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Gd7T3 (T=Rh, Pd) intermetallics crystal growth
ARTICLE IN PRESS
Journal of Crystal Growth 283 (2005) 547–552 www.elsevier.com/locate/jcrysgro
Gd7T3 (T ¼ Rh, Pd) intermetallics crystal growth E. Talika,, M. Klimczaka, A. Winiarskia, R. Troc´b b
a Institute of Physics, University of Silesia, Uniwersytecka 4, 40-007 Katowice, Poland Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Oko´lna 2, 50-422 Wroc!aw, Poland
Received 28 April 2005; accepted 14 June 2005 Available online 5 August 2005 Communicated by P. Rudolph
Abstract Single crystals of intermetallic compounds Gd7Pd3 and Gd7Rh3 were grown by the Czochralski method from a levitated melt. The X-ray Berg–Barrett topography confirmed good quality of the obtained crystals. They grow in a single Th7Fe3-type hexagonal phase. Magnetic susceptibility measurements of Gd7Pd3 revealed a strong anisotropy in the examined compound. r 2005 Elsevier B.V. All rights reserved. PACS: 71.20.Lp; 61.10.Nz Keywords: A1. X-ray diffraction; A1. X-ray topography; A2. Czochralski method; A2. Growth from melt; A2. Single crystal growth; B1. Rare earth compounds
1. Introduction Recently, magnetic properties of the polycrystalline R7Pd3 (R ¼ light rare earth elements) intermetallic compounds were investigated [1]. These compounds are interesting due to their complex magnetic behaviour. In Ref. [1] they were obtained in polycrystalline form by melting of the constituent elements of 99.9% purity for rare earth and 99.95% purity for Pd under argon atmosphere in Corresponding author. Tel.: +48 32 359 1187;
fax: +48 32 2588 431. E-mail address: [email protected] (E. Talik).
an arc furnace. After melting, the samples were rapidly cooled. Then the ingots were sealed in an evacuated quartz tube at 600 1C for a week to homogenization. During this step the samples crystallized in the hexagonal Th7Fe3 type with space group P63mc. For La7Pd3 the temperature dependence of the electrical resistivity is anomalous and exhibits a negative curvature, with a tendency to saturation at higher temperatures. This material has a superconducting transition at 6 K. Ce7Pd3 has an antiferromagnetic transition at 5 K, while Pr7Pd3 has such transition at only 1.7 K. Nd7Pd3 shows two characteristic temperatures: the first one at 33 K related to a ferromagnetic–
0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.06.020
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E. Talik et al. / Journal of Crystal Growth 283 (2005) 547–552
antiferromagnetic transition and the second at 38 K related to an antiferromagnetic–paramagnetic transition. Sm7Pd3 exhibits two sharp peaks: the first one is connected with an antiferromagnetic–ferrimagnetic transition at 18.4 K and the second one at 167 K is connected with a transition from ferrimagnetic state to paramagnetic. All measured properties are sensitive to pressure. Magnetocaloric properties of polycrystalline Gd7Pd3 were investigated by Canepa et al. [2]. They found the ferromagnetic transition for Gd7Pd3 at 323 K. The sample was prepared by melting of the constituents Gd (99.9% purity) and Pd (99.95% purity) in stoichiometric amounts pressed in the form of a pellet in a high-frequency induction furnace on a water-cooled tantalum heart under argon atmosphere. The addition of a second phase was less than 3%. The refrigerant capacity was estimated as 100 J/kg at an applied field of 2 T and as 380 J/kg at 5 T. These samples crystallized in the hexagonal Th7Fe3 type. The lattice parameters were a ¼ 9.985 A˚ and c ¼ 6.272 A˚. Earlier, Moreau and Parthe [3] found lattice parameters of Gd7Pd3 to be a ¼ 10.0008 A˚ and c ¼ 6.2873 A˚. Such a small discrepancy of the lattice parameters may be connected with purity of the samples, stoichiometry deviation or procedure of calculation of the lattice parameters. Giant magnetoresistance in the magnetically ordered state and in a relatively high temperature above 77 K for the Gd7Rh3 compound was recently described by Sengupta et al. [4]. This sample was polycrystalline and obtained by arcmelting of stoichiometric amounts of high-purity (499.9%) elements in an inert atmosphere. The X-ray diffraction confirmed the single phase state. Gd7Rh3 crystallizes in the hexagonal Th7Fe3-type structure. The aim of this work is to grow good quality Gd7Pd3 (T ¼ Rh, Pd) single crystals by the Czochralski growth from a levitating melt for measurements of the electronic and crystal structure, magnetic and transport properties in order to investigate a role of hybridization between the Gd 5d states and d band of transition metal [5]. This crucibleless method allows growing pure single crystals of variety intermetallics, even using very reactive starting materials.
