- Email: [email protected]
Specific heat of CeCoGe single crystal
ARTICLE IN PRESS
Physica B 378–380 (2006) 797–798 www.elsevier.com/locate/physb
Specific heat of CeCoGe single crystal J. Vejpravova´a, J. Proklesˇ kaa, B. Janousˇ ova´a, A.V. Andreevb,, V. Sechovsky´a a
Department of Electronic Structures, Faculty of Mathematics and Physics, Charles University in Prague, Ke Karlovu 5, 121 16 - Prague 2, Czech Republic b Academy of Sciences, Institute of Physics, Na Slovance 2, 182 21 - Prague 8, Czech Republic
Abstract We have grown a CeCoGe single crystal and studied the specific heat as a function of temperature and magnetic field. At low temperatures also AC susceptibility and magnetization isotherms were measured. Clear attributes of the antiferromagnetic ordering below 5:5 K ð¼ T N Þ have been observed. The low-temperature specific heat (and the magnetic entropy) were considered when formulating a scenario of magnetic ordering in CeCoGe. r 2006 Elsevier B.V. All rights reserved. PACS: 75.50.Ee; 65.40.Gr Keywords: CeCoGe; Specific heat; Antiferromagnetism
The magnetization measurements on CeCoGe polycrystals and powder neutron diffraction experiments revealed antiferromagnetic (AF) ordering below the Ne´el temperature T N ¼ 5:5 K and indicates of a metamagnetic transition above 5 T [1]. We have grown a single crystal of CeCoGe by Czochralski technique and studied its magnetic properties and specific heat. The powder X-ray diffraction analysis of a crushed representative part of the single crystal confirmed presence of a single phase having the CeFeSi-type structure. The determined lattice parameters are in excellent agreement with Ref. [2]. Magnetic and specific-heat measurements were performed using the PPMS (9 and 14 T) devices (quantum design). The temperature dependence of the AC susceptibility (w0 —real, w00 —imaginary part) is depicted in Fig. 1a. The w0 ðTÞ curve shows a symmetric maximum at 5.2 K where a rapid change of slope of the w00 ðTÞ curve is observed. Origin of the considerable increase of w00 with further decreasing temperature is unclear in this stage. The specific heat (C p ) as a function of temperature (see Fig. 1b) exhibits a sharp twinned anomaly pinned at 5.5 K, which is in agreement with the T N value claimed in Ref. [1]. When a magnetic field up to 5 T is applied along the c-axis (Fig. 1b), the peak Corresponding author. Tel.: +420 2 21911352; fax: +420 2 21911617.
E-mail address: [email protected] (A.V. Andreev). 0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.01.290
is shifted continuously to lower temperatures while the peak profile remains essentially intact. With further increasing field the T N -related anomaly gradually falls apart around 7 T and becomes completely smeared out in 9 T. This development is closely connected with the magnetization behavior at low temperatures. The magnetization curve measured at 2 K in the field parallel to the caxis (Fig. 2) reveals a metamagnetic transition (MT) at 6 T that in conjunction with the temperature dependence of the AC susceptibility and the field-induced evolution of the specific heat corroborates the scenario of an AF ground state of CeCoGe and T N ¼ 5:5 K. The magnetic moment in 14 T yields 1:34mB =f:u: in consistency with the saturatedmoment value expected for the first excited doublet of the Ce3þ ion. This result does not explicitly substantiate the claim of the very reduced Ce ordered moment determined from the neutron diffraction experiment [1]. To test arguments for a Kondo-type screening scenario, we analyzed the temperature dependence of the specific heat measured in absence of magnetic field as a sum of the magnetic (C mag ), phonon (C ph ) and the electron (C e ) contribution, respectively. LaCoGe, the non-magnetic isostructural analog of CeCoGe served as a reference material for the determination of C ph . The magnetic contribution to the specific heat C mag has been obtained by subtracting C ph þ C e ðC e was determined by the
ARTICLE IN PRESS 798
J. Vejpravova´ et al. / Physica B 378–380 (2006) 797–798
1.6
M (µB / f.u.)
1.2
0.8
0.4 B c 0.0
0
4
8
12
µ0H (T) Fig. 2. Magnetization curve at 2 K for field parallel to the c-axis.
