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Magnetic properties of the (CoxMn1-x)4Nb2O9 solid solution series
ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 322 (2010) L1–L3
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Letter to the Editor
Magnetic properties of the ðCox Mn1x Þ4 Nb2 O9 solid solution series B. Schwarz a,, D. Kraft b, R. Theissmann c, H. Ehrenberg a a b c
Leibniz-Institute for Solid State and Materials Research Dresden (IFW Dresden), Institute for Complex Materials, Helmholzstrasse 20, P.O. Box 270116, D-01171 Dresden, Germany Institute for Materials Science, Darmstadt University of Technology, Petersenstrasse 23, D-64287 Darmstadt, Germany Institute for Nano Structures and Technology (NST), University of Duisburg-Essen, Bismarckstr. 81, D-47057 Duisburg, Germany
a r t i c l e in fo
abstract
Article history: Received 9 January 2008 Received in revised form 23 September 2009 Available online 9 October 2009
Co4 Nb2 O9 and Mn4 Nb2 O9 order collinear antiferromagnetically with the same magnetic spin structure type below 30 and 125 K, respectively. Magnetization measurements on powder samples of the solid solution series ðCo; MnÞ4 Nb2 O9 prepared by arc melting reveal a linear progression of the Ne´eltemperature with Co/Mn ratio. Powder neutron diffraction experiments performed for a selected composition confirm the existence of the same magnetic structure type as found for the end members. ðCo; MnÞ4 Nb2 O9 samples prepared by subsolidus reaction and comparably much lower cooling rates after tempering contain very small amounts of additional ðCo; MnÞ3 O4 spinel phases with strongly varying transition temperatures as a function of the Co/Mn ratio. & 2009 Elsevier B.V. All rights reserved.
PACS: 61.66.Fn 75.25.-j Keywords: Solid solution oxide Neutron diffraction XRD Magnetic phase transition Spinel
1. Introduction
2. Sample preparation and experimental results
The isostructural compounds Me4 A2 O9 (Me ¼ Co, Mn, Fe, Mg; A ¼ Nb, Ta) were investigated by Bertaut et al. [1–3] concerning their crystal and magnetic structure at room and low temperature. All compounds crystallize in a trigonal crystal structure, SG P3c1, that can be derived from the corundum ðaAl2 O3 Þ structure. The Co4 Nb2 O9 unit cell consists of two formula units and can thought to be built up by three unit cells of the corundum type. The Coand the Nb-ions are distributed on the Al sites with a ratio of 2:1 and the Co ions occupy two non-equivalent crystallographic sites. Fig. 1 shows schematically the two oxygen layers A and B of the hexagonal lattice and the occupation of the two non-equivalent octahedral sites lying inbetween by Co1 and Co2 . Co4 Nb2 O9 and Mn4 Nb2 O9 order antiferromagnetically with the same spin structure below the Ne´el temperature of 30 and 125 K, respectively. The magnetic moments of the Co-, respectively Mnatoms, are parallel to the c-direction and form chains along the lines 13 23 z ð þ spinsÞ and 23 13 z ðspinsÞ, resulting in an antiferromagnetic spin structure with a magnetic unit cell identical with the chemical one.
For all syntheses appropriate amounts of the oxides MnO (Aldrich, 99%), CoO (Aldrich, 99.99%) and Nb2 O5 (Aldrich, 99.99%) are intimately mixed in an agate mortar under acetone. The mixture of the oxides is tempered at 920 K for 20 h in order to incorporate the Mn in a stable precursor (orthorhombic ðCo; MnÞNb2 O6 with columbite-type structure [4]). This treatment prevents that MnO, that is volatile already at about 1900 K, is lost during the synthesis in the arc furnace at temperatures higher than 2500 K. The phase composition of the powder samples is investigated with Rietveld refinement [5,6] based on X-ray diffraction data. For the samples prepared by arc melting only the ðCo; MnÞ4 Nb2 O9 phase can be verified. The linear dependence of the ðCo; MnÞ4 Nb2 O9 cell parameters with increasing Mn amount testifies to the high degree of correct cation incorporation into this phase in agreement with the cation ratio of the reactants (Table 1). This is confirmed by the results of fluorescent X-ray analysis. The magnetic properties of the samples are determined by temperature dependent magnetization measurements (SQUID) and neutron diffraction experiments. The magnetization measurements of the samples prepared by arc melting only show one transition from paramagnetic to antiferromagnetic each. The Ne´el temperature obeys a linear dependence from stoichiometry
Corresponding author. Tel.: + 49 351 4659 846; fax: + 49 351 4659 452.
