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A STUDY OF THE MIXED SYSTEM (Fex, Mn1-x) WO4USING NEUTRON DIFFRACTION AND
MÖSSBAUER SPECTROSCOPY
Ch. Klein, R. Geller
To cite this version:
Ch. Klein, R. Geller. A STUDY OF THE MIXED SYSTEM (Fex, Mn1-x) WO4USING NEUTRON DIFFRACTION AND MÖSSBAUER SPECTROSCOPY. Journal de Physique Colloques, 1974, 35 (C6), pp.C6-589-C6-593. �10.1051/jphyscol:19746126�. �jpa-00215739�
A STUDY OF THE MIXED SYSTEM (Fe,, Mn,-,) WO,
USING NEUTRON DIFFRACTION AND MOSSBAUER SPECTROSCOPY
Ch. KLEIN and R. GELLER
Institut fiir Kristallographie der Universitat Tiibingen D 7400 Tiibingen, Charlottenstr. 33, Germany
R6sum6. - Les caractkristiques les plus intkressantes dans le comportement magnktique du systkme de cristaux mixtes (Fe,, Mn I - ~ ) W O ~ proviennent du fait que, bien que posskdant la meme structure cristallographique, les composks extremes ont des structures magnktiques diffkrentes et que les interactions d'kchange impliqukes dans I'ordre magnktique sont trks diffkrentes. Le mkcanisme de cette interaction a kt6 ktudik en utilisant la diffraction de neutrons et la spectroscopie Mossbauer.
Les rksultats sont discutks sur la base d'un modkle pour un cristal magnktiquement dksordonnk.
Abstract. - The most interesting features in the magnetic behaviour of the mixed crystal system (Fe,, Mnl-,)W04 are that the end members, although possessing the same crystal structure, have different magnetic structures and that the exchange interactions involved in the magnetic ordering are quite different. The mechanism of this interaction has been investigated using neutron diffraction and Mossbauer spectroscopy. The results are discussed on the basis of a model for a magnetically disordered crystal.
1. Introduction. - The mixed crystal
shows some interesting characteristics in its magnetic properties, which have already been discussed in part in earlier works [l-41. They originate from the fact that the end-members FeWO, and MnWO, of the binary system possess quite different magnetic struc- tures, whereas their crystallographic structures, as with the mixed crystals themselves, are identical.
The aim in this work is to help develop further the earlier models concerned with the magnetic interaction mechanism by using elastic neutron scattering and Mossbauer techniques.
The following observations were made :
(i) The magnetization in the mixed crystal shows an unusual temperature dependence. In the samples with small Fe content, the neutron diffraction peaks corresponding t o both magnetic structures are seen simultaneously.
(ii) Using the Mossbauer effect an additional line broadening is found beyond the temperature for magnetic ordering. These results are discussed on the basis of a model for magnetic interactions in mixed crystals.
2. Crystallographic and magnetic structure. - Fer- berite, FeWO,, and huebnerite, MnWO,, the pure substances in the series of mixed crystals (Fe,, Mn, -,)WO,, both possess a wolframite struc- ture with only small differences in lattice constants and
atomic parameters [5, 61. Both are monoclinic, the space group is P 2/c. The oxygen ions form a distorted hexagonal close-packed structure such that one half of the octahedra holes is filled with Fe, Mn-ions, and the other half with W-ions, respectively. Hence, in the b-c plane, layers of Fe- and Mn-ions alternate with layers of W-ions, so that only in the z-direction the Fe-and Mn-ions are so close together that the usual exchange or superexchange can take place. The magnetic structures of FeWO, and MnWO,, however, are different. In the case of FeWO, the moments of successive planes have antiparallel directions (TJ).
The magnetic unit cell is doubled in the a-direction in comparison to the chemical unit cell. FeWO, belongs to the Schubnikow-group Pa 2/c. In the case of MnWO, the magnetic unit cell is quadrupled in the a-direction and doubled in the b, c-directions (fTJJ) (Fig. 1).
