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Heat capacity of NdB6
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 310 (2007) e595–e597 www.elsevier.com/locate/jmmm
Heat capacity of NdB6 Marian Reiffersa,, Josef Sˇebekb, Eva Sˇantava´b, Natasha Shitsevalovac, Slavomir Gaba´nia, Gabriel Prista´sˇ a, Karol Flachbarta a
Institute of Experimental Physics, Slovak Academy of Sciences, Watsonova 47, SK-043 53 Kosˇice, Slovakia b Institute of Physics, V Holesˇovicˇka´ch 2, 180 00 Praha, Czech Republic c Institute for Problems of Materials Science, Academy of Sciences of Ukraine, UA-252680 Kiev, Ukraine Available online 13 November 2006
Abstract We present the study of heat capacity C(T) of single-crystalline NdB6 prepared by zone floating which orders in an A-type collinear antiferromagnetic structure below TNE8 K. In zero field, we have observed a magnetic transition at TN ¼ 7.45 K in a magnetic field of 3 T, this transition is shifted to 7.6 K. From the observed results, we determined the magnetic entropy and the magnetic part of C(T) by subtracting the phonon and electronic part of the heat capacity of isostructural LaB6. r 2006 Elsevier B.V. All rights reserved. PACS: 65.40.Ba; 71.70.Ch; 75.47.Np Keywords: Heat capacity; NdB6; Magnetic contribution
1. Introduction Rare-earth hexaborides with simple cubic CaB6-type structure have been attracting much attention because of their variety of electronic and magnetic properties. Among them, NdB6 is interesting due to its complicated magnetic transport properties, namely due to the anomalous large variation of the Hall coefficient in the neighborhood of the critical temperature [1,2]. According to the previous experimental investigations [3], NdB6 orders in an A-type collinear antiferromagnetic structure below TNE8 K. The ground state of Nd+3 ions (with J ¼ 9/2) in crystalline electric field is the G(2) 8 quartet [4]. However, there are no results available related to the magnetic field influence on the heat capacity of NdB6. Also the magnetic entropy has not yet been determined. In order to receive more information about this compound and about the influence of magnetic field on its properties, we measured the heat capacity C(T) of a single crystal of NdB6 in applied magnetic field up to 3 T. From the observed result we determined the magnetic entropy and the magnetic part of Corresponding author. Tel.: +421 55 622 8158; fax: +421 55 633 6292.
E-mail address: [email protected] (M. Reiffers). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.587
heat capacity of NdB6 by subtracting the heat capacity of isostructural LaB6 using the method described in work [5]. 2. Experimental The single crystal of NdB6 has been prepared by zone floating. Details of sample preparation and its characterization are described in Ref.[6]. Heat capacity measurements were performed by PPMS commercial device (Quantum Design) in the temperature range 2–300 K using the two-t model of the relaxation method. We have measured in zero field and in applied magnetic field of 3 T. Due to the high magnetic moment of NdB6, we could not proceed to higher magnetic fields. 3. Results Fig. 1 shows the temperature dependence of the heat capacity of NdB6 in the temperature range 2–300 K and in zero magnetic field. We have observed a sharp peak at TN ¼ 7.45 K which is connected with the second-order transition into the antiferromagnetically ordered state. Our results are in a good agreement with that obtained in Ref.[7]. The Schottky anomaly is in received results present
ARTICLE IN PRESS e596
M. Reiffers et al. / Journal of Magnetism and Magnetic Materials 310 (2007) e595–e597 100
C(T) [J.mol-1.K-1]
80
60
NdB6
40 C(T) 20
0 0
50
100
150
200
250
300
T [K]
Fig. 1. Temperature dependence of the heat capacity C(T) of NdB6 single crystal. The inset shows the magnetic field influence on the transition into the magnetically ordered state.
25 NdB6 Cmag(T) [J.mol-1.K-1]
20
Cmag(T)
15
10
5
0 0
25
50
75
100
125
150
T [K]
Fig. 2. Temperature dependence of the magnetic contribution Cmag(T) to heat capacity of NdB6.
3 NdB6
2 S(T)/R
as a broad maximum at about 50 K. In the inset of Fig. 1 one can see the influence of magnetic field 3 T. In this case the peak is shifted to TN ¼ 7.6 K. Measurements at higher magnetic fields could not been performed due to the high magnetic moment of the sample. In order to determine the magnetic contribution Cmag(T) to the heat capacity of NdB6, we have subtracted the heat capacity of LaB6 which is an nonmagnetic isostructural compound containing only the electronic and phonon contribution [8]. In Fig. 2, the temperature dependence of Cmag(T) is shown. One can now see more clearly the Schottky maximum at 50 K. Using the standard procedure of Schottky anomaly fitting to determine the hexagonal CEF level splitting [9], we obtained the values W ¼ 3.71 K and x ¼ 0.91. This yields to the 3-level CEF scheme with energies 101.4 and 276.59 K. The values are in a qualitative agreement with results in Ref.[4]. In Fig. 3, the plot of the temperature dependence of the magnetic entropy S(T)/R determined from the magnetic contribution Cmag(T) is shown. At high temperatures, the ratio S(T)/R is reaching a value of 2.88. This value is, however, higher than the theoretically expected value ln 10 (J ¼ 9/2). We suppose that this discrepancy occurs probably due to improper data of the heat capacity C(T) of LaB6 [8]. This could also explain the small difference between our obtained CEF parameters and that from Ref. [4]. Therefore, further measurements of C(T) of LaB6 will be necessary.