E-mail address: [email protected] (B. Schwarz). 0304-8853/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2009.10.008
ARTICLE IN PRESS L2
B. Schwarz et al. / Journal of Magnetism and Magnetic Materials 322 (2010) L1–L3
ampoules. The samples are heated at a rate of 300–1300 K and after 16 h cooled down to room temperature at a rate of 180 K/h, i.e. a relatively low cooling rate in comparison with that realized by arc melting. Each of the samples prepared in the Pt/Rh ampoules contain considerable amounts of additional ðCo; MnÞNb2 O6 phases with columbite-type structure and for certain Mn/Co ratios also the ðCo; MnÞ3 O4 phase with spinel-type structure can be verified (see Table 1). From magnetization vs. temperature data the existence of additional ðCo; MnÞ3 O4 phases for samples of the complete solid solution series can be inferred from the existence of two distinct magnetic phase transitions. The temperatures of the transitions Tt vs. averaged stoichiometry /xS are represented in Fig. 3. The transition temperatures of the ðCo; MnÞ4 Nb2 O9 main phase show a linear progression from about 30 K for /xS ¼ 0 to about 125 K for /xS ¼ 4, in accordance with the results already found for the single-phase samples prepared by arc melting. Those of the ðCo; MnÞ3 O4 spinels are first observable from /xS 0:4 on with a transition temperature of about 185 K decreasing monotonically to about 40 K for /xS ¼ 4. The transition temperatures fit very well with those of the stoichiometric compounds Mn3 O4 [7,8] (tetragonal, Tt ¼ 40 K), CoMn2 O4 [9] (tetragonal, 103.5(5) K) and Co2 MnO4 [10] (cubic, 185(1) K) suggesting that the additional ferrimagnetic spinel phases possess the Mn/Co ratio of the reactants, respectively.
x: TN ðxÞ ¼ x 23 K þ 30:5 K. Exemplarily, a Co3:2 Mn0:8 Nb2 O9 sample is measured at the powder diffractometer E2 (BENSC, Hahn˚ Fig. 2 shows Meitner-Institut Berlin) with wavelength l ¼ 2:41 A. sections of the measured neutron powder diffractograms, simulated diffractograms for the nuclear and magnetic (only for 8 K data) structure and difference plots of the measured and simulated intensities for a variety of temperatures. The diffractogram at 8 K shows considerable changes in relative Bragg intensities compared to those measured at 90 and 250 K, clearly visible by comparing the relative intensity of the 100reflection. A Rietveld analysis confirms that this is due to magnetic contributions by the magnetic structure type already found for the end members Co4 Nb2 O9 and Mn4 Nb2 O9 [1,2] with magnetic moments forming ferromagnetic chains along 13 ; 23 ; z and 23 ; 13 ; z with antiparallel interchain magnetic coupling (Fig. 1). Note that this magnetic structure is an example for an antiferromagnetic structure with coinciding nuclear and magnetic Bragg reflections. The refined ordered magnetic moment of 3:1 mB at 8 K is slightly larger than the spin-only magnetic moment mCo2 þ ¼ 3 mB for Co2 þ SCo2 þ ¼ 32 determined for Co4 Nb2 O9 [1] but is still below the linear combination (LC) of Co2 þ and Mn2 þ ðSMn2 þ ¼ 52Þ spin-only values for this composition mLC ¼ 3:4 mB . For a variety of Mn/Co ratios ðCo; MnÞ4 Nb2 O9 solid solution crystals are also prepared by subsolidus reaction in Pt/Rh ð90 : 10Þ
3. Conclusions The ðCo; MnÞ4 Nb2 O9 powder samples prepared by arc melting do not contain additional phases verifiable with laboratory X-ray diffraction. The temperature dependent magnetization measurements revealed antiferromagnetic ordering for the whole solid solution series with a linear progression of the Ne´el temperature. Mainly due to the high cooling rates realized during the arc melting, diffusion dependent precipitation of additional phases is suppressed. In contrast, the samples prepared in the Pt/Rh ampoules with a cooling rate of 180 K/h, build up considerable amounts of the additional ðCo; MnÞNb2 O6 phases with columbitetype structure and for specific compositions the ðCo; MnÞ3 O4 phase with spinel-type structure is also detectable with X-ray diffraction. From magnetization measurements the existence of these spinel phases can be verified for the complete solid solution series.
