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Magnetism and electronic structure of Gd5Bi3 and Gd4Bi3
Journal of Alloys and Compounds 299 (2000) 72–78
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Magnetism and electronic structure of Gd 5 Bi 3 and Gd 4 Bi 3 J. Szade*, M. Drzyzga Institute of Physics, University of Silesia, Uniwersytecka 4, 40 -007 Katowice, Poland Received 28 October 1999; accepted 24 November 1999
Abstract Gd 5 Bi 3 and Gd 4 Bi 3 were synthesised using induction melting. The AC and DC magnetic susceptibility and the electrical resistivity were measured over a wide temperature range. The electronic structure of the bismuthides was studied with the use of X-ray photoemission spectroscopy. The properties of Gd 5 Bi 3 have been described for the first time. A complex magnetic ordering was found for this compound at temperatures below 110 K. A relation between the electronic structure and magnetic properties of the bismuthides is discussed. 2000 Published by Elsevier Science S.A. All rights reserved. Keywords: Gadolinium bismuthides; Magnetic susceptibility; Electrical resistivity; Photoemission
1. Introduction Rare-earth compounds have been the subject of extensive research mostly due to the interesting magnetic properties and increasing number of applications. Gd with its pure spin momentum in the unfilled 4f shell is the best element to study basic indirect exchange phenomena which are far from being fully understood. Gd compounds with nonmagnetic elements give the opportunity to study the variation of magnetic and other properties with variations in electronic structure. A correlation has been found between the experimental information on the electronic structure, as obtained from photoelectron spectroscopy, and the magnetic and electrical properties for several groups of Gd–M compounds. Here M is a nonmagnetic metal or Si [1,2]. Ferromagnetic Gd compounds with T C close to the room temperature are interesting also because of possible applications. Such a compound is Gd 4 Bi 3 . Some of its properties were investigated many years ago [3]. Gd forms with Bi three compounds in a narrow composition range: GdBi, Gd 4 Bi 3 and Gd 5 Bi 3 . Recently many investigations, including X-ray photoemission spectroscopy (XPS), were performed for a single crystal of GdBi [4–6]. It was found to order antiferromagnetically at relatively low temperature and its electronic structure was
*Corresponding author. Tel.: 148-32-587-978; fax: 148-32-588-431. E-mail address: [email protected] (J. Szade)
discussed and compared to the multiplet calculation. For Gd 5 Bi 3 only the crystallographic properties were studied and two different types of structure were found for this compound [7,8]. In the present work we have prepared polycrystalline samples of Gd 4 Bi 3 and Gd 5 Bi 3 and studied their structure, magnetism and electrical resistivity. We have also obtained experimental information on their electronic structure. The properties of Gd 5 Bi 3 are to our knowledge reported for the first time. The discussion is focused on the relations between the various properties of two studied bismuthides and on the comparison to GdBi.
2. Experimental Gd 4 Bi 3 and Gd 5 Bi 3 were obtained using induction melting in a two-step process. In the first step the pieces of Gd (99.9%) and Bi (99.999%) were melted together using a levitation coil described elsewhere [9]. Due to the high vapour pressure of Bi, its starting weight was taken to exceed the stoichiometric one by 5–15%, depending on the sample and its further processing. Melting was performed in an argon atmosphere at an overpressure of about 400 mbar. In the second step the obtained porous mixture was remelted several times. During that process the weight loss was 1–3 wt.% and the final weight of a sample was close to the assumed stoichiometry. A part of the samples was annealed in the induction furnace for 30 min to 1 h at temperatures about 10008C. Samples of Gd 5 Bi 3 were
0925-8388 / 00 / $ – see front matter 2000 Published by Elsevier Science S.A. All rights reserved. PII: S0925-8388( 99 )00799-9
J. Szade, M. Drzyzga / Journal of Alloys and Compounds 299 (2000) 72 – 78
annealed for 9 days at 10008C, in vacuum. The weight loss during the annealing was negligible. A crystal was also grown by the Czochralski method from a levitated melt using a relatively high pulling speed to keep the process short. The XPS measurements were performed with a Physical Electronics PHI 5700 ESCA spectrometer, using monochromatised Al Ka radiation. The vacuum level during the measurements was about 10 210 Torr. SEM and AES studies were done with a Physical Electronics PHI 660 spectrometer. The samples were cleaved in the UHV chamber just before taking the spectra. To ensure that the surface be free of contaminants the samples were also sputtered with a low energy Ar ion beam (up to 1.5 kV). The influence of sputtering on chemical composition and on the shape of the peaks was checked. The level of oxygen and carbon contamination was controlled during the measurements by monitoring C1s and O1s photoemission levels or O and C Auger transitions. Standard X-ray powder diffraction was used to check the crystal structure. The DC magnetic susceptibility measurements were performed using the Faraday method in the temperature range 4.2–900 K, under an atmosphere of helium. The AC magnetic susceptibility was measured in the range 4.2–360 K, using a magnetic field with a frequency of 1 kHz and an amplitude of about 1 Oe. The electrical resistivity was measured by the van der Pauw method in the range 4.2–350 K.
