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Magnetic properties of RAuGe compounds (R=Pr, Nd, Tb–Er)
Journal of Alloys and Compounds 282 (1999) L6–L8
L
Letter
Magnetic properties of RAuGe compounds (R5Pr, Nd, Tb–Er) a a b a, ´ B. Penc , S. Baran , M. Slaski , A. Szytuła * a
b
´ , Poland Institute of Physics, Jagellonian University, Reymonta 4, 30 -059 Krakow School of Physics and Space Research, University of Birmingham, Edgbaston, Birmingham, UK Received 4 August 1998
Abstract Experimental studies of the magnetic susceptibility and magnetization for RAuGe (R5Pr, Nd, Tb–Er) compounds are reported. The measurements reveal that the compounds order antiferromagnetically at 8.8, 7.6, 6.1, 7.6 and 5.7 K for R5Nd, Tb, Dy, Ho and Er, respectively. PrAuGe is a paramagnet down to 4.2 K. The magnetization does not saturate in a magnetic field of H5120 kOe and T54.2 K. 1999 Elsevier Science S.A. All rights reserved. Keywords: Magnetic susceptibility; Magnetization; Rare earth gold germinides; Ternary rare earth intermetallics
1. Introduction Ternary RAuGe (R5La–Nd, Sm, Gd–Eu, Y, Sc) intermetallic compounds crystallize in the hexagonal CaIn 2 type structure with random distribution of Au and Ge atoms [1]. On the other hand, X-ray single crystal data favour the occurrence of the hexagonal LiGaGe-type crystal structure with Au and Ge atoms localized in separate lattice sites [2,3]. The magnetic properties of these compounds strongly depend on the nature of the R element. ScAuGe, LuAuGe and YAuGe are diamagnets [2,3], whereas LaAuGe is a paramagnet [3]. In CeAuGe, ferromagnetic ordering at 10 K was observed, while ´ GdAuGe and YbAuGe are antiferromagnets with Neel temperatures of 16 K [4] and 2.35 K [5], respectively. Here we report on results of the magnetic susceptibility and magnetization measurements of RAuGe (R5Pr, Nd, Tb–Er) compounds.
2. Experimental details The samples (each of mass about 1 g) were prepared by melting of the metals (enclosed in an argon atmosphere) in an arc furnace. The samples were subsequently annealed in vacuum for 1 week at 8008C. The samples thus obtained were examined by powder X-ray diffraction and confirmed to be of a single phase *Corresponding author. Tel.: 148-12-33-6377 / 546; fax: 148-12-3370-86; e-mail: [email protected]
with the hexagonal crystal structure. The indexing of the diffraction lines was verified by computing the intensities using data from Refs. [1–3]. The determined lattice constants are in good agreement with the data published earlier by Rossi et al. [1]. The magnetic susceptibility and magnetization of polycrystalline samples were determined with a Cryogenics S100 SQUID susceptometer that operates up to 1000 Oe and an Oxford Instruments VSM 12 T magnetometer working between 4.2 and 300 K.
3. Results Fig. 1 shows the temperature dependence of the magnetic susceptibility and reciprocal magnetic susceptibility of RAuGe compounds within the temperature range 4.2–300 K. In the low temperature range, for the RAuGe (R5Nd, Tb–Er) compounds, a maximum of an antiferroto paramagnetic transition is observed. The determined ´ temperatures are listed in Table 1. The values of the Neel magnetic susceptibility of PrAuGe has paramagnetic character down to 4.2 K. ´ temperature the reciprocal magnetic Above the Neel susceptibility curves of all compounds obey the Curie– Weiss law: nm 2eff x 5 ]]]] 3k B (T 1 up ) where n is the number of paramagnetic atoms per mol, meff the effective magnetic moment and up the paramagnetic
0925-8388 / 99 / $ – see front matter 1999 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 98 )00843-3
B. Penc et al. / Journal of Alloys and Compounds 282 (1999) L6 –L8
L7
Fig. 1. Magnetic susceptibility and reciprocal magnetic susceptibility (measured in an external field of H510 Oe)–temperature curves for RAuGe compounds: (a) R5Pr, Nd and (b) R5Tb–Er.
Curie temperature. Numerical fits to the experimental data give negative values of the paramagnetic Curie temperatures and show that the effective magnetic moments are near the R31 free ion values (see Table 1). The magnetization curves at different temperatures in external magnetic fields up to 120 kOe are shown in Fig. 2. These curves have no typical character for magnetic materials and do not saturate in a magnetic field of H5120 kOe. The values of the magnetic moments at T54.2 K and H5120 kOe are smaller than the free R31 ion values (see Table 1).
