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The magnetic structure of Cu5Bi2B4O14: a neutron scattering study
ARTICLE IN PRESS
Physica B 350 (2004) e1043–e1046
The magnetic structure of Cu5Bi2B4O14: a neutron scattering study G.A. Petrakovskiia, A.M. Vorotynova,*, K.A. Sablinaa, L.V. Udoda, A.I. Pankratsa, C. Ritterb a
L.V.Kirenskii Institute of Physics SB RAS, 660036 Krasnoyarsk, Russia b Laue–Langevin Institute, Grenoble, France
Abstract Neutron-scattering measurements are performed on powders of Cu5Bi2B4O14 to investigate their magnetic structure at temperatures ranging from 2 to 35 K. The ferrimagnetic easy-axis structure is confirmed with spin canting for one of the sublattices. No doubling of the crystal unit cell below the transition temperature TN=25 K is observed. r 2004 Elsevier B.V. All rights reserved. PACS: 75.25.+z Keywords: Neutron scattering; Magnetic structure; Canting
1. Introduction The discovery of high-temperature superconductivity led to active research on oxocuprates. Although oxocuprates are not superconductors, and not even metallic conductors, their crystal structures involve fragments that are similar to those found in high-Tc superconductors. These determine the magnetic properties of the compounds in the case of superexchange interactions. In his monograph, Wells [1] noted that of all chemical elements, bivalent copper should form the largest variety of magnetic structures. This high diversity of magnetic properties of copper*Corresponding author. E-mail address: [email protected] (A. Vorotynov).
oxide compounds can be illustrated with oxocuprates that we studied previously. In particular, CuGeO3 is a chain spin-Peierls magnet with a transition temperature TSP=14 K [2], LiCu2O2 is a two-dimensional antiferromagnet with a damaged ladder structure and a magnetic transition at TN=24 K [3], and Bi2CuO4 is a three-dimensional antiferromagnet characterized by the four-spin exchange interaction and TN=41 K [4]. In CuB2O4 [5–8], the magnetic states for T below 20 K form a complex phase diagram, including a transition between commensurate and incommensurate structures. Recently, the structural, magnetic, and resonance properties of the new compound Cu5Bi2B4O14 were studied [9]. The magnetic phase transition was found at TN=24.5 K. Above TN,
0921-4526/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2004.03.286
ARTICLE IN PRESS e1044
G.A. Petrakovskii et al. / Physica B 350 (2004) e1043–e1046
all the above measurements indicate that Cu5Bi2B4O14 is a paramagnet with an effective magnetic moment meff ¼ 0:81mB and a paramagnetic Curie temperature y¼ 17:4 K: Below TN, the experimental data suggest a ferrimagnetic structure with magnetic moments parallel to the crystal n˜-axis and a saturation magnetization of s=17.1 emu/g. The theoretical saturation magnetization for such a magnetic structure is s=18.1 emu/g. This 6% discrepancy between theory and experiment might suggest some canting of the magnetic moments on either one or both sublattices. Moreover, it is important to directly test the proposed magnetic structure with neutron scattering. To this effect, we grew small, 11B-enriched, Cu5Bi2B4O14 single crystals. Results of neutronscattering experiments to explore the low-temperature magnetic structure of Cu5Bi2B4O14 are reported here.
2. Sample preparation The preparation of Cu5Bi2B4O14 has been described elsewhere [9]. In the present case, B2O3 with 11B isotope was used in order to eliminate the strong neutron absorption of natural boron. Darkgreen crystals of different shapes were mechanically removed from the crucible. A powder sample for neutron scattering was obtained by chafing single crystals each with a size of about 2 2 2 mm3. X-ray diffraction analysis was performed on powders of grown crystals on a DRON-2 diffractometer. Small-size high-quality single crystals were also measured on a CAD4 automated diffractometer using MoKa radiation. It was found [9] that the crystals have a triclinic symmetry with space group P1. The unit-cell ( , b = 9.385 A, ( c= parameters are: a = 10.132 A ( a=105.443 , b=97.405 , g=107.784 , 3.485 A, and Z ¼ 1 (Fig. 1).
