the electrical resistivity measured in a constant magnetic field. One may notice a peak at around 1 T at 0.7 and 2.4 kbar, but its origin is not clear at present. Neglecting this, we find a monotonic decrease in E g with increasing magnetic field. It is clear that the reduction of the gap energy by magnetic fields explains the negative magnetoresistance observed in Fig. 1(a). Then, the problem to be addressed now is whether the gap-energy reduction is inherent to SmS or not. Provided that the black SmS is nonmagnetic, such a large magnetic field effect on the gap will not be expected. By contrast, the assumption that the gap deduced from the transport properties is intrinsic, leads us to suggest that the black SmS is not a simple nonmagnetic semiconductor. We hope that the present result stimulates a further theoretical investigation such as a (molecular) magnetic exciton model [6].
In conclusion, we report the magnetoresistivity and Hall resistivity experiments under pressure up to 3.7 kbar. We observed positive magnetoresistance at low temperatures and negative magnetoresistance in a wide temperature range, consistent with the previous reports. The former seems to be ascribed to a magnetic field effect on impurity conduction, and the latter can be attributed to the gap-energy reduction by magnetic fields. We also detected the hysteresis in the T-dependence of the electrical resistivity under magnetic field, but it remains to be resolved if the anomaly is of bulk origin. This work was partially supported by a Grant-in-Aid from the Ministry of Education, Culture, Sports, Science and Technology, Japan, and Arai Science and Technology foundation. K.M. is supported by a Grant-in-Aid for JSPS Fellows.
References [1] A. Jayaraman, V. Narayanamurti, E. Bucher, R.G. Maines, Phys. Rev. Lett. 25 (1970) 1430. [2] M.B. Maple, D. Wohleben, Phys. Rev. B 7 (1973) 4986. [3] M. Konczykowski, J. Morillo, J.C. Portal, J. Galibert, J.P. Senateur, in: P. Wachter, H. Boppart (Eds.), Valence Instabilities, NorthHolland, Amsterdam, 1982, p.447. [4] M. Konczykowski, P. Haen, J.C. Portal, J.P. Senateur, J. Magn. Magn. Mater. 47 & 48 (1985) 455. [5] F. Lapierre, M. Ribault, F. Holtzberg, J. Flouquet, Solid State Commun. 40 (1981) 347. [6] T. Kasuya, J. Phys. Colloq. 37 (1976) C4–261.
Journal of Magnetism and Magnetic Materials 310 (2007) e54–e56 www.elsevier.com/locate/jmmm
Magnetoresistivity and Hall effect investigations under pressure on SmS K. Imuraa,, K. Matsubayashia, H.S. Suzukib, K. Deguchia, N.K. Satoa a
Department of Physics, Graduate School of Science, Nagoya University, Nagoya 464-8602, Japan b Nanomaterial Laboratory, National Institute for Materials Science, Tsukuba 305-0047, Japan Available online 10 November 2006
Abstract We report magnetoresistivity and Hall resistivity measurements on the black SmS. We observed positive magnetoresistance at low temperatures and negative magnetoresistance in a wide temperature range. The former seems to be ascribed to a magnetic field effect on impurity conduction, and the latter can be attributed to gap-energy reduction by magnetic fields. We also detected hysteresis in the Tdependence of the electrical resistivity under magnetic field. r 2006 Elsevier B.V. All rights reserved. PACS: 71.28.þd; 72.15.Gd Keywords: SmS; Magnetoresistivity; Hall effect
SmS is a well known mixed-valence compound showing a first-order phase transition at a critical pressure Pc (6 kbar at room temperature), accompanied by an isostructural large volume collapse of DV 8% [1]. In a low pressure phase below Pc , called the black phase, it is believed that SmS is a nonmagnetic semiconductor; temperature dependence of magnetic susceptibility can be explained by van Vleck paramagnetism of a stable Sm2þ ion (4f 6 , S ¼ 3, L ¼ 3, J ¼ 0) [2], and the electrical resistivity shows an activation-type temperature dependence with a narrow energy gap E g (0:1 eV at ambient pressure). On the other hand, a strong magnetic-field effect on the electrical resistivity was observed; at ambient pressure, the electrical resistivity decreases drastically with increasing magnetic field in a wide temperature range ð64 KpTp280 KÞ [3]. An increase of carrier density with magnetic field was also found at T ¼ 77 K. Furthermore, a positive magnetoresistance and a hysteresis phenomenon were detected in a low temperature region ðTp23 KÞ at 3.3 kbar [4]. An impurity effect, such as a bound magnetic polaron model, was proposed to explain these unusual transport phenomena in magnetic fields. However, it is not clear yet whether all of these observations are ascribed to Corresponding author. Tel.: +81 52 789 2889; fax: +81 52 789 2933.