Table 1 Magnetic data for RAuGe (R5Pr, Nd, Tb–Er) compounds R
Pr Nd Tb Dy Ho Er a
T N (K)
8.8 7.6 6.1 7.6 5.7
up (K)
212.4 23.0 216.2 26.1 25.1 25.4
meff (m B )
ms (m B )
Exp.
Theor.
Exp.a
Theor.
4.0 3.95 9.77 10.6 10.1 9.2
3.58 3.62 9.72 10.65 10.61 9.58
1.4 1.8 4.6 8.5 6.9 5.8
3.2 3.27 9.0 10.0 10.0 9.0
Magnetization measurements in H5120 kOe at T54.2 K.
Fig. 2. RAuGe: magnetization versus magnetic field strength function at T54.2 K for (a) R5Pr, Nd and (b) R5Tb–Er in fields up to 120 kOe.
4. Discussion The RAuGe compounds investigated in this paper crystallize in the hexagonal structure which is an ordered variant of the hexagonal CaIn 2 -type [2,3]. The atomic arrangement leads to two different kinds of layers in the unit cell: R layers at z¯0 and 0.5 and Au–Ge layers at z¯0.25 and 0.75. The crystal structure of RAuGe is shown in Fig. 3. The interatomic distance between the rare earths ˚ (a-axis) in the basal plane and |3.7 A ˚ (c / 2) is |4.4 A between the planes. This suggests that the magnetic properties should be strongly anisotropic. The large interatomic distances and metallic character of the electric resistivity [3] indicate that the observed antiferromagnetic ordering at low temperatures results from an indirect coupling via conduction electrons (RKKY model). In the RKKY model, the ordering temperatures are proportional to the de Gennes factor ( gJ 2 1)2 J(J 1 1) [6]. ´ temperature on the rare earth The dependence of the Neel atom R in RAuGe, when viewed in terms of the experimental data and the data resulting from theory, is
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B. Penc et al. / Journal of Alloys and Compounds 282 (1999) L6 –L8
Fig. 3. Crystal structure of RAuGe.
´ temperature is shown in Fig. 4. A maximum of the Neel
observed for GdAuGe, which is in agreement with the de ´ Gennes scaling. The experimental values of the Neel temperature for other compounds are do not agree with the de Gennes function. The present results for light rare earths (Ce, Nd) show that there are additional interactions between the Ce or Nd ions which are responsible for the higher T N values than expected from the de Gennes scaling. Such deviations from the de Gennes scaling are widely encountered in rare earth intermetallic compounds [7]. There are two approaches to explain the phenomena: the 4f electric quadrupole interaction or the Kondo effect [8]. The de Gennes scaling is also not obeyed for heavier rare earths.
Acknowledgements This work was supported by the State Committee for Scientific Research of Poland via grant No. 2 P03B 107 15.
References
Fig. 4. The ordering temperatures of RAuGe compounds (T N ) plotted against the number of 4f electrons localized on the R31 ion. (———) The de Gennes function for RAuGe normalized with respect to GdAuGe.
[1] D. Rossi, R. Marazza, R. Ferro, J. Alloys Comp. 187 (1992) 267. ¨ [2] R. Pottgen, H. Borrmann, C. Felser, O. Jepsen, R. Henn, R.K. Kremer, A. Simon, J. Alloys Comp. 235 (1996) 170. ¨ [3] W. Schnelle, R. Pottgen, R.K. Kremer, E. Gmelin, O. Jepsen, J. Phys. Condensed Matter 9 (1997) 1435. ¨ [4] E.A. Gorlich, Electronic and Magnetic Properties of Ternary Rare ´ Earth Intermetallic Phases, Wydawnictwo Uniwersytetu Jagiellonskiego, Krakow, 1997. ¨ [5] R. Pottgen, H. Borrmann, R.K. Kremer, J. Magn. Magn. Mater. 152 (1996) 196. [6] P.G. de Gennes, J. Phys. Radium 23 (1962) 510, 630. [7] A. Szytuła, J. Leciejewicz, Handbook of Crystal Structure and Magnetic Properties of Rare Earth Intermetallics, CRC Press, Boca Raton, FL, 1994. [8] E.V. Sampathkumaran, Z. Phys. B 92 (1993) 191.