2. Experimental procedure The single crystals were grown from a small amount (about 1 g) of the starting materials. The starting materials Gd (99.9% purity), Pd (Specpure Johnson Matthey Chemical Ltd.) or Rh (99.99% purity) which were melted together on a water-cooled eight-segment conical coil with diameter 14 mm in stoichiometric amounts under very pure argon atmosphere in a Czochralski apparatus
Fig. 1. Sketch of the Czochralski apparatus: (1) levitation coil, (1a) mica plates, (1b) a segment of the levitation coil, (2) RF induction coil, (3) silica tube, (4) phototransistor, (5) molybdenum spike.
ARTICLE IN PRESS E. Talik et al. / Journal of Crystal Growth 283 (2005) 547–552
[6] (Fig. 1). The obtained ingots were remelted several times to homogenize them. The RF electromagnetic field causes a levitation of metal, which conducts the electric current much better than oxides collected on a surface of the bottom part of a melt drop. Moreover, the strong stirring of the melt ensured homogenization during the growth process. The crystal growth process was started by immersion of a molybdenum spike in the melt. The crystal is pulled from the pure area of the top of the melt drop. After necking the crystal was pulled out with a constant velocity less than 0.1 mm/min. Such a growth rate was adapted to the growth of thin single crystals. No significant evaporation of the melt during the crystal growth was observed and a planar growth front was maintained by observation with an eyepiece. The obtained crystals were cylindrical with a diameter of about 2 mm and a length of 60 mm. The phase diagrams show a peritectic formation of the Gd7Pd3 and Gd7Rh3 compounds [7]. In case the separation between liquidus and peritectic line is narrow enough, crystallization of the required compound is observed. Several growth processes were performed by changing the growth rate. Better quality of the crystals, without growth patterns, was for the rate of order 0.05 mm/min. The real structure of the as-grown crystal was examined by the Berg–Barrett topography using Fe Ka radiation. The sensitivity of the method to the misorientation is 10 . X-ray reflections from (h k l) planes were recorded in the range of 2Y equal to 70–1001 on Agfa-Gevaert Structurix D2 film, which was placed close to the crystal surface. The distance between the crystal and the film was about 1 mm to obtain a good pattern. These examinations were performed to check the quality of the obtained single crystals. The spatial resolution was 10 mm. Some fragments of the crystals were pulverized and identified by X-ray powder diffraction with Cu Ka radiation using a Siemens D-5000 diffractometer. The XPS spectra of the Gd7T3 single crystals were measured with monochromatized Al Ka radiation (1486.6 eV) at room temperature using a PHI 5700/660 ESCA spectrometer. The energy spectra of the electrons were determined by a
549
hemispherical mirror analyser with an energy resolution of about 0.3 eV. Magnetic measurements were performed within temperature range 2–400 K in magnetic field 0.1 T using a Quantum Design SQUID magnetometer in Wroc"aw.
