- Email: [email protected]
Magnetic excitations of CuNb2O6
Journal of Physics and Chemistry of Solids 60 (1999) 1129–1132
Magnetic excitations of CuNb2O6 K. Kodama a,c,*, H. Harashina a,c, S. Sasaki a, M. Kanada a, M. Kato a, M. Sato a,c, K. Kakurai b,c, M. Nishi b a
Department of Physics, Division of Material Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan b Neutron Scattering Laboratory, ISSP, The University of Tokyo, Shirakata 106-1, Tokai 319-1195, Japan c CREST, Japan Science and Technology Corporation (JST), Japan
Abstract Dynamical magnetic properties of the quasi-one-dimensional spin-gap system CuNb2O6 have been studied by means of neutron inelastic scattering. The dispersion curve of the singlet–triplet excitation indicates that the system has ferromagnetic– antiferromagnetic (F–AF) alternating bond chains, which is the origin of the spin gap. The magnetic excitation and the macroscopic properties are analyzed by the F–AF alternating bond model. The impurity effect of this system is also studied. q 1999 Elsevier Science Ltd. All rights reserved. Keywords: A. Magnetic materials; B. Crystal growth; C. Neutron scattering; D. Magnetic properties
CuNb2O6 has zig–zag chains of edge-sharing CuO6 octahedra [1]. The structure of the Cu–O chain is schematically shown in Fig. 1. The chain runs along the c-axis. The Cu atoms have a valency of 1 2 and form a quasi-one-dimensional quantum spin system with the spin S 1/2. For this compound, two polymorphs, orthorhombic (O-) and monoclinic (M-) phases have been found [2,3]. In Fig. 2, the spin susceptibility x of the O- and M-phases are plotted against the temperature T, where the Curie term and the T-independent contributions are subtracted from the data for the Mphase, while for the O-phase, the raw data are shown. As is clearly expected from the data, M-CuNb2O6 has been found to have a spin-gapped ground state with the gap value D , 20 K, while O-CuNb2O6 undergoes the transition to the antiferromagnetic ordered state [4,5]. The difference in the magnetic properties between the two phases can be attributed to the slight difference in the linkages of the CuO6 octahedra between them: The spin system of the M-phase is considered to be an alternating bond chain, while that of the O-phase is considered to be a uniform Heisenberg chain. We have studied the dynamical magnetic properties of
* Corresponding author. Tel.: 1 81-052-789-2852; fax: 1 81052-789-2933.
M-CuNb2O6 by means of neutron inelastic scattering [6], and by using information obtained there, its macroscopic properties are analyzed. Then, based on the results of the studies, the mechanism of the spin-gap formation is discussed. The effects of the Zn-substitution for Cu, are also discussed. A single crystal of M-Cu12xZnxNb2O6 with x , 0.1 was used in the neutron measurements. We have not succeeded in growing crystals with x smaller than 0.1. Because the Znsubstitution does not destroy the spin-gapped state, but only reduces the absolute values of the exchange parameters [7] as mentioned later, the mechanism of the spin-gap formation in this system can be discussed based on the results obtained for the present Zn-doped sample. The neutron measurements were carried out on the triple-axis spectrometer ISSP-PONTA at JRR-3M of JAERI in Tokai. In Fig. 3(a), the excitation energies of the observed isolated branches are plotted as functions of l. The bottom of the dispersion curve of the lower-energy branch (plotted by closed circles) is located at the wave vector Q with l 0.5. This result indicates that the system has a F–AF alternating bond chain, which can explain the spin-gap formation of this system. (For the AF–AF one, the dispersion bottom should be located at the Q-points with l 1.0, because there are two Cu atoms in the unit cell length c within a chain, as shown in Fig. 1.) The l-dependences of the scattering intensities of the isolated branches are shown in Fig. 3(b). The
0022-3697/99/$ - see front matter q 1999 Elsevier Science Ltd. All rights reserved. PII: S0022-369 7(99)00074-8
1130
K. Kodama et al. / Journal of Physics and Chemistry of Solids 60 (1999) 1129–1132
Fig. 1. Structure of the Cu–O chains is schematically shown. The filled and open circles indicate the Cu and O atoms, respectively.
intensity of the lower-energy branch has a peak structure at l , 0.6, which can also be expected by considering that the system has the F–AF alternating bond chain. The F–AF alternating bond chain can be described by the following Hamiltonian. 8 9
Fig. 2. T-dependence of x of O-(filled) and M-(open) CuNb2O6. The solid line is the result of the fitting by using the formula reported in Ref. [9].