3. Results and discussion
3.1. Structural study According to the only existing phase diagram [8] both compounds are formed by a peritectic reaction. This is the reason that the preparation of single-phase samples was difficult. An additional problem is the high vapour pressure of Bi and the high temperatures of compound formation. All the X-ray diffraction spectra exhibited some peaks due to metallic Bi. We have investigated the samples of both compounds with a scanning electron microscope (SEM) and by Auger electron spectroscopy (AES). The samples were fractured in the ultra high vacuum (UHV). We have found no metallic Bi on the freshly fractured surfaces studied. The presence of metallic Bi in the XRD spectra may be related to the powdering process. A very small amount of Bi metal, present at the sample surface or in the grain boundaries, may cover the grains of a compound with a thin Bi film during the powdering process. The samples oxidised very quickly and the presence of metallic Bi may also be related to the decomposition of the compound and formation of Gd oxide on the sample surface. The obtained diffraction patterns were compared to the crystallographic data from the literature.
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For Gd 4 Bi 3 the best results were obtained for the samples annealed for 30 min at 10008C and then fast cooled by switching off the r.f. power. A very small amount of another phase was, however, always present in the diffraction pattern. It may be identified as the hexagonal phase of Gd 5 Bi 3 , described by Hohnke and Parthe´ [10]. The structure of the main Gd 4 Bi 3 phase was found to be consistent with the cubic anti-P4 Th 3 -type of structure reported earlier. We found the structure became unstable at high temperatures. The samples that were slowly cooled after annealing and measured at high temperatures in the magnetic balance exhibited much more of the hexagonal Gd 5 Bi 3 phase. The temperature dependence of the magnetic susceptibility will be described later in the text. Our result is consistent with the reported attempts to obtain thin films of Gd 4 Bi 3 [11]. The best diffraction spectra for Gd 5 Bi 3 were observed for the samples obtained with the Czochralski method and after annealling for 9 days at 10008C. The rod-shaped sample was not single-crystalline. The diffraction pattern was consistent with the orthorhombic structure of the Y 5 Bi 3 -type (space group Pnma), reported earlier for the RE 51x Bi 3 compounds by Yoshihara et al. [7] and for Gd 5 Bi 3 by Abulhajev [8]. The position of the peaks agreed with those calculated from the lattice constants given in the ˚ b59.526 A, ˚ c512.11 A. ˚ SEM and last paper: a58.23 A, AES investigations of the Gd 5 Bi 3 , fractured in UHV, revealed uniform composition for the annealed samples. For one of the samples that was not annealed we found at least two-phase composition, with one of these phases rich in Bi. The composition of that phase was close to GdBi 2 but an attempt to obtain a sample with such composition was unsuccessful. The formation of the compound GdBi 2 was suggested by Abulhajev [8].
3.2. Magnetic susceptibility and electrical resistivity Figs. 1 and 2 show that Gd 4 Bi 3 is a ferromagnet having one of the highest T C values (335 K) among the compounds in which RE is combined with a nonmagnetic element. The DC susceptibility increases with decreasing temperature below the transition but the AC measurements indicate that ferromagnetism is stable down to 4.2 K. A small and sharp peak at T C is, however, unusual for a simple ferromagnet. The absorption part of the AC susceptibility shows a small step at T C but no significant changes could be found below T C . The transition temperature is in agreement with previous studies [3]. In the paramagnetic range strong indications for degradation of the cubic ferromagnetic sample were obtained, although the measurements were performed in a helium atmosphere. The hysteresis, visible in the inverse susceptibility, is related to the decrease in amount of the cubic ferromagnetic phase in the sample when the temperature exceeds 800 K. After returning from high temperatures the magnetisation was
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Fig. 1. Temperature dependence of magnetic susceptibility for Gd 4 Bi 3 . The magnetic field was 150 Oe for temperatures below 400 K.
much smaller, although it was measured in the same magnetic field. For the initial heating, the Curie–Weiss law is obeyed in a narrow temperature range, 450–750 K. The effective moment derived from this region is 7.36 mB which is much lower than the theoretical one for the free Gd 31 ion. Low values of the effective moments were found in some other Gd compounds, mostly those exhibiting ferromagnetic order. The negative curvature of the inverse susceptibility, observed for the temperature range 350–450 K, was also found in other Gd compounds with relatively high T C : Gd 5 Si 4 and Gd 2 In [2,12]. This has been attributed to the highly localised Gd 5d electrons and their temperature-dependent susceptibility.
Fig. 2. AC magnetic susceptibility versus temperature for Gd 4 Bi 3 .
Fig. 3. Temperature dependence of magnetic susceptibility for Gd 5 Bi 3 . The magnetic field was 150 for temperatures below 200 K. FC means cooling in the field of 150 Oe. ZFC, zero field cooling.
The magnetic properties of Gd 5 Bi 3 are much different from those of Gd 4 Bi 3 (Figs. 3 and 4). For temperatures above 250 K, the Curie–Weiss law is obeyed with Qp 5120 K and meff / Gd 58.43 mB . Contrary to Gd 4 Bi 3 , the effective moment exceeds the free ion value by about 0.5 mB . This fact correlates with a less ferromagnetic interaction. Analysis of the effective magnetic moments in compounds of Gd with non-magnetic metals indicates that there is some correlation between the strength of the ferromagnetic interactions and the value of the effective moment. Usually
Fig. 4. AC magnetic susceptibility versus temperature for Gd 5 Bi 3 .