3. Experimental results The very high-intensity powder diffractometer ( was used to D20 with the wavelength 2.42097 A collect neutron spectra. First, data in the para-
Fig. 1. Cu5Bi2B4O14 crystal structure.
magnetic state at T=35 K up to angles 2y ¼ 120 were acquired. All the observed peaks coincide with X-ray peaks. These results are presented in Fig. 2. Then, the neutron spectrum in the magnetically ordered state at T=2 K, and thermal scans between 2 and 35 K, were obtained. An attempt to refine the data at T=2 K did not work properly, as the magnetic moments were not very stable. This is not surprising as the nuclear symmetry is very low and only a small magnetic signal is observed on the background of the nuclear signal. We therefore decided to refine the difference spectrum obtained by subtracting the 35 K data set from the 2 K data set. The fit was limited to the angular region up to 80 as the intensity of magnetic scattering decreases fast at high angles. This gave the satisfactory results shown in Fig. 3. The plots of the refinement contain the observed, calculated, and difference spectra as well as the position of the reflections shown by tick marks. Table 1 presents the results obtained after refinement of the neutron spectrum shown in Fig. 3. The notations for copper positions are from Fig. 1. Ma, Mb, Mc and M are the magnetic moment projections on the triclinic axes x; y; z and its module, respectively. The numbers in parentheses are the fitting errors. The theoretical magnetic moment used for fitting equals 1mB.
ARTICLE IN PRESS G.A. Petrakovskii et al. / Physica B 350 (2004) e1043–e1046
e1045
Fig. 2. Neutron spectrum of the Cu5Bi2B4O14 compound at T=35 K. The observed spectrum (thin line), the calculated one (thick line), and the difference are shown. The peak positions are indicated with tick marks.
Fig. 3. Magnetic peaks obtained after subtracting neutron spectra at 35 and 2 K. The observed spectrum (thin line), the calculated one (thick line), and the difference are shown. The peak positions are indicated with tick marks.
4. Conclusions In conclusion, we want to call the attention to the accuracy of our results since the main
question we would like to answer is whether the observed canting is a real feature or a fitting artifact. One can see from Table 1 that the error bars are quite large for the Mb components as was
ARTICLE IN PRESS G.A. Petrakovskii et al. / Physica B 350 (2004) e1043–e1046
e1046 Table 1 Copper position
Ma, mB
Mb, mB
Mc, mB
M, mB
Cu(4) Cu(1) Cu(3) Cu(2), Cu(2)
0.00(0) 0.00(0) 0.65(1) 0.00(0)
0.00(0) 0.00(0) 0.3(2) 0.31(2)
0.95(6) 0.71(1) 0.95(1) 0.72(1)
0.95(0) 0.71(1) 1.23(1) 0.86(1)
We also argue for the presence of some canting. If the magnetic moments are fixed in the cdirection, an increase of the R factor from 3% to 5% is obtained. More importantly, one clearly sees that the second magnetic peak at about 16.3 (see Fig. 3) appears only if the magnetic moments are somewhat non-collinear. Another fact is that the theoretical saturation magnetization value for the strongly collinear ferrimagnetic structure is 18.1 emu/g, slightly larger than the observed one, 17.1 emu/g. Taking canting into account and using the obtained projections for Mi (Table 1), the saturation magnetization volume becomes s=17.32 emu/g which is almost equal to the experimental one. Further single-crystal measurements will be necessary to clarify this question, as demonstrated in the case of copper metaborate, CuB2O4 [ ref. x1 of the report: M. Boehm et al. PRB 68 (2003) 024405].
References
Fig. 4. Comparison of nuclear and magnetic signal intensities in Cu5Bi2B4O14 compound.
as for module M. We believe that the differences between theoretical and experimental values are very reasonable, owing to the difficulties in extracting a very small magnetic signal from the overall diffraction pattern. An example of the magnetic and nuclear signal intensity ratio is presented in Fig. 4. Visible intensity changes happen only for the peaks near 6 and 22 below the transition temperature.
[1] A. Wells, Structural Inorganic Chemistry, Clarendon, Oxford, 1984; Mir, Moscow, 1987. [2] G.A. Petrakovskii, K.A. Sablina, A.M. Vorotynov, et al., JETP 71 (1990) 772. [3] A.M. Vorotynov, A.I. Pankrats, G.A. Petrakovskii, et al., JETP 86 (1998) 1020. [4] G.A. Petrakovskii, K.A. Sablina, A.I., Pankrats, et al., J. Magn. Magn. Mater. 140–144 (1995) 1991. [5] G. Petrakovskii, D. Velikanov, A. Vorotinov, et al., J. Magn. Magn. Mater. 205 (1) (1999) 105. [6] A.I. Pankrats, G.A. Petrakovskii, N.V. Volkov, Phys. Solid State 42 (2000) 96. [7] G. Petrakovskii, A.D. Balaev, A.M. Vorotynov, Phys. Solid State 42 (2000) 321. [8] B. Roessli, J. Schefer, G. Petrakovskii, et al., Phys. Rev. Lett. 86 (9) (2001) 1885. [9] G.A. Petrakovskii, K.A. Sablina, A.I. Pankrats, et al., Phys. Solid State 44 (7) (2002) 1339.