Fig. 1. (a) Real (w0 ) and imaginary (w00 ) part of the AC susceptibility. (b) Evolution of the specific heat C p of CeCoGe in magnetic fields parallel to the c-axis. (c) Specific heat C p (open circles) in comparison with the Schottky contribution C Sch (dotted line) and specific heat of LaCoGe C eþph (dashed line). The dash–dot line corresponds to the calculated magnetic entropy Smag , the solid line shows the fit according to Ref. [3].
procedure described lower) from the CeCoGe specific-heat data. The calculated magnetic entropy S mag (dash–dot line) at T N amounts to 4 J mol1 K2 that is somewhat lower than the expected R ln 2 value. In context with the reduced jump of C p at T N (contrary to the expected value for the doublet ground state 12.5 J mol1 K2 obtained from the mean-field approach), Kondo-type interaction with a characteristic temperature T K 6 K might be considered in this compound (for details, see Ref. [3]). However,
neither the reduction of S, nor that of DC p are really strong and are rather caused by short range AF-fluctuation above T N and broadening of the transition. The temperature dependence of the magnetic entropy Smag indicates that the first and the second excited levels are placed well above the ground state. The value of R ln 2 is reached at 16 K and a change of slope of S mag ðTÞ can be observed above 80 K (indicating position of the first excited state consistently with the Smag R ln 4, not shown). The contribution of AF spin waves to C mag was determined within the model proposed in Ref. [4] and points to a gap in the AF spin wave dispersion 7 K together with the Sommerfeld coefficient g ¼ 60 m J mol1 K2 , see Fig. 1c (solid line). We are aware that the model is relevant only well below T N and the temperature interval used for the analysis was actually too narrow to obtain more precise values. In summary, our investigation of the CeCoGe single crystal confirmed the AF ordering below T N ¼ 5:5 K and with MT at 6 T (at 2 K) observed for the field kc. Contrary to the previous results [1] no convincing evidence for the presence of Kondo-type interaction causing reduction of the Ce magnetic moment in CeCoGe has been find out in our specific heat and magnetization data. This work is a part of the research plan MSM 0021620834 financed by the Ministry of Education of the Czech Republic. References [1] [2] [3] [4]
B. Chevalier, et al., Solid State Commun. 130 (2004) 711. J. Welter, et al., J. Alloys Compound 201 (1993) 191. K.D. Schotte, et al., Phys. Lett. A 55 (1975) 38. M.A. Continentino, et al., Phys. Rev. B 64 (2001) 012404.
Physica B 378–380 (2006) 797–798 www.elsevier.com/locate/physb
Specific heat of CeCoGe single crystal J. Vejpravova´a, J. Proklesˇ kaa, B. Janousˇ ova´a, A.V. Andreevb,, V. Sechovsky´a a
Department of Electronic Structures, Faculty of Mathematics and Physics, Charles University in Prague, Ke Karlovu 5, 121 16 - Prague 2, Czech Republic b Academy of Sciences, Institute of Physics, Na Slovance 2, 182 21 - Prague 8, Czech Republic
Abstract We have grown a CeCoGe single crystal and studied the specific heat as a function of temperature and magnetic field. At low temperatures also AC susceptibility and magnetization isotherms were measured. Clear attributes of the antiferromagnetic ordering below 5:5 K ð¼ T N Þ have been observed. The low-temperature specific heat (and the magnetic entropy) were considered when formulating a scenario of magnetic ordering in CeCoGe. r 2006 Elsevier B.V. All rights reserved. PACS: 75.50.Ee; 65.40.Gr Keywords: CeCoGe; Specific heat; Antiferromagnetism
The magnetization measurements on CeCoGe polycrystals and powder neutron diffraction experiments revealed antiferromagnetic (AF) ordering below the Ne´el temperature T N ¼ 5:5 K and indicates of a metamagnetic transition above 5 T [1]. We have grown a single crystal of CeCoGe by Czochralski technique and studied its magnetic properties and specific heat. The powder X-ray diffraction analysis of a crushed representative part of the single crystal confirmed presence of a single phase having the CeFeSi-type structure. The determined lattice parameters are in excellent agreement with Ref. [2]. Magnetic and specific-heat measurements were performed using the PPMS (9 and 14 T) devices (quantum design). The temperature dependence of the AC susceptibility (w0 —real, w00 —imaginary part) is depicted in Fig. 1a. The w0 ðTÞ curve shows a symmetric maximum at 5.2 K where a rapid change of slope of the w00 ðTÞ curve is observed. Origin of the considerable increase of w00 with further decreasing temperature is unclear in this stage. The specific heat (C p ) as a function of temperature (see Fig. 1b) exhibits a sharp twinned anomaly pinned at 5.5 K, which is in agreement with the T N value claimed in Ref. [1]. When a magnetic field up to 5 T is applied along the c-axis (Fig. 1b), the peak Corresponding author. Tel.: +420 2 21911352; fax: +420 2 21911617.