J. Szade, M. Drzyzga / Journal of Alloys and Compounds 299 (2000) 72 – 78
the highest excess of the effective moment per Gd atom is observed in compounds with antiferromagnetic order. The value of the paramagnetic Curie temperature obtained for Gd 5 Bi 3 correlates with the temperature of the main transition observed for both the DC and the AC susceptibility runs. A precursor peak is observed at about 140 K and it is followed by the main transition at 110 K. The value of the susceptibility at the maximum is relatively high, but the magnetic ordering is probably not simply ferromagnetic. The observed maxima in the DC and AC susceptibility are shifted by about 30 K, and this can be related to the influence of magnetic field. A strong decrease of the AC susceptibility below 60 K may indicate that, at very low field, a compensation of magnetic moments takes place, due to the antiferromagnetic canting or spin-glass like freezing. A weak thermomagnetic effect was found below 50 K in the DC run. Moreover, the presence of a peak in the absorption part of the AC susceptibility may indicate that a spin reorientation or freezing takes place at about 60–80 K. The magnetic ordering process in Gd 5 Bi 3 is complicated and it may be related to the possible atomic disorder and its temperature dependence. All susceptibility runs were, however, reversible with temperature changes, excluding a thermomagnetic effect at low temperatures. The magnetic transitions are manifested in the electrical resistivity, presented in Fig. 5. The relatively high value of the resistivity found in Gd 5 Bi 3 is expected in a compound with a low density of states at the Fermi level, which can be inferred from the XPS results (Fig. 6). Contrary to Gd 4 Bi 3, there is a temperature range where the resistivity of Gd 5 Bi 3 decreases linearly (200–300 K). Some change of the slope is visible at about 200 K which corresponds to the slight anomaly in the susceptibility. Next, the resistivity drops at about 100 K, close to the main magnetic transition. Below
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Fig. 6. XPS spectra in the region close to the Fermi level. Spectra of Gd and its bismuthides are normalised with respect to the intensity of Gd 4f level.
this transition the curvature is negative and some anomaly is visible at about 30 K. The absence of a residual resistivity plateau at low temperatures is anomalous and it may indicate that a magnetic disorder contribution to the resistivity is very large at low temperatures. This suggests that the order is still not established at 4.2 K. It corresponds to the decreasing AC susceptibility below 60 K. The resistivity of Gd 4 Bi 3 slightly increases when the temperature decreases down to 260 K and a negative curvature is observed for lower temperatures. The magnetic transition is manifested only by a slight change of the slope. The resistivity increase below T C is anomalous for a ferromagnetic transition. Our result agrees qualitatively with the temperature dependence of the resistivity obtained by Holtzberg et al. [3], although the residual resisitivity of our sample is much lower, indicating to higher atomic order. The temperature dependence of resistivity for both measured bismuthides is very different from the behaviour obtained for GdBi [6], which is linear in a temperature range of over more than 200 K.
3.3. X-Ray photoemission
Fig. 5. Temperature dependence of electrical resistivity for two Gd bismuthides.
The XPS spectra of the valence band are shown in Fig. 6, where the spectra for the pure elements are added. All spectra are obtained with the same spectrometer and under similar conditions. Both bismuthides exhibit a peak at about 2.5 eV, which appears to be characteristic for Gd–Bi compounds, as Yamada et al. [4] found a similar feature in
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GdBi. For the ferromagnetic Gd 4 Bi 3 an additional feature at a binding energy of 0.5 eV is visible. We suppose that one can relate this feature with a part of the exchange split 5d band originating from the Gd atoms. This feature is not visible in Gd 5 Bi 3 and it was not found in antiferromagnetic GdBi [4]. The temperature dependence of the photoemission in the region close to the Fermi edge may be an additional indication for this explanation. For Gd 5 Bi 3 , a slight increase of the mentioned feature at 0.5 eV is visible at the lowest achieved temperature in our spectrometer, 150 K (Fig. 7). A temperature dependence of the exchange splitting in the Gd conduction band was observed experimentally [13,14] and we suppose that, for Gd 5 Bi 3 , this mechanism leads to the changes in the spectra at temperatures close to a magnetic transition. The spectra of pure Bi were obtained with a single-crystalline sample cleaved in UHV. The valence band and the position of some levels were found to be slightly different from those given by Nascimento et al. [15]. Two peaks visible in the region close to the Fermi level are better resolved in our spectra, with the low binding energy feature situated at about 1.5 eV instead of at 2.3 eV, as found by Nascimento et al. The position of the second feature is the same, 3.5 eV. Our results for the Bi 5d and 4f levels are, however, in good agreement with the data reported in Ref. [16]. A characteristic feature of both bismuthides is the relatively low density of states at the Fermi level. It is worth noting that a similar relation between the electronic structure and magnetism was found for the Gd 5 (Si,Ge) 4 system [2]. The ferromagnetic compounds, Gd 5 Si 4 and Gd 5 Si 2 Ge 2 , exhibit a characteristic bump just below the Fermi level, whereas there is no such feature for the antiferromagnetic Gd 5 Ge 4 . Ferromagnetic compounds with high T C have probably a part of the exchange split 5d band just below the Fermi
Fig. 7. XPS spectra for Gd 5 Bi 3 obtained at 300 and 150 K.
level. The broad band located at a binding energy of about 2 eV may be related to the bonding Gd–Bi or Gd–Si(Ge) electrons. It is worth noting that the ferromagnetic Gd 4 Bi 3 and Gd 5 Si 4 compounds show not only similar valence band in the vicinity of the Fermi level but also very similar temperature dependence of the resistivity. Additionally, both compounds exhibit a non-linear inverse susceptibility in a part of the paramagnetic region and have a low value of the effective moment derived from the Curie–Weiss region. For Gd 5 Si 4 it is 7.70 mB / Gd. Fig. 8 shows the valence band spectra in a broad binding energy range. A shift in binding energy of the Gd 4f line is well visible. The observed value of the shift of about 1 eV is relatively large in comparison to other Gd metallic compounds [1,2]. Similar values of the shift were observed for other Gd multiplets, e.g., 4d and 3d, but the estimation of the shift is problematic due to the different shape of the corresponding multiplets in Gd metal and in the bismuthides (Figs. 9 and 10). The position of the Bi lines is hardly changed in relation to Bi metal. There is a small negative shift. The large positive chemical shift in the Gd spectra is related to the charge transfer from Gd to Bi (Table 1). However, the additional charge does not influence much the potential at the Bi sites. The effect is similar for both investigated compounds and this is consistent with the conclusion reached in previous studies of various Gd–M groups of compounds (where M is a nonmagnetic element) [1,2]. Charge redistribution deduced from the XPS results is characteristic for a particular group
Fig. 8. XPS valence band spectra in a broad binding energy range. Gd and Bi spectra are added for comparison.