L
Letter
Magnetic properties of RAuGe compounds (R5Pr, Nd, Tb–Er) a a b a, ´ B. Penc , S. Baran , M. Slaski , A. Szytuła * a
b
´ , Poland Institute of Physics, Jagellonian University, Reymonta 4, 30 -059 Krakow School of Physics and Space Research, University of Birmingham, Edgbaston, Birmingham, UK Received 4 August 1998
Abstract Experimental studies of the magnetic susceptibility and magnetization for RAuGe (R5Pr, Nd, Tb–Er) compounds are reported. The measurements reveal that the compounds order antiferromagnetically at 8.8, 7.6, 6.1, 7.6 and 5.7 K for R5Nd, Tb, Dy, Ho and Er, respectively. PrAuGe is a paramagnet down to 4.2 K. The magnetization does not saturate in a magnetic field of H5120 kOe and T54.2 K. 1999 Elsevier Science S.A. All rights reserved. Keywords: Magnetic susceptibility; Magnetization; Rare earth gold germinides; Ternary rare earth intermetallics
1. Introduction Ternary RAuGe (R5La–Nd, Sm, Gd–Eu, Y, Sc) intermetallic compounds crystallize in the hexagonal CaIn 2 type structure with random distribution of Au and Ge atoms [1]. On the other hand, X-ray single crystal data favour the occurrence of the hexagonal LiGaGe-type crystal structure with Au and Ge atoms localized in separate lattice sites [2,3]. The magnetic properties of these compounds strongly depend on the nature of the R element. ScAuGe, LuAuGe and YAuGe are diamagnets [2,3], whereas LaAuGe is a paramagnet [3]. In CeAuGe, ferromagnetic ordering at 10 K was observed, while ´ GdAuGe and YbAuGe are antiferromagnets with Neel temperatures of 16 K [4] and 2.35 K [5], respectively. Here we report on results of the magnetic susceptibility and magnetization measurements of RAuGe (R5Pr, Nd, Tb–Er) compounds.
2. Experimental details The samples (each of mass about 1 g) were prepared by melting of the metals (enclosed in an argon atmosphere) in an arc furnace. The samples were subsequently annealed in vacuum for 1 week at 8008C. The samples thus obtained were examined by powder X-ray diffraction and confirmed to be of a single phase *Corresponding author. Tel.: 148-12-33-6377 / 546; fax: 148-12-3370-86; e-mail: [email protected]
with the hexagonal crystal structure. The indexing of the diffraction lines was verified by computing the intensities using data from Refs. [1–3]. The determined lattice constants are in good agreement with the data published earlier by Rossi et al. [1]. The magnetic susceptibility and magnetization of polycrystalline samples were determined with a Cryogenics S100 SQUID susceptometer that operates up to 1000 Oe and an Oxford Instruments VSM 12 T magnetometer working between 4.2 and 300 K.
3. Results Fig. 1 shows the temperature dependence of the magnetic susceptibility and reciprocal magnetic susceptibility of RAuGe compounds within the temperature range 4.2–300 K. In the low temperature range, for the RAuGe (R5Nd, Tb–Er) compounds, a maximum of an antiferroto paramagnetic transition is observed. The determined ´ temperatures are listed in Table 1. The values of the Neel magnetic susceptibility of PrAuGe has paramagnetic character down to 4.2 K. ´ temperature the reciprocal magnetic Above the Neel susceptibility curves of all compounds obey the Curie– Weiss law: nm 2eff x 5 ]]]] 3k B (T 1 up ) where n is the number of paramagnetic atoms per mol, meff the effective magnetic moment and up the paramagnetic
0925-8388 / 99 / $ – see front matter 1999 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 98 )00843-3
B. Penc et al. / Journal of Alloys and Compounds 282 (1999) L6 –L8
L7
Fig. 1. Magnetic susceptibility and reciprocal magnetic susceptibility (measured in an external field of H510 Oe)–temperature curves for RAuGe compounds: (a) R5Pr, Nd and (b) R5Tb–Er.
Curie temperature. Numerical fits to the experimental data give negative values of the paramagnetic Curie temperatures and show that the effective magnetic moments are near the R31 free ion values (see Table 1). The magnetization curves at different temperatures in external magnetic fields up to 120 kOe are shown in Fig. 2. These curves have no typical character for magnetic materials and do not saturate in a magnetic field of H5120 kOe. The values of the magnetic moments at T54.2 K and H5120 kOe are smaller than the free R31 ion values (see Table 1).
