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Structure and magnetic properties of layered (FexCo1 − x)Ta2O6 compounds
Journal of
ELSEVIER
Journal of Magnetism and Magnetic Materials 196-197 (1999) 846-847
magnetic materials
Structure and magnetic properties of layered (FexCo _ x)Ta206 compounds V.D. Mello, L.I. Zawislak, J.B. Marimon da Cunha, E.J. Kinast, J.B. Soares, C.A. dos Santos* lnstituto de Fisica-UFRGS, At;. Bento Goncalves 9500. C.P. 15051-Campus do f2de, 91501-970 Porto Alegre, RS, Brazil
Abstract Samples of (FexCo1_x)Ta206 were prepared from pure FeTa20~, and CoTa206. From X-ray diffraction (XRD) measurements followed by Rietveld refinement, it is demonstrated that the solid solution obeys the Vegard's law. M6ssbauer spectroscopy (MS) measurements at 4.2 and 300 K show that for x ~> 0.8 the Fe --+ Co substitution has no apparent effect on the hyperfine parameters. ~) !999 Elsevier Science B.V. All rights reserved. K e v w o r d s : Antiferromagnet: Rietveld refinement; M6ssbauer spectroscopy
In the tetragonal isomorphous ATa206 compounds (A = Fe, Co, Ni), the sub[attice formed by the 3d transition metal ions have the same symmetry as the Ni sublattice in KzNiF4, the well-known two-dimensional Heisenberg antiferromagnet [-1,2]. Recent M6ssbauer spectroscopy (MS) measurements [-3] have shown that from room temperature (RT) to about 15 K, FeTa20~, is clearly in the paramagnetic regime. The onset of longrange order is manifested near 10 K by the emerging hyperfine magnetic field. Magnetization curves [4], for temperatures between 1.4 and 5 K, display a noticeable change in slope, suggesting a spin-flop transition induced by a field higher than !0 T. Similar results were obtained for CoTazO6 [-2], but the magnetic structure, as suggested by powder neutron diffraction taken at 4.2 K [-5], is much more complex than for FeTa206. A tow-cone axis helical spin structure was proposed. Therefore, random mixtures of the type (FexCoi x)Ta206 are very appropriate to investigations on the effect of the competing anisotropies on the magnetic-phase diagram.
*Corresponding author. Tel.: + 55-51-3166424; fax: + 5551-3191762; e-mail: [email protected].
In the present work we report, for the first time, powder X-ray diffraction (XRD) and MS measurements on samples (FexCol .0Ta20~,, with x = 0, 0.4, 0.8 and 1. FeTa206 was prepared in vacuum, as described in Ref. [4]. CoTa206 was prepared in a similar way, but in air, instead of vacuum. For (FexCol x)TazO 6 samples, appropriate amounts of FeTa20~ and CoTa2Oo were mixed and processed as for the FeTa2Ot, preparation. XRD patterns were obtained in a Bragg-Brentano parafocusing geometry, under experimental setup as described in Ref. [4]. Structural parameters were refined with the program Fullprof [6]. The MS spectra were obtained at room temperature (RT) and at 4.2 K using a conventional spectrometer. The hyperfine parameters were obtained by a least-squares procedure. STCo in rhodium was used at room temperature as a source, with nominal activity of 50 mCi. The XRD patterns for all the samples were indexed to the space group P42/mnm, as expected for a trirutile structure. Typical pattern is shown in Fig. 1 for Feo.4COo.6Ta206. The small Bragg factor obtained for all the Rietveld refinements and the absence of spurious reflections on the XRD diagrams are strong indications that we have prepared well-crystallised single-phase samples, with lattice parameters typical of the tapiolite-mossite system (a ~ 0.47 nm, c ~ 0.92 nm). In Fig. 2 the
0304-8853/99/$ - see front matter ~!'~ 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 8 ) 0 0 9 6 9 - X
KD. Mello et al. / Journal of Magnetism and Magnetic Materials 196-197 (1999) 846-847
1.01 1.00 0.99 0.98 ~ 0.97
¢.20
30
40
50
60
2O
,
.
,
•
,
II
0.96
70
0.95 -4
Fig. 1. Room temperature X-ray powder diffraction pattern for (Fe0.4Co0.6)Ta206.
•
847
•
,
.
,
.
,
•
0.9200
Velocity (mm/s) Fig. 3. Room temperature M6ssbauer spectrum for (Fe0.sCo0.2)Ta206. AEo = 3.02 mm/s, ~Fe = 1.13 mm/s. Typical errors are + 3%.