J. Szade, M. Drzyzga / Journal of Alloys and Compounds 299 (2000) 72 – 78
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Table 1 Chemical shift (in eV) of the most prominent photoemission lines for two bismuthides, the experimental error is about 60.1 eV a Compound
Gd 4f 8.1 eV
Bi 5d 5 / 2 24.1 eV
Bi 4f 7 / 2 157.0 eV
Gd 4 Bi 3 Gd 5 Bi 3
1.0 1.1
20.1 20.1
20.1 20.2
a
Fig. 9. Gd 4d and Bi 4f XPS spectra of Gd 4 Bi 3 , Gd 5 Bi 3 , Gd and Bi.
of compounds. The models of chemical shift, which require charge neutrality or charge correlation, are not applicable here. Due to the interaction with a half-filled 4f shell and its spin momentum, the photoemission spectra of all Gd core levels are exchange split or form complex multiplets when
The binding energies are given for pure elements.
spin–orbit interactions are included. For the Gd 4d multiplet the splitting of the first main line was observed in Gd metal and some of its metallic compounds [17]. The splitting is not visible in the spectra of bismuthides (Fig. 9). We suppose that this is related to the increase of the natural line-width of the component lines of the multiplet. The reason for that increase is probably a weak screening of a photo-hole in the compounds with low density of states at the Fermi level. Weaker screening may lead to a shorter life-time of the excited states. The observed linewidth of the multiplet components is a result of many factors, including quality of the sample and instrumental broadening. In our previous studies we found, however, that in the ionic compound GdF 3 there is no clear splitting of the 4d multiplet although the sample was a single crystal cleaved in the UHV. It suggests that lack of splitting of this multiplet should be related to the short life-time of the photo-hole when screening is weak or there is no screening. A similar effect is expected for the Gd 3d photoemission, shown in Fig. 10. However, due to the weaker exchange interaction, the structure of the multiplet is much less resolved than in the 4d photoemission, even for Gd metal. The chemical shift is similar to the one observed for the shallow 4f level. The satellite features at about 1195 eV vary slightly between the spectra from Gd metal and bismuthides. The variation is, however, not clear enough to discuss the relation of the satellites to the valence band features, as it was done by Yamada et al. [4].
4. Conclusion
Fig. 10. Gd 3d XPS spectra of Gd 4 Bi 3 , Gd 5 Bi 3 and Gd.
Gd 4 Bi 3 and Gd 5 Bi 3 exhibit different magnetic, transport and spectroscopic properties, although their stoichiometries are close to each other. Gd 4 Bi 3 is a ferromagnet with a high T C , whereas magnetic ordering in Gd 5 Bi 3 is complex and takes place at much lower temperatures. Besides the ferromagnetic-like transition at about 100 K, a spin reorientation or freezing was observed below 60 K. Further studies are necessary to solve the magnetic structure of Gd 5 Bi 3 . XPS results confirm conclusions reached from the previous systematic studies of various Gd compounds. Bonding in Gd compounds is related with a charge redistribution, which is characteristic for each system, also for the investigated bismuthides. The values of chemical
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shift and the shape of the valence band are very similar in all bismuthides, including the results reported earlier for GdBi. Our study indicates that there is a correlation between the shape of the valence band close to the Fermi level and the magnetic properties. Gd photoemission multiplets of core levels differ from those of Gd metal and the difference may be related to a weak screening in the bismuthides and life-time effects.
Acknowledgements Financial support from KBN under project 2 P03B 129 14 is gratefully acknowledged. We wish to thank J. Kubacki for the assistance in the AC susceptibility measurements and J. Kapusta for performing the X-ray diffraction measurements.
References [1] J. Szade, M. Neumann, J. Phys.: Condensed Matter 11 (1999) 3887. [2] J. Szade, G. Skorek, J. Magn. Magn. Mater. 196–197 (1999) 699. [3] F. Holtzberg, T.R. McGuire, S. Methfessel, J.C. Suits, J. Appl. Phys. 35 (1964) 1033.
