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Magnetic properties of the ternary sulfide Na6MnS4
Journal of Alloys and Compounds 338 (2002) 116–120
L
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Magnetic properties of the ternary sulfide Na 6 MnS 4 ¨ ¨ W. Bronger*, M. Bohmer, P. Muller ¨ Anorganische Chemie der TH Aachen, Professor-Pirlet-Strasse 1, D-52056 Aachen, Germany Institut f ur Received 1 November 2001; accepted 7 December 2001
Abstract The characteristic structural feature of the ternary sulfide Na 6 MnS 4 is isolated, hardly distorted [MnS 4 ]-tetrahedrons (space group P63 mc). Measurements of the magnetic susceptibilities down to low temperatures yield a behaviour according to the Curie-law ( m 55.93(2) mB ). Only below 4 K can deviations be detected indicating anti-ferromagnetic interactions. However, neutron diffraction experiments at 1.7 K did not show a magnetic ordering. Additional measurements under the influence of an external magnetic field (.10 kOe) lead to a ferromagnetic spin structure. An experiment, in which the external magnetic field of 50 kOe was turned off at a constantly kept temperature of 1.7 K, indicates the formation of an anti-ferromagnetic spin structure, with a unit cell volume six times larger than ´ temperature of this anti-ferromagnetic phase was determined to be 4.7(2) K. After cooling, that of the crystallographic structure. The Neel it was observed that the anti-ferromagnetic spin structure, once again, could only be detected after a ferromagnetic order was induced in advance. 2002 Elsevier Science B.V. All rights reserved. Keywords: Ternary sulfides; Magnetically ordered materials; Magnetic measurements; Neutron diffraction
1. Introduction The ternary chalcogenides of the general composition A 6 MX 4 with A being Na or K, M being Mn, Fe or Co and X being S, Se or Te [1–4], crystallize isotypically in the atomic arrangement of Na 6 ZnO 4 (space group P63 mc) [5]. The characteristic structural feature of these compounds is isolated, hardly distorted [MX 4 ]-tetrahedra that are oriented parallel with the direction of the c-axis (Fig. 1). As far as the cobalt compounds Na 6 CoS 4 and Na 6 CoSe 4 are concerned, we were able to analyze the anti-ferromagnetic interactions between the transition metal atoms at low temperatures via neutron diffraction experiments [6]. We found that the crystallographic unit cell in which the transition metal atoms occupy a two-fold point position, is not maintained in the spin structure. Instead, an orthorhombic cell exists having a volume four times larger than the hexagonal one. To answer the question, if the orbital moment of the cobalt atoms does have the decisive influence we included the isotypic manganese compounds
*Corresponding author. E-mail address: [email protected] (W. Bronger).
Na 6 MnS 4 and Na 6 MnSe 4 into our experiments, since in this case, there are no such orbital moment influences on the magnetic structure due to the d 5 -configuration of the manganese ions. The results of the investigations concerning the sulfide Na 6 MnS 4 will be discussed here.
Fig. 1. The hexagonal structure of Na 6 MnS 4 , shown as a projection along the c-axis. The small dark spheres are the positions of the sodium atoms, the large spheres those of the sulfur atoms. The masket manganese atoms centre the sulfur tetrahedrons, their z-parameters being 1 / 4 and 3 / 4, respectively. For the other atoms the z-parameters are given in the picture (values from X-ray experiments on single crystals [1]).
0925-8388 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 02 )00224-4
W. Bronger et al. / Journal of Alloys and Compounds 338 (2002) 116 – 120
2. Susceptibility measurements Na 6 MnS 4 was synthesized by heating a mixture of sodium carbonate, manganese and sulfur at a temperature of 1200 K in a stream of hydrogen [1]. Since the product contained small amounts of Na 2 S due to the excess of sodium carbonate in the original mixture, a complete analysis was carried out to correct the susceptibility values with regard to the manganese content (ICP-OES-Spectrometer VISTA RL, Varian). The susceptibility measurements were carried out with a SQUID-magnetometer (Quantum Design) in the temperature range between room temperature and 1.8 K. The weight of the sample was 3.805 mg. The vessel material was quartz (Suprasil, Fa. Heraeus). For each temperature setting, measurements were done at 11 different field strengths in a range between 50 Oe and 50 kOe. In Fig. 2, the 1 /xmol 2 T-diagram is shown. Down to low temperatures, a behaviour according to the Curie-law can be observed. Only below 4 K can deviations from the linear course be detected. The paramagnetic moment m at higher temperatures is 5.93(2) mB and thus matches exactly the expected value for the existing d 5 configuration of a manganese(II) compound. In addition, two more observations seem to be of interest: (i) The susceptibility values are in a range between 300 K and ca. 5 K independent of the field strength. At lower temperatures they show a weak field dependence at a field strength below 10 kOe. At higher field strengths however, they are much more dependent.