Fig. 1. Schematic representation of the octahedral sites’ occupation by metal ions in Co4 Nb2 O9 . The arrows symbolize the magnetic moments located on the Co ions.
Table 1 Results of Rietveld refinement based on X-ray diffraction data for ðCo; MnÞ4 Nb2 O9 powder samples prepared by subsolidus reaction in Pt/Rh (90:10) ampoules and by arc melting. Stoichiometry x
Arc melting
Pt/Rh (90:10) ampoule
Unit cell (exper.)
0.0 0.4 1.2 1.6 2.0 2.8 3.2 3.6 4.0 n
Additional phases (% w/w)
Unit cell (exper.) ˚ a (A)
˚ c (A)
Columbite
3 5.175 5.202 5.215 5.229 5.272 5.287 5.316 5.332
3 14.148 14.209 14.210 14.255 14.296 14.289 14.314 14.322
3 6.4 10.1 3 23.4 32.7 24.8 19.8 25.1
˚ a (A)
˚ c (A)
Columbite
Spinel
5.169 5.183 5.206 5.226 5.238 5.270 5.292 5.306 5.322
14.127 14.165 14.223 14.246 14.271 14.300 14.302 14.288 14.304
n
n
n
n
n
n
n
n
n
n
n
n
n
n
: not verifiable, 3: not determined.
n
3
n n
Additional phases (% w/w) Spinel 3 n n n n
0.7 1.1 3 3
ARTICLE IN PRESS B. Schwarz et al. / Journal of Magnetism and Magnetic Materials 322 (2010) L1–L3
L3
Fig. 3. Temperatures of magnetic phase transitions Tt vs. averaged stoichiometry /xS of the Co4x Mnx Nb2 O9 samples prepared by subsolidus reaction in Pt/Rh (90:10) ampoules and moderate cooling rate. The magnetic transitions of the Co4x Mnx Nb2 O9 main phase show a linear progression with /xS. Those of the ðCo; MnÞ3 O4 spinels are first observable from /xS 0:4 on with a transition temperature of about 185 K decreasing monotonically to about 40 K for /xS ¼ 4. The transition temperatures of the ferrimagnetic spinels Mn3 O4 [7,8] (tetragonal, Tt ¼ 40 K), CoMn2 O4 [9] (tetragonal, 103.5(5) K) and Co2 MnO4 [10] (cubic, 185(1) K) are marked with gray rhombs.
Acknowledgments Financial support by the Deutsche Forschungsgemeinschaft (EH 183/4) is gratefully acknowledged. References [1] E.F. Bertaut, L. Corliss, F. Forrat, R. Ale´onard, R. Pauthenet, J. Phys. Chem. Solids 21 (1961) 234. [2] E.F. Bertaut, L. Corliss, F. Forrat, Comptes rendus hebdomadaires des se´ances de l’Acade´mie des Sciences 251 (1960) 1733. [3] R. Ale´onard, R. Pauthenet, M.L. Ne´el, Comptes rendus hebdomadaires des se´ances de l’Acade´mie des Sciences 251 (1960) 1730. [4] H.J. Sturdivant, Z. Krist. 75 (1930) 88. [5] H.M. Rietveld, J. Appl. Cryst. 2 (1969) 65. [6] J. Rodriguez-Carvajal, Physica B 55 (1993) 192. [7] G.B. Jensen, O.V. Nielsen, J. Phys. C: Solid State Phys. 7 (1974) 409. [8] B. Chardon, F. Vigneron, J. Magn. Magn. Mater. 58 (1986) 128. [9] S. Tamura, Phyisca B 190 (1993) 150. [10] S. Tamura, J. Phys. Soc. Jpn. 61 (1992) 752.
nuclear magnetic
Fig. 2. Sections of neutron powder diffractograms of a Co3:2 Mn0:8 Nb2 O9 sample prepared by arc melting measured at 250, 90 and 8 K, respectively, at the powder diffractometer E2 (BENSC, Hahn-Meitner-Institut Berlin) with wavelength ˚ Simulated diffractograms for the nuclear and magnetic (only for 8 K l ¼ 2:41 A. data) structure together with difference plots of the measured and simulated data were obtained by Rietveld refinement. The vertical markers give the positions of the nuclear (upper marks) and magnetic (lower marks, only for 8 K data) Bragg reflections, respectively.