3. Results and discussion The Berg–Barrett X-ray topography of Gd7Pd3 and Gd7Rh3 shows that the investigated crystals grew without mosaic structure (Figs. 2, 3). The powder diffraction spectra show that the single crystals grow in the hexagonal Th7Fe3 phase (Figs. 4, 5). The spectra show the Th7Fe3 phase only. Using the Powder Cell 2.3 program published by Kraus and Nolze (BAM Berlin) [8], the spectra
Fig. 2. Single crystal of Gd7Pd3: (a) as-grown photo, (b) Berg–Barrett topography of the crystal fragment.
Fig. 3. Single crystal of Gd7Rh3: (a) as-grown photo, (b) Berg–Barrett topography of the fragment of the crystal.
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Fig. 4. X-ray powder diffraction spectrum of Gd7Pd3 (above) compared with the calculated one (below).
Fig. 6. XPS spectrum of a Gd7Pd3 single crystal in the energy range 0–1400 eV.
Fig. 5. X-ray powder diffraction spectrum of Gd7Rh3 (above) compared with calculated one (below).
the intensity of the (2 0 2) line is much higher than the calculated one (Fig. 5). The XPS spectrum of the Gd7Pd3 single crystal broken under UHV conditions and measured in a wide energy range shows no contaminations with carbon and oxygen, which for polycrystalline samples have been often occurring (Fig. 6). Moreover, using the XPS spectra, the stoichiometry was checked. It was in agreement with the nominal one and stable in different crystal fragments, as well as for crystals grown in different growth processes. Recently, wide investigations of the magnetic properties and temperature dependences of the lattice parameters of Gd7Pd3 single crystals were performed [5]. A strongly anisotropic behaviour of the magnetic properties was observed. Fig. 7 shows the magnetic susceptibility of the Gd7Pd3 single crystal measured along the principal directions. Along the a-axis an increase of the magnetic susceptibility with decreasing temperature is observed. Along the c-axis there is an insignificant increase of the magnetic susceptibility with decreasing temperature, indicating a much stronger non-collinear coupling along this direction. The results of magnetic measurements show any transition connected with gadolinium precipitation. Such a transition was observed for the eutectic Gd–Gd7Pd3 [9]. The width of transition is 15 K, being similar to those for polycrystalline
were calculated to compare them with the experimental ones. The calculated parameters of Gd7Pd3 were a ¼ 9.988 A˚ and c ¼ 6.288 A˚ for polycrystalline sample and a ¼ 9.9807 A˚ and c ¼ 6.2790 A˚ [5] for a single crystal. The measured reflections from the (h h 0) planes were a bit more intensive than the calculated ones (Fig. 4). It relates to partly selfarrangement of magnetic powder grains. The lattice parameters were also calculated from the Gd7Rh3 powder X-ray spectra to be a ¼ 9.868 A˚ and c ¼ 6.208 A˚. A partial powder grain orientation is visible in the spectra because
ARTICLE IN PRESS E. Talik et al. / Journal of Crystal Growth 283 (2005) 547–552 100
Gd7Pd3
80 χ (emu/mole)
H // a
H = 0.1 T
60
40
TC = 334 K H // c
20
0 0
100
200 T (K)
300
400
Fig. 7. Magnetic susceptibility of a Gd7Pd3 crystal along the principal directions.
