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NMR study of the magnetic properties of the ordered perovskite Sr2Cu(Re0.69Ca0.31)O6
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 272–276 (2004) 818–819
NMR study of the magnetic properties of the ordered perovskite Sr2 CuðRe0:69Ca0:31ÞO6 Y. Uchida*, M. Isobe, E. Takayama-Muromachi Superconducting Materials Center, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
Abstract NMR measurements have been performed on the ordered perovskite Sr2 CuðRe0:69 Ca0:31 ÞO6 which shows ferromagnetic (ferrimagnetic) properties below Tc B440 K with spontaneous magnetization of about 0:25 mB per formula unit at 5 K: Two kinds of NMR lines showing large enhancement of NMR signals were observed at 5 K with H ¼ 0:5 T at the frequency range from 58 to 180 MHz; and each line was attributed to the Cu1 and Cu2 site. r 2003 Elsevier B.V. All rights reserved. PACS: 75:50: y; 76:60: k Keywords: Correlated electron system; Ferromagnetism; NMR; Perovskite structure; Transition metal oxides
1. Introduction The ordered perovskite Sr2 CuðRe0:69 Ca0:31 ÞO6 was recently found to show insulating ferromagnetic (ferrimagnetic) properties below Tc B440 K with spontaneous magnetization of about 0:25 mB per formula unit at 5 K [1]. The crystal structure of the compound belongs to the space group Pm–3m (No. 221) and the unit cell consists of four formula units. There are two different kinds of Cu site: Cu1 occupies at the 1b site having the Oh symmetry and Cu2 at the 3c site with the D4h symmetry as shown in Fig. 1. The Re ion occupying at the 3d site is considered a nonmagnetic Re7þ state on the basis of the results of electron energy loss spectroscopy (EELS) measurements [1]. Bond valence sum calculations suggest the Cu3þ state (3d8 ; S ¼ 1) for the Cu1 site and Cu2þ (3d9 ; S ¼ 12) for Cu2. The Cu1 site is surrounded by six neighboring Cu2þ ions while the Cu2 site by two Cu3þ and four Re7þ ions. Recently, the ordered double perovskites A2 B0 BO6 (A=Ca, Sr,Ba; B0 ¼ Fe; Cr; B=Mo, Re) have attracted *Corresponding author. Tel.: +81-29-860-4508; fax: +8129-860-4674. E-mail address: [email protected] (Y. Uchida).
much attention because of their interesting physical properties such as the half-metallic ferrimagnet [2]. In these compound, the transition-metal sites are occupied alternatively by B0 and B. In Sr2 CuðRe0:69 Ca0:31 ÞO6 ; B0 is formally expressed as ðCu2þ 0:75 Ca0:25 Þ and B as ðCu3þ Re Ca Þ: 0:69 0:06 0:25
2. Results and discussion The spin echo NMR spectra for a powder sample of Sr2 CuðRe0:69 Ca0:31 ÞO6 showing large enhancement of NMR signals were observed at 5 K with a magnetic field H of 0:5 T as shown by solid circles in Fig. 2. The field of 0:5 T is sufficient to saturate the magnetization of the sample. At each frequency, we have adjusted the RF pulse levels (i.e., around 10 V peak-to-peak) to obtain the maximum spin echo signal. The well resolved resonance lines with the full-width at half-maximum of 8 MHz have been observed at 163 and 174 MHz; the ratio of which agrees well with that of the nuclear gyromagnetic ratio of the 65;63 Cu nuclei ð65 g=63 g ¼ 1:071Þ: Then the two lines are clearly identified as the 63 Cu=65 Cu NMR lines and this frequency corresponds to the hyperfine field of 14:4 T: The spin echo decay at 163 MHz was almost single exponential with the spin
0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.1280
ARTICLE IN PRESS Y. Uchida et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 818–819
819 63
T=5K
Cu
Cu2
O2 Cu1
O1
H=1.0T
65
Cu
×5
60
O3 Ca
Spin Echo Intensity
H=0.5T
Sr
Re
80
100
120
140
160
180
Frequency (MHz)
Fig. 1. Crystal structure of Sr2 CuðRe0:69 Ca0:31 ÞO6 : 8% of the Re sites are shared with the Ca ions.