Table 1 Magnetic data for RAuGe (R5Pr, Nd, Tb–Er) compounds R
Pr Nd Tb Dy Ho Er a
T N (K)
8.8 7.6 6.1 7.6 5.7
up (K)
212.4 23.0 216.2 26.1 25.1 25.4
meff (m B )
ms (m B )
Exp.
Theor.
Exp.a
Theor.
4.0 3.95 9.77 10.6 10.1 9.2
3.58 3.62 9.72 10.65 10.61 9.58
1.4 1.8 4.6 8.5 6.9 5.8
3.2 3.27 9.0 10.0 10.0 9.0
Magnetization measurements in H5120 kOe at T54.2 K.
Fig. 2. RAuGe: magnetization versus magnetic field strength function at T54.2 K for (a) R5Pr, Nd and (b) R5Tb–Er in fields up to 120 kOe.
4. Discussion The RAuGe compounds investigated in this paper crystallize in the hexagonal structure which is an ordered variant of the hexagonal CaIn 2 -type [2,3]. The atomic arrangement leads to two different kinds of layers in the unit cell: R layers at z¯0 and 0.5 and Au–Ge layers at z¯0.25 and 0.75. The crystal structure of RAuGe is shown in Fig. 3. The interatomic distance between the rare earths ˚ (a-axis) in the basal plane and |3.7 A ˚ (c / 2) is |4.4 A between the planes. This suggests that the magnetic properties should be strongly anisotropic. The large interatomic distances and metallic character of the electric resistivity [3] indicate that the observed antiferromagnetic ordering at low temperatures results from an indirect coupling via conduction electrons (RKKY model). In the RKKY model, the ordering temperatures are proportional to the de Gennes factor ( gJ 2 1)2 J(J 1 1) [6]. ´ temperature on the rare earth The dependence of the Neel atom R in RAuGe, when viewed in terms of the experimental data and the data resulting from theory, is
L8
B. Penc et al. / Journal of Alloys and Compounds 282 (1999) L6 –L8
Fig. 3. Crystal structure of RAuGe.
´ temperature is shown in Fig. 4. A maximum of the Neel
observed for GdAuGe, which is in agreement with the de ´ Gennes scaling. The experimental values of the Neel temperature for other compounds are do not agree with the de Gennes function. The present results for light rare earths (Ce, Nd) show that there are additional interactions between the Ce or Nd ions which are responsible for the higher T N values than expected from the de Gennes scaling. Such deviations from the de Gennes scaling are widely encountered in rare earth intermetallic compounds [7]. There are two approaches to explain the phenomena: the 4f electric quadrupole interaction or the Kondo effect [8]. The de Gennes scaling is also not obeyed for heavier rare earths.
Acknowledgements This work was supported by the State Committee for Scientific Research of Poland via grant No. 2 P03B 107 15.
References
Fig. 4. The ordering temperatures of RAuGe compounds (T N ) plotted against the number of 4f electrons localized on the R31 ion. (———) The de Gennes function for RAuGe normalized with respect to GdAuGe.
[1] D. Rossi, R. Marazza, R. Ferro, J. Alloys Comp. 187 (1992) 267. ¨ [2] R. Pottgen, H. Borrmann, C. Felser, O. Jepsen, R. Henn, R.K. Kremer, A. Simon, J. Alloys Comp. 235 (1996) 170. ¨ [3] W. Schnelle, R. Pottgen, R.K. Kremer, E. Gmelin, O. Jepsen, J. Phys. Condensed Matter 9 (1997) 1435. ¨ [4] E.A. Gorlich, Electronic and Magnetic Properties of Ternary Rare ´ Earth Intermetallic Phases, Wydawnictwo Uniwersytetu Jagiellonskiego, Krakow, 1997. ¨ [5] R. Pottgen, H. Borrmann, R.K. Kremer, J. Magn. Magn. Mater. 152 (1996) 196. [6] P.G. de Gennes, J. Phys. Radium 23 (1962) 510, 630. [7] A. Szytuła, J. Leciejewicz, Handbook of Crystal Structure and Magnetic Properties of Rare Earth Intermetallics, CRC Press, Boca Raton, FL, 1994. [8] E.V. Sampathkumaran, Z. Phys. B 92 (1993) 191.