Choosing a new system of axes the magnetic cell can be described by the vectors [201], [020] and 10021. The Schubnikow-group then is A, 2/a [7].
3. Neutron measurements. - With neutron scatter- ing such mixed crystals were investigated, whose reflection patterns show the magnetic reflections of both magnetic structures in the magnetically ordered state.
In the 2 0-range between 50 and 100 the ( t 00) reflection for the magnetic ferberite structure and the ( 6 4 +) reflection for the huebnerite structure were measured in steps of 0.10 with neutrons of wave- length 1 = 1.08
A.
Both magnetic reflections are neither disturbed by nuclear Bragg reflections, nor byArticle published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19746126
C6-590 Ch. KLEIN AND R. GELLER
magnetic exchange mechanism in these mixed crys- tals, a part of the Mn-spins is built into the magnetic ferberite structure and this explains why the intensity of the
(3
00) reflection once more noticeably increases around 12 K. At about 11 K the Mn-spins order themselves into a huebnerite structure and the intensity of the( $ 3 4)
reflection rises steeply around 9 K.At this same temperature the intensity of the
(3;
00) reflection shows a clear maximum and then falls at lower temperatures to about 65%
of this value.Neutron scattering on the samples with x = 0.16 and x = 0.28 confirms the above results, if, at the same time, the effect was not so pronounced.
FIG. 1. - Magnetic structure of FeW04, ferberite,and MnW04, huebnerite. For MnW04 only a half of the unit cell is shown.
The moments of the next layer in z-direction are antiparallel.
the other magnetic structure, nor by 112 contributions.
The integral intensity was measured to an accuracy of
*
5%.
Powder samples of concentrations x = 0.16, x = 0.20 and x = 0.28 were used for the measu- rements.
In figure 2 the integral intensities are shown for the
(4
00) and(4
)4)
reflections for x = 0.20.The ordering temperature is about 23 K and is defined by the setting in of the long range ordering of the Fe-spins. Using a model of Klein [3] for the
-4 -3 -2 -1 0 1 2 3 4
Doppler velocity (mrn/s) FIG. 3. -Examples of Mossbauer spectra of a (Fe0.2 (*I,
M f ; 0 . 8 ) ~ ~ 4 powder absorber in the paramagnetic and antifer- romagnetic region.
(*) Fe is enriched to 50% with Fe57.
temperature (KI
FIG. 2. - Integral intensities of magnetic powder-reflexes as a function of temperature : a) (+ OO), magnetic ferberite-structure,
b) (g
+
+), magnetic huebnerite structure.4. Mossbauer studies.
-
Because of the high wol- fram content for the Mossbauer studies a compound enriched with Fe57 had to be made so as to provide an analogue to the neutron diffraction measurements (I).For this purpose a stoichiometric mixture of MnO, FeO (50
%
Fe57) and W 0 3 was annealed for two days at 1 000 OC and finally quenched. The quenching has the effect of freezing the equipartition distribution of the Fe- and Mn-ions in the lattice. The lattice constants were then determined with a Guinier camera to see whether the reaction had undergone the full cycle and, subsequently, whether it had resulted in the desired mixture (2). The absolute error in the concentration x was estimated at+
3%.
The thickness of the powder absorber was 10 mg/cm2. The substance was ground, then diluted with about twice as much powdered silicid acid and finally put into a plastic container.In figure 3 a representative series of measured iMossbauer spectra is seen. The P ~ ( C O ~ ~ ) source was at room temperature for all measurements. The
Mossbauer spectra show a large quadrupole split- temperature (K) ting (QS) in the paramagnetic region and a resolved
hyperfine spectrum in the antiferromagnetic one. The FIG. 4. -Temperature dependence of H e ~ f in (Feo.;! (*), Mno.s)W04.
high temperature parameter and the temperature
dependen; QS (see Table I) show that the ~ e ~ + - i o n s (*) Fe is enriched to 50% with Fes7.