samples measured in a magnetic field of 0.05 T as was shown in Ref. [2]. The difference in the value of the transition temperature may be related to deviation from stoichiometry or purity of the sample. Such complicated magnetic behaviour of the Gd7T3 compounds is due to the geometrical frustration in the triangular net formed by the gadolinium ions in the Th7Fe3 structure with three non-equivalent Gd sites. The gadolinium ions form tetrahedra stacked in chains along the c-axis. The strong molecular field produced by the gadolinium sublattice may polarize the palladium d band which is hybridized with the gadolinium 5d states. A similar situation took place for the RMn2 and R3T compounds. The XPS electronic structure measurements using the single crystals of the above-mentioned compounds confirmed such a model [10–12]. The hybridization leads to the strong coupling between sublattices and complicated magnetic structures. Recently, a very similar complicated magnetic behaviour due to frustration effect was observed for GdPdAl and GdPdNi too [13–15]. GdNiAl and GdPdAl belong to the ternary intermetallics with unstable crystal structure. These crystals, grown by the same technique, crystallized in the hexagonal ZrNiAl-type structure. The thermal variation of the lattice parameters, electrical resistivity and magnetic susceptibility gave the evidence of an isostructural phase transition from a high-temperature phase
551
modification HTM I to a low-temperature phase modification HTM II at 180 K. The jump, related to this transition, caused a decrease of the lattice parameter a of 0.7% and an increase of the lattice parameter c of 1.3% during the cooling process. In result, the contraction of the unit cell volume was 0.2%. Besides the structural phase transition, a magnetic ordering of the Gd sublattice was observed at 48 K. At about 20 K a spin reorientation process occurred. For GdNiAl, an isostructural phase transition from the high-temperature modification HTM I to HTM II at 220 K was found. The jump related to this transition caused a decrease of the lattice parameter a of 0.6% and an increase of the lattice parameter c of 1% during the cooling process. Such an interaction mechanism as for RMn2, R3T and RTX could be assumed for Gd7T3 crystals too. After Sengupta et al. [4] single crystals are necessary to explain whether there is an unusual role of the grain boundaries influencing the anomalous properties of this compound or another mechanism.
4. Summary The measurements of the electronic structure for the described intermetallics, showing hybridization between the rare earth 5d states and d band of transition metals, were obtained thanks to the good single crystals (for all these materials, to our best knowledge, for the first time). Especially, for RMn2 the measurements of the XPS valence band were possible only using single crystals grown by the Czochralski method. These materials are very reactive and compact single crystals that enable such measurements after breaking them under UHV condition only. For polycrystalline samples the XPS measurements of e.g. RMn2 were not possible due to the very big contamination. In earlier examined by our group intermetallics and those described in the present paper appears a frustration effect due to triangular net of rare earth. All these compounds show similarity of magnetic properties and electronic structure. The aim of our work was the better understanding of
ARTICLE IN PRESS 552
E. Talik et al. / Journal of Crystal Growth 283 (2005) 547–552
the role of hybridization in different intermetallics. This may be realized only with good crystals.
Acknowledgement The authors thank Mr. R. Gorzelniak, Institute of Low Temperature and Structure Research Polish Academy of Sciences, for the SQUID measurements. References [1] H. Kadomatsu, K. Kuwano, K. Umeo, Y. Itoh, T. Tokunaga, J. Magn. Magn. Mater. 189 (1998) 335. [2] F. Canepa, M. Napoletano, S. Cirafici, Intermetallics 10 (2002) 731. [3] J.M. Moreau, E. Parthe, J. Less-Common Met. 32 (1973) 91. [4] K. Sengupta, S. Rayaprol, E.V. Sampathkumaran, Europhys. Lett. 69 (2005) 454.