Fig. 2. The spin echo NMR spectra in Sr2 CuðRe0:69 Ca0:31 ÞO6 at 5 K with H ¼ 0:5 T (solid circle) and H ¼ 1:0 T (open circle).
echo decay time T2 of 0:8 ms: The other NMR line is weak and very broad spreading over the wide frequency range from 58 to 148 MHz: The spin echo decay did not show any spin echo modulation caused by the quadrupole interaction, but also nearly single exponential and T2 at 86 MHz was about 0:4 ms: The results of EELS measurements indicate that the Re ion is a nonmagnetic Re7þ state. By the search for NMR signals made up to the maximum frequency of 380 MHz; we could not observe any other spin echo signals. It is reasonable to assume that the Cu NMR lines observed at 163 and 174 MHz are attributed to the Cu1 site (Cu3þ ; S ¼ 1) having the Oh symmetry and the broad NMR line to the Cu2 site (Cu2þ ; S ¼ 12) surrounded by two Cu3þ and four Re7þ ions. The 63 Cu NMR line at 163 MHz with H ¼ 0:5 T shifts to 169 MHz when H ¼ 1:0 T as shown by open circles in Fig. 2. The difference of 6 MHz agrees well with the estimated value from 63 g ¼ 11:285 MHz=T: This assures the magnetic field of 0:5 T sufficiently strong to saturate the magnetization of the sample. The small spontaneous magnetization per formula unit at 5 K and a large antiferromagnetic superexchange interaction expected between Cu1 and Cu2 sites via the Cu– O–Cu 180 bond channel in the perovskite structure suggest a ferrimagnetic ground state for this compound. Since there are one Cu1 site and three Cu2 sites in the unit cell, the magnetic moment of Cu1 has an opposite direction to the spontaneous magnetization. The in-
crease of the resonance frequency of Cu1 with increasing H indicates that the hyperfine field is negative, i.e. antiparallel to the magnetic moment of Cu1. When H ¼ 1:0 T; the sides of the broad line shift to the higher frequency direction: the increase of 3:8 MHz is observed near 85 MHz and 4 MHz near 140 MHz: It is known that the hyperfine interaction of a Cu2þ ion with octahedral coordination is very anisotropic. The Cu NMR lines for a single crystal of K2 CuF4 in the ferromagnetic state have been observed at the frequency range from about 30–200 MHz [3]. The broad spectrum spreading from 58 to 148 MHz is understood as the strongly anisotropic character of the hyperfine interaction and the electric quadrupole interaction of the Cu2 site. In these cases it is hard to obtain the information about the hyperfine field of Cu2 from the H dependence of NMR spectra of a powder sample.
References [1] E. Takayama-Muromachi, T. Drezen, M. Isobe, N.D. Zhigadlo, K. Kimoto, Y. Matsui, E. Kita, J. Solid State Chem. 175 (2003) 366. [2] K.-I. Kobayashi, T. Kimura, H. Sawada, K. Terakura, Y. Tokura, Nature 395 (1998) 677. [3] M. Fujii, F. Wakai, H. Abe, A. Hirai, J. Phys. Soc. Jpn. 50 (1981) 1109.
Journal of Magnetism and Magnetic Materials 272–276 (2004) 818–819
NMR study of the magnetic properties of the ordered perovskite Sr2 CuðRe0:69Ca0:31ÞO6 Y. Uchida*, M. Isobe, E. Takayama-Muromachi Superconducting Materials Center, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
Abstract NMR measurements have been performed on the ordered perovskite Sr2 CuðRe0:69 Ca0:31 ÞO6 which shows ferromagnetic (ferrimagnetic) properties below Tc B440 K with spontaneous magnetization of about 0:25 mB per formula unit at 5 K: Two kinds of NMR lines showing large enhancement of NMR signals were observed at 5 K with H ¼ 0:5 T at the frequency range from 58 to 180 MHz; and each line was attributed to the Cu1 and Cu2 site. r 2003 Elsevier B.V. All rights reserved. PACS: 75:50: y; 76:60: k Keywords: Correlated electron system; Ferromagnetism; NMR; Perovskite structure; Transition metal oxides
1. Introduction The ordered perovskite Sr2 CuðRe0:69 Ca0:31 ÞO6 was recently found to show insulating ferromagnetic (ferrimagnetic) properties below Tc B440 K with spontaneous magnetization of about 0:25 mB per formula unit at 5 K [1]. The crystal structure of the compound belongs to the space group Pm–3m (No. 221) and the unit cell consists of four formula units. There are two different kinds of Cu site: Cu1 occupies at the 1b site having the Oh symmetry and Cu2 at the 3c site with the D4h symmetry as shown in Fig. 1. The Re ion occupying at the 3d site is considered a nonmagnetic Re7þ state on the basis of the results of electron energy loss spectroscopy (EELS) measurements [1]. Bond valence sum calculations suggest the Cu3þ state (3d8 ; S ¼ 1) for the Cu1 site and Cu2þ (3d9 ; S ¼ 12) for Cu2. The Cu1 site is surrounded by six neighboring Cu2þ ions while the Cu2 site by two Cu3þ and four Re7þ ions. Recently, the ordered double perovskites A2 B0 BO6 (A=Ca, Sr,Ba; B0 ¼ Fe; Cr; B=Mo, Re) have attracted *Corresponding author. Tel.: +81-29-860-4508; fax: +8129-860-4674. E-mail address: [email protected] (Y. Uchida).
