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XANES and magnetic studies of Ca1−xSrxRuO3 itinerant ferromagnetic system
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 272–276 (2004) 479–481
XANES and magnetic studies of Ca1 xSrxRuO3 itinerant ferromagnetic system B.N. Lin, C.Y. Lin, Y.S. Wu, H.C. Ku* Department of Physics, National Tsing Hua University, Hsinchu 300, Taiwan, ROC
Abstract X-ray absorption near-edge structure (XANES) and magnetic results on the itinerant ferromagnetic system Ca1 xSrxRuO3 (0pxp1) are reported. Ru L3-edge and O K-edge XANES as well as powder X-ray Rietveld analysis indicate stoichiometric 113 composition with tetravalent Ru4+ and larger RuO6 octahedral distortion near the exchange-enhanced paramagnetic metal CaRuO3 region. Larger absorption intensity with increasing Sr substitution indicates larger density of states near the Fermi level eF toward the itinerant ferromagnetic metal SrRuO3. Anomalous field-dependent behavior with small, unsaturated magnetic moment was observed in the ferromagnetic region (x>0.2). Ferromagnetic spin fluctuation was observed in the paramagnetic region (xp0.2) from low temperature M(Ba) hysteresis studies. Time-dependent magnetic relaxation data M(t) show anomalous behavior. r 2004 Elsevier B.V. All rights reserved. PACS: 74.25.Ha; 74.72.Yg; 78.70.Dm Keywords: XANES; Itinerant ferromagnetism; Ca1 xSrxRuO3 system
Since the discovery of p-wave triplet superconductivity in quasi-two-dimensional Sr2RuO4 [1], ruthenates have attracted much interest due to the appearance of wide range of ground states. For example, a fieldinduced quantum critical point was observed in Sr3Ru2O7. On the other hand, the three-dimensional SrRuO3 is an itinerant ferromagnetic metal with Curie temperature TCB160 K, while isostructural CaRuO3 is an exchange-enhanced paramagnetic metal. The Ca1 xSrxRuO3 phase diagram remains unclear due to contradicting experimental results [2–5]. Since X-ray absorption near-edge structure (XANES) data can supply vital information on the variation of Ru 4d unoccupied bands, we present here the results of XANES, magnetic and structural data for the Ca1 xSrxRuO3 system. Samples with nominal composition were synthesized by solid-state reaction and annealed at 1150 C in air. The magnetic susceptibility and magnetization data were *Corresponding author. Tel.: +886-3-5715928; fax: +886-35723052. E-mail address: [email protected] (H.C. Ku).
collected using a Quantum Design MPMS SQUID magnetometer. XANES measurements were performed at Hsinchu National Synchrotron Radiation Research Center (NSRRC). The Ru L3-edge XANES fluorescence mode data were collected in BL15B beamline and the O K-edge XANES data were collected in BL11A beamline. The normalized Ru L3-edge XANES spectrum for the Ca1 xSrxRuO3 system at room temperature are shown in Fig. 1. The A peak is the transition from 2p3/2 to empty 4d–t2g state, and B peak is the transition to empty 4d–eg state. Almost identical threshold edge energies E0 for all Ca1 xSrxRuO3 samples as compared with E0 of RuO2 standard indicate that all samples are indeed very close to the stoichiometric 113 composition with tetravalent Ru4+. Similar shape for the Ca1 xSrxRuO3 system reflects similar RuO6 symmetry in the orthorhombic space group Pbnm while increasing peak intensity with increasing Sr substitution indicates larger density of states near Fermi level eF and smaller lattice distortion. Powder X-ray Rietveld analysis show that a smaller RuO6 distortion with c-axis tilting angle of B10 was observed for SrRuO3 and larger distortion with tilting angle of B20 for CaRuO3. The electronic
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.11.192
ARTICLE IN PRESS 480
B.N. Lin et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 479–481
Fig. 1. Ru L3-edge XANES for Ca1 xSrxRuO3 (x=0, 0.2, 0.5, 0.8 and 1) system at room temperature. The threshold edge energy of RuO2 standard is also indicated.
Fig. 2. Magnetic hysteresis loop M(Ba) for Ca0.8Sr0.2RuO3 at 5, 20, 70 and 100 K.
state and magnetic properties are strongly correlated to the lattice distortion. The O K-edge XANES (1s–2p transition) for more-distorted CaRuO3 indicates smaller 4d t2g–eg band overlap as compared with less-distorted SrRuO3. Anomalous field-dependent behavior with small, unsaturated magnetic moment was observed in the ferromagnetic region. Since the controversial ferromagnetic-paramagnetic transition is believed close to xB0.2 for the Ca1 xSrxRuO3 system where spin glass behavior was observed near the Ca-rich region [2–5], the magnetic hysteresis loop for Ca0.8Sr0.2RuO3 at various temperatures are shown in Fig. 2 to check the magnetic behavior in this region. Ferromagnetic-like hysteresis loop is observed clearly up to 70 K. A small loop can still be observed at 100 K. For comparison, similar ferromagnetic-like hysteresis loop is also observed up to 20 K in Sr-free CaRuO3 prepared in air as well as annealed in Ar or oxygen. This excludes the possible ferromagnetic SrRuO3 contribution due to inhomogeneous Ca/Sr distribution in the sample. However, whether this
Fig. 3. Logarithmic time-dependent magnetization for paramagnetic CaRuO3. Procedure 1: ZFC/100 G relaxation followed by FC/0 G relaxation. Procedure 2: ZFC relaxation only.
