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Magnetic circular dichroism in CeRh3B2
Physica B 259—261 (1999) 1142—1143
Magnetic circular dichroism in CeRh B W. Grange *, J.-P. Kappler , E. Dartyge, P. Haen, P. Lejay, A. Rogalev IPCMS-GEMME, 23 rue du Loess, 67037 Strasbourg, France LURE, B.P. 34, 91898 Orsay, Cedex, France CRTBT, B.P. 166, 38042 Grenoble, Cedex, France ESRF, B.P. 220, 38043 Grenoble, Cedex, France
Abstract We report a magnetic circular dichroism study at the Ce-2p core level X-ray absorption spectra on the ferromagnet CeRh B . The Ce-¸ spectra measured on a single crystal are compared to those obtained on CeRu Ge and CeFe , taken as reference systems. The shape of the circular dichroism signal, that witnesses the 4f hybridisation, supports the delocalised picture of the 4f electrons in CeRh B . 1999 Elsevier Science B.V. All rights reserved. Keywords: Ce; Ferromagnetism; Magnetic dichroism
1. Introduction The study of the ternary boride CeRh B is of great importance because of its exceptional ferromagnetic state. One of the most surprising feature is that the Curie temperature is unusually large (¹ "115 K) for a Ce compound without 3d magnetic constituents [1]. Magnetisation measurements show that the saturation moment is only about 0.5—0.4l /fu [2]: this is much smaller than the free-ion value and has been attributed to large crystal field effects and/or a Kondo-type interaction [3,4]. The structural properties of CeRh B seem also anomalous: CeRh B crystallises in a distorted hexa gonal P6/mmm CeCo B -like structure, with a"5.484 As and c"3.087 As ; this latter value is the shortest ever observed in intermetallic Ce compounds. The linear thermal expansion is large and highly anisotropic: along the a-axis the expansion is nearly temperature independent whereas along the c-axis the expansion is three times larger than in an usual metal [5,6]. To precise the role played by the Ce-5d and Rh-4d electrons in the magnetic properties of CeRh B , we present in this paper X-ray absorption spectroscopy * Corresponding author. Tel.: 33388-107-069; fax: 33388-107249; e-mail: [email protected].
(XAS) and X-ray magnetic circular dichroism (XMCD) experiments performed at the Ce and Rh ¸ edges. The comparison of the spectra with those obtained from some reference systems evidences the itinerant behaviour of the 4f electrons.
2. Results and discussion The Ce and Rh ¸ XAS measurements were carried out at the ESRF beamline ID 12A in the total fluorescence detection mode. The XMCD signal was monitored by keeping the helicity of the incoming X-rays fixed and flipping the direction of the applied magnetic field. The XAS spectra presented here are recorded with the incoming light parallel to the direction of the applied magnetic field (2 T) and perpendicular to the c-axis, i.e. parallel to the easy axis of the magnetisation, at ¹"10 K. As a first approach in the understanding of the magnetic properties of CeRh B , one could expect that the high Curie temperature results from the Rh-4d electrons. However, our own XMCD experiments performed at the Rh-¸ edges exclude this assumption (M )0.004l ). 0 This result is in agreement with both valence-band photoemission and recent polarised neutron diffraction measurements [7].
0921-4526/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 8 ) 0 0 7 3 6 - 4
W. Grange et al. / Physica B 259—261 (1999) 1142—1143
Fig. 1. Ce-¸ edges in CeRh B , CeRu Ge and CeFe (from Ref. [10]). The arrow indicates the position of the 4f contribution.
We now discuss results obtained from measurements at the Ce-¸ edges and show that the character of the 4f states is intermediate to that found in strong localised and well-delocalised Ce compounds. In highly correlated systems the Ce-¸ absorption edges may present a satellite located at about 10 eV above the main white line. This structure can be interpreted either as a “shake up” or the signature of a 4f configuration in the ground state (the 4f electrons interact strongly with the 2p core holes leading to 2p4f5dL> and 2p4f5dL> final states [8]). The interpretation of the satellite is far from trivial; however, the Ce-M edges also show a weak satellite [9]. This sug gests that the structure (not observed for trivalent Ce compounds such as CeRu Ge ) results from a 4f config uration. Consequently, the 5d—4f hybridisation is certainly non-negligible in CeRh B (Fig. 1). Fig. 2 illustrates the XMCD response at the Ce-¸ edges for CeRu Ge (¹ "8 K), CeRh B (¹ "110 K) and CeFe (¹ "220 K). The presence of a signal reveals the existence of a 5d polarisation. The large palette of magnetic signals observed shows that the 5d XMCD is very sensitive to the Ce ground state. For CeRu Ge , where the 4f localised character is undisputed, the XMCD signals are characterised by a negative (positive) signal at the ¸ (¸ ) edge. The sign of the integrated intensity is opposite to that expected from the one-particle model. Moreover, the branching ratio (!5), is far away from !2 i.e., the value expected from band structure calculations and corresponding to a pure spin contribution. Such dichroic signals, always observed for well-localised 4f systems, are taken as a reference for weak Kondo temperature systems [10,11]. In contrast, for CeFe both the sign and the branching ratio of XMCD are in accordance with the one-particle model, in agreement with the small Ce-4f moment contribution
1143
Fig. 2. XMCD at the Ce-¸ edges for CeRu Ge , CeRh B and CeFe (from Ref. [10]). Normalised absorption unit corre sponds to 1% of the edge.
