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Investigation of Eu6C60 magnetic properties
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 272–276 (2004) 544–545
Investigation of Eu6 C60 magnetic properties T. Shirokaa,*, M. Ricco" a, O. Ligabuea, F. Bolzonib, G. Concasc, G. Spanoc Dip.to di Fisica and INFM, Universita" di Parma, Parco Area delle Scienze 7/a, Parma 43100, Italy b Istituto Maspec-CNR, Parco Area delle Scienze, Loc. Fontanini, Parma 43010, Italy c Dip.to di Fisica and INFM, Univ. di Cagliari, S.P. Monserrato-Sestu, Monserrato 09042 (CA), Italy a
Abstract A representative rare earth doped fulleride, Eu6 C60 ; has been studied using several spectroscopic and magnetometry techniques. We find that europium ions are present both in their magnetic bivalent state and nonmagnetic trivalent state. The Eu2þ =Eu3þ ratio seems to critically depend on sample preparation conditions. Muon relaxation rates show a large magnetic field distribution near the ordered phase, whereas 13 C NMR lineshapes at room temperature are consistent with dipolar field broadening (B100 G) from neighbouring Eu2þ magnetic moments. r 2003 Elsevier B.V. All rights reserved. PACS: 75.50.y; 71.20.Tx; 75.20.Hr; 76.30.Kg Keywords: Eu6 C60 ; Rare earth fullerides; Magnetically ordered materials
Rare earth doped fullerides are expected to exhibit interesting magnetic properties, due to the presence of 4f electrons in lanthanide atoms and their tendency to form coordination compounds. A representative compound, such as Eu6 C60 ; which is known to display both ferromagnetic behaviour [1] as well as giant magnetoresistance [2], has been extensively studied in this work using several experimental techniques including: SQUID magnetometry, muon spin rotation, 13 C NMR and 151 . Eu Mossbauer spectroscopy. The samples were prepared using standard solid state synthesis techniques, by mixing stoichiometric amounts of fullerene and europium and heating the mixture at 6003 C in a controlled environment (o1 ppm O2 and H2 O). It is known that generally europium can be found either in the form of magnetic bivalent ion Eu2þ ; characterized by a 7 mB magnetic moment, or as a nonmagnetic trivalent ion Eu3þ : Although previous works have shown the presence of both type of ions in Eu6 C60 [1,3], recent studies [2,4] claim the bivalent state to be the only present in this rare earth fulleride.
*Corresponding author. Tel.: +39-0521-905803; fax: +390521-905223. E-mail address: toni.shiroka@fis.unipr.it (T. Shiroka).
SQUID magnetometry confirmed the presence of a transition to a ferromagnetic state, observed near TB16 K and of a second transition (whose nature is still being investigated) occurring near T K. Beside that, measurements of saturation magnetisation allowed us to evaluate the concentration of magnetic Eu2þ ions. Systematic studies of magnetism in several samples, prepared in different heat treatment conditions (annealing time from several hours to several weeks), show a strong correlation between the maximum achievable magnetization and the annealing time. As depicted in Fig. 1 both very short or very prolonged heating will cause a decrease in the magnetic moment, by favouring the appearance of nonmagnetic Eu3þ ions. Measurements at a microscopic scale, including the . use of mSR, NMR and Mossbauer spectroscopies, were carried out in two different samples: A and B, with annealing times of 85 and 1000 h respectively. Since sample A shows a higher value of saturation moment, it is expected to be richer in Eu2þ ions. A steep increase in the zero-field muon relaxation rates when approaching the magnetically ordered phase at Tc C16 K; was observed in both samples. The rather fast relaxation rate (B5 ms1 ), together with the longitudinal field repolarization curves in the magnetic phase, indicate a wide distribution of static internal
0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.397
ARTICLE IN PRESS T. Shiroka et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 544–545 6
85
A
Bohr magnetons/Eu ion
75 5
70
4.5
B
65 60
Percentage of Eu2+ ions
80
5.5
4 55
Annealing temperature: 600˚C
3.5
50 0
10
1
10
2
10
3
10
Annealing time (hours)
Fig. 1. Evolution of magnetic moment (left) and of the Eu2þ content (right) with annealing time. Notice the broad maximum occurring after B100 h of annealing at 6003 C. Squares . represent Mosbbauer data for samples A and B.
