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Magnetic properties of single crystal EuCo2Ge2
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 264 (2003) 142–145
Magnetic properties of single crystal EuCo2Ge2 Z. Hossain*, C. Geibel Max-Planck Institute for Chemical Physics of Solids, Materials Development Group, Nothnitzer Str. 40, 01187 Dresden, Germany . Received 20 November 2002; received in revised form 21 January 2003
Abstract We present resistivity, magnetization, specific heat and magnetoresistance data on single crystals of EuCo2Ge2 which undergo antiferromagnetic transition below TN ¼ 23 K. Linear magnetization and positive magnetoresistance for low field in the ordered state are consistent with the antiferromagnetic nature of the magnetic ordering. The compound exhibits metamagnetic transition. In contrast to the previous report on polycrystalline sample (J. Phys. Chem. Solids 39 (1978) 767), our findings suggest no moment on Co. r 2003 Elsevier Science B.V. All rights reserved. PACS: 75.20.En; 75.30.Kz; 75.50.Ee Keywords: Eu-compound; Antiferromagnetism; Metamagnetic transition; magnetic moment
1. Introduction RCo2Ge2 (R=rare earth) series of compounds have interesting magnetic properties, for example CeCo2Ge2 shows valence fluctuation, YbCo2Ge2 exhibits heavy fermion behavior, TbCo2Ge2 undergoes commensurate–incommensurate magnetic phase transition [1–3]. Except EuCo2Ge2, Co was reported to be nonmagnetic in all the other RCo2Ge2 compounds. From magnetic susceptibility measurement Felner et al. claimed a substantial moment on Co [4]. Even though Co was found to be magnetic in some Co-based intermetallic compounds such as RCo3B2, RCo2 [5,6]; since none of the RT2X2 (T=transition element, X=Si, Ge) except those with T ¼ Mn; has moment on the *Corresponding author. Tel.: +49-351-4646-2249; fax: +49351-4646-2262. E-mail address: [email protected] (Z. Hossain).
transition element, this was a surprising result. An interesting case of appearance of moment on Co driven by Eu-valence change was found in EuCo2P2 under externally applied pressure [7]. Polycrystalline EuCo2Ge2 was investigated before by means of magnetic susceptibility and . Mossbauer spectroscopy [4]. Europium ions were found to be in stable divalent state which orders magnetically below 23 K. Here we present the resistivity, magnetic susceptibility, specific heat and magnetoresistance results on single crystals of EuCo2Ge2.
2. Experimental We have grown single crystals of EuCo2Ge2 using self-flux method [8]. The single crystals formed as platelets with c-axis perpendicular to
0304-8853/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0304-8853(03)00177-X
ARTICLE IN PRESS Z. Hossain, C. Geibel / Journal of Magnetism and Magnetic Materials 264 (2003) 142–145
the plane of the plate like crystals. The single crystalline nature and the orientation of the crystals were checked with Laue method; the phase purity and lattice parameters were obtained using powder X-ray diffraction of crushed single crystals. Magnetization measurements were carried out using a commercial superconducting quantum interference device magnetometer. Because the demagnetizing field is very small in comparison to the externally applied field, demagnetization correction was not applied to the magnetic susceptibility and magnetization data presented here. The error involved in not considering the demagnetization factor is estimated to be less than 1%. Resistivity, magnetoresistance and heat capacity measurements were carried out using AC transport and heat capacity options of a commercial physical property measurement system (quantum design).
3. Results and discussions Powder X-ray diffraction pattern of crushed single crystals revealed that the sample was a single phase which forms in ThCr2Si2-type tetragonal ( structure with lattice parameters a ¼ 4:0322ð6Þ A ( The lattice parameters of our and c ¼ 10:447ð3Þ A. ( and polycrystalline samples are a ¼ 4:0330ð5Þ A ( which are in close agreement c ¼ 10:4503ð21Þ A with those reported for the polycrystalline samples in Ref. [4]. The lattice volume of the flux grown single crystal is nearly similar to the polycrystalline samples. The magnetic susceptibility, measured in a field of 50 kG, follow Curie–Weiss behavior in the temperature range 100–300 K with effective magnetic moment meff ¼ 7:81 mB, Weiss temperature yP ¼ 8:4 K for HJc and meff ¼ 7:93 mB, yP ¼ 11:5 K for HJab: The effective moment is very close to the value expected (7.94 mB) for divalent Eu2+ ions. In contrast to Ref. [4] we do not find any evidence of moment on Co in our single crystal. The susceptibility measurements on our single-phase polycrystalline sample also did not show any evidence of moment on Co. The susceptibility data measured in a field of 1 kG exhibit a peak at 23 K for HJab (Fig. 1) which is characteristic of an
143
Fig. 1. Temperature dependent magnetic susceptibility (w) as a function of temperature for EuCo2Ge2 measured in a field of 1 kG.