and consequently with the itinerant 5d band character, without any orbital contribution. The unusual XMCD observed in CeRh B , which can be interpreted as a combination of that observed in CeFe and CeRu Ge , demonstrates the importance of the 4f hybridisation in the peculiar properties of CeRh B . Indeed, the arrows in Fig. 2 illustrate a possible correspondence of the main XMCD structures of CeFe and those of CeRh B .
3. Conclusions In conclusion, the XMCD signals in CeRh B suggest that the 4f states do not reach the high degree of localisation observed in CeRu Ge due to 5d—4f hybridisation. This result is in agreement with the additional features observed in Ce-M absorption edges.
References [1] S.K. Dhar et al., J. Phys. C 14 (1981) L321. [2] M. Kasaya et al., J. Magn. Magn. Mater. 76 & 77 (1988) 347. [3] K. Yamaguchi et al., Phys. Rev. B 46 (1992) 9845. [4] K. Yamaguchi et al., Phys. Rev. B 51 (1992) 13 952. [5] A.C. Lawson, A. Williams, J.G. Huber, J. Less-common Met. 136 (1987) 87. [6] E.V. Sampathkumaran et al., Phys. Rev. B 31 (1985) 3185. [7] J.A. Alonso et al., J. Magn. Magn. Mater. 177—181 (1998) 1048. [8] J. Ro¨hler, Handbook of the Physics and Chemistry of Rare Earths, North-Holland, Amsterdam, 1987. [9] J.Ph. Schilli et al., Phys. Rev. B 50 (1994) 2985. [10] Ch. Giorgetti et al., Phys. Rev. B 48 (1993) 12 732. [11] M. Arend et al., Phys. Rev. B 57 (1998) 2174.
Magnetic circular dichroism in CeRh B W. Grange *, J.-P. Kappler , E. Dartyge, P. Haen, P. Lejay, A. Rogalev IPCMS-GEMME, 23 rue du Loess, 67037 Strasbourg, France LURE, B.P. 34, 91898 Orsay, Cedex, France CRTBT, B.P. 166, 38042 Grenoble, Cedex, France ESRF, B.P. 220, 38043 Grenoble, Cedex, France
Abstract We report a magnetic circular dichroism study at the Ce-2p core level X-ray absorption spectra on the ferromagnet CeRh B . The Ce-¸ spectra measured on a single crystal are compared to those obtained on CeRu Ge and CeFe , taken as reference systems. The shape of the circular dichroism signal, that witnesses the 4f hybridisation, supports the delocalised picture of the 4f electrons in CeRh B . 1999 Elsevier Science B.V. All rights reserved. Keywords: Ce; Ferromagnetism; Magnetic dichroism
1. Introduction The study of the ternary boride CeRh B is of great importance because of its exceptional ferromagnetic state. One of the most surprising feature is that the Curie temperature is unusually large (¹ "115 K) for a Ce compound without 3d magnetic constituents [1]. Magnetisation measurements show that the saturation moment is only about 0.5—0.4l /fu [2]: this is much smaller than the free-ion value and has been attributed to large crystal field effects and/or a Kondo-type interaction [3,4]. The structural properties of CeRh B seem also anomalous: CeRh B crystallises in a distorted hexa gonal P6/mmm CeCo B -like structure, with a"5.484 As and c"3.087 As ; this latter value is the shortest ever observed in intermetallic Ce compounds. The linear thermal expansion is large and highly anisotropic: along the a-axis the expansion is nearly temperature independent whereas along the c-axis the expansion is three times larger than in an usual metal [5,6]. To precise the role played by the Ce-5d and Rh-4d electrons in the magnetic properties of CeRh B , we present in this paper X-ray absorption spectroscopy * Corresponding author. Tel.: 33388-107-069; fax: 33388-107249; e-mail: [email protected].
(XAS) and X-ray magnetic circular dichroism (XMCD) experiments performed at the Ce and Rh ¸ edges. The comparison of the spectra with those obtained from some reference systems evidences the itinerant behaviour of the 4f electrons.
