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THE USE OF CIRCULARLY POLARIZED
MÖSSBAUER γ-RAYS TO DETERMINE THE SIGNS OF CO-EXISTENT HYPERFINE FIELDS
J. Williams, J. Brooks
To cite this version:
J. Williams, J. Brooks. THE USE OF CIRCULARLY POLARIZED MÖSSBAUERγ-RAYS TO DE- TERMINE THE SIGNS OF CO-EXISTENT HYPERFINE FIELDS. Journal de Physique Colloques, 1974, 35 (C6), pp.C6-693-C6-695. �10.1051/jphyscol:19746155�. �jpa-00215771�
JOURNAL DE PHYSIQUE Colloque C6, supplkment au no 12, Tome 35, De'cembre 1974, page C6-693
THE USE OF CIRCULARLY POLARIZED MOSSBAUER r-RAYS TO DETERMINE THE SIGNS OF CO-EXISTENT HYPERFINE FIELDS
J. M. WILLIAMS and J. S. BROOKS (*) Department of Physics, University of Sheffield, England
R6sum6. - On montre comment des rayons y Mossbauer polarisks circulairement peuvent &re utilisks pour dkterminer sans ambiguitk le signe de deux champs hyperfins qui peuvent coexister dans un Cchantillon unique. Une telle situation est frkquemment rencontrk quand, pour des raisons inkvitables, un noyau sonde se trouve occuper deux sites diffkrents.
Ceci est le cas pour I'alliage de type Heusler CozTiSn. Le spectre Mossbauer d'absorption obtenu avec une source de "9Sn montre qu'en plus du spectre magnktique attribuB aux atomes d'ktain occupant les sites prkvus X de la structure de Heusler (AzBX) il y a une composante magnktique additionnelle due ii des atomes d'ktain occupant un autre site ou phase. On montre que les deux champs hyperfins sont positifs, leurs valeurs Btant 78,s et 12,2 kG respectivement.
Abstract. - It is demonstrated how circularly polarized Mossbauer y-rays may be used to obtain an unambiguous determination of the signs of two hyperfine fields which may exist together in a single sample. Such a situation is often encountered when, for unavoidable reasons, a probe nucleus is found to occupy two different sites.
This is the case for the Heusler type alloy Co2TiSn. The Mossbauer absorption spectrum using a 119Sn source shows that in addition to the magnetic spectrum attributed to Sn atoms occupying the expected X sites in the Heusler (AzBX) structure, there is an additional magnetic component due to Sn atoms occupying some other site or phase. It is shown that both hyperfine fields are positive having values of 78.8 and 12.2 kG respectively.
1. Introduction. - Heusler alloys are chemically ordered with the Strukturbericht type L2, structure [I]
having the typical composition A2MnX, where A is normally a transition metal such as Co, Pd, Ni, etc., and X is a metal such as Al, In, Sn, Sb, etc. Most Heuslers order with a magnetic moment near to 4 p, this being entirely carried by the Mn atoms [2]. Recent interest has been shown in Heusler alloys in which the Mn atoms have been replaced by Ti or V [3], and some of these alloys have also been shown to be magnetically ordered.
Occasionally it proves difficult, if not impossible, to prepare some of these alloys with the complete single HeusIer structure and a Mossbauer study of hyperfine fields is hampered by the existence of multiple spectra.
A typical example of this situation is found in the case of Co2TiSn, where it is found that the Mossbauer absorption spectrum obtained using a '"Sn source consists of the superposition of two magnetically split spectra with widely differing hyperfine field values.
Using circularly polarized Mossbauer y-rays it is shown that the signs of both hyperfine fields may be determin- ed unambiguously.
2. Sample preparation. - The sample of Co,TiSn investigated was that used by Fujita et al. [3], who
(*) Present address : Department of Applied Physics, Sheffield Polytechnic, Sheffield, England.
showed that the alloy was ferromagnetic with a Curie temperature of 373 K and a magnetic moment of 1.9 pB. They were prepared by melting together the appropriate quantities of spectrographically pure elements in an argon arc furnace, and then homo- genizing at 800 OC for 1 hour in a vacuum. An X-ray powder diffraction study using CuK, radiation was undertaken, and the measured diffraction lines were compared with the theoretical relative intensities computed assuming the L2, structure. The results are tabulated in table I, and the existence of type B and C superlattice lines show the sample to be well ordered with a lattice parameter of 6.062 A which is in good agreement with the values of 6.06A obtained by Fujita et al. [3], and 6.059 A by Kripyakevich and Markiv [4].
3. Experimental method and results. - The Moss- bauer absorption spectrum obtained using a '19Sn source (Pd, 'l9Sn) was measured at 4.2 K using a conventional constant acceleration spectrometer. The spectrum is shown in figure 1. The main absorption peaks were fitted satisfactorily with a 6-line magnetic spectrum yielding sensible area ratios and line widths.
Initially the (( central doublet )) was fitted with two lines only, but the assumed Lorentzian line shape did not fit the experimental data well. The central peaks were then fitted with another 6-line spectrum, having a much smaller value for the hyperfine field than the main
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19746155
C6-694 J. M. WILLIAMS AND J. S. BROOKS spectrum, and a good fit was obtained as shown by the
solid line in figure 1. Table I1 lists the measured para- meters as obtained from the final fit.
