MÖSSBAUER STUDIES OF THE ISOSTRUCTURAL COMPOUNDS FeGe, FeSn AND CoSn

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MÖSSBAUER STUDIES OF THE ISOSTRUCTURAL COMPOUNDS FeGe, FeSn AND CoSn

L. Häggström, T. Ericsson, R. Wäppling, E. Karlsson, K. Chandra

To cite this version:

L. Häggström, T. Ericsson, R. Wäppling, E. Karlsson, K. Chandra. MÖSSBAUER STUDIES OF

THE ISOSTRUCTURAL COMPOUNDS FeGe, FeSn AND CoSn. Journal de Physique Colloques,

1974, 35 (C6), pp.C6-603-C6-607. �10.1051/jphyscol:19746129�. �jpa-00215742�

JOURNAL DE PHYSIQUE

Colloque C6, supple'ment au no 12, Tome 35, De'cembre 1974, page C6-603

MOSSBAUER STUDIES

OF THE ISOSTRUCTURAL COMPOUNDS FeGe, FeSn AND CoSn

L. HAGGSTROM, T. ERICSSON, R. WAPPLING E. KARLSSON and K. CHANDRA

Institute of Physics, University of Uppsala, Upspala, Sweden

Resume.

Des etudes de spectroscopie Mossbauer du compose antiferromagnetique hexagonal FeGe ont

entreprises dans un domaine de temperature allant de

A partir des valeurs de

au voisinage de la temperature d'ordre on a obtenu l'exposant critique P

et la temp&

rature de Nee1

L'analyse des spectres Mossbauer de 57Fe et de 119Sn dans FeSn suggere une structure de spin antiferromagnetique plus complexe. L'ordre ferromagnetique dans les plans de base semble &tre partiellement detruit et la maille Blementaire magnetique est alors quatre fois plus grande qtie la maille chimique. La variation avec la temperature du champ hyperfin reduit pour un site de l'etain s'ecarte notablement de la quantite correspondante trouvee pour le fer.

On trouve

et on obtient un exposant critique de

Aucun ordre magnetique n'est apparu dans CoSn au-dessus de

Abstract. - Mossbauer spectroscopic studies of the hexagonal antiferromagnet FeGe have been undertaken in the temperature range

From the values of

close to the ordering tempe- rature the critical exponent p

and the Nkel temperature T N

were obtained. From the analysis of 57Fe and ll9Sn Mossbauer spectra in FeSn, a more complex antiferromagnetic spin structure is suggested. The ferromagnetic ordering in the basal planes seems to be partly destroyed and the magnetic unit cell is then four times as large as the chemical unit cell. The tempe- rature variation of the reduced hyperfine field for one tin position strongly deviates from the corresponding quantity found for iron.

is found to be

and the critical exponent obtained is

In CoSn no magnetic ordering has been revealed above

1. Introduction. -Hexagonal FeGe, FeSn and CoSn all have the same crystal structure, B 35. In the unit cell there is a threefold position, occupied by iron and cobalt, while germanium and tin occupy one singlefold and one twofold position (Fig. 1). From susceptibility measurements it was found that hexagonal FeGe was an antiferromagnet with a NCel temperature of 400 K [I]. Later it was deduced from a neutron diffrac- tion study [2] that the spins of the iron atoms were directed parallel to the hexagonal axis. The magnetic cell was thus twice as big as the chemical unit cell, being doubled along the c-axis.

In a neutron diffraction study of FeSn by Yama- guchi et al. [3], they revealed an antiferromagnetic ordering in the c-direction with the iron moments in the basal plane, either in the [loo] or in the [210] direc- tion. The magnetic unit cell would then even in this case be twice as big as the chemical one, being doubled along the c-axis.

The first Mossbauer measurements on FeGe (B 35) were reported by Nikolaev et al. [4] and Tomiyoshi et al. [5]. The first group found the N6el temperature to be 411(2) K, while the second group found the Ntel temperature to be 400(4) K and the hyperfine field at 79 K to be 15.5(5) T.

In FeSn, Yamamoto [6] determined the NCel tempe- rature from Mossbauer effect measurements to be about 370 K. Trumpy et al. [7] interpreted the 57Fe Mossbauer spectrum, recorded at 77 K, as a super- position of two six line patterns with an intensity ratio of 1

2. However, in another Mossbauer study, Ligenza [8] interpreted the iron spectrum in terms of two sets of lines, with an intensity ratio of 1

1, ori- ginating from two magnetically different positions.