117
(ii) At temperatures below ca. 4.5 K we observed a small, but measurable, magnetic hysteresis independent of the field strength (50 Oe–50 kOe) at alternating cooling and heating steps. Both observations indicate complex interactions of the paramagnetic centres at low temperatures, possibly a metamagnetic behaviour.
3. Neutron diffraction experiments Neutron diffraction experiments were carried out at the research reactor BER II at the Hahn-Meitner-Institut (Berlin) and showed that the crystal structure determined by X-ray analysis at room temperature is preserved, even at 1.7 K. In Table 1, some details about the measurements on a powdered sample are given, as well as the summarized evaluation of the diagrams at 293 and 1.7 K. The lattice constants and atomic positions at room temperature correspond very well with the values we obtained from X-ray experiments on single crystals [1] (see Fig. 1). Unexpectedly, the measurement at 1.7 K did not indicate a magnetic ordering. Therefore, we did additional measurements at that temperature under the influence of differently strong external magnetic fields (E6-Diffractometer, neutron flux at the sample: 5?10 6 n / cm 2 ?s; cryomagnet VM-3 with a maximal field strength of 50 kOe). Fig. 3 shows ten measurements. The field strength starting at 0 kOe (lowest graph) was increased by 5 kOe for each measurement so that the upper graph is to be assigned to an external field
Fig. 2. Na 6 MnS 4 . Temperature dependence of the reciprocal molar susceptibilities at a field strength of 5 kOe. The upper inset shows the behaviour at low temperature.
W. Bronger et al. / Journal of Alloys and Compounds 338 (2002) 116 – 120
118
Table 1 Refinement results on neutron powder diffraction data for Na 6 MnS 4 ; space group P63 mc, Z52, profile function Gauss, refinements by program ˚a FULLPROF, l 52.4357 A Temperature 2u range Lattice constants
293 K 5.5–100.58 ˚ a58.996(1) A ˚ c56.951(1) A
Atomic parameters
x
y
z
˚ 2) Biso. (A
x
y
z
˚ 2) Biso. (A
Na1(6c) Na2(6c) Mn(2b) S1(2b) S2(6c) R Bragg -value
0.145(2) 0.533(1) 1/3 1/3 0.184(1)
20.145(2) 20.533(1) 2/3 2/3 20.184(1)
0.542(4) 0.378(4) 1/4 0.601(4) 0.147(4) 0.030
1.2(7) 0.3(4) 2.1(8) 2.5(9) 0.1(5)
0.141(2) 0.533(1) 1/3 1/3 0.182(2)
20.141(2) 20.533(1) 2/3 2/3 20.182(2)
0.537(4) 0.370(5) 1/4 0.600(8) 0.145(6) 0.027
0.6 a 0.6 a 0.6 a 0.6 a 0.6 a
a
1.7 K 9–858 ˚ a58.892(2) A ˚ c56.848(2) A
Isotropic temperature factor kept constant.
strength of 50 kOe. Evaluation of the obtained data showed that the spin structure can be described by the crystallographically found unit cell. The increasing intensities of the original reflections with increasing field strength, including those reflections that are not detectable due to their low intensity, clearly show that there is a ferromagnetic arrangement. The observation of the ferromagnetic spin structure at field strengths above 10 kOe corresponds with the results from susceptibility measurements mentioned above. At low temperatures, they show that a clear
dependence of the x -values on the field strength above 10 kOe exists (see (ii)). For the determination of the magnetic structure, the atomic positions of the room temperature scan, as shown in Table 1, were used and kept constant. The spin structure was determined using the measurement at 50 kOe, since the saturation is almost reached at that field according to the intensity course of the diffraction reflections. With nine variables, we were able to refine the crystallographic structure to a R Bragg -value of 0.049 and the magnetic
Fig. 3. Na 6 MnS 4 . Neutron diffraction diagrams at 1.7 K depending on an external magnetic field. The diagram at the bottom is associated with a field of 0 kOe, that at the top of 50 kOe. The steps represent 5 kOe each.