S(T) 1
4. Conclusions We have investigated the heat capacity of single-crystalline NdB6. We have observed a sharp peak at TN ¼ 7.45 K which is connected with the second-order transition into the antiferromagnetically ordered state. The Schottky
0 0
50
100
150
200
T [K]
Fig. 3. Magnetic entropy S(T)/R of NdB6.
250
300
ARTICLE IN PRESS M. Reiffers et al. / Journal of Magnetism and Magnetic Materials 310 (2007) e595–e597
anomaly is present as a broad maximum at about 50 K. The magnetic field of 3 T shifts the C(T) peak to TN ¼ 7.45 K. Moreover, we have determined the temperature dependence of the magnetic contribution Cmag(T) to heat capacity and the temperature dependence of magnetic entropy ratio S(T)/R. Acknowledgments This work has been supported partly by the COST–ECOM project P16, by Science and Technology Assistance Agency—contract No. APVT-51-031704, by contract No. I/2/2003 of the Slovak Academy of Sciences for the Centers
e597
of Excellence and by INTAS—3036. US Steel Kosˇ ice sponsored a part of liquid nitrogen needed for experiments. References [1] M. Sera, et al., J. Phys. Soc. Jpn. 67 (1998) 629. [2] J. Stankiewicz, et al., Phys. Rev. B 71 (2005) 134426. [3] C.M. McCarthy, C.W. Tompson, J. Phys. Chem. Solids 41 (1980) 1319. [4] M. Loewenhaupt, M. Prager, Z. Phys. B 62 (1986) 195. [5] S. Kunii, et al., J. Solid State Chem. 154 (2000) 275. [6] E.S. Konovalova, et al., Sov. Inorg. Mater. 26 (1990) 1218. [7] E.F. Westrum Jr., et al., J. Chem. Thermod. 34 (2002) 239. [8] H.G. Smith, et al., Solid Stat Commun. 53 (1985) 15. [9] A. Czopnik, et al., J. Solid State Chem. 177 (2004) 507.
Journal of Magnetism and Magnetic Materials 310 (2007) e595–e597 www.elsevier.com/locate/jmmm
Heat capacity of NdB6 Marian Reiffersa,, Josef Sˇebekb, Eva Sˇantava´b, Natasha Shitsevalovac, Slavomir Gaba´nia, Gabriel Prista´sˇ a, Karol Flachbarta a
Institute of Experimental Physics, Slovak Academy of Sciences, Watsonova 47, SK-043 53 Kosˇice, Slovakia b Institute of Physics, V Holesˇovicˇka´ch 2, 180 00 Praha, Czech Republic c Institute for Problems of Materials Science, Academy of Sciences of Ukraine, UA-252680 Kiev, Ukraine Available online 13 November 2006
Abstract We present the study of heat capacity C(T) of single-crystalline NdB6 prepared by zone floating which orders in an A-type collinear antiferromagnetic structure below TNE8 K. In zero field, we have observed a magnetic transition at TN ¼ 7.45 K in a magnetic field of 3 T, this transition is shifted to 7.6 K. From the observed results, we determined the magnetic entropy and the magnetic part of C(T) by subtracting the phonon and electronic part of the heat capacity of isostructural LaB6. r 2006 Elsevier B.V. All rights reserved. PACS: 65.40.Ba; 71.70.Ch; 75.47.Np Keywords: Heat capacity; NdB6; Magnetic contribution
1. Introduction Rare-earth hexaborides with simple cubic CaB6-type structure have been attracting much attention because of their variety of electronic and magnetic properties. Among them, NdB6 is interesting due to its complicated magnetic transport properties, namely due to the anomalous large variation of the Hall coefficient in the neighborhood of the critical temperature [1,2]. According to the previous experimental investigations [3], NdB6 orders in an A-type collinear antiferromagnetic structure below TNE8 K. The ground state of Nd+3 ions (with J ¼ 9/2) in crystalline electric field is the G(2) 8 quartet [4]. However, there are no results available related to the magnetic field influence on the heat capacity of NdB6. Also the magnetic entropy has not yet been determined. In order to receive more information about this compound and about the influence of magnetic field on its properties, we measured the heat capacity C(T) of a single crystal of NdB6 in applied magnetic field up to 3 T. From the observed result we determined the magnetic entropy and the magnetic part of Corresponding author. Tel.: +421 55 622 8158; fax: +421 55 633 6292.
