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MAGNETIC PROPERTIES OF Fe5SiB2 AND
RELATED COMPOUNDS
R. Wäppling, T. Ericsson, L. Häggström, Y. Andersson
To cite this version:
JOURNAL DE PHYSIQUE ColIoque C6, supplirnent au no 12, Tome 37, Dkcembre 1976, page C6-591
MAGNETIC PROPERTIES OF Fe,SiB2 AND RELATED COMPOUNDS
R. WAPPLING, T. ERICSSON, L. HAGGSTROM and Y. ANDERSSON (*)
Institute of Physics, Uppsala University, Box 530, S-751 21 Uppsala, Sweden
R6sum6.
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Les rksultats Mossbauer sur les propriktks magndtiques de FesSiBz sont prksentks et compards avec les mesures de Fe5PBz, MnsPBz et MnsSiBz. L'introduction de bore dans les sites de silicium produit une augmentation de champ magnetique hyperfin de noyaux de fer voisins.Abstract.
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Mossbauer data on the magnetic properties of FesSiBz are presented and compared with measurements on Fe5PB2, MnsPBz and MnsSiBz. The introduction of boron on the silicon position leads to an increase in the magnetic hyperfine field at neighbouring iron nuclei.1. Introduction.
-
In an realier study, the magnetic and crystallographic properties of Fe,PB2 were investigated [I]. Fe5PB2 was shown to be a simple ferromagnet with the tetragonal axis as the direction of easy magnetization. Furthermore, the Mossbauer data indicated that in non-stoichiometric Fe5PB2 part of the phosphorus positions are occupied by boron atoms and this has a rather large effect on the atomic moment on neighbouring iron atoms. A similar increase in atomic moment, and consequently magnetic hyperfine field at the nucleus, was found for iron in Fe,P, -,B, [2] and in order to see if the same situation prevails for Si/B substitution it was decided to investigate non- stoichiometric Fe5SiB2, which is isostructural with Fe5PB2 [3]. The results are compared with recently available magnetic data for the corresponding manga- nese compounds.2. Sample preparation.
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The following starting materials were used :Iron, spectroscopically standardized iron rod, Johnson, Matthey & Co. Ltd., London.
Boron chips with claimed purity 99.8
%,
Koch Light.Silicon, semiconductor grade, General Electric Co, England.
Fe2B and FeB master alloys were prepared by melting appropriate mixtures of iron and boron in an argon-filled arc furnace. An FeSi master alloy (ana- lyzed composition Fe, .,,Si, ) .,, prepared in a similar manner was kindly put to our disposal by Dr. M. Richardson, Institute of Chemistry, Uppsala. The Fe5SiB2 sample used for the Mossbauer measu- rements was prepared from the master alloys in the following manner. An initial alloy was synthetized by arc melting a mixture of the Fe,B and FeSi alloys. It was homogenized by crushing and subsequent
(*) Institute of Chemistry, Uppsala University, Box 531,
S-751 21 Uppsala, Sweden.
annealing at 1 000 OC for a few days in an evacuated silica tube. The product was examined by X-ray powder diffraction. The composition of the sample was adjusted by adding small amounts of FeB, followed by another homogenizing treatment. This procedure was repeated until a final Fe5SiB2 alloy, containing only trace amounts of Fe,B and Fe2Sio,Bo., [3], was obtained. Powder diffraction patterns were recorded in a Hagg-Guinier-type camera using CrKa, radiation and silicon SRM 640 [4] (a = 5.430 880 A) as the calibration standard. Lattice parameters were refined by the least-squares method using the local program CELNE [5] on an IBM 1800 computer. The unit cell dimensions obtained at 24 OC for Fe,SiB, in the final sample were :
a = 5.549 8(2)
A
and c = 10.332 4(5)(number in parentheses following numerical values are the calculated standard deviations referring to the least significant digits).