W. Bronger et al. / Journal of Alloys and Compounds 338 (2002) 116 – 120
structure to 0.027. Although the hexagonal unit cell could be kept for the magnetic structure, the symmetry gets lost. Therefore, calculations were made within the space group P1. For the magnetic moment, a value of 5.1(2) mB was determined which is in accordance with the expected value of 5.0 mB . The moment is oriented along a face diagonal [101] or [011]. Due to the hexagonal metrics, it is not possible to differentiate between the directions [101] and [011] from powdered samples because the corresponding reflections match. A surprise was an experiment in which the external magnetic field of 50 kOe was turned off at a constantly kept temperature of 1.7 K. We found a diagram that showed that the intensities caused by the magnetic field totally disappeared, but instead, new, very strong reflections occurred. As we found, they indicate the formation of an anti-ferromagnetic spin structure. In order to index all reflections, it was necessary to make the original unit cell volume six times larger. The hexagonal metrics, however, remain with a mag. 5 a cryst. ? œ3 and c mag. 5 2 ? c cryst. . For the calculation of the spin structure, the decrease of symmetry was once again permitted down to the triclinic space group P1. With 12 manganese atoms per-unit-cell, 462 models result [7] for a colinear spin arrangement. With these models, intensity calculations where performed until an arrangement was found that matched the experimentally determined pattern of intensities. For the refinement of the structure model, the atomic parameters gained from the single crystal measurements were kept constant. The lattice constants, as well as the profile parameters, were refined both for the crystallographic and the magnetic structure according to their relation above. Results are summarized in Table 2. The relatively high R Bragg -value of the magnetic structure is given by the superposition of reflections caused by the large unit cell. Fig. 4 sketches the correlations between crystal and magnetic structure. For the manganese atoms, a moment of 4.4(3) mB was found in the anti-ferromagnetic structure. The orientation is along [101] or [011]. Once again, due to the hexagonal metrics, it is not possible to differentiate between both directions on the basis of powdered samples. Fig. 5 shows the results of a series of measurements at constant temperature, in which one spectrum was done in an external Table 2 Refinement results on neutron powder diffraction data for Na 6 MnS 4 after an external magnetic field of 50 kOe was turned off at a constantly kept temperature of 1.6 K; profile function Gauss, refinements by program ˚ FULLPROF, l 52.4397 A Unit cell (crystallographic) Lattice constants
Formula units 2u range Atomic parameters R Bragg -value
˚ a58.911(1) A ˚ c56.857(1) A
Unit cell (magnetic)
˚ a515.434(1) A ˚ b515.434(1) A ˚ c513.714(2) A a 5 b 5908, g 51208 Z52 Z512 8.58–84.58 See Table 1, measurement at 1.7 K 0.056 0.198
119
Fig. 4. Na 6 MnS 4 . (a) Interrelation between the unit cells of the crystal structure and of the magnetic structure. (b) and (c) The spin structure of the twelve manganese atoms inside the magnetic unit cell.
magnetic field and one without, the field strength progressively increased. With the external field turned off, the anti-ferromagnetic structure can be detected only with preceding magnetization above 40 kOe. Obviously, the technical restriction to 50 kOe does not allow an entire formation of the anti-ferromagnetic structure. This is probably the reason why we found a slightly too low magnetic moment for the manganese atoms. In an additional experiment, the sample with the antiferromagnetic structure was slowly heated, and by intensi´ ty measurements of the magnetic reflections, the Neel temperature was determined to be 4.7(2) K.
W. Bronger et al. / Journal of Alloys and Compounds 338 (2002) 116 – 120
120
Fig. 5. Na 6 MnS 4 . Neutron diffraction diagrams at 1.7 K recorded alternating with and without an increasing external magnetic field.