antiferromagnetic transition. For HJc the susceptibility data show a tendency to saturation. Similar behavior of susceptibility was found in EuNi2Ge2 [9]. In order to get a better understanding of the low temperature magnetic properties, magnetization (M) versus field (H) data were collected at 2 and 35 K. The results of the M vs. H for HJab and HJc are presented in Fig. 2. The M vs. H for HJab shows linear dependence at low field region and a field induced metamagnetic transition at higher field. The magnetization at 50 kG correspond to B5.5 mB/Eu. For HJc the M vs. H does not show any anomaly. Nearly linear behavior of magnetization for low field and the presence of metamagnetic transition in the ordered state are consistent with antiferromagnetic nature of the magnetic transition. The specific heat shows a large anomaly (B16 J/ mol K) with a peak at 22.5 K associated with the magnetic transition (Fig. 3). The magnetic entropy just above TN (at 25 K) is B0.9R ln 8 which is close to the expected R ln 8 corresponding to J ¼ 7=2 for divalent Eu ions. The in-plane resistivity shows metallic behavior. The room temperature resistivity is B100 mO cm and the residual resistivity ratio (RRR) is 3.5. The resistivity shows a large drop below TN due to loss of spin disorder scattering. Since the compound exhibits metamagnetic transitions for HJab; the magnetoresistance was measured as a function of
ARTICLE IN PRESS 144
Z. Hossain, C. Geibel / Journal of Magnetism and Magnetic Materials 264 (2003) 142–145
Fig. 4. Zero-field in-plane resistivity of EuCo2Ge2.
Fig. 2. Magnetization (M) versus applied field (H) of EuCo2Ge2 at 2 and 35 K.
Fig. 5. Normalized longitudinal EuCo2Ge2 at 2 and 35 K.
Fig. 3. Specific heat of EuCo2Ge2 as a function of temperature. Large anomaly due to magnetic transition is seen at low temperature.
field (IJHJab) at constant temperature above and below TN : In the ordered state the resistivity increases as the field is increased up to 27 kG above which it starts to decrease. The field at
magnetoresistance
of
which resistivity is maximum corresponds well with the field at which metamagnetic transition is observed in the magnetization data. In the paramagnetic state the magnetoresistance is negative (4% at T ¼ 35 K and H ¼ 50 kG) which is possibly due to reduction of spin disorder scattering on application of external magnetic field as in the case HoNi2B2C [10] (Figs. 4 and 5). Since Eu in EuCo2Si2 is trivalent and nonmagnetic, sufficiently high pressure would suppress the magnetic transition in EuCo2Ge2 and drive the system to the valence fluctuating state. We plan to investigate the pressure response on magnetism of this compound in future.
ARTICLE IN PRESS Z. Hossain, C. Geibel / Journal of Magnetism and Magnetic Materials 264 (2003) 142–145
4. Conclusions The resistivity, magnetoresistance, magnetization and specific heat data provide conclusive evidence for the presence of antiferromagnetic transitions below 23 K in EuCo2Ge2. The Ne! el temperature is same as that of the previously reported polycrystalline sample. Magnetization versus field isotherms in ab plane reveal the metamagnetic transition. There is no moment on Co in EuCo2Ge2.
References [1] H. Fujii, E. Ueda, Y. Uwatoko, T. Shigeoka, J. Magn. Magn. Mater. 76–77 (1988) 179.
145
[2] O. Trovarelli, C. Geibel, M. Grosche, R. Schleser, R. Borth, G. Sparn, F. Steglich, Physica B 259–261 (1999) 140. [3] B. Penc, M. Hofmann, J. Leciejwicz, A. Szytu"a, J. Phys.: Condens. Matter 11 (1999) 7579. [4] I. Felner, I. Nowik, J. Phys. Chem. Solids 39 (1978) 767. [5] H. Ido, O. Nashima, T. Ito, T. Kaneda, Y. Saito, J. Appl. Phys. 85 (1999) 4865. [6] N.H. Duc, T.D. Hien, P.E. Brommer, J.J.M. Franse, J. Magn. Magn. Mater. 104–107 (1992) 1252. [7] M. Chefki, M.M. Abd-Elmeguid, H. Micklitz, C. Huhnt, W. Schlabitz, Phys. Rev. Lett. 80 (1998) 802. [8] P.C. Canfield, Z. Fisk, Philos. Mag. B 65 (1992) 1117. [9] S.L. Bu.ıko, Z. Islam, T.A. Wiener, I.R. Fisher, A.H. Lacerda, P.C. Canfield, J. Magn. Magn. Mater. 205 (1999) 53. [10] I.R. Fisher, J.R. Cooper, P.C. Canfield, Phys. Rev. B 56 (1997) 10820.
