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Submitted on 1 Jan 1974
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SPIN ORIENTATION IN FeB
H. Bunzel, E. Kreber, U. Gonser
To cite this version:
H. Bunzel, E. Kreber, U. Gonser. SPIN ORIENTATION IN FeB. Journal de Physique Colloques, 1974, 35 (C6), pp.C6-609-C6-610. �10.1051/jphyscol:19746131�. �jpa-00215745�
JOURNAL DE PHYSIQUE ColZoque C6, supplkment au no 12, Tome 35, Ddcembre 1974, page
SPIN ORIENTATION IN FeB
H. BUNZEL, E. KREBER and U. GONSER
Fachbereich Angewandte Physik, Werkstoffphysik und Werkstofftechnologie Universitat des Saarlandes, Saarbriicken, Germany
R6sum6. - La dkpendance angulaire de l'interaction hyperfine a permis de dkterminer l'orienta- tion de spin dans FeB. On a trouve que les spins forment un angle de 200 avec l'axe 6.
La structure magnktique doit &re dkcrite comme un ferromagnktique non linkaire.
Abstract. - From the angular dependence of the hyperfine interaction the spin orientation of FeB was determined. It was found that the spins form an angle of about 20° to the 6 axis.
The magnetic structure has to be described as canted ferromagnetism.
Introduction. - Iron boride, FeB, is considered ferromagnetic at room temperature with a Curie temperature of 594 K. The axes of the orthorhombic unit cell containing 4 formulae are a = 4.053 A,
b = 5.495 A, and c = 2.946 A [I, 21. The crystal structure can be described as an arrangement of prisms with boron situated in the center while the edges are
occupied by iron atoms (Fig. 1). This causes in c-direc-
T
a iron boron
FIG. 1. -Arrangement of prisms forming the FeB crystal structure.
tion a helical chain of borons while the iron atoms are colinearly arranged to the c-axis. Figure 2 represents the projection of the FeB lattice onto the a-b plane and shows that neither boron nor iron atoms are arranged in straight lines parallel to the a or the b axis.
jron of c = 1/4 o boron
@ jron at c = 3/4
o boron
FIG. 2. -Projection of EeB crystal structure onto a-b plane.
1. Experiments. - A single crystal of this inter- metallic compound has been inbedded in araldite and ground t o a thickness of less than 0.05 mm (less than 0.7 mg FeS7/cm2). The crystal sample was mounted into a Buergers precession camera where the crystal axes have been determined with an accuracy better than 1 degree. Room temperature Mossbauer measure- ments have been performed on this absorber varying systematically its orientation with respect t o the propagation direction of the y-rays, y.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19746131
C6~610 H. BUNZEL, E. KREBER AND U. GONSER We used a single line source ( C O ~ ~ in Cu) moving in 3
constant acceleration mode and a multichannel ana- lyzer recording the spectra in 200 channels.
Results and discussion. - In our Mossbauer experi- ments we found an internal field
a quadrupole splitting of 0.13 (+ 0.005) mm/s and an isomer shift of 0.3 (+ 0.7) mm/s in good agreement to the values of reference [3].
Changing the orientation of the FeB single crystal sample systematically in steps of 5 degrees the lines 2 and 5 (Am 2 0) never vanished completely, instead we obtained in experiments with y parallel to the crystallo- graphic b-axis (y )I b) a minimum 'in the intensity ratio of the lines 2 and 5 (Am = 0) to the lines 1 and 6 (Am = + 1) with values in the range of 0.08 to 0.10 (Fig. 3). Experiments with y parallel to crystallo-
FIG. 3. -, Mossbauer spectrum $of a FeB single crystal (Y 11 b) obtained with a C057 CU source both at room temperature.
Solid line indicate computer analysis.
graphic c-axis (y 11 c) showed, considering the angular dependence ofjhe hyperline interaction, a spin orien- tation close to the a-b plane was found. The fact that we were not able to obtain from a ferromagnetic single crystal a spectrum with zero intensity of the lines 2 and 5 has to be explained by the presence,of different spin orientations in the a-b plane. Synthesizing theoretically MSssbauer spectra (see ref. [4]) $aking into account the experimentally found value; of the hyperfine interac- tions we obtained results shown in figure 4. Curve (a) illustrates the dependence of the intensity ratio as a function of the angle formed by y and Hi,, assuming ,a unique spin 'orientation. Our experimental results 'indicate that our spectrum (y 11 b) wikh intensity ratio of about 0.08 would correspond to an angle of 20 degrees
FIG. 4. - Calculated angular dependence of intensity ratio of line 2 to line 1 (see text).
4
0
z z 9
eci- w
I- l-4
m-0 20
Z d - -
l-4 Z
9 2-
between y and Hint. Assuming two different spin orien- tations in the a-b plane forming an angle of 40 degrees and rotating around in the a-b plane the angle between the bisector and y we calculated the intensity ratio as shown in figure 46. One can see that in this case the intensity ratio is for all orientations greater than 0.08.
The experimentally determined ratios follow very closely the (6) curve and thus indicates the validity of our assumption that the spins form an angle of 20 degrees to the b-axis. Inspecting figure 2 one observes that the Fe-Fe bond forms an angle of 20 degrees to the b-axis.
In conclusion our experimental results indicate that the spins in FeB are close to the a-b plane and are not colinear but rather deviate from the b-axis by an angle of 20 degrees. Thus, one has to describe the magnetic structure as canted ferromagnetism. Although the spins are determined to form an angle 'of 20 degrees to the b-direction as the Fe-pairs in the chains (see Fig. 2), the precise spin structure that is the sequence of the spins cannot be determined by means of our experi- ments.
dl- d . M 9.00 8.00 L2.00 16.00 20.00
Acknowledgment. - We gratefully acknowledge Dr. R. S. Perkins, BBC Ltd. Baden, Switzerland, for discussions and supplying us kindly the single crystal specimen. We wish to express our thanks to Dip1.- Chem. F. Henschel for performance and evaluation of X-ray measurements.
DEGREE
References
[I] NIGGLI, P., EWALD, P. P., FAJANS, K. and V. LAUE, M., [3] COOPER, J. D., GIBB, T. C., GREENWOOD, N. N. and PARISH, Strukturberichte 3 (1937) 12. R. V . , Trans. Favaday Soc. 60 (1964) 2097.
[2] BJURSTROM, T. and ARNFELT, H., Z. Kristallogr. 74 (1930) [4] KUNDIG, W., Nucl. Instrum. and Methods 48 (1967) 219 and
517. KUNDIG, W., private communication.