4. Final comments The result that the anti-ferromagnetic spin structure does not provide a neutron diffraction diagram without preceding magnetization at a field strength of .40 kOe down to low temperatures, shows that there is, under these conditions, only a short-range order, not a long range order. The susceptibility measurements confirm the assumption that anti-ferromagnetic interactions exist at temperatures below 4.7 K even without preceding magnetization. The mechanism that leads to the formation of the long range order of the anti-ferromagnetic interactions after the development of a ferromagnetic order and the following switching off of the external magnetic field is still unknown. Additional investigations will be necessary to answer this question. It is worth mentioning that corresponding experiments concerning the isotypic selenide Na 6 MnSe 4 showed analogous phenomena. Here, we were not able to detect a spin structure down to a temperature of 0.3 K, although susceptibility measurements indicate that below ca. 4.5 K spin–spin interactions exist. The anti-ferromagnetic structure is still unknown, although a preceding magnetization at 50 kOe induces a ferromagnetic order according to the isotypic sulfide, but after the switching off of the field, there is no anti-ferromagnetic structure detectable. Obvi-
ously, the magnetic field is not strong enough for that purpose.
Acknowledgements We thank the Fonds der Chemischen Industrie and the BMBF for financial support of this work, BENSC in Berlin for supporting the neutron diffraction experiments, Dr Gudrun Auffermann for the analytical work and Holger Bronger for assistance in preparing the manuscript.
References [1] W. Bronger, H. Balk-Hardtdegen, Z. Anorg. Allg. Chem. 574 (1989) 89. [2] W. Bronger, H. Balk-Hardtdegen, U. Ruschewitz, Z. Anorg. Allg. Chem. 616 (1992) 14. [3] K. Klepp, W. Bronger, Z. Naturforsch. 38B (1983) 12. [4] W. Bronger, C. Bomba, W. Koelman, Z. Anorg. Allg. Chem. 621 (1995) 409. [5] P. Kastner, R. Hoppe, Z. Anorg. Allg. Chem. 409 (1983) 69. ¨ [6] W. Bronger, W. Koelman, P. Muller, Z. Anorg. Allg. Chem. 621 (1995) 412. [7] W. Prandl, Neutron diffraction, in: H. Dachs (Ed.), Topics in Current Physics, Springer Verlag, Berlin, Heidelberg, New York, 1978, p. 113.
L
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Magnetic properties of the ternary sulfide Na 6 MnS 4 ¨ ¨ W. Bronger*, M. Bohmer, P. Muller ¨ Anorganische Chemie der TH Aachen, Professor-Pirlet-Strasse 1, D-52056 Aachen, Germany Institut f ur Received 1 November 2001; accepted 7 December 2001
Abstract The characteristic structural feature of the ternary sulfide Na 6 MnS 4 is isolated, hardly distorted [MnS 4 ]-tetrahedrons (space group P63 mc). Measurements of the magnetic susceptibilities down to low temperatures yield a behaviour according to the Curie-law ( m 55.93(2) mB ). Only below 4 K can deviations be detected indicating anti-ferromagnetic interactions. However, neutron diffraction experiments at 1.7 K did not show a magnetic ordering. Additional measurements under the influence of an external magnetic field (.10 kOe) lead to a ferromagnetic spin structure. An experiment, in which the external magnetic field of 50 kOe was turned off at a constantly kept temperature of 1.7 K, indicates the formation of an anti-ferromagnetic spin structure, with a unit cell volume six times larger than ´ temperature of this anti-ferromagnetic phase was determined to be 4.7(2) K. After cooling, that of the crystallographic structure. The Neel it was observed that the anti-ferromagnetic spin structure, once again, could only be detected after a ferromagnetic order was induced in advance. 2002 Elsevier Science B.V. All rights reserved. Keywords: Ternary sulfides; Magnetically ordered materials; Magnetic measurements; Neutron diffraction
1. Introduction The ternary chalcogenides of the general composition A 6 MX 4 with A being Na or K, M being Mn, Fe or Co and X being S, Se or Te [1–4], crystallize isotypically in the atomic arrangement of Na 6 ZnO 4 (space group P63 mc) [5]. The characteristic structural feature of these compounds is isolated, hardly distorted [MX 4 ]-tetrahedra that are oriented parallel with the direction of the c-axis (Fig. 1). As far as the cobalt compounds Na 6 CoS 4 and Na 6 CoSe 4 are concerned, we were able to analyze the anti-ferromagnetic interactions between the transition metal atoms at low temperatures via neutron diffraction experiments [6]. We found that the crystallographic unit cell in which the transition metal atoms occupy a two-fold point position, is not maintained in the spin structure. Instead, an orthorhombic cell exists having a volume four times larger than the hexagonal one. To answer the question, if the orbital moment of the cobalt atoms does have the decisive influence we included the isotypic manganese compounds
*Corresponding author. E-mail address: [email protected] (W. Bronger).