Physica B 350 (2004) e1043–e1046
The magnetic structure of Cu5Bi2B4O14: a neutron scattering study G.A. Petrakovskiia, A.M. Vorotynova,*, K.A. Sablinaa, L.V. Udoda, A.I. Pankratsa, C. Ritterb a
L.V.Kirenskii Institute of Physics SB RAS, 660036 Krasnoyarsk, Russia b Laue–Langevin Institute, Grenoble, France
Abstract Neutron-scattering measurements are performed on powders of Cu5Bi2B4O14 to investigate their magnetic structure at temperatures ranging from 2 to 35 K. The ferrimagnetic easy-axis structure is confirmed with spin canting for one of the sublattices. No doubling of the crystal unit cell below the transition temperature TN=25 K is observed. r 2004 Elsevier B.V. All rights reserved. PACS: 75.25.+z Keywords: Neutron scattering; Magnetic structure; Canting
1. Introduction The discovery of high-temperature superconductivity led to active research on oxocuprates. Although oxocuprates are not superconductors, and not even metallic conductors, their crystal structures involve fragments that are similar to those found in high-Tc superconductors. These determine the magnetic properties of the compounds in the case of superexchange interactions. In his monograph, Wells [1] noted that of all chemical elements, bivalent copper should form the largest variety of magnetic structures. This high diversity of magnetic properties of copper*Corresponding author. E-mail address: [email protected] (A. Vorotynov).
oxide compounds can be illustrated with oxocuprates that we studied previously. In particular, CuGeO3 is a chain spin-Peierls magnet with a transition temperature TSP=14 K [2], LiCu2O2 is a two-dimensional antiferromagnet with a damaged ladder structure and a magnetic transition at TN=24 K [3], and Bi2CuO4 is a three-dimensional antiferromagnet characterized by the four-spin exchange interaction and TN=41 K [4]. In CuB2O4 [5–8], the magnetic states for T below 20 K form a complex phase diagram, including a transition between commensurate and incommensurate structures. Recently, the structural, magnetic, and resonance properties of the new compound Cu5Bi2B4O14 were studied [9]. The magnetic phase transition was found at TN=24.5 K. Above TN,
0921-4526/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2004.03.286
ARTICLE IN PRESS e1044
G.A. Petrakovskii et al. / Physica B 350 (2004) e1043–e1046
all the above measurements indicate that Cu5Bi2B4O14 is a paramagnet with an effective magnetic moment meff ¼ 0:81mB and a paramagnetic Curie temperature y¼ 17:4 K: Below TN, the experimental data suggest a ferrimagnetic structure with magnetic moments parallel to the crystal n˜-axis and a saturation magnetization of s=17.1 emu/g. The theoretical saturation magnetization for such a magnetic structure is s=18.1 emu/g. This 6% discrepancy between theory and experiment might suggest some canting of the magnetic moments on either one or both sublattices. Moreover, it is important to directly test the proposed magnetic structure with neutron scattering. To this effect, we grew small, 11B-enriched, Cu5Bi2B4O14 single crystals. Results of neutronscattering experiments to explore the low-temperature magnetic structure of Cu5Bi2B4O14 are reported here.
2. Sample preparation The preparation of Cu5Bi2B4O14 has been described elsewhere [9]. In the present case, B2O3 with 11B isotope was used in order to eliminate the strong neutron absorption of natural boron. Darkgreen crystals of different shapes were mechanically removed from the crucible. A powder sample for neutron scattering was obtained by chafing single crystals each with a size of about 2 2 2 mm3. X-ray diffraction analysis was performed on powders of grown crystals on a DRON-2 diffractometer. Small-size high-quality single crystals were also measured on a CAD4 automated diffractometer using MoKa radiation. It was found [9] that the crystals have a triclinic symmetry with space group P1. The unit-cell ( , b = 9.385 A, ( c= parameters are: a = 10.132 A ( a=105.443 , b=97.405 , g=107.784 , 3.485 A, and Z ¼ 1 (Fig. 1).