Journal of Magnetism and Magnetic Materials 264 (2003) 142–145
Magnetic properties of single crystal EuCo2Ge2 Z. Hossain*, C. Geibel Max-Planck Institute for Chemical Physics of Solids, Materials Development Group, Nothnitzer Str. 40, 01187 Dresden, Germany . Received 20 November 2002; received in revised form 21 January 2003
Abstract We present resistivity, magnetization, specific heat and magnetoresistance data on single crystals of EuCo2Ge2 which undergo antiferromagnetic transition below TN ¼ 23 K. Linear magnetization and positive magnetoresistance for low field in the ordered state are consistent with the antiferromagnetic nature of the magnetic ordering. The compound exhibits metamagnetic transition. In contrast to the previous report on polycrystalline sample (J. Phys. Chem. Solids 39 (1978) 767), our findings suggest no moment on Co. r 2003 Elsevier Science B.V. All rights reserved. PACS: 75.20.En; 75.30.Kz; 75.50.Ee Keywords: Eu-compound; Antiferromagnetism; Metamagnetic transition; magnetic moment
1. Introduction RCo2Ge2 (R=rare earth) series of compounds have interesting magnetic properties, for example CeCo2Ge2 shows valence fluctuation, YbCo2Ge2 exhibits heavy fermion behavior, TbCo2Ge2 undergoes commensurate–incommensurate magnetic phase transition [1–3]. Except EuCo2Ge2, Co was reported to be nonmagnetic in all the other RCo2Ge2 compounds. From magnetic susceptibility measurement Felner et al. claimed a substantial moment on Co [4]. Even though Co was found to be magnetic in some Co-based intermetallic compounds such as RCo3B2, RCo2 [5,6]; since none of the RT2X2 (T=transition element, X=Si, Ge) except those with T ¼ Mn; has moment on the *Corresponding author. Tel.: +49-351-4646-2249; fax: +49351-4646-2262. E-mail address: [email protected] (Z. Hossain).
transition element, this was a surprising result. An interesting case of appearance of moment on Co driven by Eu-valence change was found in EuCo2P2 under externally applied pressure [7]. Polycrystalline EuCo2Ge2 was investigated before by means of magnetic susceptibility and . Mossbauer spectroscopy [4]. Europium ions were found to be in stable divalent state which orders magnetically below 23 K. Here we present the resistivity, magnetic susceptibility, specific heat and magnetoresistance results on single crystals of EuCo2Ge2.
2. Experimental We have grown single crystals of EuCo2Ge2 using self-flux method [8]. The single crystals formed as platelets with c-axis perpendicular to
0304-8853/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0304-8853(03)00177-X
ARTICLE IN PRESS Z. Hossain, C. Geibel / Journal of Magnetism and Magnetic Materials 264 (2003) 142–145
the plane of the plate like crystals. The single crystalline nature and the orientation of the crystals were checked with Laue method; the phase purity and lattice parameters were obtained using powder X-ray diffraction of crushed single crystals. Magnetization measurements were carried out using a commercial superconducting quantum interference device magnetometer. Because the demagnetizing field is very small in comparison to the externally applied field, demagnetization correction was not applied to the magnetic susceptibility and magnetization data presented here. The error involved in not considering the demagnetization factor is estimated to be less than 1%. Resistivity, magnetoresistance and heat capacity measurements were carried out using AC transport and heat capacity options of a commercial physical property measurement system (quantum design).
3. Results and discussions Powder X-ray diffraction pattern of crushed single crystals revealed that the sample was a single phase which forms in ThCr2Si2-type tetragonal ( structure with lattice parameters a ¼ 4:0322ð6Þ A ( The lattice parameters of our and c ¼ 10:447ð3Þ A. ( and polycrystalline samples are a ¼ 4:0330ð5Þ A ( which are in close agreement c ¼ 10:4503ð21Þ A with those reported for the polycrystalline samples in Ref. [4]. The lattice volume of the flux grown single crystal is nearly similar to the polycrystalline samples. The magnetic susceptibility, measured in a field of 50 kG, follow Curie–Weiss behavior in the temperature range 100–300 K with effective magnetic moment meff ¼ 7:81 mB, Weiss temperature yP ¼ 8:4 K for HJc and meff ¼ 7:93 mB, yP ¼ 11:5 K for HJab: The effective moment is very close to the value expected (7.94 mB) for divalent Eu2+ ions. In contrast to Ref. [4] we do not find any evidence of moment on Co in our single crystal. The susceptibility measurements on our single-phase polycrystalline sample also did not show any evidence of moment on Co. The susceptibility data measured in a field of 1 kG exhibit a peak at 23 K for HJab (Fig. 1) which is characteristic of an
143
Fig. 1. Temperature dependent magnetic susceptibility (w) as a function of temperature for EuCo2Ge2 measured in a field of 1 kG.