Na 6 MnS 4 and Na 6 MnSe 4 into our experiments, since in this case, there are no such orbital moment influences on the magnetic structure due to the d 5 -configuration of the manganese ions. The results of the investigations concerning the sulfide Na 6 MnS 4 will be discussed here.
Fig. 1. The hexagonal structure of Na 6 MnS 4 , shown as a projection along the c-axis. The small dark spheres are the positions of the sodium atoms, the large spheres those of the sulfur atoms. The masket manganese atoms centre the sulfur tetrahedrons, their z-parameters being 1 / 4 and 3 / 4, respectively. For the other atoms the z-parameters are given in the picture (values from X-ray experiments on single crystals [1]).
0925-8388 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 02 )00224-4
W. Bronger et al. / Journal of Alloys and Compounds 338 (2002) 116 – 120
2. Susceptibility measurements Na 6 MnS 4 was synthesized by heating a mixture of sodium carbonate, manganese and sulfur at a temperature of 1200 K in a stream of hydrogen [1]. Since the product contained small amounts of Na 2 S due to the excess of sodium carbonate in the original mixture, a complete analysis was carried out to correct the susceptibility values with regard to the manganese content (ICP-OES-Spectrometer VISTA RL, Varian). The susceptibility measurements were carried out with a SQUID-magnetometer (Quantum Design) in the temperature range between room temperature and 1.8 K. The weight of the sample was 3.805 mg. The vessel material was quartz (Suprasil, Fa. Heraeus). For each temperature setting, measurements were done at 11 different field strengths in a range between 50 Oe and 50 kOe. In Fig. 2, the 1 /xmol 2 T-diagram is shown. Down to low temperatures, a behaviour according to the Curie-law can be observed. Only below 4 K can deviations from the linear course be detected. The paramagnetic moment m at higher temperatures is 5.93(2) mB and thus matches exactly the expected value for the existing d 5 configuration of a manganese(II) compound. In addition, two more observations seem to be of interest: (i) The susceptibility values are in a range between 300 K and ca. 5 K independent of the field strength. At lower temperatures they show a weak field dependence at a field strength below 10 kOe. At higher field strengths however, they are much more dependent.
117
(ii) At temperatures below ca. 4.5 K we observed a small, but measurable, magnetic hysteresis independent of the field strength (50 Oe–50 kOe) at alternating cooling and heating steps. Both observations indicate complex interactions of the paramagnetic centres at low temperatures, possibly a metamagnetic behaviour.
3. Neutron diffraction experiments Neutron diffraction experiments were carried out at the research reactor BER II at the Hahn-Meitner-Institut (Berlin) and showed that the crystal structure determined by X-ray analysis at room temperature is preserved, even at 1.7 K. In Table 1, some details about the measurements on a powdered sample are given, as well as the summarized evaluation of the diagrams at 293 and 1.7 K. The lattice constants and atomic positions at room temperature correspond very well with the values we obtained from X-ray experiments on single crystals [1] (see Fig. 1). Unexpectedly, the measurement at 1.7 K did not indicate a magnetic ordering. Therefore, we did additional measurements at that temperature under the influence of differently strong external magnetic fields (E6-Diffractometer, neutron flux at the sample: 5?10 6 n / cm 2 ?s; cryomagnet VM-3 with a maximal field strength of 50 kOe). Fig. 3 shows ten measurements. The field strength starting at 0 kOe (lowest graph) was increased by 5 kOe for each measurement so that the upper graph is to be assigned to an external field
Fig. 2. Na 6 MnS 4 . Temperature dependence of the reciprocal molar susceptibilities at a field strength of 5 kOe. The upper inset shows the behaviour at low temperature.