3. Experimental results The very high-intensity powder diffractometer ( was used to D20 with the wavelength 2.42097 A collect neutron spectra. First, data in the para-
Fig. 1. Cu5Bi2B4O14 crystal structure.
magnetic state at T=35 K up to angles 2y ¼ 120 were acquired. All the observed peaks coincide with X-ray peaks. These results are presented in Fig. 2. Then, the neutron spectrum in the magnetically ordered state at T=2 K, and thermal scans between 2 and 35 K, were obtained. An attempt to refine the data at T=2 K did not work properly, as the magnetic moments were not very stable. This is not surprising as the nuclear symmetry is very low and only a small magnetic signal is observed on the background of the nuclear signal. We therefore decided to refine the difference spectrum obtained by subtracting the 35 K data set from the 2 K data set. The fit was limited to the angular region up to 80 as the intensity of magnetic scattering decreases fast at high angles. This gave the satisfactory results shown in Fig. 3. The plots of the refinement contain the observed, calculated, and difference spectra as well as the position of the reflections shown by tick marks. Table 1 presents the results obtained after refinement of the neutron spectrum shown in Fig. 3. The notations for copper positions are from Fig. 1. Ma, Mb, Mc and M are the magnetic moment projections on the triclinic axes x; y; z and its module, respectively. The numbers in parentheses are the fitting errors. The theoretical magnetic moment used for fitting equals 1mB.
ARTICLE IN PRESS G.A. Petrakovskii et al. / Physica B 350 (2004) e1043–e1046
e1045
Fig. 2. Neutron spectrum of the Cu5Bi2B4O14 compound at T=35 K. The observed spectrum (thin line), the calculated one (thick line), and the difference are shown. The peak positions are indicated with tick marks.
Fig. 3. Magnetic peaks obtained after subtracting neutron spectra at 35 and 2 K. The observed spectrum (thin line), the calculated one (thick line), and the difference are shown. The peak positions are indicated with tick marks.
4. Conclusions In conclusion, we want to call the attention to the accuracy of our results since the main
question we would like to answer is whether the observed canting is a real feature or a fitting artifact. One can see from Table 1 that the error bars are quite large for the Mb components as was
ARTICLE IN PRESS G.A. Petrakovskii et al. / Physica B 350 (2004) e1043–e1046
e1046 Table 1 Copper position
Ma, mB
Mb, mB
Mc, mB
M, mB
Cu(4) Cu(1) Cu(3) Cu(2), Cu(2)
0.00(0) 0.00(0) 0.65(1) 0.00(0)
0.00(0) 0.00(0) 0.3(2) 0.31(2)
0.95(6) 0.71(1) 0.95(1) 0.72(1)
0.95(0) 0.71(1) 1.23(1) 0.86(1)
We also argue for the presence of some canting. If the magnetic moments are fixed in the cdirection, an increase of the R factor from 3% to 5% is obtained. More importantly, one clearly sees that the second magnetic peak at about 16.3 (see Fig. 3) appears only if the magnetic moments are somewhat non-collinear. Another fact is that the theoretical saturation magnetization value for the strongly collinear ferrimagnetic structure is 18.1 emu/g, slightly larger than the observed one, 17.1 emu/g. Taking canting into account and using the obtained projections for Mi (Table 1), the saturation magnetization volume becomes s=17.32 emu/g which is almost equal to the experimental one. Further single-crystal measurements will be necessary to clarify this question, as demonstrated in the case of copper metaborate, CuB2O4 [ ref. x1 of the report: M. Boehm et al. PRB 68 (2003) 024405].
References
Fig. 4. Comparison of nuclear and magnetic signal intensities in Cu5Bi2B4O14 compound.
as for module M. We believe that the differences between theoretical and experimental values are very reasonable, owing to the difficulties in extracting a very small magnetic signal from the overall diffraction pattern. An example of the magnetic and nuclear signal intensity ratio is presented in Fig. 4. Visible intensity changes happen only for the peaks near 6 and 22 below the transition temperature.
[1] A. Wells, Structural Inorganic Chemistry, Clarendon, Oxford, 1984; Mir, Moscow, 1987. [2] G.A. Petrakovskii, K.A. Sablina, A.M. Vorotynov, et al., JETP 71 (1990) 772. [3] A.M. Vorotynov, A.I. Pankrats, G.A. Petrakovskii, et al., JETP 86 (1998) 1020. [4] G.A. Petrakovskii, K.A. Sablina, A.I., Pankrats, et al., J. Magn. Magn. Mater. 140–144 (1995) 1991. [5] G. Petrakovskii, D. Velikanov, A. Vorotinov, et al., J. Magn. Magn. Mater. 205 (1) (1999) 105. [6] A.I. Pankrats, G.A. Petrakovskii, N.V. Volkov, Phys. Solid State 42 (2000) 96. [7] G. Petrakovskii, A.D. Balaev, A.M. Vorotynov, Phys. Solid State 42 (2000) 321. [8] B. Roessli, J. Schefer, G. Petrakovskii, et al., Phys. Rev. Lett. 86 (9) (2001) 1885. [9] G.A. Petrakovskii, K.A. Sablina, A.I. Pankrats, et al., Phys. Solid State 44 (7) (2002) 1339.