3. Results.
-
As is clearly seen in figure 1 the room temperature Mossbauer spectra of Fe5SiB2 and Fe5PB2 are very similar. This means that Si/B substitution produces the same effect on the magnetic properties as P/B substitution. The spectra of Fe5SiB, can, therefore, be analysed in an analogous way as those of Fe5PB2 [l]. In particular, the assignment of the components in the Mossbauer spectra to the crystallographic positions will be according to figure 1 where Fe(1) and Fe(2) denotes the 16 1 and 4 c crys- tallographic positions respectively. Fe(1)a has an ideal near surrounding while for Fe(1)b one of the neighbouring silicon atoms is replaced by a boron atom.From the temperature variation of the magnetic hyperfine fields a Curie temperature of 784(1) K was obtained. This value, although accurate, is not to be taken as the Curie temperature of stoichiometric
- 4 - 3 -2 -1 0 e l +2 +3 + A m m l s
FIG. 1. -Room temperature Mossbauer spectra of FesSiBz (top) and FesPBz (bottom).
Fe5SiB2 since, in analogy to Fe5PB2 [6], the ordering temperature depends on the exact composition of the sample.
4. Discussion.
-
The close similarity between Fe5SiB2 and Fe5PB2 makes a direct comparison of the Mdssbauer results of the two reasonable. Furthermore, the results of a recent spin echo NMR study of the corresponding manganese compounds [7] can be included in the discussion. As is seen in figure 1 and in table I the magnetic hyperfine field B,, on the Fe(2) position is higher than that on the Fe(1) position in both iron compounds. The ratio between the Bhf values of Fe(2) and of Fe(1)a is-
1.35 for both Fe5SiB2 and Fe5PB2. One can also directly see the effect of replacing a near silicon or phosphorus atom by a boron atom since this can be expressed as the ratio between the B,, values for Fe(1)b and Fe(1)a.This ratio is 1 .I7 for Fe5SiB2 and 1.22 for Fe5PB2. The substitution has no visible effect on Fe(2) proba- bly due to the larger interatomic distances involved. The Curie temperatures of Fe5SiB2 and Fe,PB2 can be compared directly since the samples used have about the same degree of nonstoichiometry as judged from the intensity of the Fe(1)b component. Thus, the Curie temperature of Fe5SiB2 is about 25
%
higher than the value found for Fe,PB2 (628(11) K) indicating a corresponding difference in magnetic interaction strength.If we now turn to the manganese compounds wee se that the Curie temperature of Mn5SiB2 (411 K) is
-
30%
higher than the value for Mn5PB2 (312 K) [7] in conformity with the trend above. In his interpre- tation of the 55Mn NMR spectra, Kasaya [7] assumes that the signals observed are due to manganese atoms on the 16 1 positions only. The structures observed inthe spectra are interpreted as due to resonances in the interior and at the edge of the domain walls as suggested by Butler [8]. The same way of analysis has been applied to the NMR spectrum of Fe,P [8] where our own measurements show that it is inapplicable since the same structure is found also in the Mossbauer spectrum of Fe,P [9]. It is, therefore, interesting to see whether we can arrive at an alternative interpretation of the manganese NMR spectra of Mn5PB2 and Mn5SiB2. Parts of the NMR spectra are shown in figure 2 and display one unsplit and one quadrupole split resonance line in both cases.
If we in e. g. Mn5PB2 have the same situation as in Fe5PB2 we would expect to find, in the stoichiometric case, one intense line with a very small quadrupole interaction and, at about 35
%
higher frequency, a line with an appreciable quadrupole splitting. The experimental spectrum does indeed display this structure and we suggest the quadrupole split line at 185 MHz to be interpreted as originating from manga- nese nuclei on the 4 c crystallographic position. Similarily, the line at 184 MHz in Mn5SiB2 can be interpreted as arising from manganese nuclei on the 4 c position. The quintuplet spacing is smaller in Mn5SiB2 (1.6 MHz) than in Mn5PB2 (2.8 MHz) in conformityResults of the Mossbauer measurements of Fe5SiB2 and Fe,PB2 at 295 K. Bhf is the magnetic
hyperfine field in T, 6 the isomer shift in mm/s relative to iron metal at room temperature, A the electric quadrupole splitting in mm/s
1 3 cos2 0
-
1+
q sin2 0 cos 2 cp(A = 2 e ~ ~ , ,
.