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Letter to the Editor
Magnetic properties of the ðCox Mn1x Þ4 Nb2 O9 solid solution series B. Schwarz a,, D. Kraft b, R. Theissmann c, H. Ehrenberg a a b c
Leibniz-Institute for Solid State and Materials Research Dresden (IFW Dresden), Institute for Complex Materials, Helmholzstrasse 20, P.O. Box 270116, D-01171 Dresden, Germany Institute for Materials Science, Darmstadt University of Technology, Petersenstrasse 23, D-64287 Darmstadt, Germany Institute for Nano Structures and Technology (NST), University of Duisburg-Essen, Bismarckstr. 81, D-47057 Duisburg, Germany
a r t i c l e in fo
abstract
Article history: Received 9 January 2008 Received in revised form 23 September 2009 Available online 9 October 2009
Co4 Nb2 O9 and Mn4 Nb2 O9 order collinear antiferromagnetically with the same magnetic spin structure type below 30 and 125 K, respectively. Magnetization measurements on powder samples of the solid solution series ðCo; MnÞ4 Nb2 O9 prepared by arc melting reveal a linear progression of the Ne´eltemperature with Co/Mn ratio. Powder neutron diffraction experiments performed for a selected composition confirm the existence of the same magnetic structure type as found for the end members. ðCo; MnÞ4 Nb2 O9 samples prepared by subsolidus reaction and comparably much lower cooling rates after tempering contain very small amounts of additional ðCo; MnÞ3 O4 spinel phases with strongly varying transition temperatures as a function of the Co/Mn ratio. & 2009 Elsevier B.V. All rights reserved.
PACS: 61.66.Fn 75.25.-j Keywords: Solid solution oxide Neutron diffraction XRD Magnetic phase transition Spinel
1. Introduction
2. Sample preparation and experimental results
The isostructural compounds Me4 A2 O9 (Me ¼ Co, Mn, Fe, Mg; A ¼ Nb, Ta) were investigated by Bertaut et al. [1–3] concerning their crystal and magnetic structure at room and low temperature. All compounds crystallize in a trigonal crystal structure, SG P3c1, that can be derived from the corundum ðaAl2 O3 Þ structure. The Co4 Nb2 O9 unit cell consists of two formula units and can thought to be built up by three unit cells of the corundum type. The Coand the Nb-ions are distributed on the Al sites with a ratio of 2:1 and the Co ions occupy two non-equivalent crystallographic sites. Fig. 1 shows schematically the two oxygen layers A and B of the hexagonal lattice and the occupation of the two non-equivalent octahedral sites lying inbetween by Co1 and Co2 . Co4 Nb2 O9 and Mn4 Nb2 O9 order antiferromagnetically with the same spin structure below the Ne´el temperature of 30 and 125 K, respectively. The magnetic moments of the Co-, respectively Mnatoms, are parallel to the c-direction and form chains along the lines 13 23 z ð þ spinsÞ and 23 13 z ðspinsÞ, resulting in an antiferromagnetic spin structure with a magnetic unit cell identical with the chemical one.
For all syntheses appropriate amounts of the oxides MnO (Aldrich, 99%), CoO (Aldrich, 99.99%) and Nb2 O5 (Aldrich, 99.99%) are intimately mixed in an agate mortar under acetone. The mixture of the oxides is tempered at 920 K for 20 h in order to incorporate the Mn in a stable precursor (orthorhombic ðCo; MnÞNb2 O6 with columbite-type structure [4]). This treatment prevents that MnO, that is volatile already at about 1900 K, is lost during the synthesis in the arc furnace at temperatures higher than 2500 K. The phase composition of the powder samples is investigated with Rietveld refinement [5,6] based on X-ray diffraction data. For the samples prepared by arc melting only the ðCo; MnÞ4 Nb2 O9 phase can be verified. The linear dependence of the ðCo; MnÞ4 Nb2 O9 cell parameters with increasing Mn amount testifies to the high degree of correct cation incorporation into this phase in agreement with the cation ratio of the reactants (Table 1). This is confirmed by the results of fluorescent X-ray analysis. The magnetic properties of the samples are determined by temperature dependent magnetization measurements (SQUID) and neutron diffraction experiments. The magnetization measurements of the samples prepared by arc melting only show one transition from paramagnetic to antiferromagnetic each. The Ne´el temperature obeys a linear dependence from stoichiometry
Corresponding author. Tel.: + 49 351 4659 846; fax: + 49 351 4659 452.