[5] E. Talik, M. Klimczak, J. Kusz, R. Troc´, T. Mydlarz, A. Winiarski, A. Damm, to be published. [6] E. Talik, J. Szade, J. Heimann, A. Winiarska, J. Winiarski, A. Che"kowski, J. Less-Common Met. 138 (1988) 129. [7] T. Massalski, CD ROM: Binary Alloy Phase Diagrams, ASM Intern., Ohio, USA, 1996. [8] W. Kraus, G. Noltze, Powder Cell for Windows. 2.4, Federal Institute for Materials Research and. Testing, Berlin, Germany, 2000. [9] F. Canepa, S. Cirafici, M. Napoletano, C. Ciccarelli, C. Belfortini, Solid State Commun. 133 (2005) 241. [10] E. Talik, M. Kulpa, T. Mydlarz, J. Kusz, H. Bo¨hm, A. Winiarski, Physica B 271 (1999) 265. [11] E. Talik, W. Witas, J. Kusz, A. Winiarski, T. Mydlarz, M. Neumann, H. Bo¨hm, Physica B 293 (2000) 75. [12] E. Talik, M. Neumann, T. Mydlarz, J. Kusz, H. Bo¨hm, A. Winiarski, A. Gilewski, J. Condens. Matter 10 (1998) 581. [13] E. Talik, M. Skutecka, J. Kusz, H. Bo¨hm, J. Jarosz, T. Mydlarz, A. Winiarski, J. Alloys Comp. 325 (2001) 42. [14] J. Jarosz, E. Talik, T. Mydlarz, J. Kusz, H. Bo¨hm, A. Winiarski, J. Magn. Magn. Mater. 208 (2000) 169. [15] J. Kusz, H. Bo¨hm, E. Talik, M. Skutecka, J. Deniszczyk, J. Alloys Comp. 348 (2003) 65.
Journal of Crystal Growth 283 (2005) 547–552 www.elsevier.com/locate/jcrysgro
Gd7T3 (T ¼ Rh, Pd) intermetallics crystal growth E. Talika,, M. Klimczaka, A. Winiarskia, R. Troc´b b
a Institute of Physics, University of Silesia, Uniwersytecka 4, 40-007 Katowice, Poland Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Oko´lna 2, 50-422 Wroc!aw, Poland
Received 28 April 2005; accepted 14 June 2005 Available online 5 August 2005 Communicated by P. Rudolph
Abstract Single crystals of intermetallic compounds Gd7Pd3 and Gd7Rh3 were grown by the Czochralski method from a levitated melt. The X-ray Berg–Barrett topography confirmed good quality of the obtained crystals. They grow in a single Th7Fe3-type hexagonal phase. Magnetic susceptibility measurements of Gd7Pd3 revealed a strong anisotropy in the examined compound. r 2005 Elsevier B.V. All rights reserved. PACS: 71.20.Lp; 61.10.Nz Keywords: A1. X-ray diffraction; A1. X-ray topography; A2. Czochralski method; A2. Growth from melt; A2. Single crystal growth; B1. Rare earth compounds
1. Introduction Recently, magnetic properties of the polycrystalline R7Pd3 (R ¼ light rare earth elements) intermetallic compounds were investigated [1]. These compounds are interesting due to their complex magnetic behaviour. In Ref. [1] they were obtained in polycrystalline form by melting of the constituent elements of 99.9% purity for rare earth and 99.95% purity for Pd under argon atmosphere in Corresponding author. Tel.: +48 32 359 1187;
fax: +48 32 2588 431. E-mail address: [email protected] (E. Talik).