E-mail address: [email protected] (M. Reiffers). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.587
heat capacity of NdB6 by subtracting the heat capacity of isostructural LaB6 using the method described in work [5]. 2. Experimental The single crystal of NdB6 has been prepared by zone floating. Details of sample preparation and its characterization are described in Ref.[6]. Heat capacity measurements were performed by PPMS commercial device (Quantum Design) in the temperature range 2–300 K using the two-t model of the relaxation method. We have measured in zero field and in applied magnetic field of 3 T. Due to the high magnetic moment of NdB6, we could not proceed to higher magnetic fields. 3. Results Fig. 1 shows the temperature dependence of the heat capacity of NdB6 in the temperature range 2–300 K and in zero magnetic field. We have observed a sharp peak at TN ¼ 7.45 K which is connected with the second-order transition into the antiferromagnetically ordered state. Our results are in a good agreement with that obtained in Ref.[7]. The Schottky anomaly is in received results present
ARTICLE IN PRESS e596
M. Reiffers et al. / Journal of Magnetism and Magnetic Materials 310 (2007) e595–e597 100
C(T) [J.mol-1.K-1]
80
60
NdB6
40 C(T) 20
0 0
50
100
150
200
250
300
T [K]
Fig. 1. Temperature dependence of the heat capacity C(T) of NdB6 single crystal. The inset shows the magnetic field influence on the transition into the magnetically ordered state.
25 NdB6 Cmag(T) [J.mol-1.K-1]
20
Cmag(T)
15
10
5
0 0
25
50
75
100
125
150
T [K]
Fig. 2. Temperature dependence of the magnetic contribution Cmag(T) to heat capacity of NdB6.
3 NdB6
2 S(T)/R
as a broad maximum at about 50 K. In the inset of Fig. 1 one can see the influence of magnetic field 3 T. In this case the peak is shifted to TN ¼ 7.6 K. Measurements at higher magnetic fields could not been performed due to the high magnetic moment of the sample. In order to determine the magnetic contribution Cmag(T) to the heat capacity of NdB6, we have subtracted the heat capacity of LaB6 which is an nonmagnetic isostructural compound containing only the electronic and phonon contribution [8]. In Fig. 2, the temperature dependence of Cmag(T) is shown. One can now see more clearly the Schottky maximum at 50 K. Using the standard procedure of Schottky anomaly fitting to determine the hexagonal CEF level splitting [9], we obtained the values W ¼ 3.71 K and x ¼ 0.91. This yields to the 3-level CEF scheme with energies 101.4 and 276.59 K. The values are in a qualitative agreement with results in Ref.[4]. In Fig. 3, the plot of the temperature dependence of the magnetic entropy S(T)/R determined from the magnetic contribution Cmag(T) is shown. At high temperatures, the ratio S(T)/R is reaching a value of 2.88. This value is, however, higher than the theoretically expected value ln 10 (J ¼ 9/2). We suppose that this discrepancy occurs probably due to improper data of the heat capacity C(T) of LaB6 [8]. This could also explain the small difference between our obtained CEF parameters and that from Ref. [4]. Therefore, further measurements of C(T) of LaB6 will be necessary.
S(T) 1
4. Conclusions We have investigated the heat capacity of single-crystalline NdB6. We have observed a sharp peak at TN ¼ 7.45 K which is connected with the second-order transition into the antiferromagnetically ordered state. The Schottky
0 0
50
100
150
200
T [K]
Fig. 3. Magnetic entropy S(T)/R of NdB6.
250
300
ARTICLE IN PRESS M. Reiffers et al. / Journal of Magnetism and Magnetic Materials 310 (2007) e595–e597
anomaly is present as a broad maximum at about 50 K. The magnetic field of 3 T shifts the C(T) peak to TN ¼ 7.45 K. Moreover, we have determined the temperature dependence of the magnetic contribution Cmag(T) to heat capacity and the temperature dependence of magnetic entropy ratio S(T)/R. Acknowledgments This work has been supported partly by the COST–ECOM project P16, by Science and Technology Assistance Agency—contract No. APVT-51-031704, by contract No. I/2/2003 of the Slovak Academy of Sciences for the Centers
e597
of Excellence and by INTAS—3036. US Steel Kosˇ ice sponsored a part of liquid nitrogen needed for experiments. References [1] M. Sera, et al., J. Phys. Soc. Jpn. 67 (1998) 629. [2] J. Stankiewicz, et al., Phys. Rev. B 71 (2005) 134426. [3] C.M. McCarthy, C.W. Tompson, J. Phys. Chem. Solids 41 (1980) 1319. [4] M. Loewenhaupt, M. Prager, Z. Phys. B 62 (1986) 195. [5] S. Kunii, et al., J. Solid State Chem. 154 (2000) 275. [6] E.S. Konovalova, et al., Sov. Inorg. Mater. 26 (1990) 1218. [7] E.F. Westrum Jr., et al., J. Chem. Thermod. 34 (2002) 239. [8] H.G. Smith, et al., Solid Stat Commun. 53 (1985) 15. [9] A. Czopnik, et al., J. Solid State Chem. 177 (2004) 507.