0.9195 ~ ,~ 0.9190 ~.~0.9185 o 0.91S0 0.9175
i , ' 0'.0 012 01.4' 016 01.8 ].0 x (mol %)
Fig. 2. Variation in lattice parameter c with chemical composition.
c parameter is plotted as a function of chemical composition. The linear dependence reflects the Vegard's law validity for solid solubility. Another evidence of the good quality of the (FexC01-x)Ta206 samples comes from the MS measurements, as illustrated in Fig. 3 for the Feo.sCoo.2Ta206 sample at RT. All the spectra at RT were fitted to a quadrupole doublet with linewidth at half-height (F) in the range 0.26-0.27 mm/s. As the iron foil calibration resulted in F = 0.26mm/s, it is reasonable to suppose no significant impurity effect on the M6ssbauer spectra of the present samples. F r o m the crystallographic data it becomes evident that Fe 2 + (Co 2 +) and Ta 5 + cations are surrounded by 0 2 - octahedra, and that F e - O ( T a - O ) planes are well defined in the trirutile structure of (FexC01 x)Ta206. Successive F e - O ( C o - O ) planes are
separated by two T a - O planes. The hyperfine parameters are constant as a function of chemical composition for x >~ 0.8. This trend is observed at 300 and at 4.2 K. In conclusion, the tetragonal (FexCOl -x)TaeO6 system is a continuous solid solution, for which the effect of the Fe,--*Co substitution on the lattice parameters a and c is consistent with the Vegard's law. On the other hand F e ~ C o substitution have no effect on the hyperfine parameters for iron-rich samples. This work was supported in part by the Brazilian agencies CAPES, C N P q , F A P E R G S and F I N E P .
References
[1] S.M. Eicher, J.E. Greedan, K.J. Lushington, J. Solid State Chem. 62 (1986) 220. [2] R.K. Kremer, J.E. Greedan, J. Solid State Chem. 73 (1988) 579. [3] L.I. Zawislak, J.B.M. da Cunha, A. Vasquez, C.A. dos Santos, Solid State Commun. 94 (1995) 345. [4] L.I. Zawislak, G.L.F. Fraga, J.B.M. da Cunha, D. Schmitt, A.S. Carri~o, C.A. dos Santos, J. Phys.: Condens. Matter 9 (1997) 2295. [5] J.N. Reimers, J.E. Greedan, C.V. Stager, R. Kremer, J. Solid State Chem. 83 (1989) 20. [6] J. Rodriguez-Carvajal, Short reference guide of the program FullProf. Version 3.2, available in "pub/divers/fullp" of the anonymous ftp area of the LLB unix cluster.
ELSEVIER
Journal of Magnetism and Magnetic Materials 196-197 (1999) 846-847
magnetic materials
Structure and magnetic properties of layered (FexCo _ x)Ta206 compounds V.D. Mello, L.I. Zawislak, J.B. Marimon da Cunha, E.J. Kinast, J.B. Soares, C.A. dos Santos* lnstituto de Fisica-UFRGS, At;. Bento Goncalves 9500. C.P. 15051-Campus do f2de, 91501-970 Porto Alegre, RS, Brazil
Abstract Samples of (FexCo1_x)Ta206 were prepared from pure FeTa20~, and CoTa206. From X-ray diffraction (XRD) measurements followed by Rietveld refinement, it is demonstrated that the solid solution obeys the Vegard's law. M6ssbauer spectroscopy (MS) measurements at 4.2 and 300 K show that for x ~> 0.8 the Fe --+ Co substitution has no apparent effect on the hyperfine parameters. ~) !999 Elsevier Science B.V. All rights reserved. K e v w o r d s : Antiferromagnet: Rietveld refinement; M6ssbauer spectroscopy
In the tetragonal isomorphous ATa206 compounds (A = Fe, Co, Ni), the sub[attice formed by the 3d transition metal ions have the same symmetry as the Ni sublattice in KzNiF4, the well-known two-dimensional Heisenberg antiferromagnet [-1,2]. Recent M6ssbauer spectroscopy (MS) measurements [-3] have shown that from room temperature (RT) to about 15 K, FeTa20~, is clearly in the paramagnetic regime. The onset of longrange order is manifested near 10 K by the emerging hyperfine magnetic field. Magnetization curves [4], for temperatures between 1.4 and 5 K, display a noticeable change in slope, suggesting a spin-flop transition induced by a field higher than !0 T. Similar results were obtained for CoTazO6 [-2], but the magnetic structure, as suggested by powder neutron diffraction taken at 4.2 K [-5], is much more complex than for FeTa206. A tow-cone axis helical spin structure was proposed. Therefore, random mixtures of the type (FexCoi x)Ta206 are very appropriate to investigations on the effect of the competing anisotropies on the magnetic-phase diagram.
*Corresponding author. Tel.: + 55-51-3166424; fax: + 5551-3191762; e-mail: [email protected].