an arc furnace. After melting, the samples were rapidly cooled. Then the ingots were sealed in an evacuated quartz tube at 600 1C for a week to homogenization. During this step the samples crystallized in the hexagonal Th7Fe3 type with space group P63mc. For La7Pd3 the temperature dependence of the electrical resistivity is anomalous and exhibits a negative curvature, with a tendency to saturation at higher temperatures. This material has a superconducting transition at 6 K. Ce7Pd3 has an antiferromagnetic transition at 5 K, while Pr7Pd3 has such transition at only 1.7 K. Nd7Pd3 shows two characteristic temperatures: the first one at 33 K related to a ferromagnetic–
0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.06.020
ARTICLE IN PRESS 548
E. Talik et al. / Journal of Crystal Growth 283 (2005) 547–552
antiferromagnetic transition and the second at 38 K related to an antiferromagnetic–paramagnetic transition. Sm7Pd3 exhibits two sharp peaks: the first one is connected with an antiferromagnetic–ferrimagnetic transition at 18.4 K and the second one at 167 K is connected with a transition from ferrimagnetic state to paramagnetic. All measured properties are sensitive to pressure. Magnetocaloric properties of polycrystalline Gd7Pd3 were investigated by Canepa et al. [2]. They found the ferromagnetic transition for Gd7Pd3 at 323 K. The sample was prepared by melting of the constituents Gd (99.9% purity) and Pd (99.95% purity) in stoichiometric amounts pressed in the form of a pellet in a high-frequency induction furnace on a water-cooled tantalum heart under argon atmosphere. The addition of a second phase was less than 3%. The refrigerant capacity was estimated as 100 J/kg at an applied field of 2 T and as 380 J/kg at 5 T. These samples crystallized in the hexagonal Th7Fe3 type. The lattice parameters were a ¼ 9.985 A˚ and c ¼ 6.272 A˚. Earlier, Moreau and Parthe [3] found lattice parameters of Gd7Pd3 to be a ¼ 10.0008 A˚ and c ¼ 6.2873 A˚. Such a small discrepancy of the lattice parameters may be connected with purity of the samples, stoichiometry deviation or procedure of calculation of the lattice parameters. Giant magnetoresistance in the magnetically ordered state and in a relatively high temperature above 77 K for the Gd7Rh3 compound was recently described by Sengupta et al. [4]. This sample was polycrystalline and obtained by arcmelting of stoichiometric amounts of high-purity (499.9%) elements in an inert atmosphere. The X-ray diffraction confirmed the single phase state. Gd7Rh3 crystallizes in the hexagonal Th7Fe3-type structure. The aim of this work is to grow good quality Gd7Pd3 (T ¼ Rh, Pd) single crystals by the Czochralski growth from a levitating melt for measurements of the electronic and crystal structure, magnetic and transport properties in order to investigate a role of hybridization between the Gd 5d states and d band of transition metal [5]. This crucibleless method allows growing pure single crystals of variety intermetallics, even using very reactive starting materials.
2. Experimental procedure The single crystals were grown from a small amount (about 1 g) of the starting materials. The starting materials Gd (99.9% purity), Pd (Specpure Johnson Matthey Chemical Ltd.) or Rh (99.99% purity) which were melted together on a water-cooled eight-segment conical coil with diameter 14 mm in stoichiometric amounts under very pure argon atmosphere in a Czochralski apparatus
Fig. 1. Sketch of the Czochralski apparatus: (1) levitation coil, (1a) mica plates, (1b) a segment of the levitation coil, (2) RF induction coil, (3) silica tube, (4) phototransistor, (5) molybdenum spike.
ARTICLE IN PRESS E. Talik et al. / Journal of Crystal Growth 283 (2005) 547–552
[6] (Fig. 1). The obtained ingots were remelted several times to homogenize them. The RF electromagnetic field causes a levitation of metal, which conducts the electric current much better than oxides collected on a surface of the bottom part of a melt drop. Moreover, the strong stirring of the melt ensured homogenization during the growth process. The crystal growth process was started by immersion of a molybdenum spike in the melt. The crystal is pulled from the pure area of the top of the melt drop. After necking the crystal was pulled out with a constant velocity less than 0.1 mm/min. Such a growth rate was adapted to the growth of thin single crystals. No significant evaporation of the melt during the crystal growth was observed and a planar growth front was maintained by observation with an eyepiece. The obtained crystals were cylindrical with a diameter of about 2 mm and a length of 60 mm. The phase diagrams show a peritectic formation of the Gd7Pd3 and Gd7Rh3 compounds [7]. In case the separation between liquidus and peritectic line is narrow enough, crystallization of the required compound is observed. Several growth processes were performed by changing the growth rate. Better quality of the crystals, without growth patterns, was for the rate of order 0.05 mm/min. The real structure of the as-grown crystal was examined by the Berg–Barrett topography using Fe Ka radiation. The sensitivity of the method to the misorientation is 10 . X-ray reflections from (h k l) planes were recorded in the range of 2Y equal to 70–1001 on Agfa-Gevaert Structurix D2 film, which was placed close to the crystal surface. The distance between the crystal and the film was about 1 mm to obtain a good pattern. These examinations were performed to check the quality of the obtained single crystals. The spatial resolution was 10 mm. Some fragments of the crystals were pulverized and identified by X-ray powder diffraction with Cu Ka radiation using a Siemens D-5000 diffractometer. The XPS spectra of the Gd7T3 single crystals were measured with monochromatized Al Ka radiation (1486.6 eV) at room temperature using a PHI 5700/660 ESCA spectrometer. The energy spectra of the electrons were determined by a
549
hemispherical mirror analyser with an energy resolution of about 0.3 eV. Magnetic measurements were performed within temperature range 2–400 K in magnetic field 0.1 T using a Quantum Design SQUID magnetometer in Wroc"aw.