Fig. 3. (a) Excitation energies of the isolated branches are shown along (00l). The solid and broken lines show the calculated results for the lower- and higher-energy branches, respectively. (b) Intensities of the isolated branches are shown along (00l). The solid and broken lines show the calculated results for the lower- and higherenergy branches, respectively.
are shown by solid lines. They can reproduce the observed ones very well. Their calculation also suggests the existence of the higher-energy branch. It was confirmed by the present measurement, as shown in Fig. 3(a) and (b) by open circles, where the calculated dispersion of the branch is also shown by the broken line in the figures. Although the calculated dispersion curve can roughly reproduce the observed one, the calculated intensity does not exhibit satisfactory agreement with the observed one. As mentioned above, the characteristics of the magnetic excitation of M-CuNb2O6 indicate that the system has the F–AF alternating bond chains. For such systems, the spin susceptibility x can be fitted by the formula presented by Borras-Almenar et al. [9]. The result of the fitting is shown by the solid line in Fig. 2. The fitting is found to be satisfactory. The obtained values of the exchange parameters are J 51.4 K and b 22.36. The value of D(,20 K) is
K. Kodama et al. / Journal of Physics and Chemistry of Solids 60 (1999) 1129–1132
1131
Fig. 4. T-dependence of x of M-Cu12xZnxNb2O6 for various x-values. In the inset, the T-dependence of x res of M-Cu12x Znx Nb2 O6 is shown for various x-values.
reasonable for these parameters. The specific heat C of MCuNb2O6 can also be reproduced by the results of the exact diagonalization obtained for the above exchange parameters [8]. The T-dependence of x of M-Cu12xZnxNb2O6 with various x-values is shown in Fig. 4. With increasing x, the Curie contribution (or the Curie–Weiss one with the Weiss temperature smaller than 1 K) increases. The anomaly associated with the magnetic ordering was not observed in the x –T curves. It was not observed in the T-dependences of C of M-Cu12xZnxNb2O6, either. In the inset, the T-dependence of x res obtained by subtracting the Curie (or the Curie Weiss) contribution from x , is shown. The x res –T curves can roughly be fitted by the formula mentioned above. The obtained exchange parameters and spin density monotonously decrease with increasing x. Results of the detailed analyses of the Curie contribution suggest that the effect of Zn impurities is only to induce the isolated spins with S 1/ 2 at their nearest neighbor sites connected with the AF interaction, and the other sites remain in the spin-gapped state. The similar impurity effects are observed in Haldane systems (e.g., see Ref. [10]. The presently observed reduction of the exchange parameters seems to be due to the changes of the structural parameters caused by the Znsubstitution.
In summary, the magnetic excitation of M-CuNb2O6 has been studied by neutron inelastic scattering, the results of which clearly indicate that the system has the F–AF alternating chains. The dynamical behavior observed by neutrons as well as the macroscopic properties can be well explained by using the parameters deduced within the framework of the F–AF bond chain model.
References [1] M.G.B. Drew, R.J. Hobson, V.T. Padayatcy, J. Mater. Chem. 3 (1993) 889. [2] E. Wahlstrom, B.-O. Marinder, Inorg. Nucl. Chem. Lett. 13 (1977) 559. [3] M.G.B. Drew, R.J. Hobson, V.T. Padayatcy, J. Mater. Chem. 5 (1995) 1779. [4] K. Kodama, T. Fukamachi, H. Harashina, M. Kanada, Y. Kobayashi, M. Kasai, H. Sasaki, M. Sato, K. Kakurai, J. Phys. Soc. Jpn. 67 (1998) 57. [5] T. Fukamachi, Y. Kobayashi, M. Kanada, M. Kasai, Y. Yasui, M. Sato, J. Phys. Soc. Jpn. 67 (1998) 2107. [6] K. Kodama, H. Harashina, H. Sasaki, M. Kato, M. Sato, K. Kakurai, M. Nishi, submitted for publication.
1132
K. Kodama et al. / Journal of Physics and Chemistry of Solids 60 (1999) 1129–1132
[7] T. Nishikawa, M. Kato, M. Kanada, T. Fukamachi, K. Kodama, H. Harashina, M. Sato, J. Phys. Soc. Jpn. 67 (1998) 1988. [8] S. Watanabe, H. Yokoyama, in preparation.
[9] J.J. Borras-Almenar, E. Corondo, J. Curely, R. Georges, J.C. Gianduzzo, Inorg. Chem. 33 (1994) 5171. [10] Y. Ajiro, T. Asano, F. Masui, M. Maekata, H. Aruga-Katori, T. Goto, H. Kikuchi, Phys. Rev. B 51 (1995) 9399.