fields. Probably due to the large quadrupolar moment of the Eu nucleus no direct NMR signal could be observed from the Eu ions. Nevertheless, the 13 C NMR resonance was easier to detect and investigate. More importantly, recent studies on the Sr-doped Eu6 C60 [2], suggest that C60 units provide an indirect exchange among the magnetic Eu ions, a circumstance we expect to be reflected in the 13 C signal. Taking into account the known structure of Eu6 C60 [2], at room temperature we expect a B90 G internal field at carbon sites, due to neighbouring Eu2þ dipolar moments. This value was found to be in good agreement with the measured B105 G experimental broadening. Measurements of carbon linewidth as a function of temperature show an anomalous peak around 16 K, where the transition to the magnetically ordered phase occurs; on the other hand a very steep increase in FWHM was observed in the range 60 – 100 K. . Mossbauer spectroscopy represents a direct method for checking, at a microscopic level, the presence of both
545
type of ions in Eu6 C60 ; since the difference of the isomer shifts of Eu2þ and Eu3þ is much larger than the linewidth. The experiments were carried out in a transmission geometry using a 151 SmF3 source (activity 3.7 GBq) and the resulting spectra were fitted with a superposition of two Lorentzian curves. We find that both Eu2þ as well as Eu3þ ions are present in our samples; the Eu2þ is identified by an absorption peak at a velocity value of about 12 mm/s and Eu3þ by a peak at about 0.5 mm/s. There is also a strong dependence of the Eu2þ /Eu3þ ratio on the sample annealing time. Thus, the approximative 6:1 ratio for Eu2þ /Eu3þ in sample A, becomes 2:1 in sample B, showing a decrease in Eu2þ content when the annealing time is protracted. Both results are consistent with the magnetization measurements as shown in Fig. 1. We emphasize that similar results were obtained also in . recent Mossbauer experiments carried out in Eu3 C60 samples, prepared either through solid state synthesis [1] or by following a liquid ammonia route [3]. Taking into account the large number of samples considered, the different preparation techniques employed and the variety of heat treatments, it seems reasonable to conclude that the coexistence of bivalent and trivalent europium ions seems to be the rule rather than the exception in the europium fullerides. The magnetic behaviour of compounds with different Eu2þ /Eu3þ ratios could be rationalized in terms of the coexistence of paramagnetic regions (characterized by low Eu2þ concentrations) with ferromagnetic regions (where Eu ions are exclusively in the 2+ oxidation state). More details will be given in a forthcoming extended paper.
References [1] [2] [3] [4]
Y. Ksari, et al., J. Phys. Chem. Solids 58 (1997) 1771. K. Ishii, et al., Phys. Rev. B 65 (2002) 134431. A. V!ertes, et al., J. Phys. Chem. Solids 61 (2000) 2013. I. Margiolaki, et al., J. Am. Chem. Soc. 24 (2002) 11288.
Journal of Magnetism and Magnetic Materials 272–276 (2004) 544–545
Investigation of Eu6 C60 magnetic properties T. Shirokaa,*, M. Ricco" a, O. Ligabuea, F. Bolzonib, G. Concasc, G. Spanoc Dip.to di Fisica and INFM, Universita" di Parma, Parco Area delle Scienze 7/a, Parma 43100, Italy b Istituto Maspec-CNR, Parco Area delle Scienze, Loc. Fontanini, Parma 43010, Italy c Dip.to di Fisica and INFM, Univ. di Cagliari, S.P. Monserrato-Sestu, Monserrato 09042 (CA), Italy a
Abstract A representative rare earth doped fulleride, Eu6 C60 ; has been studied using several spectroscopic and magnetometry techniques. We find that europium ions are present both in their magnetic bivalent state and nonmagnetic trivalent state. The Eu2þ =Eu3þ ratio seems to critically depend on sample preparation conditions. Muon relaxation rates show a large magnetic field distribution near the ordered phase, whereas 13 C NMR lineshapes at room temperature are consistent with dipolar field broadening (B100 G) from neighbouring Eu2þ magnetic moments. r 2003 Elsevier B.V. All rights reserved. PACS: 75.50.y; 71.20.Tx; 75.20.Hr; 76.30.Kg Keywords: Eu6 C60 ; Rare earth fullerides; Magnetically ordered materials
Rare earth doped fullerides are expected to exhibit interesting magnetic properties, due to the presence of 4f electrons in lanthanide atoms and their tendency to form coordination compounds. A representative compound, such as Eu6 C60 ; which is known to display both ferromagnetic behaviour [1] as well as giant magnetoresistance [2], has been extensively studied in this work using several experimental techniques including: SQUID magnetometry, muon spin rotation, 13 C NMR and 151 . Eu Mossbauer spectroscopy. The samples were prepared using standard solid state synthesis techniques, by mixing stoichiometric amounts of fullerene and europium and heating the mixture at 6003 C in a controlled environment (o1 ppm O2 and H2 O). It is known that generally europium can be found either in the form of magnetic bivalent ion Eu2þ ; characterized by a 7 mB magnetic moment, or as a nonmagnetic trivalent ion Eu3þ : Although previous works have shown the presence of both type of ions in Eu6 C60 [1,3], recent studies [2,4] claim the bivalent state to be the only present in this rare earth fulleride.