2. Sample preparations and Mossbauer measu- rements.

Hexagonal FeGe was prepared by Dr. M. Richardson using the halogen transport method [9]. Dr. Richardson also made the FeSn sample by isothermal precipitation of solid FeSn from a tin- rich melt at 670 0C. After rapid cooling to room temperature, the product was freed from the tin matrix by reaction with chlorine. Phase analysis and characte- risation were carried out by X-ray powder diffraction, using a Gunier-Hagg type focussing camera. The CoSn sample used for the '19Sn measurements was made by Fulmer Research Institute, England from 99.999 %

electrolytic cobalt and 99.999 % tin. A 57CoSn source was made at our institute from CoCI,. 6 H,O of small specific activity and high purity cobalt and tin. X-ray

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19746129

FeSn hex

835

FIG. 1. - Crystal structure of FeSn with the magnetic ordering as given by neutron diffraction. In the figure the spins are drawn

parallel to [210].

FeGe 835 TN=398(1) K

025

,

, ' \

0 I

0 TIT,

0 25 0 50 075 100

FIG. 2. -The magnetic hyperfine field at the iron nuclei in hexagonal FeGe as a function of temperature (solid line).

Brillouin curves for S ⁼ 1 and S ⁼ 3 are also included.

FeGe 835

diffraction studies of the two samples showed only small traces of CoSn, besides the lines from CoSn.

After thorough grinding, the samples were mixed with boron nitride powder and pressed to circular plates with an average thickness of - ⁵ mg/cm2 of iron.

With the absorbers in a furnace or in a flow cryostat, transmission Mossbauer spectra were recorded utiliz- ing a constant acceleration spectrometer and room temperature sources (57FePd and BaiigSnO,). The temperatures of the absozers were controlled to within f 0.3 degrees.

3. Results.

FeGe.

The Ntel temperature of FeGe (B 35), found in the present investigation, is 398(1) K, in conformity with the results in reference [I].

The temperature variation of the magnetic hyperfine field, Bhf at the iron atoms is shown in figure 2, together with the Brillouin functions for

+ ^{and S}

^{1. In}

the vicinity of the transition point it is possible to deduce a value for the critical exponent P [lo], defined through

TN

for 0.8 TN < T < TN

-L -3 -2 -1 0 1 +2 -3 +L mmls

provided that Bh, is proportional to the sublattices

magnetization. The value P

0.33(2), although of

MOSSBAUER STUDIES O F THE ISOSTRUCTURAL COMPOUNDS FeGe, FeSn AND CoSn C6-605

limited accuracy, is in agreement with what one would

expect for a 3-dimensional antiferromagnet. Using the full hyperfine Hamiltonian in the analysis of the Moss- bauer spectra it is found that the electric quadrupole interaction is temperature independent. The tempe- rature variation of the centroid shift is compatible with a second order Doppler shift indicating a cons- tancy also for the electron density at the nucleus. The angle 0 between

and the largest component V,, of the EFG was found t o be 890(3), in agreement with a point charge lattice sum calculation which gave 0

900. Representative spectra are shown in figure 3 and the experimental results are presented in table I.

Results from the fittings of the j7Fe Mossbauer spectra. 6 is the centroid shift in mm/s versus natural iron at room temperature. A

e2 qQ/2 in mm/s for T < TN, while A

+.e2 qQ(l + y2/3)'I2 for T > TN.q is the asymmetry parameter.

is the magnetic hf Jield in Tesla. For FeSn the

values are the lowest and highest value obtained for the six patterns. The temperature is in degrees Kelvin.

Temperature

-

19 20 120 120 295 295 295 395 400

Sample - FeGe FeSn FeGe FeSn FeGe FeSn 57CoSn FeSn FeGe

b)

FeSn. - The l19Sn Mossbauer spectra of FeSn recorded above the Ntel temperature showed two quadrupole split doublets, with an intensity ratio of 1

2, which could be correlated with the two tin posi- tions. At lower temperatures a magnetic pattern, attributed to Sn(l), was observed. The pattern originat- ing from Sn(2) was unchanged except for a gradual increase in centroid shift (Table 11). Concerning the 57Fe Mijssbauer spectra a well resolved electric qua- drupole split doublet was noticed above the Ntel temperature. Below the transition temperature compli- cated magnetic spectra were observed (Fig. 4). In the low temperature spectra, it was not possible to get reasonable fits with less than six sets of lines. Toreduce the number of variables in the fitting, the electric quadrupole coupling, the asymmetry parameter, the centroid shift and the intensities were restricted to be the same for all six sets of lines. As the six measured fields had the same temperature dependence, it was possible to deduce a value for the critical exponent

p

0.34(2) and the NCel temperature TN

368(1) K . c) CoSn.

The li9Sn Mossbauer spectra showed two quadrupole split doublets with the intensity ratio

1

2. The electric quadrupole splittings stay essentially constant over the temperature range investigated. The values for the isomer shifts in table 2 fall in the range (1.3-3.0 mm/s) usually reported for metals and alloys [Ill. The 57Co source spectrum, recorded at room temperature, was a symmetric double with a quadrupole splitting close to the value for 57Fe in FeSn.

F I ~ . 4.