Magnetic excitations of CuNb2O6 K. Kodama a,c,*, H. Harashina a,c, S. Sasaki a, M. Kanada a, M. Kato a, M. Sato a,c, K. Kakurai b,c, M. Nishi b a
Department of Physics, Division of Material Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan b Neutron Scattering Laboratory, ISSP, The University of Tokyo, Shirakata 106-1, Tokai 319-1195, Japan c CREST, Japan Science and Technology Corporation (JST), Japan
Abstract Dynamical magnetic properties of the quasi-one-dimensional spin-gap system CuNb2O6 have been studied by means of neutron inelastic scattering. The dispersion curve of the singlet–triplet excitation indicates that the system has ferromagnetic– antiferromagnetic (F–AF) alternating bond chains, which is the origin of the spin gap. The magnetic excitation and the macroscopic properties are analyzed by the F–AF alternating bond model. The impurity effect of this system is also studied. q 1999 Elsevier Science Ltd. All rights reserved. Keywords: A. Magnetic materials; B. Crystal growth; C. Neutron scattering; D. Magnetic properties
CuNb2O6 has zig–zag chains of edge-sharing CuO6 octahedra [1]. The structure of the Cu–O chain is schematically shown in Fig. 1. The chain runs along the c-axis. The Cu atoms have a valency of 1 2 and form a quasi-one-dimensional quantum spin system with the spin S 1/2. For this compound, two polymorphs, orthorhombic (O-) and monoclinic (M-) phases have been found [2,3]. In Fig. 2, the spin susceptibility x of the O- and M-phases are plotted against the temperature T, where the Curie term and the T-independent contributions are subtracted from the data for the Mphase, while for the O-phase, the raw data are shown. As is clearly expected from the data, M-CuNb2O6 has been found to have a spin-gapped ground state with the gap value D , 20 K, while O-CuNb2O6 undergoes the transition to the antiferromagnetic ordered state [4,5]. The difference in the magnetic properties between the two phases can be attributed to the slight difference in the linkages of the CuO6 octahedra between them: The spin system of the M-phase is considered to be an alternating bond chain, while that of the O-phase is considered to be a uniform Heisenberg chain. We have studied the dynamical magnetic properties of
* Corresponding author. Tel.: 1 81-052-789-2852; fax: 1 81052-789-2933.
M-CuNb2O6 by means of neutron inelastic scattering [6], and by using information obtained there, its macroscopic properties are analyzed. Then, based on the results of the studies, the mechanism of the spin-gap formation is discussed. The effects of the Zn-substitution for Cu, are also discussed. A single crystal of M-Cu12xZnxNb2O6 with x , 0.1 was used in the neutron measurements. We have not succeeded in growing crystals with x smaller than 0.1. Because the Znsubstitution does not destroy the spin-gapped state, but only reduces the absolute values of the exchange parameters [7] as mentioned later, the mechanism of the spin-gap formation in this system can be discussed based on the results obtained for the present Zn-doped sample. The neutron measurements were carried out on the triple-axis spectrometer ISSP-PONTA at JRR-3M of JAERI in Tokai. In Fig. 3(a), the excitation energies of the observed isolated branches are plotted as functions of l. The bottom of the dispersion curve of the lower-energy branch (plotted by closed circles) is located at the wave vector Q with l 0.5. This result indicates that the system has a F–AF alternating bond chain, which can explain the spin-gap formation of this system. (For the AF–AF one, the dispersion bottom should be located at the Q-points with l 1.0, because there are two Cu atoms in the unit cell length c within a chain, as shown in Fig. 1.) The l-dependences of the scattering intensities of the isolated branches are shown in Fig. 3(b). The
0022-3697/99/$ - see front matter q 1999 Elsevier Science Ltd. All rights reserved. PII: S0022-369 7(99)00074-8
1130
K. Kodama et al. / Journal of Physics and Chemistry of Solids 60 (1999) 1129–1132
Fig. 1. Structure of the Cu–O chains is schematically shown. The filled and open circles indicate the Cu and O atoms, respectively.
intensity of the lower-energy branch has a peak structure at l , 0.6, which can also be expected by considering that the system has the F–AF alternating bond chain. The F–AF alternating bond chain can be described by the following Hamiltonian. 8 9
Fig. 2. T-dependence of x of O-(filled) and M-(open) CuNb2O6. The solid line is the result of the fitting by using the formula reported in Ref. [9].