[4] H. Yamada, T. Fukawa, T. Muro, Y. Tanaka, S. Imada, S. Suga, D.X. Li, T. Suzuki, J. Phys. Soc. Japan 65 (1996) 1000. [5] D.X. Li, Y. Haga, H. Shida, T. Suzuki, Y.S. Kwon, G. Kido, J. Phys.: Condens. Matt. 9 (1997) 10777. [6] D.X. Li, Y. Haga, H. Shida, T. Suzuki, Y.S. Kwon, Phys. Rev. B 54 (1996) 10483. [7] K. Yoshihara, J.B. Taylor, L.D. Calvert, J.G. Despault, J. Less Common Met. 41 (1975) 329. [8] W.D. Abulhajev, Metally 1 (1993) 187. [9] E. Talik, J. Szade, J. Heimann, A. Winiarska, A. Winiarski, A. Chelkowski, J. Less-Common Met. 138 (1988) 129. ´ J. Less Common Met. 17 (1968) 291. [10] D. Hohnke, E. Parthe, [11] K. Baba, H. Ishii, I. Yamagouchi, O. Nakamura, T. Takeda, in: Growth, Characterization and Properties of Ultrathin Magnetic Films and Multilayers, San Diego, CA, 25–28 April 1989, Pittsburg, PA, USA, Mater. Res. Soc., 1989, p. 289. [12] J. Szade, J. Magn. Magn. Mater. 170 (1997) 228. [13] D. Li, J. Zhang, P.A. Dowben, M. Onellion, Phys. Rev. 45 (1992) 7272. [14] B. Kim, A.B. Andrews, J.L. Erskine, K.J. Kim, B.N. Harmon, Phys. Rev. Lett. 68 (1992) 1931. [15] V.B. Nascimento, V.E. de Carvalho, R. Paniago, E.A. Soares, L.O. Ladeira, H.D. Pfannes, J. Electron Spectr. Rel. Phen. 104 (1999) 99. [16] J. Chastain, R.C. KingJr. (Eds.), Handbook of X-ray Photoelectron Spectroscopy, Physical Electronics, Eden Prairie, USA, 1995. [17] J. Szade, J. Lachnitt, M. Neumann, Phys. Rev. B 55 (1997) 1430.
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Magnetism and electronic structure of Gd 5 Bi 3 and Gd 4 Bi 3 J. Szade*, M. Drzyzga Institute of Physics, University of Silesia, Uniwersytecka 4, 40 -007 Katowice, Poland Received 28 October 1999; accepted 24 November 1999
Abstract Gd 5 Bi 3 and Gd 4 Bi 3 were synthesised using induction melting. The AC and DC magnetic susceptibility and the electrical resistivity were measured over a wide temperature range. The electronic structure of the bismuthides was studied with the use of X-ray photoemission spectroscopy. The properties of Gd 5 Bi 3 have been described for the first time. A complex magnetic ordering was found for this compound at temperatures below 110 K. A relation between the electronic structure and magnetic properties of the bismuthides is discussed. 2000 Published by Elsevier Science S.A. All rights reserved. Keywords: Gadolinium bismuthides; Magnetic susceptibility; Electrical resistivity; Photoemission
1. Introduction Rare-earth compounds have been the subject of extensive research mostly due to the interesting magnetic properties and increasing number of applications. Gd with its pure spin momentum in the unfilled 4f shell is the best element to study basic indirect exchange phenomena which are far from being fully understood. Gd compounds with nonmagnetic elements give the opportunity to study the variation of magnetic and other properties with variations in electronic structure. A correlation has been found between the experimental information on the electronic structure, as obtained from photoelectron spectroscopy, and the magnetic and electrical properties for several groups of Gd–M compounds. Here M is a nonmagnetic metal or Si [1,2]. Ferromagnetic Gd compounds with T C close to the room temperature are interesting also because of possible applications. Such a compound is Gd 4 Bi 3 . Some of its properties were investigated many years ago [3]. Gd forms with Bi three compounds in a narrow composition range: GdBi, Gd 4 Bi 3 and Gd 5 Bi 3 . Recently many investigations, including X-ray photoemission spectroscopy (XPS), were performed for a single crystal of GdBi [4–6]. It was found to order antiferromagnetically at relatively low temperature and its electronic structure was
*Corresponding author. Tel.: 148-32-587-978; fax: 148-32-588-431. E-mail address: [email protected] (J. Szade)
discussed and compared to the multiplet calculation. For Gd 5 Bi 3 only the crystallographic properties were studied and two different types of structure were found for this compound [7,8]. In the present work we have prepared polycrystalline samples of Gd 4 Bi 3 and Gd 5 Bi 3 and studied their structure, magnetism and electrical resistivity. We have also obtained experimental information on their electronic structure. The properties of Gd 5 Bi 3 are to our knowledge reported for the first time. The discussion is focused on the relations between the various properties of two studied bismuthides and on the comparison to GdBi.
2. Experimental Gd 4 Bi 3 and Gd 5 Bi 3 were obtained using induction melting in a two-step process. In the first step the pieces of Gd (99.9%) and Bi (99.999%) were melted together using a levitation coil described elsewhere [9]. Due to the high vapour pressure of Bi, its starting weight was taken to exceed the stoichiometric one by 5–15%, depending on the sample and its further processing. Melting was performed in an argon atmosphere at an overpressure of about 400 mbar. In the second step the obtained porous mixture was remelted several times. During that process the weight loss was 1–3 wt.% and the final weight of a sample was close to the assumed stoichiometry. A part of the samples was annealed in the induction furnace for 30 min to 1 h at temperatures about 10008C. Samples of Gd 5 Bi 3 were
0925-8388 / 00 / $ – see front matter 2000 Published by Elsevier Science S.A. All rights reserved. PII: S0925-8388( 99 )00799-9
J. Szade, M. Drzyzga / Journal of Alloys and Compounds 299 (2000) 72 – 78
annealed for 9 days at 10008C, in vacuum. The weight loss during the annealing was negligible. A crystal was also grown by the Czochralski method from a levitated melt using a relatively high pulling speed to keep the process short. The XPS measurements were performed with a Physical Electronics PHI 5700 ESCA spectrometer, using monochromatised Al Ka radiation. The vacuum level during the measurements was about 10 210 Torr. SEM and AES studies were done with a Physical Electronics PHI 660 spectrometer. The samples were cleaved in the UHV chamber just before taking the spectra. To ensure that the surface be free of contaminants the samples were also sputtered with a low energy Ar ion beam (up to 1.5 kV). The influence of sputtering on chemical composition and on the shape of the peaks was checked. The level of oxygen and carbon contamination was controlled during the measurements by monitoring C1s and O1s photoemission levels or O and C Auger transitions. Standard X-ray powder diffraction was used to check the crystal structure. The DC magnetic susceptibility measurements were performed using the Faraday method in the temperature range 4.2–900 K, under an atmosphere of helium. The AC magnetic susceptibility was measured in the range 4.2–360 K, using a magnetic field with a frequency of 1 kHz and an amplitude of about 1 Oe. The electrical resistivity was measured by the van der Pauw method in the range 4.2–350 K.