much attention because of their interesting physical properties such as the half-metallic ferrimagnet [2]. In these compound, the transition-metal sites are occupied alternatively by B0 and B. In Sr2 CuðRe0:69 Ca0:31 ÞO6 ; B0 is formally expressed as ðCu2þ 0:75 Ca0:25 Þ and B as ðCu3þ Re Ca Þ: 0:69 0:06 0:25
2. Results and discussion The spin echo NMR spectra for a powder sample of Sr2 CuðRe0:69 Ca0:31 ÞO6 showing large enhancement of NMR signals were observed at 5 K with a magnetic field H of 0:5 T as shown by solid circles in Fig. 2. The field of 0:5 T is sufficient to saturate the magnetization of the sample. At each frequency, we have adjusted the RF pulse levels (i.e., around 10 V peak-to-peak) to obtain the maximum spin echo signal. The well resolved resonance lines with the full-width at half-maximum of 8 MHz have been observed at 163 and 174 MHz; the ratio of which agrees well with that of the nuclear gyromagnetic ratio of the 65;63 Cu nuclei ð65 g=63 g ¼ 1:071Þ: Then the two lines are clearly identified as the 63 Cu=65 Cu NMR lines and this frequency corresponds to the hyperfine field of 14:4 T: The spin echo decay at 163 MHz was almost single exponential with the spin
0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.1280
ARTICLE IN PRESS Y. Uchida et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 818–819
819 63
T=5K
Cu
Cu2
O2 Cu1
O1
H=1.0T
65
Cu
×5
60
O3 Ca
Spin Echo Intensity
H=0.5T
Sr
Re
80
100
120
140
160
180
Frequency (MHz)
Fig. 1. Crystal structure of Sr2 CuðRe0:69 Ca0:31 ÞO6 : 8% of the Re sites are shared with the Ca ions.
Fig. 2. The spin echo NMR spectra in Sr2 CuðRe0:69 Ca0:31 ÞO6 at 5 K with H ¼ 0:5 T (solid circle) and H ¼ 1:0 T (open circle).
echo decay time T2 of 0:8 ms: The other NMR line is weak and very broad spreading over the wide frequency range from 58 to 148 MHz: The spin echo decay did not show any spin echo modulation caused by the quadrupole interaction, but also nearly single exponential and T2 at 86 MHz was about 0:4 ms: The results of EELS measurements indicate that the Re ion is a nonmagnetic Re7þ state. By the search for NMR signals made up to the maximum frequency of 380 MHz; we could not observe any other spin echo signals. It is reasonable to assume that the Cu NMR lines observed at 163 and 174 MHz are attributed to the Cu1 site (Cu3þ ; S ¼ 1) having the Oh symmetry and the broad NMR line to the Cu2 site (Cu2þ ; S ¼ 12) surrounded by two Cu3þ and four Re7þ ions. The 63 Cu NMR line at 163 MHz with H ¼ 0:5 T shifts to 169 MHz when H ¼ 1:0 T as shown by open circles in Fig. 2. The difference of 6 MHz agrees well with the estimated value from 63 g ¼ 11:285 MHz=T: This assures the magnetic field of 0:5 T sufficiently strong to saturate the magnetization of the sample. The small spontaneous magnetization per formula unit at 5 K and a large antiferromagnetic superexchange interaction expected between Cu1 and Cu2 sites via the Cu– O–Cu 180 bond channel in the perovskite structure suggest a ferrimagnetic ground state for this compound. Since there are one Cu1 site and three Cu2 sites in the unit cell, the magnetic moment of Cu1 has an opposite direction to the spontaneous magnetization. The in-
crease of the resonance frequency of Cu1 with increasing H indicates that the hyperfine field is negative, i.e. antiparallel to the magnetic moment of Cu1. When H ¼ 1:0 T; the sides of the broad line shift to the higher frequency direction: the increase of 3:8 MHz is observed near 85 MHz and 4 MHz near 140 MHz: It is known that the hyperfine interaction of a Cu2þ ion with octahedral coordination is very anisotropic. The Cu NMR lines for a single crystal of K2 CuF4 in the ferromagnetic state have been observed at the frequency range from about 30–200 MHz [3]. The broad spectrum spreading from 58 to 148 MHz is understood as the strongly anisotropic character of the hyperfine interaction and the electric quadrupole interaction of the Cu2 site. In these cases it is hard to obtain the information about the hyperfine field of Cu2 from the H dependence of NMR spectra of a powder sample.
References [1] E. Takayama-Muromachi, T. Drezen, M. Isobe, N.D. Zhigadlo, K. Kimoto, Y. Matsui, E. Kita, J. Solid State Chem. 175 (2003) 366. [2] K.-I. Kobayashi, T. Kimura, H. Sawada, K. Terakura, Y. Tokura, Nature 395 (1998) 677. [3] M. Fujii, F. Wakai, H. Abe, A. Hirai, J. Phys. Soc. Jpn. 50 (1981) 1109.