*Corresponding author. Tel.: +39-0521-905803; fax: +390521-905223. E-mail address: toni.shiroka@fis.unipr.it (T. Shiroka).
SQUID magnetometry confirmed the presence of a transition to a ferromagnetic state, observed near TB16 K and of a second transition (whose nature is still being investigated) occurring near T K. Beside that, measurements of saturation magnetisation allowed us to evaluate the concentration of magnetic Eu2þ ions. Systematic studies of magnetism in several samples, prepared in different heat treatment conditions (annealing time from several hours to several weeks), show a strong correlation between the maximum achievable magnetization and the annealing time. As depicted in Fig. 1 both very short or very prolonged heating will cause a decrease in the magnetic moment, by favouring the appearance of nonmagnetic Eu3þ ions. Measurements at a microscopic scale, including the . use of mSR, NMR and Mossbauer spectroscopies, were carried out in two different samples: A and B, with annealing times of 85 and 1000 h respectively. Since sample A shows a higher value of saturation moment, it is expected to be richer in Eu2þ ions. A steep increase in the zero-field muon relaxation rates when approaching the magnetically ordered phase at Tc C16 K; was observed in both samples. The rather fast relaxation rate (B5 ms1 ), together with the longitudinal field repolarization curves in the magnetic phase, indicate a wide distribution of static internal
0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.397
ARTICLE IN PRESS T. Shiroka et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 544–545 6
85
A
Bohr magnetons/Eu ion
75 5
70
4.5
B
65 60
Percentage of Eu2+ ions
80
5.5
4 55
Annealing temperature: 600˚C
3.5
50 0
10
1
10
2
10
3
10
Annealing time (hours)
Fig. 1. Evolution of magnetic moment (left) and of the Eu2þ content (right) with annealing time. Notice the broad maximum occurring after B100 h of annealing at 6003 C. Squares . represent Mosbbauer data for samples A and B.
fields. Probably due to the large quadrupolar moment of the Eu nucleus no direct NMR signal could be observed from the Eu ions. Nevertheless, the 13 C NMR resonance was easier to detect and investigate. More importantly, recent studies on the Sr-doped Eu6 C60 [2], suggest that C60 units provide an indirect exchange among the magnetic Eu ions, a circumstance we expect to be reflected in the 13 C signal. Taking into account the known structure of Eu6 C60 [2], at room temperature we expect a B90 G internal field at carbon sites, due to neighbouring Eu2þ dipolar moments. This value was found to be in good agreement with the measured B105 G experimental broadening. Measurements of carbon linewidth as a function of temperature show an anomalous peak around 16 K, where the transition to the magnetically ordered phase occurs; on the other hand a very steep increase in FWHM was observed in the range 60 – 100 K. . Mossbauer spectroscopy represents a direct method for checking, at a microscopic level, the presence of both
545
type of ions in Eu6 C60 ; since the difference of the isomer shifts of Eu2þ and Eu3þ is much larger than the linewidth. The experiments were carried out in a transmission geometry using a 151 SmF3 source (activity 3.7 GBq) and the resulting spectra were fitted with a superposition of two Lorentzian curves. We find that both Eu2þ as well as Eu3þ ions are present in our samples; the Eu2þ is identified by an absorption peak at a velocity value of about 12 mm/s and Eu3þ by a peak at about 0.5 mm/s. There is also a strong dependence of the Eu2þ /Eu3þ ratio on the sample annealing time. Thus, the approximative 6:1 ratio for Eu2þ /Eu3þ in sample A, becomes 2:1 in sample B, showing a decrease in Eu2þ content when the annealing time is protracted. Both results are consistent with the magnetization measurements as shown in Fig. 1. We emphasize that similar results were obtained also in . recent Mossbauer experiments carried out in Eu3 C60 samples, prepared either through solid state synthesis [1] or by following a liquid ammonia route [3]. Taking into account the large number of samples considered, the different preparation techniques employed and the variety of heat treatments, it seems reasonable to conclude that the coexistence of bivalent and trivalent europium ions seems to be the rule rather than the exception in the europium fullerides. The magnetic behaviour of compounds with different Eu2þ /Eu3þ ratios could be rationalized in terms of the coexistence of paramagnetic regions (characterized by low Eu2þ concentrations) with ferromagnetic regions (where Eu ions are exclusively in the 2+ oxidation state). More details will be given in a forthcoming extended paper.
References [1] [2] [3] [4]
Y. Ksari, et al., J. Phys. Chem. Solids 58 (1997) 1771. K. Ishii, et al., Phys. Rev. B 65 (2002) 134431. A. V!ertes, et al., J. Phys. Chem. Solids 61 (2000) 2013. I. Margiolaki, et al., J. Am. Chem. Soc. 24 (2002) 11288.