3. Results and discussion
3.1. Structural study According to the only existing phase diagram [8] both compounds are formed by a peritectic reaction. This is the reason that the preparation of single-phase samples was difficult. An additional problem is the high vapour pressure of Bi and the high temperatures of compound formation. All the X-ray diffraction spectra exhibited some peaks due to metallic Bi. We have investigated the samples of both compounds with a scanning electron microscope (SEM) and by Auger electron spectroscopy (AES). The samples were fractured in the ultra high vacuum (UHV). We have found no metallic Bi on the freshly fractured surfaces studied. The presence of metallic Bi in the XRD spectra may be related to the powdering process. A very small amount of Bi metal, present at the sample surface or in the grain boundaries, may cover the grains of a compound with a thin Bi film during the powdering process. The samples oxidised very quickly and the presence of metallic Bi may also be related to the decomposition of the compound and formation of Gd oxide on the sample surface. The obtained diffraction patterns were compared to the crystallographic data from the literature.
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For Gd 4 Bi 3 the best results were obtained for the samples annealed for 30 min at 10008C and then fast cooled by switching off the r.f. power. A very small amount of another phase was, however, always present in the diffraction pattern. It may be identified as the hexagonal phase of Gd 5 Bi 3 , described by Hohnke and Parthe´ [10]. The structure of the main Gd 4 Bi 3 phase was found to be consistent with the cubic anti-P4 Th 3 -type of structure reported earlier. We found the structure became unstable at high temperatures. The samples that were slowly cooled after annealing and measured at high temperatures in the magnetic balance exhibited much more of the hexagonal Gd 5 Bi 3 phase. The temperature dependence of the magnetic susceptibility will be described later in the text. Our result is consistent with the reported attempts to obtain thin films of Gd 4 Bi 3 [11]. The best diffraction spectra for Gd 5 Bi 3 were observed for the samples obtained with the Czochralski method and after annealling for 9 days at 10008C. The rod-shaped sample was not single-crystalline. The diffraction pattern was consistent with the orthorhombic structure of the Y 5 Bi 3 -type (space group Pnma), reported earlier for the RE 51x Bi 3 compounds by Yoshihara et al. [7] and for Gd 5 Bi 3 by Abulhajev [8]. The position of the peaks agreed with those calculated from the lattice constants given in the ˚ b59.526 A, ˚ c512.11 A. ˚ SEM and last paper: a58.23 A, AES investigations of the Gd 5 Bi 3 , fractured in UHV, revealed uniform composition for the annealed samples. For one of the samples that was not annealed we found at least two-phase composition, with one of these phases rich in Bi. The composition of that phase was close to GdBi 2 but an attempt to obtain a sample with such composition was unsuccessful. The formation of the compound GdBi 2 was suggested by Abulhajev [8].
3.2. Magnetic susceptibility and electrical resistivity Figs. 1 and 2 show that Gd 4 Bi 3 is a ferromagnet having one of the highest T C values (335 K) among the compounds in which RE is combined with a nonmagnetic element. The DC susceptibility increases with decreasing temperature below the transition but the AC measurements indicate that ferromagnetism is stable down to 4.2 K. A small and sharp peak at T C is, however, unusual for a simple ferromagnet. The absorption part of the AC susceptibility shows a small step at T C but no significant changes could be found below T C . The transition temperature is in agreement with previous studies [3]. In the paramagnetic range strong indications for degradation of the cubic ferromagnetic sample were obtained, although the measurements were performed in a helium atmosphere. The hysteresis, visible in the inverse susceptibility, is related to the decrease in amount of the cubic ferromagnetic phase in the sample when the temperature exceeds 800 K. After returning from high temperatures the magnetisation was
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Fig. 1. Temperature dependence of magnetic susceptibility for Gd 4 Bi 3 . The magnetic field was 150 Oe for temperatures below 400 K.
much smaller, although it was measured in the same magnetic field. For the initial heating, the Curie–Weiss law is obeyed in a narrow temperature range, 450–750 K. The effective moment derived from this region is 7.36 mB which is much lower than the theoretical one for the free Gd 31 ion. Low values of the effective moments were found in some other Gd compounds, mostly those exhibiting ferromagnetic order. The negative curvature of the inverse susceptibility, observed for the temperature range 350–450 K, was also found in other Gd compounds with relatively high T C : Gd 5 Si 4 and Gd 2 In [2,12]. This has been attributed to the highly localised Gd 5d electrons and their temperature-dependent susceptibility.
Fig. 2. AC magnetic susceptibility versus temperature for Gd 4 Bi 3 .