antiferromagnetic transition. For HJc the susceptibility data show a tendency to saturation. Similar behavior of susceptibility was found in EuNi2Ge2 [9]. In order to get a better understanding of the low temperature magnetic properties, magnetization (M) versus field (H) data were collected at 2 and 35 K. The results of the M vs. H for HJab and HJc are presented in Fig. 2. The M vs. H for HJab shows linear dependence at low field region and a field induced metamagnetic transition at higher field. The magnetization at 50 kG correspond to B5.5 mB/Eu. For HJc the M vs. H does not show any anomaly. Nearly linear behavior of magnetization for low field and the presence of metamagnetic transition in the ordered state are consistent with antiferromagnetic nature of the magnetic transition. The specific heat shows a large anomaly (B16 J/ mol K) with a peak at 22.5 K associated with the magnetic transition (Fig. 3). The magnetic entropy just above TN (at 25 K) is B0.9R ln 8 which is close to the expected R ln 8 corresponding to J ¼ 7=2 for divalent Eu ions. The in-plane resistivity shows metallic behavior. The room temperature resistivity is B100 mO cm and the residual resistivity ratio (RRR) is 3.5. The resistivity shows a large drop below TN due to loss of spin disorder scattering. Since the compound exhibits metamagnetic transitions for HJab; the magnetoresistance was measured as a function of
ARTICLE IN PRESS 144
Z. Hossain, C. Geibel / Journal of Magnetism and Magnetic Materials 264 (2003) 142–145
Fig. 4. Zero-field in-plane resistivity of EuCo2Ge2.
Fig. 2. Magnetization (M) versus applied field (H) of EuCo2Ge2 at 2 and 35 K.
Fig. 5. Normalized longitudinal EuCo2Ge2 at 2 and 35 K.
Fig. 3. Specific heat of EuCo2Ge2 as a function of temperature. Large anomaly due to magnetic transition is seen at low temperature.
field (IJHJab) at constant temperature above and below TN : In the ordered state the resistivity increases as the field is increased up to 27 kG above which it starts to decrease. The field at
magnetoresistance
of
which resistivity is maximum corresponds well with the field at which metamagnetic transition is observed in the magnetization data. In the paramagnetic state the magnetoresistance is negative (4% at T ¼ 35 K and H ¼ 50 kG) which is possibly due to reduction of spin disorder scattering on application of external magnetic field as in the case HoNi2B2C [10] (Figs. 4 and 5). Since Eu in EuCo2Si2 is trivalent and nonmagnetic, sufficiently high pressure would suppress the magnetic transition in EuCo2Ge2 and drive the system to the valence fluctuating state. We plan to investigate the pressure response on magnetism of this compound in future.
ARTICLE IN PRESS Z. Hossain, C. Geibel / Journal of Magnetism and Magnetic Materials 264 (2003) 142–145
4. Conclusions The resistivity, magnetoresistance, magnetization and specific heat data provide conclusive evidence for the presence of antiferromagnetic transitions below 23 K in EuCo2Ge2. The Ne! el temperature is same as that of the previously reported polycrystalline sample. Magnetization versus field isotherms in ab plane reveal the metamagnetic transition. There is no moment on Co in EuCo2Ge2.
References [1] H. Fujii, E. Ueda, Y. Uwatoko, T. Shigeoka, J. Magn. Magn. Mater. 76–77 (1988) 179.
145
[2] O. Trovarelli, C. Geibel, M. Grosche, R. Schleser, R. Borth, G. Sparn, F. Steglich, Physica B 259–261 (1999) 140. [3] B. Penc, M. Hofmann, J. Leciejwicz, A. Szytu"a, J. Phys.: Condens. Matter 11 (1999) 7579. [4] I. Felner, I. Nowik, J. Phys. Chem. Solids 39 (1978) 767. [5] H. Ido, O. Nashima, T. Ito, T. Kaneda, Y. Saito, J. Appl. Phys. 85 (1999) 4865. [6] N.H. Duc, T.D. Hien, P.E. Brommer, J.J.M. Franse, J. Magn. Magn. Mater. 104–107 (1992) 1252. [7] M. Chefki, M.M. Abd-Elmeguid, H. Micklitz, C. Huhnt, W. Schlabitz, Phys. Rev. Lett. 80 (1998) 802. [8] P.C. Canfield, Z. Fisk, Philos. Mag. B 65 (1992) 1117. [9] S.L. Bu.ıko, Z. Islam, T.A. Wiener, I.R. Fisher, A.H. Lacerda, P.C. Canfield, J. Magn. Magn. Mater. 205 (1999) 53. [10] I.R. Fisher, J.R. Cooper, P.C. Canfield, Phys. Rev. B 56 (1997) 10820.