TABLE I
Parameters of hfs interactions at selected temperatures
T
1s
(n) QS (b) He,,(K) - (mm/s> - (mm/s)
- (kOe)
-
298 0.950
+
0.002 1.590+
0.003 -200 1.026
+
0.002 1.730+
0.003 -150 1.059
+
0.002 1.823 0.004 -30 1.093
+
0.006-
1.87+
0.05 38 320 1.103+0.006 -1.81 k 0 . 0 5 4 8 + 3 10 1.107
+
0.006 - 1.81+
0.05 51+
54.2 1.112 +_ 0.005 - 1.81 +_ 0.05 54 ) 5 (") IS relative to the ~ d ( C o ~ ~ ) source.
(b) QS =
3
e2qQ(l+
$ y2)'/'.exist in high spin configuration. The strong broadening of the lines at room temperature is typical of Fez+
high spin compounds with cation randomisation and indicates a local variation in the hyperfine interaction.
The course of He,, with temperature (see Fig. 4) was found from a fit of the low temperature spectra with the 4 energy values of the excited state and the splitting of the ground state as the fit parameters. If we assume an axial symmetric EFG tensor, then we arrive at the values for the QS splitting and these are represented in
table I. From about 50 K an additional broadening of the lines is observed which exhibits a weak maximum around 25 K. Apart from the two weak lines, which, because of the large relative error, do not provide any significant argument, the broadening is about the same for all lines. In addition, below around 100 K, both the QS lines have the same intensity but show an asymmetrical profile. The half-widths of the lines corresponding to the most positive and most negative velocities are shown in figure 5.
(1) It was manufactured in the same manner as the earlier
(Feo.2, Mno.~)W04 specimens one for neutron and suscepti- I
bility measurements. 100
1
J
150' 300 (2) However we did not succeed in manufacturing probes temperature (K) --, with the same mixing ratio, because the one with natural Fe and
the other enriched with Fel did not have the same order tem- FIG. 5. - Half line widths of hf lines corresponding to the
perature. most pos. (a) and most neg. (b) Doppler velocity.
C6-592 Ch. KLEIN AND R. GELLER
5. Discussion. - The peculiarities in the magnetic behaviour of the wolframite mixed crystal system (Fe,, Mn,-,) WO, are based mainly on the fact that the end-members FeWO, and MnWO,, although possessing the same crystal structure, have different magnetic structures. The exchange integrals can be estimated from the paramagnetic Curie and NCel temperatures [3]. As a result we find that the Fe-Fe exchange interactions are much stronger than the Mn-Mn and the Fe-Mn exchange interactions (JF,_,, z 76 K, J,,.,, E 14 K, JFe.,, r 8 K). The magnetic structure of the mixed crystals with high Fe content (x > 0.35) is identical with that of pure FeWO, at all temperatures.
The irregular course of the magnetization, as first observed by Weitzel [3, 41, can be explained by building Mn-spins at temperatures T < TN into the magnetic order of the Fe-spins. This process comes into play if the strength of the weak Fe-Mn exchange interactions suffice to couple the Mn-spins partly with the already long range ordered magnetic ferberite structure. A calculation from Klein [3] on the basis of a modified effective field model yields the course of the magnetiza- tion with temperature for the mixed crystals with high Fe concentration under the assumption that the Fe- and Mn-ions are statistically distributed on the magnetic ion sites (see Fig. 6). The simultaneous
0 I I I I I
i,
I0 10 20 30 40 50 60 70 80 temperature fKI FIG. 6. -Calculated temperature dependence of the total spontaneous magnetization of ( F ~ o . ~ , Mno.s)wO~. The points
represent experimental values of Weitzel.
appearance of the reflections of both magnetic struc- tures in the neutron diffraction studies of the mixed crystal (Fe,
,,
Mn,.,)WO, as well as the anomalous decrease in the integral intensity of the (4 00) reflection at low temperatures is explained by this model in the following way : Mn-spins are partly first built into the magnetic ferberite structure because of a local pre- dominantly Fe neighbourhood but at lower tempera- tures they change from this structure to the magnetic huebnerite structure as soon as the latter is formed from the remaining and previously disordered Mn-spins.The picture that the Mn-spins in crystals with lower Fe content can firstly be built into the magnetic ferberite structure, but later, at lower temperatures, however, into the magnetic huebnerite structure, means that different magnetic phases can coexist in the sample (Fe,,,, Mn,.,) WO,. The possibility of such a coexis- tence of different magnetic phases has been theoreti- cally investigated and ratified by Wegner [8].