ferromagnetic signal is intrinsic or due to other impurities is not clear. The temperature dependence of magnetic susceptibility in CaRuO3 show similar reported anomalous spinglass-like behavior. In order to gather more information of this anomalous behavior, time-dependent magnetic relaxation M(t) at 100 and 0 G for CaRuO3 are shown in Fig. 3. Sample was zero-field-cooled (ZFC) to 5 K and applied a small field of 100 G (magnetization= 1.53 10 4 mB/Ru) and wait for relaxation. A very small relaxation from 1.5246 10 4 to 1.5331 10 4 mB/Ru (B0.55%) can be detected in the present time-scale up to 5 h. The small relaxation is like the reported rate at 6 K [5]. After that, sample was heated to 100 K (procedure 1) and field-cooled (FC) back to 5 K, turn off the field (with FC residual magnetization of 1.72 10 4 mB/Ru) and wait for zero-field relaxation. Again, a small relaxation can be observed for 3 h. In procedure 2, field is turned off which results with a much smaller residual magnetization of 2.1 10 5 mB/Ru. In much higher 1-T field, the field-dependent behavior for CaRuO3 in two relaxation procedures show identical results as expected from simple paramagnetic behavior. On the other hand, the anomalous FC residual magnetization was also observed in ferromagnetic SrRuO3. In conclusion, magnetic and XANES data show a strong correlation between magnetic properties, electronic structure and RuO6 distortion in the Ca1 xSrxRuO3 system. Ferromagnetic spin fluctuation with anomalous time and field dependent behavior was observed near the Ca-rich paramagnetic region with larger lattice distortion. Research supported by under NSC91-2119-M007005.
ARTICLE IN PRESS B.N. Lin et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 479–481
References [1] Y. Maeno, et al., Nature 372 (1995) 532. [2] G. Cao, et al., Phys. Rev. B 56 (1997) 321.
[3] K. Yoshimura, et al., Phys. Rev. Lett. 83 (1999) 4397. [4] I. Felner, et al., Phys. Rev B 62 (2000) 11332. [5] K. Yoshii, H. Abe, Phys. B 63 (2002) 172403.
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Journal of Magnetism and Magnetic Materials 272–276 (2004) 479–481
XANES and magnetic studies of Ca1 xSrxRuO3 itinerant ferromagnetic system B.N. Lin, C.Y. Lin, Y.S. Wu, H.C. Ku* Department of Physics, National Tsing Hua University, Hsinchu 300, Taiwan, ROC
Abstract X-ray absorption near-edge structure (XANES) and magnetic results on the itinerant ferromagnetic system Ca1 xSrxRuO3 (0pxp1) are reported. Ru L3-edge and O K-edge XANES as well as powder X-ray Rietveld analysis indicate stoichiometric 113 composition with tetravalent Ru4+ and larger RuO6 octahedral distortion near the exchange-enhanced paramagnetic metal CaRuO3 region. Larger absorption intensity with increasing Sr substitution indicates larger density of states near the Fermi level eF toward the itinerant ferromagnetic metal SrRuO3. Anomalous field-dependent behavior with small, unsaturated magnetic moment was observed in the ferromagnetic region (x>0.2). Ferromagnetic spin fluctuation was observed in the paramagnetic region (xp0.2) from low temperature M(Ba) hysteresis studies. Time-dependent magnetic relaxation data M(t) show anomalous behavior. r 2004 Elsevier B.V. All rights reserved. PACS: 74.25.Ha; 74.72.Yg; 78.70.Dm Keywords: XANES; Itinerant ferromagnetism; Ca1 xSrxRuO3 system
Since the discovery of p-wave triplet superconductivity in quasi-two-dimensional Sr2RuO4 [1], ruthenates have attracted much interest due to the appearance of wide range of ground states. For example, a fieldinduced quantum critical point was observed in Sr3Ru2O7. On the other hand, the three-dimensional SrRuO3 is an itinerant ferromagnetic metal with Curie temperature TCB160 K, while isostructural CaRuO3 is an exchange-enhanced paramagnetic metal. The Ca1 xSrxRuO3 phase diagram remains unclear due to contradicting experimental results [2–5]. Since X-ray absorption near-edge structure (XANES) data can supply vital information on the variation of Ru 4d unoccupied bands, we present here the results of XANES, magnetic and structural data for the Ca1 xSrxRuO3 system. Samples with nominal composition were synthesized by solid-state reaction and annealed at 1150 C in air. The magnetic susceptibility and magnetization data were *Corresponding author. Tel.: +886-3-5715928; fax: +886-35723052. E-mail address: [email protected] (H.C. Ku).