3. Results and discussion The Berg–Barrett X-ray topography of Gd7Pd3 and Gd7Rh3 shows that the investigated crystals grew without mosaic structure (Figs. 2, 3). The powder diffraction spectra show that the single crystals grow in the hexagonal Th7Fe3 phase (Figs. 4, 5). The spectra show the Th7Fe3 phase only. Using the Powder Cell 2.3 program published by Kraus and Nolze (BAM Berlin) [8], the spectra
Fig. 2. Single crystal of Gd7Pd3: (a) as-grown photo, (b) Berg–Barrett topography of the crystal fragment.
Fig. 3. Single crystal of Gd7Rh3: (a) as-grown photo, (b) Berg–Barrett topography of the fragment of the crystal.
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Fig. 4. X-ray powder diffraction spectrum of Gd7Pd3 (above) compared with the calculated one (below).
Fig. 6. XPS spectrum of a Gd7Pd3 single crystal in the energy range 0–1400 eV.
Fig. 5. X-ray powder diffraction spectrum of Gd7Rh3 (above) compared with calculated one (below).
the intensity of the (2 0 2) line is much higher than the calculated one (Fig. 5). The XPS spectrum of the Gd7Pd3 single crystal broken under UHV conditions and measured in a wide energy range shows no contaminations with carbon and oxygen, which for polycrystalline samples have been often occurring (Fig. 6). Moreover, using the XPS spectra, the stoichiometry was checked. It was in agreement with the nominal one and stable in different crystal fragments, as well as for crystals grown in different growth processes. Recently, wide investigations of the magnetic properties and temperature dependences of the lattice parameters of Gd7Pd3 single crystals were performed [5]. A strongly anisotropic behaviour of the magnetic properties was observed. Fig. 7 shows the magnetic susceptibility of the Gd7Pd3 single crystal measured along the principal directions. Along the a-axis an increase of the magnetic susceptibility with decreasing temperature is observed. Along the c-axis there is an insignificant increase of the magnetic susceptibility with decreasing temperature, indicating a much stronger non-collinear coupling along this direction. The results of magnetic measurements show any transition connected with gadolinium precipitation. Such a transition was observed for the eutectic Gd–Gd7Pd3 [9]. The width of transition is 15 K, being similar to those for polycrystalline
were calculated to compare them with the experimental ones. The calculated parameters of Gd7Pd3 were a ¼ 9.988 A˚ and c ¼ 6.288 A˚ for polycrystalline sample and a ¼ 9.9807 A˚ and c ¼ 6.2790 A˚ [5] for a single crystal. The measured reflections from the (h h 0) planes were a bit more intensive than the calculated ones (Fig. 4). It relates to partly selfarrangement of magnetic powder grains. The lattice parameters were also calculated from the Gd7Rh3 powder X-ray spectra to be a ¼ 9.868 A˚ and c ¼ 6.208 A˚. A partial powder grain orientation is visible in the spectra because
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Gd7Pd3
80 χ (emu/mole)
H // a
H = 0.1 T
60
40
TC = 334 K H // c
20
0 0
100
200 T (K)
300
400
Fig. 7. Magnetic susceptibility of a Gd7Pd3 crystal along the principal directions.