The simultaneous appearance of the reflections corresponding to the different magnetic structures could possibly be explained by the existence of a miscibility gap. The observed intensity maximum of the
(3
00) reflection does not, however, support this.In addition to this it has been shown that samples sintered at different temperatures produce the same results in both susceptibility and neutron diffraction measurements, so that the influence of any possible miscibility gap may be considered as a secondary effect [4]. With the aid of Mossbauer spectroscopy it should be further tested and explained whether the anomalous course of the integral intensity of the (+ 00) reflection comes from the changing of a part of the Mn-spins from the one magnetic structure to the other.
In contrast to the space spin correlations which are responsible for the magnetic neutron scattering, using Mossbauer spectroscopy, time correlations are observed. The mixed crystals with low Fe content can be viewed as paramagnetically diluted for the Fe-ions because the neighbourhood of the Fe-ions consists mainly of Mn-ions and the mixed interaction is very small in comparison to the Fe-Fe interaction.
In such systems the ionic spin fluctuations may be sufficiently slow for observation [9, 101. Below 50 K a broadening, independent of line position, is observed in the spectra.
According to F. van der Woude et al. [9] this points to slow fluctuations. The difference in the order tempe- ratures found from neutron diffraction and Mossbauer spectroscopy can be explained by the experimental error assumed for the mixing ratios in the sample, prepared on the one hand with natural Fe for neutron measurements, and on the other enriched with Fe57 for Mossbauer measurements. The weak maximum in the line-widths can be explained by relaxations, which are caused by the building in of Mn spins in the Fe- structure. In this temperature range the course of He,, does not exhibit any irregularity within the experimen- tal accuracy. This indicates that the Mn-spins built in the Fe structure have an influence on the relaxations times but only a negligible contribution to He,,.
From this it must be concluded that the maximum in the course of the (4 00) intensity arises from the coupling of the Mn spins to the magnetic ferberite structure and their changing to the magnetic huebnerite structure at lower temperatures.
Acknowledgments. - The authors wish to thank the Gesellschaft fiir Kernforschung Karlsruhe, espe-
cially the RB department for the availability of neutron Prof. H. Schrocke and Dr. A. Trumm, Institut fur diffraction facilities at the FR 2 Pile. We are indebted Kristallographie und Mineralogie, Munchen, for to Prof. H. Dachs for his continuous support and to preparing the mixed crystals used in these studies.
References
[I] WEITZEL, H., N. Jahrb. Miner. Abh. 113 (1970) 13. [71 DACHS, H., Solid State Commun. 7 (1969) 1015.
. .
[2] WEITZEL, H., 2. Kristallogr. 131 (1970) 289. [8] WEGNER, F., Solid State Commun. 12 (1973) 785.
[3] KLEIN, Ch., SolidState Commcm. 12 (1973) 773.
[4] OBERMAYER, H. A., DACHS, H. and SCHRGCKE, H., Solid 191 VAN DER WOUDE, F., BLAAUW, ' - 7 DEKKER, A- I., PrOc.
State Commun. 12 (1973) 779. Conf. on Appl. of the Mossbauer effect, Tihany (1969).
[5] ULKU, D., 2. Kristallogr. 124 (1967) 192. [lo] DE WAARD, H. and HOUSLEY, R. M., Hyperfine Interactions [6] DACHS, H., STOLL, E. and WEITZEL, H., Solid State (Academic Press Inc.) 1967, Ed. by A. J. Freeman and
Commun. 4 (1966) 473. R. B. Frankel.