2. Results and discussion The Ce and Rh ¸ XAS measurements were carried out at the ESRF beamline ID 12A in the total fluorescence detection mode. The XMCD signal was monitored by keeping the helicity of the incoming X-rays fixed and flipping the direction of the applied magnetic field. The XAS spectra presented here are recorded with the incoming light parallel to the direction of the applied magnetic field (2 T) and perpendicular to the c-axis, i.e. parallel to the easy axis of the magnetisation, at ¹"10 K. As a first approach in the understanding of the magnetic properties of CeRh B , one could expect that the high Curie temperature results from the Rh-4d electrons. However, our own XMCD experiments performed at the Rh-¸ edges exclude this assumption (M )0.004l ). 0 This result is in agreement with both valence-band photoemission and recent polarised neutron diffraction measurements [7].
0921-4526/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 8 ) 0 0 7 3 6 - 4
W. Grange et al. / Physica B 259—261 (1999) 1142—1143
Fig. 1. Ce-¸ edges in CeRh B , CeRu Ge and CeFe (from Ref. [10]). The arrow indicates the position of the 4f contribution.
We now discuss results obtained from measurements at the Ce-¸ edges and show that the character of the 4f states is intermediate to that found in strong localised and well-delocalised Ce compounds. In highly correlated systems the Ce-¸ absorption edges may present a satellite located at about 10 eV above the main white line. This structure can be interpreted either as a “shake up” or the signature of a 4f configuration in the ground state (the 4f electrons interact strongly with the 2p core holes leading to 2p4f5dL> and 2p4f5dL> final states [8]). The interpretation of the satellite is far from trivial; however, the Ce-M edges also show a weak satellite [9]. This sug gests that the structure (not observed for trivalent Ce compounds such as CeRu Ge ) results from a 4f config uration. Consequently, the 5d—4f hybridisation is certainly non-negligible in CeRh B (Fig. 1). Fig. 2 illustrates the XMCD response at the Ce-¸ edges for CeRu Ge (¹ "8 K), CeRh B (¹ "110 K) and CeFe (¹ "220 K). The presence of a signal reveals the existence of a 5d polarisation. The large palette of magnetic signals observed shows that the 5d XMCD is very sensitive to the Ce ground state. For CeRu Ge , where the 4f localised character is undisputed, the XMCD signals are characterised by a negative (positive) signal at the ¸ (¸ ) edge. The sign of the integrated intensity is opposite to that expected from the one-particle model. Moreover, the branching ratio (!5), is far away from !2 i.e., the value expected from band structure calculations and corresponding to a pure spin contribution. Such dichroic signals, always observed for well-localised 4f systems, are taken as a reference for weak Kondo temperature systems [10,11]. In contrast, for CeFe both the sign and the branching ratio of XMCD are in accordance with the one-particle model, in agreement with the small Ce-4f moment contribution
1143
Fig. 2. XMCD at the Ce-¸ edges for CeRu Ge , CeRh B and CeFe (from Ref. [10]). Normalised absorption unit corre sponds to 1% of the edge.
and consequently with the itinerant 5d band character, without any orbital contribution. The unusual XMCD observed in CeRh B , which can be interpreted as a combination of that observed in CeFe and CeRu Ge , demonstrates the importance of the 4f hybridisation in the peculiar properties of CeRh B . Indeed, the arrows in Fig. 2 illustrate a possible correspondence of the main XMCD structures of CeFe and those of CeRh B .
3. Conclusions In conclusion, the XMCD signals in CeRh B suggest that the 4f states do not reach the high degree of localisation observed in CeRu Ge due to 5d—4f hybridisation. This result is in agreement with the additional features observed in Ce-M absorption edges.
References [1] S.K. Dhar et al., J. Phys. C 14 (1981) L321. [2] M. Kasaya et al., J. Magn. Magn. Mater. 76 & 77 (1988) 347. [3] K. Yamaguchi et al., Phys. Rev. B 46 (1992) 9845. [4] K. Yamaguchi et al., Phys. Rev. B 51 (1992) 13 952. [5] A.C. Lawson, A. Williams, J.G. Huber, J. Less-common Met. 136 (1987) 87. [6] E.V. Sampathkumaran et al., Phys. Rev. B 31 (1985) 3185. [7] J.A. Alonso et al., J. Magn. Magn. Mater. 177—181 (1998) 1048. [8] J. Ro¨hler, Handbook of the Physics and Chemistry of Rare Earths, North-Holland, Amsterdam, 1987. [9] J.Ph. Schilli et al., Phys. Rev. B 50 (1994) 2985. [10] Ch. Giorgetti et al., Phys. Rev. B 48 (1993) 12 732. [11] M. Arend et al., Phys. Rev. B 57 (1998) 2174.