Fig. 3. Temperature dependence of magnetic susceptibility for Gd 5 Bi 3 . The magnetic field was 150 for temperatures below 200 K. FC means cooling in the field of 150 Oe. ZFC, zero field cooling.
The magnetic properties of Gd 5 Bi 3 are much different from those of Gd 4 Bi 3 (Figs. 3 and 4). For temperatures above 250 K, the Curie–Weiss law is obeyed with Qp 5120 K and meff / Gd 58.43 mB . Contrary to Gd 4 Bi 3 , the effective moment exceeds the free ion value by about 0.5 mB . This fact correlates with a less ferromagnetic interaction. Analysis of the effective magnetic moments in compounds of Gd with non-magnetic metals indicates that there is some correlation between the strength of the ferromagnetic interactions and the value of the effective moment. Usually
Fig. 4. AC magnetic susceptibility versus temperature for Gd 5 Bi 3 .
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the highest excess of the effective moment per Gd atom is observed in compounds with antiferromagnetic order. The value of the paramagnetic Curie temperature obtained for Gd 5 Bi 3 correlates with the temperature of the main transition observed for both the DC and the AC susceptibility runs. A precursor peak is observed at about 140 K and it is followed by the main transition at 110 K. The value of the susceptibility at the maximum is relatively high, but the magnetic ordering is probably not simply ferromagnetic. The observed maxima in the DC and AC susceptibility are shifted by about 30 K, and this can be related to the influence of magnetic field. A strong decrease of the AC susceptibility below 60 K may indicate that, at very low field, a compensation of magnetic moments takes place, due to the antiferromagnetic canting or spin-glass like freezing. A weak thermomagnetic effect was found below 50 K in the DC run. Moreover, the presence of a peak in the absorption part of the AC susceptibility may indicate that a spin reorientation or freezing takes place at about 60–80 K. The magnetic ordering process in Gd 5 Bi 3 is complicated and it may be related to the possible atomic disorder and its temperature dependence. All susceptibility runs were, however, reversible with temperature changes, excluding a thermomagnetic effect at low temperatures. The magnetic transitions are manifested in the electrical resistivity, presented in Fig. 5. The relatively high value of the resistivity found in Gd 5 Bi 3 is expected in a compound with a low density of states at the Fermi level, which can be inferred from the XPS results (Fig. 6). Contrary to Gd 4 Bi 3, there is a temperature range where the resistivity of Gd 5 Bi 3 decreases linearly (200–300 K). Some change of the slope is visible at about 200 K which corresponds to the slight anomaly in the susceptibility. Next, the resistivity drops at about 100 K, close to the main magnetic transition. Below
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Fig. 6. XPS spectra in the region close to the Fermi level. Spectra of Gd and its bismuthides are normalised with respect to the intensity of Gd 4f level.
this transition the curvature is negative and some anomaly is visible at about 30 K. The absence of a residual resistivity plateau at low temperatures is anomalous and it may indicate that a magnetic disorder contribution to the resistivity is very large at low temperatures. This suggests that the order is still not established at 4.2 K. It corresponds to the decreasing AC susceptibility below 60 K. The resistivity of Gd 4 Bi 3 slightly increases when the temperature decreases down to 260 K and a negative curvature is observed for lower temperatures. The magnetic transition is manifested only by a slight change of the slope. The resistivity increase below T C is anomalous for a ferromagnetic transition. Our result agrees qualitatively with the temperature dependence of the resistivity obtained by Holtzberg et al. [3], although the residual resisitivity of our sample is much lower, indicating to higher atomic order. The temperature dependence of resistivity for both measured bismuthides is very different from the behaviour obtained for GdBi [6], which is linear in a temperature range of over more than 200 K.
3.3. X-Ray photoemission
Fig. 5. Temperature dependence of electrical resistivity for two Gd bismuthides.
The XPS spectra of the valence band are shown in Fig. 6, where the spectra for the pure elements are added. All spectra are obtained with the same spectrometer and under similar conditions. Both bismuthides exhibit a peak at about 2.5 eV, which appears to be characteristic for Gd–Bi compounds, as Yamada et al. [4] found a similar feature in
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GdBi. For the ferromagnetic Gd 4 Bi 3 an additional feature at a binding energy of 0.5 eV is visible. We suppose that one can relate this feature with a part of the exchange split 5d band originating from the Gd atoms. This feature is not visible in Gd 5 Bi 3 and it was not found in antiferromagnetic GdBi [4]. The temperature dependence of the photoemission in the region close to the Fermi edge may be an additional indication for this explanation. For Gd 5 Bi 3 , a slight increase of the mentioned feature at 0.5 eV is visible at the lowest achieved temperature in our spectrometer, 150 K (Fig. 7). A temperature dependence of the exchange splitting in the Gd conduction band was observed experimentally [13,14] and we suppose that, for Gd 5 Bi 3 , this mechanism leads to the changes in the spectra at temperatures close to a magnetic transition. The spectra of pure Bi were obtained with a single-crystalline sample cleaved in UHV. The valence band and the position of some levels were found to be slightly different from those given by Nascimento et al. [15]. Two peaks visible in the region close to the Fermi level are better resolved in our spectra, with the low binding energy feature situated at about 1.5 eV instead of at 2.3 eV, as found by Nascimento et al. The position of the second feature is the same, 3.5 eV. Our results for the Bi 5d and 4f levels are, however, in good agreement with the data reported in Ref. [16]. A characteristic feature of both bismuthides is the relatively low density of states at the Fermi level. It is worth noting that a similar relation between the electronic structure and magnetism was found for the Gd 5 (Si,Ge) 4 system [2]. The ferromagnetic compounds, Gd 5 Si 4 and Gd 5 Si 2 Ge 2 , exhibit a characteristic bump just below the Fermi level, whereas there is no such feature for the antiferromagnetic Gd 5 Ge 4 . Ferromagnetic compounds with high T C have probably a part of the exchange split 5d band just below the Fermi
Fig. 7. XPS spectra for Gd 5 Bi 3 obtained at 300 and 150 K.