FIG. 1. - The ll9Sn Mossbauer absorption spectrum of CozTiSn measured at 4.2K in zero applied field.
R"I IT,\ ,-
The measured parameters obtained from the C02TiSn Mossbauer spectrum
V F l O t l T Y ( m m > - 1 )
Hyperfine field Relative Isomer shift (**)
Alloy kG (*I Absorption mm s-1
- - -
CozTiSn 78.8 0.85 - 0.20
at 4.2 K 12.2 0.15 4- 0.15
- 1 1
-10 - 8 6 - 4 -2 0 2 4 6 8 10 *
(*) Calculated from the magnetic moments given by Brooks et al. [5].
(* *) Isomer shifts measured relative to a 119Sn in Pd3Sn source at room temperature.
It is seen that, although the X-ray analysis showed no appreciable evidence for a second phase, approxima- tely 15 % of the Sn nuclei occupy sites which experienc a much smaller hyperfine field than exists at the ordered Sn sites occupying the superlattice positions in the L2, structure. This would be expected if there was a small amount of a different ordered phase present, or if some Sn nuclei occupied sites other than the X sites in the L2, Heusler structure.
Hyperfine field sign determination is complicated by the existence of two fields and for this reason, and because of the possible ambiguity associated with a direct sign determination [6], a polarization technique was used. Here the Mossbauer absorption spectrum was obtained with the source and absorber in large longitudinal magnetic fields (- 60 kg) produced by a superconducting solenoid. The field is assumed to be sufficiently strong to orient the magnetic domains in the absorber so that the field at the nucleus is either parallel or antiparallel to the applied field. Only eight absorption peaks are expected in the Mossbauer spectrum for an I = 5; to I = 4 transition in such a longitudinal field, but, since the z-component of angular momentum must be conserved in any reso- nance absorption, the positions and relative intensities of the peaks will differ greatly depending on whether the nuclear field in the source is parallel or antiparallel to that in the absorber. This technique as applied to '19Sn is discussed in greater detail by Brooks [7] and by Brooks and Williams [6]. Similar techniques have alveady been applied to the 57Fe isotope.
The polarization spectrum obtained using such
The results of X-ray powder photograph analysis of Co2SiSn using CuK, radiation
Intensities Type A Type B Type C
m (exp) ( 4 hkl = 2(h' k' 1') hkl odd hkl even
THE USE OF CIRCULARLY POLARIZED MOSSBAUER y-RAYS C6-695
circularly polarised y-rays is shown in figure 2. - visual interpretation showed that the main absorption was consistant with a large positive hyperfine field. In addition to this there was again a central doublet having a large positive isomer shift. This was assumed to be the polarisation spectrum resulting from the smaller hyperfine field. The final computer fit, shown as the solid line in figure 2 consists of two superimposed
FIG. 2. -The Mossbauer polarization spectrum of CozTiSn measured at 4.2 K with source and absorber in a large longitudinal
applied field.
RELATIVE f COL'li.rS
FIG. 4. -The predicted polarization spectra computed assu- ming (a) both fields positive (continuous line) and (6) large field
positive, small negative (dotted line).
eight-line polarization spectra produced when both hyperfine fields are positive. The relative absorption intensities of the spectra are as measured from the magnitude measurement data listed in table I.
To show conclusively that bo) hyperfine fields are positive, figures 3 and 4 show the contrasting predicted spectra expected when :
the large hyperfine field is negative figure 3 the small hyperfine field is positive (dotted line)
7-
-I\- 7 I
the large hyperfine field is positive the small hyperfine field is negative
References
,
\ 1 I 1 II I E M T I Y E
'
I4 , 9 , , , 8 8 \I
[I] WEBSTER, P. J., Contemp. Phys. 10 (1969) 559-77. [5] BROOKS, J. S., WILLIAMS, J. M. and WEBSTER, P. J., J. Phys.
[2] WEBSTER, P. J., J. Phys. Chem. Sol. 32 (1971) 1221-31. 0 . 6 (1973) 1403-8.
[3] FUJITA, Y., ENDO, K., TERADA, M. and IOMURA, R., J, Phys. [6] BROOKS, J. S. and WILLIAMS, J. M., J. Phys. F. Metal Physics
Chem. Solids 33 (1972) 1443-6. 4 (1974).
[4] KRIPYAKEVICH, P. L. and MARKIV, V. Ya., Dokl. acad. nauk. [7] BROOKS, J. S., Ph. D. thesis (1973) University of Sheffield,
Ukrain. SSR 12 (1963) 1606. England.
\
_i
4. Conclusion. - We conclude that both the hyper- fine fields present in Co,TiSn are Positive. The method shows clearly how circularly polarised y-rays maybe used to obtain an unambiguous determination of the signs of two hyperfine fields which may co-exist in aYE1 OCITY ( r n m I-]) single sample.
-16 1 4 -12 -10 -8 -6 -4 - 2 0 2 4 6 8 10 12 14 16 ,
FIG. 3. -The predicted polarization spectra computed assu- Acknowledgements. - We are extremely grateful to
ming (a) both fields are positive (continuous line), and (b) large Dr. Y. Fujita of the Institute Keio Univer-
field negative, small field positive (dotted line). sity, Japan, for the Co,TiSn samples investigated.