E-mail address: [email protected] (B. Schwarz). 0304-8853/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2009.10.008
ARTICLE IN PRESS L2
B. Schwarz et al. / Journal of Magnetism and Magnetic Materials 322 (2010) L1–L3
ampoules. The samples are heated at a rate of 300–1300 K and after 16 h cooled down to room temperature at a rate of 180 K/h, i.e. a relatively low cooling rate in comparison with that realized by arc melting. Each of the samples prepared in the Pt/Rh ampoules contain considerable amounts of additional ðCo; MnÞNb2 O6 phases with columbite-type structure and for certain Mn/Co ratios also the ðCo; MnÞ3 O4 phase with spinel-type structure can be verified (see Table 1). From magnetization vs. temperature data the existence of additional ðCo; MnÞ3 O4 phases for samples of the complete solid solution series can be inferred from the existence of two distinct magnetic phase transitions. The temperatures of the transitions Tt vs. averaged stoichiometry /xS are represented in Fig. 3. The transition temperatures of the ðCo; MnÞ4 Nb2 O9 main phase show a linear progression from about 30 K for /xS ¼ 0 to about 125 K for /xS ¼ 4, in accordance with the results already found for the single-phase samples prepared by arc melting. Those of the ðCo; MnÞ3 O4 spinels are first observable from /xS 0:4 on with a transition temperature of about 185 K decreasing monotonically to about 40 K for /xS ¼ 4. The transition temperatures fit very well with those of the stoichiometric compounds Mn3 O4 [7,8] (tetragonal, Tt ¼ 40 K), CoMn2 O4 [9] (tetragonal, 103.5(5) K) and Co2 MnO4 [10] (cubic, 185(1) K) suggesting that the additional ferrimagnetic spinel phases possess the Mn/Co ratio of the reactants, respectively.
x: TN ðxÞ ¼ x 23 K þ 30:5 K. Exemplarily, a Co3:2 Mn0:8 Nb2 O9 sample is measured at the powder diffractometer E2 (BENSC, Hahn˚ Fig. 2 shows Meitner-Institut Berlin) with wavelength l ¼ 2:41 A. sections of the measured neutron powder diffractograms, simulated diffractograms for the nuclear and magnetic (only for 8 K data) structure and difference plots of the measured and simulated intensities for a variety of temperatures. The diffractogram at 8 K shows considerable changes in relative Bragg intensities compared to those measured at 90 and 250 K, clearly visible by comparing the relative intensity of the 100reflection. A Rietveld analysis confirms that this is due to magnetic contributions by the magnetic structure type already found for the end members Co4 Nb2 O9 and Mn4 Nb2 O9 [1,2] with magnetic moments forming ferromagnetic chains along 13 ; 23 ; z and 23 ; 13 ; z with antiparallel interchain magnetic coupling (Fig. 1). Note that this magnetic structure is an example for an antiferromagnetic structure with coinciding nuclear and magnetic Bragg reflections. The refined ordered magnetic moment of 3:1 mB at 8 K is slightly larger than the spin-only magnetic moment mCo2 þ ¼ 3 mB for Co2 þ SCo2 þ ¼ 32 determined for Co4 Nb2 O9 [1] but is still below the linear combination (LC) of Co2 þ and Mn2 þ ðSMn2 þ ¼ 52Þ spin-only values for this composition mLC ¼ 3:4 mB . For a variety of Mn/Co ratios ðCo; MnÞ4 Nb2 O9 solid solution crystals are also prepared by subsolidus reaction in Pt/Rh ð90 : 10Þ
3. Conclusions The ðCo; MnÞ4 Nb2 O9 powder samples prepared by arc melting do not contain additional phases verifiable with laboratory X-ray diffraction. The temperature dependent magnetization measurements revealed antiferromagnetic ordering for the whole solid solution series with a linear progression of the Ne´el temperature. Mainly due to the high cooling rates realized during the arc melting, diffusion dependent precipitation of additional phases is suppressed. In contrast, the samples prepared in the Pt/Rh ampoules with a cooling rate of 180 K/h, build up considerable amounts of the additional ðCo; MnÞNb2 O6 phases with columbitetype structure and for specific compositions the ðCo; MnÞ3 O4 phase with spinel-type structure is also detectable with X-ray diffraction. From magnetization measurements the existence of these spinel phases can be verified for the complete solid solution series.