E-mail address: [email protected] (K. Imura). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.547
an impurity effect. In this paper, we report systematic investigations of the magnetoresistance and Hall effect under pressure on the black SmS. The transverse magnetoresistance and Hall effect measurements under pressure were carried out using the DC van der Pauw method from 1.3 K to about 100 K. A singlecrystal sample of SmS with a typical dimension of 1:5 1:5 0:25 mm3 was cleaved from a bulk ingot. Pressure was applied at room temperature using a piston cylinder clamp cell, which consists of a BeCu outer cylinder and a NiCrAl inner cylinder. Daphne oil 7373 was used as a pressure-transmitting medium. Low temperature pressure was determined by a superconducting transition temperature of indium (6N-purity). Fig. 1(a) shows the temperature dependence of the electrical resistivity at 3.1 kbar under magnetic field. Below about 100 K, we find a negative magnetoresistance. Interestingly, a hysteretic behavior was observed below about T X 25 K in two curves measured at 6.8 T; FC and ZFC curves denote the electrical resistivity measured in a warming process under 6.8 T after cooled under 6.8 T and zero field, respectively. We observed the hysteresis at 2.4 and 3.7 kbar (not shown here), but failed to detect it at ambient pressure. Indeed, it is possible to ascribe the occurrence of the hysteresis to an impurity effect such as a spin glass. However, it is noteworthy that samples of
ARTICLE IN PRESS K. Imura et al. / Journal of Magnetism and Magnetic Materials 310 (2007) e54–e56
b
102
0T 1 3 6.8 6.8(FC)
(Ωcm)
101
2 e (cm2/Vs) ne(cm-3) × 10-17 (Ωcm)
a
e55
100
10-1 SmS 3.1kbar
10-2 1
10 T (K)
1 0 5
0
0
100
P = 3.1kbar T = 4.2K
50
0
2
4 H (T)
6
Fig. 1. (a) Temperature dependence of the electrical resistivity in constant magnetic fields at 3.1 kbar. Hysteresis was detected below about 25 K (marked by an arrow) in curves measured at 6.8 T (see text for detail). (b) Top panel: Magnetic field dependence of the electrical resistivity at P ¼ 3:1 kbar and T ¼ 4:2 K. Middle panel: Magnetic field dependence of carrier density at 3.1 kbar and 4.2 K. Bottom panel: Magnetic field dependence of charge mobility at 3.1 kbar and 4.2 K. The carrier density and charge mobility were calculated using a usual single band model. Note that a maximum in the magnetoresistance coincides with a minimum in the mobility.
b
a 1500
SmS ambient
0.7kbar Eg (K)
1000
2.4 500 3.1 H
3.7
0 0
2 P (kbar)
4
0
2
4 H (T)
6
Fig. 2. (a) Pressure dependence of the energy gap E g deduced from the temperature dependence of electrical resistivity and Hall resistivity. We note that the pressure was estimated at low temperature. (b) Magnetic field dependence of E g deduced from the temperature dependence of electrical resistivity in constant magnetic fields at selected pressures.