E-mail address: [email protected] (A.V. Andreev). 0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.01.290
is shifted continuously to lower temperatures while the peak profile remains essentially intact. With further increasing field the T N -related anomaly gradually falls apart around 7 T and becomes completely smeared out in 9 T. This development is closely connected with the magnetization behavior at low temperatures. The magnetization curve measured at 2 K in the field parallel to the caxis (Fig. 2) reveals a metamagnetic transition (MT) at 6 T that in conjunction with the temperature dependence of the AC susceptibility and the field-induced evolution of the specific heat corroborates the scenario of an AF ground state of CeCoGe and T N ¼ 5:5 K. The magnetic moment in 14 T yields 1:34mB =f:u: in consistency with the saturatedmoment value expected for the first excited doublet of the Ce3þ ion. This result does not explicitly substantiate the claim of the very reduced Ce ordered moment determined from the neutron diffraction experiment [1]. To test arguments for a Kondo-type screening scenario, we analyzed the temperature dependence of the specific heat measured in absence of magnetic field as a sum of the magnetic (C mag ), phonon (C ph ) and the electron (C e ) contribution, respectively. LaCoGe, the non-magnetic isostructural analog of CeCoGe served as a reference material for the determination of C ph . The magnetic contribution to the specific heat C mag has been obtained by subtracting C ph þ C e ðC e was determined by the
ARTICLE IN PRESS 798
J. Vejpravova´ et al. / Physica B 378–380 (2006) 797–798
1.6
M (µB / f.u.)
1.2
0.8
0.4 B c 0.0
0
4
8
12
µ0H (T) Fig. 2. Magnetization curve at 2 K for field parallel to the c-axis.
Fig. 1. (a) Real (w0 ) and imaginary (w00 ) part of the AC susceptibility. (b) Evolution of the specific heat C p of CeCoGe in magnetic fields parallel to the c-axis. (c) Specific heat C p (open circles) in comparison with the Schottky contribution C Sch (dotted line) and specific heat of LaCoGe C eþph (dashed line). The dash–dot line corresponds to the calculated magnetic entropy Smag , the solid line shows the fit according to Ref. [3].
procedure described lower) from the CeCoGe specific-heat data. The calculated magnetic entropy S mag (dash–dot line) at T N amounts to 4 J mol1 K2 that is somewhat lower than the expected R ln 2 value. In context with the reduced jump of C p at T N (contrary to the expected value for the doublet ground state 12.5 J mol1 K2 obtained from the mean-field approach), Kondo-type interaction with a characteristic temperature T K 6 K might be considered in this compound (for details, see Ref. [3]). However,
neither the reduction of S, nor that of DC p are really strong and are rather caused by short range AF-fluctuation above T N and broadening of the transition. The temperature dependence of the magnetic entropy Smag indicates that the first and the second excited levels are placed well above the ground state. The value of R ln 2 is reached at 16 K and a change of slope of S mag ðTÞ can be observed above 80 K (indicating position of the first excited state consistently with the Smag R ln 4, not shown). The contribution of AF spin waves to C mag was determined within the model proposed in Ref. [4] and points to a gap in the AF spin wave dispersion 7 K together with the Sommerfeld coefficient g ¼ 60 m J mol1 K2 , see Fig. 1c (solid line). We are aware that the model is relevant only well below T N and the temperature interval used for the analysis was actually too narrow to obtain more precise values. In summary, our investigation of the CeCoGe single crystal confirmed the AF ordering below T N ¼ 5:5 K and with MT at 6 T (at 2 K) observed for the field kc. Contrary to the previous results [1] no convincing evidence for the presence of Kondo-type interaction causing reduction of the Ce magnetic moment in CeCoGe has been find out in our specific heat and magnetization data. This work is a part of the research plan MSM 0021620834 financed by the Ministry of Education of the Czech Republic. References [1] [2] [3] [4]
B. Chevalier, et al., Solid State Commun. 130 (2004) 711. J. Welter, et al., J. Alloys Compound 201 (1993) 191. K.D. Schotte, et al., Phys. Lett. A 55 (1975) 38. M.A. Continentino, et al., Phys. Rev. B 64 (2001) 012404.