2,
see reference [lo]1
r
the FWHM in mm/s and I the relative intensity in percent.Fe(1) a Fe(1) b
Compound Bhf 6 A I Bni 6 A I Bhf 6 A l l
--
- --
-
--
--
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MAGNETIC PROPERTIES OF FesSiBz AND RELATED COMPOUNDS C6-593 Mn, PB, 4 2 K
-
176 188 200 160 170 180 190 200 MHz MHzFIG. 2. - NMR spectra of MnsPBz at 4 K (top) and MnsSiBz
at 77 K (bottom). After Kasaya [7].
with the situation in the iron compounds where the quadrupole interaction is twice as large in Fe5PB2 as in Fe5SiB2.
If the alternative interpretation of the NMR spectra presented above is used as a basis for further discus- sions, we note that Mn5SiB2 differs from the other compounds in one respect. The magnetic hyperfine field, and hence the magnetic moment, is smaller on the 4 c position than on the 16 1 position for this compound. This might reflect a difference in spin direction explaining the values for the magnetization found experimentally.
The absolute values of the magnetic hyperfine fields at the transition metal nuclei are all in the range 16-22 T and are characteristic for itinerant electron magnetism. Short range interactions do, however, also play an important r81e as is seen from the effects of the substitutions. The axis of easy magnetization in Fe5PB2 was found to be parallel to the c-axis and the same conclusion can be drawn from the temperature variation of the quadrupole splitting in Fe5SiB,. Magnetization along the tetragonal axis in the manga- nese compounds is suggested by Kasaya from consi- deration of line shapes 171.
Since the magnetic hyperfine field is closely propor- tional to the magnetic moment of the atom under study it is possible through the measured bulk mangetic moment, to obtain information on the actual spin ordering. In the alternative interpretation of Kasayas results presented above the average magnetic hyper- fine field at the manganese nuclei is 18.2 T and 16.5 T for Mn5SiB2 and Mn5PB2 respectively. The corres- ponding bulk magnetic moments are 1.6 pB and 1.1 pB per manganese atom [7]. The ratio between mangetic hyperfine field and magnetic moment is then for Mn5SiB2 11.4 T/pB in good agreement with what is observed for ferromagnetic intermetallic manganese compounds. The ratio for Mn5PB, 15.0 T/pB is larger indicating that some ferrimagnetic ordering might be present.
References
[I] HAGGSTROM, L., WXFTLING, R., ERICSSON, T., ANDERS-
SON, Y. and RUNDQVIST, S., J. Solid State Chem. 13
(1975) 84.
[2] WXPPLING, R., HXGGSTROM, L., KARLSSON, E. and RUNDQ-
VIST, S., J. Solid State Chem. 3 (1971) 276.
[3] ARONSSON, B., ENGSTROM, I., Acta Chem. Scand. 14 (1960) 1403.
[4] HUBBARD, C. R., SWANSON, H. E., MAUER, F. A., J.
Appl. Cryst. 8 (1975) 45.
[5] ERSSON, N.-O., Institute of Chemistry, Uppsala 1973,
unpublished.
[6] BLANC, A.-M., FRUCHART, E. and FRUCHART, R., Ann.
Chim. (Paris) 2 (1967) 251.
[7] KASAYA, M., Science Report Tohoku University LVIII
NO 1-2 (1975).
[8] BUTLER, M. A., Int. J. Magnetism 4 (1973) 131.
[9] LISHER, E., WILKINSON, C., ERICSSON, T., HAGGSTROM, L.,
LUNDGREN, L. and W~PPLING, R., J. Phys. C 7 (1974)
1344.
[lo] HAGGSTR~M, L., Uppsala University, Institute of Physics,