Fig. 1. Schematic representation of the octahedral sites’ occupation by metal ions in Co4 Nb2 O9 . The arrows symbolize the magnetic moments located on the Co ions.
Table 1 Results of Rietveld refinement based on X-ray diffraction data for ðCo; MnÞ4 Nb2 O9 powder samples prepared by subsolidus reaction in Pt/Rh (90:10) ampoules and by arc melting. Stoichiometry x
Arc melting
Pt/Rh (90:10) ampoule
Unit cell (exper.)
0.0 0.4 1.2 1.6 2.0 2.8 3.2 3.6 4.0 n
Additional phases (% w/w)
Unit cell (exper.) ˚ a (A)
˚ c (A)
Columbite
3 5.175 5.202 5.215 5.229 5.272 5.287 5.316 5.332
3 14.148 14.209 14.210 14.255 14.296 14.289 14.314 14.322
3 6.4 10.1 3 23.4 32.7 24.8 19.8 25.1
˚ a (A)
˚ c (A)
Columbite
Spinel
5.169 5.183 5.206 5.226 5.238 5.270 5.292 5.306 5.322
14.127 14.165 14.223 14.246 14.271 14.300 14.302 14.288 14.304
n
n
n
n
n
n
n
n
n
n
n
n
n
n
: not verifiable, 3: not determined.
n
3
n n
Additional phases (% w/w) Spinel 3 n n n n
0.7 1.1 3 3
ARTICLE IN PRESS B. Schwarz et al. / Journal of Magnetism and Magnetic Materials 322 (2010) L1–L3
L3
Fig. 3. Temperatures of magnetic phase transitions Tt vs. averaged stoichiometry /xS of the Co4x Mnx Nb2 O9 samples prepared by subsolidus reaction in Pt/Rh (90:10) ampoules and moderate cooling rate. The magnetic transitions of the Co4x Mnx Nb2 O9 main phase show a linear progression with /xS. Those of the ðCo; MnÞ3 O4 spinels are first observable from /xS 0:4 on with a transition temperature of about 185 K decreasing monotonically to about 40 K for /xS ¼ 4. The transition temperatures of the ferrimagnetic spinels Mn3 O4 [7,8] (tetragonal, Tt ¼ 40 K), CoMn2 O4 [9] (tetragonal, 103.5(5) K) and Co2 MnO4 [10] (cubic, 185(1) K) are marked with gray rhombs.
Acknowledgments Financial support by the Deutsche Forschungsgemeinschaft (EH 183/4) is gratefully acknowledged. References [1] E.F. Bertaut, L. Corliss, F. Forrat, R. Ale´onard, R. Pauthenet, J. Phys. Chem. Solids 21 (1961) 234. [2] E.F. Bertaut, L. Corliss, F. Forrat, Comptes rendus hebdomadaires des se´ances de l’Acade´mie des Sciences 251 (1960) 1733. [3] R. Ale´onard, R. Pauthenet, M.L. Ne´el, Comptes rendus hebdomadaires des se´ances de l’Acade´mie des Sciences 251 (1960) 1730. [4] H.J. Sturdivant, Z. Krist. 75 (1930) 88. [5] H.M. Rietveld, J. Appl. Cryst. 2 (1969) 65. [6] J. Rodriguez-Carvajal, Physica B 55 (1993) 192. [7] G.B. Jensen, O.V. Nielsen, J. Phys. C: Solid State Phys. 7 (1974) 409. [8] B. Chardon, F. Vigneron, J. Magn. Magn. Mater. 58 (1986) 128. [9] S. Tamura, Phyisca B 190 (1993) 150. [10] S. Tamura, J. Phys. Soc. Jpn. 61 (1992) 752.
nuclear magnetic
Fig. 2. Sections of neutron powder diffractograms of a Co3:2 Mn0:8 Nb2 O9 sample prepared by arc melting measured at 250, 90 and 8 K, respectively, at the powder diffractometer E2 (BENSC, Hahn-Meitner-Institut Berlin) with wavelength ˚ Simulated diffractograms for the nuclear and magnetic (only for 8 K l ¼ 2:41 A. data) structure together with difference plots of the measured and simulated data were obtained by Rietveld refinement. The vertical markers give the positions of the nuclear (upper marks) and magnetic (lower marks, only for 8 K data) Bragg reflections, respectively.