samples measured in a magnetic field of 0.05 T as was shown in Ref. [2]. The difference in the value of the transition temperature may be related to deviation from stoichiometry or purity of the sample. Such complicated magnetic behaviour of the Gd7T3 compounds is due to the geometrical frustration in the triangular net formed by the gadolinium ions in the Th7Fe3 structure with three non-equivalent Gd sites. The gadolinium ions form tetrahedra stacked in chains along the c-axis. The strong molecular field produced by the gadolinium sublattice may polarize the palladium d band which is hybridized with the gadolinium 5d states. A similar situation took place for the RMn2 and R3T compounds. The XPS electronic structure measurements using the single crystals of the above-mentioned compounds confirmed such a model [10–12]. The hybridization leads to the strong coupling between sublattices and complicated magnetic structures. Recently, a very similar complicated magnetic behaviour due to frustration effect was observed for GdPdAl and GdPdNi too [13–15]. GdNiAl and GdPdAl belong to the ternary intermetallics with unstable crystal structure. These crystals, grown by the same technique, crystallized in the hexagonal ZrNiAl-type structure. The thermal variation of the lattice parameters, electrical resistivity and magnetic susceptibility gave the evidence of an isostructural phase transition from a high-temperature phase
551
modification HTM I to a low-temperature phase modification HTM II at 180 K. The jump, related to this transition, caused a decrease of the lattice parameter a of 0.7% and an increase of the lattice parameter c of 1.3% during the cooling process. In result, the contraction of the unit cell volume was 0.2%. Besides the structural phase transition, a magnetic ordering of the Gd sublattice was observed at 48 K. At about 20 K a spin reorientation process occurred. For GdNiAl, an isostructural phase transition from the high-temperature modification HTM I to HTM II at 220 K was found. The jump related to this transition caused a decrease of the lattice parameter a of 0.6% and an increase of the lattice parameter c of 1% during the cooling process. Such an interaction mechanism as for RMn2, R3T and RTX could be assumed for Gd7T3 crystals too. After Sengupta et al. [4] single crystals are necessary to explain whether there is an unusual role of the grain boundaries influencing the anomalous properties of this compound or another mechanism.
4. Summary The measurements of the electronic structure for the described intermetallics, showing hybridization between the rare earth 5d states and d band of transition metals, were obtained thanks to the good single crystals (for all these materials, to our best knowledge, for the first time). Especially, for RMn2 the measurements of the XPS valence band were possible only using single crystals grown by the Czochralski method. These materials are very reactive and compact single crystals that enable such measurements after breaking them under UHV condition only. For polycrystalline samples the XPS measurements of e.g. RMn2 were not possible due to the very big contamination. In earlier examined by our group intermetallics and those described in the present paper appears a frustration effect due to triangular net of rare earth. All these compounds show similarity of magnetic properties and electronic structure. The aim of our work was the better understanding of
ARTICLE IN PRESS 552
E. Talik et al. / Journal of Crystal Growth 283 (2005) 547–552
the role of hybridization in different intermetallics. This may be realized only with good crystals.
Acknowledgement The authors thank Mr. R. Gorzelniak, Institute of Low Temperature and Structure Research Polish Academy of Sciences, for the SQUID measurements. References [1] H. Kadomatsu, K. Kuwano, K. Umeo, Y. Itoh, T. Tokunaga, J. Magn. Magn. Mater. 189 (1998) 335. [2] F. Canepa, M. Napoletano, S. Cirafici, Intermetallics 10 (2002) 731. [3] J.M. Moreau, E. Parthe, J. Less-Common Met. 32 (1973) 91. [4] K. Sengupta, S. Rayaprol, E.V. Sampathkumaran, Europhys. Lett. 69 (2005) 454.
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