-

4. Discussion. - The centroid shift values recorded

at room temperature for FeGe, FeSn and 57CoSn are

0.28, 0.41 and 0.36 mm/s. If we correlate the diffe-

Results from the Jittings

the l19Sn Mossbauer spectra. 6, A, Bhf and temperature are dejined as

Table

with the exception that the centroidshift is dejined versus a Ba119Sn03 source at room temperature

Temperature

-

8 10 80 80 295 295 395 395

Sample - CoSn FeSn FeSn CoSn FeSn CoSn FeSn CoSn

rences in isomer shift to the differences in unit cell 7

volume, being 87.9, 108.2 and 102.8 A3 for FeGe, FeSn

and CoSn we will get

I ^{FeSn 835}

^{m ~ n}

provided that C is independent of V in the actual interval. In this way we obtain

- 0.7 in comparison with 1.38(22) found from the experimental dependence of hydrostatic pressure on the isomer shift for iron metal [12].

The electric quadrupole interaction is nearly the same for iron in FeGe, FeSn and 57CoSn which might be expected in view of the similarity both in valence electron configurations and atomic environment.

Concerning the 19Sn measurements a close agreement is also found for the quadrupole interactions as well as for isomer shifts in FeSn and CoSn (Table 11).

From the neutron diffraction study [2], the magnetic moment M a t 77 K and at room temperature are known to be 1.67(5)

and 1.28(4)

for FeGe. The ratios

FIG. 5. - The mean value of the reduced magnetic hf field for

1 I

iron and the corresponding parameter for Sn (1) as functions of

temperature. The Brillouin curve for S = 1 is also included. FIG. 6 . - "9Sn Mossbauer spectra of FeSn.

MOSSBAUER STUDIES OF THE ISOSTRUCTURAL COMPOUNDS FeGe, FeSn AND CoSn C6-607

B,,/M are then 9.3(3) T/pB and 9.2(3) T/pB at the two temperatures. Assuming a temperature independent ratio [13], we deduced the magnetic moment at 0 K to be 1.69(5) p,. Both the ratio and the saturated average magnetic hf field in FeSn agree closely to the corres- ponding values found in FeGe.

Assuming that the antiferromagnetic ordering between the basal planes in FeSn is intact, we have to extend the magnetic unit cell also in the a-direction in order to explain the observed six sets of iron patterns.

Thus, the pure ferromagnetic ordering in the basal plane, as proposed from the neutron diffraction data, seems to be partly destroyed.

As can be seen in figure 5, the temperature depen- dence of the reduced hf field at Sn(1) strongly deviates from the corresponding mean field for iron. The transferred hf field at Sn(1) could be caused by the conduction electron polarization or from the overlap of 3d wavefunctions with the s wavefunctions of the nonmagnetic atom. The later description has been used for tin as an impurity in a cobalt matrix [14]. If the 5s-3d overlap mechanism is of importance in FeSn, the cause for having a vanishing field at Sn(2) in contrast to Sn(1) (Fig. 6) would

due to iron nearest neighbours giving a cancelled 5s electron spin polariza- tion at the Sn(2) atoms.

References

[I] KANEMATSU, K. and OHOYAMA, T., J. Phys. Soc. Japan 20 (1965) 236.

[2] WATANABE, H. and KUNITOMI, N., J. Phys. Soc. Japan 21 (1966) 1932.

[3] YAMAGUCHI, K. and WATANABE, H., J, Phys. Soc. Japan 22 (1967) 1210.

[4] NIKOLAEV, V. I., YAKIMOV, S. S., DUBOTSEV, I. A. and GAVRILOVA, Z. G., JETP Letters 2 (1965) 235.

[5] TOMIYOSHI, S., YAMAMOTO, H. and WATANABE, H., J. Phys.

Soc. Japan 21 (1966) 709.

[6] YAMAMOTO, H., J. Phys. Soc. Japan 21 (1966) 1058.

[7] TRUMPY, G., BOTH, E., DJ~GA-MARIADASSOU, C., LECOCQ, P., Phys. Rev. B 2 (1970) 3477.

[8] LIGENZA, S., Phys. Stat. Sol. (b) 44 (1971) 775

[9] RICHARDSON, M., Acta Chem. Scand. 21 (1967) 2305.

[lo] KADANOFF, L. P., GOTZE, W., HAMBLEN, D., HECHT, R., LEWIS, E. A. S., PALCIAUSKAS, V. V., RAYL, M., SWIFT, J., ASPNES, D. and KANE, J., Rev. Mod. Phys.

39 (1967) 395.

[ l l ] GREENWOOD, N. N. and GIBB, T. C., MiiSSbauer Spectro- scopy (Chapman and Hall, London) 1971 p. 421.

[12] MYOZIS, J. A. and DRICKAMER, H. G., Phys. Rev. 171 (1968) 389.

[13] FREEMAN, A. J. and WATSON, R. E., in Magnetism 11 B, G . T. Rado and H. Suhl, Eds. (Academic Press Inc., New York) 1966.

[14] CRANSHAW, T. E., J. Appl. Phys. 40 (1969) 1481.