W. Bronger et al. / Journal of Alloys and Compounds 338 (2002) 116 – 120
118
Table 1 Refinement results on neutron powder diffraction data for Na 6 MnS 4 ; space group P63 mc, Z52, profile function Gauss, refinements by program ˚a FULLPROF, l 52.4357 A Temperature 2u range Lattice constants
293 K 5.5–100.58 ˚ a58.996(1) A ˚ c56.951(1) A
Atomic parameters
x
y
z
˚ 2) Biso. (A
x
y
z
˚ 2) Biso. (A
Na1(6c) Na2(6c) Mn(2b) S1(2b) S2(6c) R Bragg -value
0.145(2) 0.533(1) 1/3 1/3 0.184(1)
20.145(2) 20.533(1) 2/3 2/3 20.184(1)
0.542(4) 0.378(4) 1/4 0.601(4) 0.147(4) 0.030
1.2(7) 0.3(4) 2.1(8) 2.5(9) 0.1(5)
0.141(2) 0.533(1) 1/3 1/3 0.182(2)
20.141(2) 20.533(1) 2/3 2/3 20.182(2)
0.537(4) 0.370(5) 1/4 0.600(8) 0.145(6) 0.027
0.6 a 0.6 a 0.6 a 0.6 a 0.6 a
a
1.7 K 9–858 ˚ a58.892(2) A ˚ c56.848(2) A
Isotropic temperature factor kept constant.
strength of 50 kOe. Evaluation of the obtained data showed that the spin structure can be described by the crystallographically found unit cell. The increasing intensities of the original reflections with increasing field strength, including those reflections that are not detectable due to their low intensity, clearly show that there is a ferromagnetic arrangement. The observation of the ferromagnetic spin structure at field strengths above 10 kOe corresponds with the results from susceptibility measurements mentioned above. At low temperatures, they show that a clear
dependence of the x -values on the field strength above 10 kOe exists (see (ii)). For the determination of the magnetic structure, the atomic positions of the room temperature scan, as shown in Table 1, were used and kept constant. The spin structure was determined using the measurement at 50 kOe, since the saturation is almost reached at that field according to the intensity course of the diffraction reflections. With nine variables, we were able to refine the crystallographic structure to a R Bragg -value of 0.049 and the magnetic
Fig. 3. Na 6 MnS 4 . Neutron diffraction diagrams at 1.7 K depending on an external magnetic field. The diagram at the bottom is associated with a field of 0 kOe, that at the top of 50 kOe. The steps represent 5 kOe each.
W. Bronger et al. / Journal of Alloys and Compounds 338 (2002) 116 – 120
structure to 0.027. Although the hexagonal unit cell could be kept for the magnetic structure, the symmetry gets lost. Therefore, calculations were made within the space group P1. For the magnetic moment, a value of 5.1(2) mB was determined which is in accordance with the expected value of 5.0 mB . The moment is oriented along a face diagonal [101] or [011]. Due to the hexagonal metrics, it is not possible to differentiate between the directions [101] and [011] from powdered samples because the corresponding reflections match. A surprise was an experiment in which the external magnetic field of 50 kOe was turned off at a constantly kept temperature of 1.7 K. We found a diagram that showed that the intensities caused by the magnetic field totally disappeared, but instead, new, very strong reflections occurred. As we found, they indicate the formation of an anti-ferromagnetic spin structure. In order to index all reflections, it was necessary to make the original unit cell volume six times larger. The hexagonal metrics, however, remain with a mag. 5 a cryst. ? œ3 and c mag. 5 2 ? c cryst. . For the calculation of the spin structure, the decrease of symmetry was once again permitted down to the triclinic space group P1. With 12 manganese atoms per-unit-cell, 462 models result [7] for a colinear spin arrangement. With these models, intensity calculations where performed until an arrangement was found that matched the experimentally determined pattern of intensities. For the refinement of the structure model, the atomic parameters gained from the single crystal measurements were kept constant. The lattice constants, as well as the profile parameters, were refined both for the crystallographic and the magnetic structure according to their relation above. Results are summarized in Table 2. The relatively high R Bragg -value of the magnetic structure is given by the superposition of reflections caused by the large unit cell. Fig. 4 sketches the correlations between crystal and magnetic structure. For the manganese atoms, a moment of 4.4(3) mB was found in the anti-ferromagnetic structure. The orientation is along [101] or [011]. Once again, due to the hexagonal metrics, it is not possible to differentiate between both directions on the basis of powdered samples. Fig. 5 shows the results of a series of measurements at constant temperature, in which one spectrum was done in an external Table 2 Refinement results on neutron powder diffraction data for Na 6 MnS 4 after an external magnetic field of 50 kOe was turned off at a constantly kept temperature of 1.6 K; profile function Gauss, refinements by program ˚ FULLPROF, l 52.4397 A Unit cell (crystallographic) Lattice constants
Formula units 2u range Atomic parameters R Bragg -value
˚ a58.911(1) A ˚ c56.857(1) A
Unit cell (magnetic)
˚ a515.434(1) A ˚ b515.434(1) A ˚ c513.714(2) A a 5 b 5908, g 51208 Z52 Z512 8.58–84.58 See Table 1, measurement at 1.7 K 0.056 0.198
119
Fig. 4. Na 6 MnS 4 . (a) Interrelation between the unit cells of the crystal structure and of the magnetic structure. (b) and (c) The spin structure of the twelve manganese atoms inside the magnetic unit cell.