different origins yield almost the same value for T X . (Note that Konczykowski et al. gave T X 23 K, as mentioned above [4].) Thermal expansion experiments under magnetic field are in progress. As shown in Fig. 1(b), the electrical resistivity rðHÞ at 4.2 K shows a maximum at around H m 1:5 T. Similar positive magnetoresistance was detectable up to about
10 K. Note that the ratio rðH m Þ=rð0Þ exceeds 2. We find that whereas the carrier density ne ðHÞ shows a nearly monotonic increase through H m , the charge mobility me ðHÞ steeply decreases up to H m before increasing with magnetic field. This clearly indicates that the positive magnetoresistance is due to the decrease in the mobility. Since variable range hopping behavior, characterized by rðTÞ / T 0:25 ,
ARTICLE IN PRESS e56
K. Imura et al. / Journal of Magnetism and Magnetic Materials 310 (2007) e54–e56
was observed in a similar temperature range, it may be possible to attribute the positive magnetoresistance to a magnetic field effect on an impurity conduction. Fig. 2(a) shows the pressure dependence of the semiconducting energy gap E g deduced from the temperature dependence of electrical resistivity and Hall resistivity measured at 1 T. Both experiments yield almost the same pressure dependence of E g . We note that E g monotonically decreases as increasing pressure, but also above Pc , in golden phase, semiconducting energy gap E g was shown in the electrical resistivity [5]. It was very difficult to reveal the accurate behavior of E g in the vicinity of Pc . It deserves a further investigation. In Fig. 2(b) shown is the magnetic field dependence of E g at a constant pressure (below Pc ), which was deduced from the temperature dependence of the electrical resistivity measured in a constant magnetic field. One may notice a peak at around 1 T at 0.7 and 2.4 kbar, but its origin is not clear at present. Neglecting this, we find a monotonic decrease in E g with increasing magnetic field. It is clear that the reduction of the gap energy by magnetic fields explains the negative magnetoresistance observed in Fig. 1(a). Then, the problem to be addressed now is whether the gap-energy reduction is inherent to SmS or not. Provided that the black SmS is nonmagnetic, such a large magnetic field effect on the gap will not be expected. By contrast, the assumption that the gap deduced from the transport properties is intrinsic, leads us to suggest that the black SmS is not a simple nonmagnetic semiconductor. We hope that the present result stimulates a further theoretical investigation such as a (molecular) magnetic exciton model [6].
In conclusion, we report the magnetoresistivity and Hall resistivity experiments under pressure up to 3.7 kbar. We observed positive magnetoresistance at low temperatures and negative magnetoresistance in a wide temperature range, consistent with the previous reports. The former seems to be ascribed to a magnetic field effect on impurity conduction, and the latter can be attributed to the gap-energy reduction by magnetic fields. We also detected the hysteresis in the T-dependence of the electrical resistivity under magnetic field, but it remains to be resolved if the anomaly is of bulk origin. This work was partially supported by a Grant-in-Aid from the Ministry of Education, Culture, Sports, Science and Technology, Japan, and Arai Science and Technology foundation. K.M. is supported by a Grant-in-Aid for JSPS Fellows.
References [1] A. Jayaraman, V. Narayanamurti, E. Bucher, R.G. Maines, Phys. Rev. Lett. 25 (1970) 1430. [2] M.B. Maple, D. Wohleben, Phys. Rev. B 7 (1973) 4986. [3] M. Konczykowski, J. Morillo, J.C. Portal, J. Galibert, J.P. Senateur, in: P. Wachter, H. Boppart (Eds.), Valence Instabilities, NorthHolland, Amsterdam, 1982, p.447. [4] M. Konczykowski, P. Haen, J.C. Portal, J.P. Senateur, J. Magn. Magn. Mater. 47 & 48 (1985) 455. [5] F. Lapierre, M. Ribault, F. Holtzberg, J. Flouquet, Solid State Commun. 40 (1981) 347. [6] T. Kasuya, J. Phys. Colloq. 37 (1976) C4–261.