Fig. 3. (a) Excitation energies of the isolated branches are shown along (00l). The solid and broken lines show the calculated results for the lower- and higher-energy branches, respectively. (b) Intensities of the isolated branches are shown along (00l). The solid and broken lines show the calculated results for the lower- and higherenergy branches, respectively.
are shown by solid lines. They can reproduce the observed ones very well. Their calculation also suggests the existence of the higher-energy branch. It was confirmed by the present measurement, as shown in Fig. 3(a) and (b) by open circles, where the calculated dispersion of the branch is also shown by the broken line in the figures. Although the calculated dispersion curve can roughly reproduce the observed one, the calculated intensity does not exhibit satisfactory agreement with the observed one. As mentioned above, the characteristics of the magnetic excitation of M-CuNb2O6 indicate that the system has the F–AF alternating bond chains. For such systems, the spin susceptibility x can be fitted by the formula presented by Borras-Almenar et al. [9]. The result of the fitting is shown by the solid line in Fig. 2. The fitting is found to be satisfactory. The obtained values of the exchange parameters are J 51.4 K and b 22.36. The value of D(,20 K) is
K. Kodama et al. / Journal of Physics and Chemistry of Solids 60 (1999) 1129–1132
1131
Fig. 4. T-dependence of x of M-Cu12xZnxNb2O6 for various x-values. In the inset, the T-dependence of x res of M-Cu12x Znx Nb2 O6 is shown for various x-values.
reasonable for these parameters. The specific heat C of MCuNb2O6 can also be reproduced by the results of the exact diagonalization obtained for the above exchange parameters [8]. The T-dependence of x of M-Cu12xZnxNb2O6 with various x-values is shown in Fig. 4. With increasing x, the Curie contribution (or the Curie–Weiss one with the Weiss temperature smaller than 1 K) increases. The anomaly associated with the magnetic ordering was not observed in the x –T curves. It was not observed in the T-dependences of C of M-Cu12xZnxNb2O6, either. In the inset, the T-dependence of x res obtained by subtracting the Curie (or the Curie Weiss) contribution from x , is shown. The x res –T curves can roughly be fitted by the formula mentioned above. The obtained exchange parameters and spin density monotonously decrease with increasing x. Results of the detailed analyses of the Curie contribution suggest that the effect of Zn impurities is only to induce the isolated spins with S 1/ 2 at their nearest neighbor sites connected with the AF interaction, and the other sites remain in the spin-gapped state. The similar impurity effects are observed in Haldane systems (e.g., see Ref. [10]. The presently observed reduction of the exchange parameters seems to be due to the changes of the structural parameters caused by the Znsubstitution.
In summary, the magnetic excitation of M-CuNb2O6 has been studied by neutron inelastic scattering, the results of which clearly indicate that the system has the F–AF alternating chains. The dynamical behavior observed by neutrons as well as the macroscopic properties can be well explained by using the parameters deduced within the framework of the F–AF bond chain model.
References [1] M.G.B. Drew, R.J. Hobson, V.T. Padayatcy, J. Mater. Chem. 3 (1993) 889. [2] E. Wahlstrom, B.-O. Marinder, Inorg. Nucl. Chem. Lett. 13 (1977) 559. [3] M.G.B. Drew, R.J. Hobson, V.T. Padayatcy, J. Mater. Chem. 5 (1995) 1779. [4] K. Kodama, T. Fukamachi, H. Harashina, M. Kanada, Y. Kobayashi, M. Kasai, H. Sasaki, M. Sato, K. Kakurai, J. Phys. Soc. Jpn. 67 (1998) 57. [5] T. Fukamachi, Y. Kobayashi, M. Kanada, M. Kasai, Y. Yasui, M. Sato, J. Phys. Soc. Jpn. 67 (1998) 2107. [6] K. Kodama, H. Harashina, H. Sasaki, M. Kato, M. Sato, K. Kakurai, M. Nishi, submitted for publication.
1132
K. Kodama et al. / Journal of Physics and Chemistry of Solids 60 (1999) 1129–1132
[7] T. Nishikawa, M. Kato, M. Kanada, T. Fukamachi, K. Kodama, H. Harashina, M. Sato, J. Phys. Soc. Jpn. 67 (1998) 1988. [8] S. Watanabe, H. Yokoyama, in preparation.
[9] J.J. Borras-Almenar, E. Corondo, J. Curely, R. Georges, J.C. Gianduzzo, Inorg. Chem. 33 (1994) 5171. [10] Y. Ajiro, T. Asano, F. Masui, M. Maekata, H. Aruga-Katori, T. Goto, H. Kikuchi, Phys. Rev. B 51 (1995) 9399.