In the present work we report, for the first time, powder X-ray diffraction (XRD) and MS measurements on samples (FexCol .0Ta20~,, with x = 0, 0.4, 0.8 and 1. FeTa206 was prepared in vacuum, as described in Ref. [4]. CoTa206 was prepared in a similar way, but in air, instead of vacuum. For (FexCol x)TazO 6 samples, appropriate amounts of FeTa20~ and CoTa2Oo were mixed and processed as for the FeTa2Ot, preparation. XRD patterns were obtained in a Bragg-Brentano parafocusing geometry, under experimental setup as described in Ref. [4]. Structural parameters were refined with the program Fullprof [6]. The MS spectra were obtained at room temperature (RT) and at 4.2 K using a conventional spectrometer. The hyperfine parameters were obtained by a least-squares procedure. STCo in rhodium was used at room temperature as a source, with nominal activity of 50 mCi. The XRD patterns for all the samples were indexed to the space group P42/mnm, as expected for a trirutile structure. Typical pattern is shown in Fig. 1 for Feo.4COo.6Ta206. The small Bragg factor obtained for all the Rietveld refinements and the absence of spurious reflections on the XRD diagrams are strong indications that we have prepared well-crystallised single-phase samples, with lattice parameters typical of the tapiolite-mossite system (a ~ 0.47 nm, c ~ 0.92 nm). In Fig. 2 the
0304-8853/99/$ - see front matter ~!'~ 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 8 ) 0 0 9 6 9 - X
KD. Mello et al. / Journal of Magnetism and Magnetic Materials 196-197 (1999) 846-847
1.01 1.00 0.99 0.98 ~ 0.97
¢.20
30
40
50
60
2O
,
.
,
•
,
II
0.96
70
0.95 -4
Fig. 1. Room temperature X-ray powder diffraction pattern for (Fe0.4Co0.6)Ta206.
•
847
•
,
.
,
.
,
•
0.9200
Velocity (mm/s) Fig. 3. Room temperature M6ssbauer spectrum for (Fe0.sCo0.2)Ta206. AEo = 3.02 mm/s, ~Fe = 1.13 mm/s. Typical errors are + 3%.
0.9195 ~ ,~ 0.9190 ~.~0.9185 o 0.91S0 0.9175
i , ' 0'.0 012 01.4' 016 01.8 ].0 x (mol %)
Fig. 2. Variation in lattice parameter c with chemical composition.
c parameter is plotted as a function of chemical composition. The linear dependence reflects the Vegard's law validity for solid solubility. Another evidence of the good quality of the (FexC01-x)Ta206 samples comes from the MS measurements, as illustrated in Fig. 3 for the Feo.sCoo.2Ta206 sample at RT. All the spectra at RT were fitted to a quadrupole doublet with linewidth at half-height (F) in the range 0.26-0.27 mm/s. As the iron foil calibration resulted in F = 0.26mm/s, it is reasonable to suppose no significant impurity effect on the M6ssbauer spectra of the present samples. F r o m the crystallographic data it becomes evident that Fe 2 + (Co 2 +) and Ta 5 + cations are surrounded by 0 2 - octahedra, and that F e - O ( T a - O ) planes are well defined in the trirutile structure of (FexC01 x)Ta206. Successive F e - O ( C o - O ) planes are
separated by two T a - O planes. The hyperfine parameters are constant as a function of chemical composition for x >~ 0.8. This trend is observed at 300 and at 4.2 K. In conclusion, the tetragonal (FexCOl -x)TaeO6 system is a continuous solid solution, for which the effect of the Fe,--*Co substitution on the lattice parameters a and c is consistent with the Vegard's law. On the other hand F e ~ C o substitution have no effect on the hyperfine parameters for iron-rich samples. This work was supported in part by the Brazilian agencies CAPES, C N P q , F A P E R G S and F I N E P .
References
[1] S.M. Eicher, J.E. Greedan, K.J. Lushington, J. Solid State Chem. 62 (1986) 220. [2] R.K. Kremer, J.E. Greedan, J. Solid State Chem. 73 (1988) 579. [3] L.I. Zawislak, J.B.M. da Cunha, A. Vasquez, C.A. dos Santos, Solid State Commun. 94 (1995) 345. [4] L.I. Zawislak, G.L.F. Fraga, J.B.M. da Cunha, D. Schmitt, A.S. Carri~o, C.A. dos Santos, J. Phys.: Condens. Matter 9 (1997) 2295. [5] J.N. Reimers, J.E. Greedan, C.V. Stager, R. Kremer, J. Solid State Chem. 83 (1989) 20. [6] J. Rodriguez-Carvajal, Short reference guide of the program FullProf. Version 3.2, available in "pub/divers/fullp" of the anonymous ftp area of the LLB unix cluster.