magnetic field and one without, the field strength progressively increased. With the external field turned off, the anti-ferromagnetic structure can be detected only with preceding magnetization above 40 kOe. Obviously, the technical restriction to 50 kOe does not allow an entire formation of the anti-ferromagnetic structure. This is probably the reason why we found a slightly too low magnetic moment for the manganese atoms. In an additional experiment, the sample with the antiferromagnetic structure was slowly heated, and by intensi´ ty measurements of the magnetic reflections, the Neel temperature was determined to be 4.7(2) K.
W. Bronger et al. / Journal of Alloys and Compounds 338 (2002) 116 – 120
120
Fig. 5. Na 6 MnS 4 . Neutron diffraction diagrams at 1.7 K recorded alternating with and without an increasing external magnetic field.
4. Final comments The result that the anti-ferromagnetic spin structure does not provide a neutron diffraction diagram without preceding magnetization at a field strength of .40 kOe down to low temperatures, shows that there is, under these conditions, only a short-range order, not a long range order. The susceptibility measurements confirm the assumption that anti-ferromagnetic interactions exist at temperatures below 4.7 K even without preceding magnetization. The mechanism that leads to the formation of the long range order of the anti-ferromagnetic interactions after the development of a ferromagnetic order and the following switching off of the external magnetic field is still unknown. Additional investigations will be necessary to answer this question. It is worth mentioning that corresponding experiments concerning the isotypic selenide Na 6 MnSe 4 showed analogous phenomena. Here, we were not able to detect a spin structure down to a temperature of 0.3 K, although susceptibility measurements indicate that below ca. 4.5 K spin–spin interactions exist. The anti-ferromagnetic structure is still unknown, although a preceding magnetization at 50 kOe induces a ferromagnetic order according to the isotypic sulfide, but after the switching off of the field, there is no anti-ferromagnetic structure detectable. Obvi-
ously, the magnetic field is not strong enough for that purpose.
Acknowledgements We thank the Fonds der Chemischen Industrie and the BMBF for financial support of this work, BENSC in Berlin for supporting the neutron diffraction experiments, Dr Gudrun Auffermann for the analytical work and Holger Bronger for assistance in preparing the manuscript.
References [1] W. Bronger, H. Balk-Hardtdegen, Z. Anorg. Allg. Chem. 574 (1989) 89. [2] W. Bronger, H. Balk-Hardtdegen, U. Ruschewitz, Z. Anorg. Allg. Chem. 616 (1992) 14. [3] K. Klepp, W. Bronger, Z. Naturforsch. 38B (1983) 12. [4] W. Bronger, C. Bomba, W. Koelman, Z. Anorg. Allg. Chem. 621 (1995) 409. [5] P. Kastner, R. Hoppe, Z. Anorg. Allg. Chem. 409 (1983) 69. ¨ [6] W. Bronger, W. Koelman, P. Muller, Z. Anorg. Allg. Chem. 621 (1995) 412. [7] W. Prandl, Neutron diffraction, in: H. Dachs (Ed.), Topics in Current Physics, Springer Verlag, Berlin, Heidelberg, New York, 1978, p. 113.