level. The broad band located at a binding energy of about 2 eV may be related to the bonding Gd–Bi or Gd–Si(Ge) electrons. It is worth noting that the ferromagnetic Gd 4 Bi 3 and Gd 5 Si 4 compounds show not only similar valence band in the vicinity of the Fermi level but also very similar temperature dependence of the resistivity. Additionally, both compounds exhibit a non-linear inverse susceptibility in a part of the paramagnetic region and have a low value of the effective moment derived from the Curie–Weiss region. For Gd 5 Si 4 it is 7.70 mB / Gd. Fig. 8 shows the valence band spectra in a broad binding energy range. A shift in binding energy of the Gd 4f line is well visible. The observed value of the shift of about 1 eV is relatively large in comparison to other Gd metallic compounds [1,2]. Similar values of the shift were observed for other Gd multiplets, e.g., 4d and 3d, but the estimation of the shift is problematic due to the different shape of the corresponding multiplets in Gd metal and in the bismuthides (Figs. 9 and 10). The position of the Bi lines is hardly changed in relation to Bi metal. There is a small negative shift. The large positive chemical shift in the Gd spectra is related to the charge transfer from Gd to Bi (Table 1). However, the additional charge does not influence much the potential at the Bi sites. The effect is similar for both investigated compounds and this is consistent with the conclusion reached in previous studies of various Gd–M groups of compounds (where M is a nonmagnetic element) [1,2]. Charge redistribution deduced from the XPS results is characteristic for a particular group
Fig. 8. XPS valence band spectra in a broad binding energy range. Gd and Bi spectra are added for comparison.
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Table 1 Chemical shift (in eV) of the most prominent photoemission lines for two bismuthides, the experimental error is about 60.1 eV a Compound
Gd 4f 8.1 eV
Bi 5d 5 / 2 24.1 eV
Bi 4f 7 / 2 157.0 eV
Gd 4 Bi 3 Gd 5 Bi 3
1.0 1.1
20.1 20.1
20.1 20.2
a
Fig. 9. Gd 4d and Bi 4f XPS spectra of Gd 4 Bi 3 , Gd 5 Bi 3 , Gd and Bi.
of compounds. The models of chemical shift, which require charge neutrality or charge correlation, are not applicable here. Due to the interaction with a half-filled 4f shell and its spin momentum, the photoemission spectra of all Gd core levels are exchange split or form complex multiplets when
The binding energies are given for pure elements.
spin–orbit interactions are included. For the Gd 4d multiplet the splitting of the first main line was observed in Gd metal and some of its metallic compounds [17]. The splitting is not visible in the spectra of bismuthides (Fig. 9). We suppose that this is related to the increase of the natural line-width of the component lines of the multiplet. The reason for that increase is probably a weak screening of a photo-hole in the compounds with low density of states at the Fermi level. Weaker screening may lead to a shorter life-time of the excited states. The observed linewidth of the multiplet components is a result of many factors, including quality of the sample and instrumental broadening. In our previous studies we found, however, that in the ionic compound GdF 3 there is no clear splitting of the 4d multiplet although the sample was a single crystal cleaved in the UHV. It suggests that lack of splitting of this multiplet should be related to the short life-time of the photo-hole when screening is weak or there is no screening. A similar effect is expected for the Gd 3d photoemission, shown in Fig. 10. However, due to the weaker exchange interaction, the structure of the multiplet is much less resolved than in the 4d photoemission, even for Gd metal. The chemical shift is similar to the one observed for the shallow 4f level. The satellite features at about 1195 eV vary slightly between the spectra from Gd metal and bismuthides. The variation is, however, not clear enough to discuss the relation of the satellites to the valence band features, as it was done by Yamada et al. [4].
4. Conclusion
Fig. 10. Gd 3d XPS spectra of Gd 4 Bi 3 , Gd 5 Bi 3 and Gd.
Gd 4 Bi 3 and Gd 5 Bi 3 exhibit different magnetic, transport and spectroscopic properties, although their stoichiometries are close to each other. Gd 4 Bi 3 is a ferromagnet with a high T C , whereas magnetic ordering in Gd 5 Bi 3 is complex and takes place at much lower temperatures. Besides the ferromagnetic-like transition at about 100 K, a spin reorientation or freezing was observed below 60 K. Further studies are necessary to solve the magnetic structure of Gd 5 Bi 3 . XPS results confirm conclusions reached from the previous systematic studies of various Gd compounds. Bonding in Gd compounds is related with a charge redistribution, which is characteristic for each system, also for the investigated bismuthides. The values of chemical
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shift and the shape of the valence band are very similar in all bismuthides, including the results reported earlier for GdBi. Our study indicates that there is a correlation between the shape of the valence band close to the Fermi level and the magnetic properties. Gd photoemission multiplets of core levels differ from those of Gd metal and the difference may be related to a weak screening in the bismuthides and life-time effects.
Acknowledgements Financial support from KBN under project 2 P03B 129 14 is gratefully acknowledged. We wish to thank J. Kubacki for the assistance in the AC susceptibility measurements and J. Kapusta for performing the X-ray diffraction measurements.
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