collected using a Quantum Design MPMS SQUID magnetometer. XANES measurements were performed at Hsinchu National Synchrotron Radiation Research Center (NSRRC). The Ru L3-edge XANES fluorescence mode data were collected in BL15B beamline and the O K-edge XANES data were collected in BL11A beamline. The normalized Ru L3-edge XANES spectrum for the Ca1 xSrxRuO3 system at room temperature are shown in Fig. 1. The A peak is the transition from 2p3/2 to empty 4d–t2g state, and B peak is the transition to empty 4d–eg state. Almost identical threshold edge energies E0 for all Ca1 xSrxRuO3 samples as compared with E0 of RuO2 standard indicate that all samples are indeed very close to the stoichiometric 113 composition with tetravalent Ru4+. Similar shape for the Ca1 xSrxRuO3 system reflects similar RuO6 symmetry in the orthorhombic space group Pbnm while increasing peak intensity with increasing Sr substitution indicates larger density of states near Fermi level eF and smaller lattice distortion. Powder X-ray Rietveld analysis show that a smaller RuO6 distortion with c-axis tilting angle of B10 was observed for SrRuO3 and larger distortion with tilting angle of B20 for CaRuO3. The electronic
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.11.192
ARTICLE IN PRESS 480
B.N. Lin et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 479–481
Fig. 1. Ru L3-edge XANES for Ca1 xSrxRuO3 (x=0, 0.2, 0.5, 0.8 and 1) system at room temperature. The threshold edge energy of RuO2 standard is also indicated.
Fig. 2. Magnetic hysteresis loop M(Ba) for Ca0.8Sr0.2RuO3 at 5, 20, 70 and 100 K.
state and magnetic properties are strongly correlated to the lattice distortion. The O K-edge XANES (1s–2p transition) for more-distorted CaRuO3 indicates smaller 4d t2g–eg band overlap as compared with less-distorted SrRuO3. Anomalous field-dependent behavior with small, unsaturated magnetic moment was observed in the ferromagnetic region. Since the controversial ferromagnetic-paramagnetic transition is believed close to xB0.2 for the Ca1 xSrxRuO3 system where spin glass behavior was observed near the Ca-rich region [2–5], the magnetic hysteresis loop for Ca0.8Sr0.2RuO3 at various temperatures are shown in Fig. 2 to check the magnetic behavior in this region. Ferromagnetic-like hysteresis loop is observed clearly up to 70 K. A small loop can still be observed at 100 K. For comparison, similar ferromagnetic-like hysteresis loop is also observed up to 20 K in Sr-free CaRuO3 prepared in air as well as annealed in Ar or oxygen. This excludes the possible ferromagnetic SrRuO3 contribution due to inhomogeneous Ca/Sr distribution in the sample. However, whether this
Fig. 3. Logarithmic time-dependent magnetization for paramagnetic CaRuO3. Procedure 1: ZFC/100 G relaxation followed by FC/0 G relaxation. Procedure 2: ZFC relaxation only.
ferromagnetic signal is intrinsic or due to other impurities is not clear. The temperature dependence of magnetic susceptibility in CaRuO3 show similar reported anomalous spinglass-like behavior. In order to gather more information of this anomalous behavior, time-dependent magnetic relaxation M(t) at 100 and 0 G for CaRuO3 are shown in Fig. 3. Sample was zero-field-cooled (ZFC) to 5 K and applied a small field of 100 G (magnetization= 1.53 10 4 mB/Ru) and wait for relaxation. A very small relaxation from 1.5246 10 4 to 1.5331 10 4 mB/Ru (B0.55%) can be detected in the present time-scale up to 5 h. The small relaxation is like the reported rate at 6 K [5]. After that, sample was heated to 100 K (procedure 1) and field-cooled (FC) back to 5 K, turn off the field (with FC residual magnetization of 1.72 10 4 mB/Ru) and wait for zero-field relaxation. Again, a small relaxation can be observed for 3 h. In procedure 2, field is turned off which results with a much smaller residual magnetization of 2.1 10 5 mB/Ru. In much higher 1-T field, the field-dependent behavior for CaRuO3 in two relaxation procedures show identical results as expected from simple paramagnetic behavior. On the other hand, the anomalous FC residual magnetization was also observed in ferromagnetic SrRuO3. In conclusion, magnetic and XANES data show a strong correlation between magnetic properties, electronic structure and RuO6 distortion in the Ca1 xSrxRuO3 system. Ferromagnetic spin fluctuation with anomalous time and field dependent behavior was observed near the Ca-rich paramagnetic region with larger lattice distortion. Research supported by under NSC91-2119-M007005.
ARTICLE IN PRESS B.N. Lin et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 479–481
References [1] Y. Maeno, et al., Nature 372 (1995) 532. [2] G. Cao, et al., Phys. Rev. B 56 (1997) 321.
[3] K. Yoshimura, et al., Phys. Rev. Lett. 83 (1999) 4397. [4] I. Felner, et al., Phys. Rev B 62 (2000) 11332. [5] K. Yoshii, H. Abe, Phys. B 63 (2002) 172403.
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