A MÖSSBAUER DETERMINATION OF A POSITIVE Heff AT Sn119 NUCLEI IN Cu2MnSn HEUSLER ALLOY

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A MÖSSBAUER DETERMINATION OF A POSITIVE Heff AT Sn119 NUCLEI IN Cu2MnSn HEUSLER

ALLOY

J. Williams

To cite this version:

J. Williams. A MÖSSBAUER DETERMINATION OF A POSITIVE Heff AT Sn119 NUCLEI IN Cu2MnSn HEUSLER ALLOY. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-790-C1-791.

�10.1051/jphyscol:19711277�. �jpa-00214109�

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JOURNAL DE PHYSIQUE Colloque C 1, supplbment au no 2-3, Tome 32, Fkvrier-Mars 1971, page C 1 - 790

A MOSSBAUER DETERMINATION OF A POSITIVE H., AT Sn119 NUCLEI IN Cu2MnSn HEUSLER ALLOY

J. M. WILLIAMS

Physics Department, University of Sheffield, England

RBsum6. - Utilisant des rayons y d'effet Mossbauer, polarises circulairement, on montre que le signe du champ nuclkaire hyperfin au niveau de Sn11Q est positifdans l'alliage de Heusler Cu2MnSn ordonn.5 ferromagnktique. Tous les champs mesurks prkckdemment au niveau de diffkrents noyaux dans divers alliages d'Heusler sont nkgatifs (a l'exception de Sn dans CooMnSn) et leur valeur en grandeur et signe (y compris la valeur positive correspondant a Sn dans CozMnSn) sont en bon accord avec les valeurs thkoriques obtenues en supposant l'existence d'une polarisation oscillante des spins des klectrons de conduction autour des atomes de Mn.

La valeur positive du champ de 260 kOe mesurke dans le cas prhent est pourtant en dksaccord flagrant avec la valeur d'environ - 300 kOe obtenue en utilisant le modhle mentionne plus haut. Les raisons possibles de ce d6saccord sont discutkes.

Abstract. - Using circularly polarized Mossbauer y-rays, the sign of the nuclear hyperfine field at SnllQ in the order ed ferromagnetic Heusler alloy Cu2MnSn is shown to be positive. All previously measured fields at various nuclei in different Heusler alloys have (with the exception of that at Sn in CozMnSn) been negative, and have all (including the positive value in CozMnSn) been in fairly good agreement regarding magnitude and sign with the teoretical predictions obtained by assuming an oscillating conduction electron spin polarization around the Mn atoms.

The presently measured positive field (I- 260 kOe) is, however, in direct disagreement with the value of approximately - 300 kOe obtained using the above mentioned theory. Possible reasons are discussed.

1. Introduction. - Heusler alloys have composi- tions near the formula Cu2MnX (X = Al, In, Sn), and are ferromagnetic in the ordered P-phase (Heus- ler [I]). The magnetic moment per molecule is always about 4 ^p, and is almost entirely carried by the Mn ions [2] [3]. The ferromagnetic coupling mechanism between the Mn moments is not entirely understood, but a model utilising the concept of resonant bound states for the 3 d electrons of Mn atoms [4] accounts for many features of these alloys including most of the hyperfine fields a t the contituent nuclei.

Previous measurements of hyperfine fields at cons- tituent nuclei in various Heusler alloys, including the all-important sign determinations, have been tabulated by Williams [5] together with the theoretical estimates obtained using the theory of Caroli and Blandin [4]

with which they are in remarkably good agreement.

The present work on CuzMnSn thus represents the first dramatic departure from the prediction obtained using this theory.

2. Experimental Method. - The magnitude and sign of the hyperfine field at Snl" nuclei in CuzMnSn were measured using circularly polarized Mossbauer y-rays in a similar way to that described in [5].

3. Results. - The Massbauer absorption spectra measured a) with no applied magnetic field, and b) with source and absorber in a large longitudinal applied field, are shown in figure 1 and 2 respec- tively. As seen in figure 1 in addition to the expected 6 line magnetically split spectrum (in this case, the lines are not fully resolved) there is a central single line. This line is attributed t o free Sn in a non-magnetic environment. The solid curve thus shows a computer fit representing the superposition of a single line and a 6-line magnetic spectrum whose relative line positions are fixed by the known Snl" nuclear g-factors for the ground and excited states.

ReIatiw counts.

H - 0

FIG. 1. - Mossbauer absorption spectrum obtained with a Snllg(Pd) source and CuzMnSn absorber in zero field at

4.2 ^OK.

In the presence of a longitudinal applied field, in addition to the &line polarization spectrum for the magnetic phase, there will be a 3-line polarization spectrum due to the non-magnetic Sn. Figure 2 shows the experimental polarization spectrum together with an 11-line (8

+

3) computer fit. The polarization spectrum depends critically on the direction of Ha relative to the applied field Hap,, and figure 3 shows the predicted spectra constructed by assuming a) He,, positive, i. e. parallel to Hap,, and b) He,, negative, i. e. opposite to Hap,. I t is

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711277

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A MOSSBAUER DETERMINATION OF A POSITIVE Hetf AT Sn119 NUCLEI IN C u z m S n HEUSLER ALLOY C 1 - ⁷⁹¹

Reloiive counts

C ^Relativcf counts

1 '

30 25 20 15 10 5 0 -5 -10 -15 -20 -25 -30' Rehtive Velocity

FIG. 2. - Mossbauer absorption spectrum obtained with both

source and absorber in a longitudinal applied field Happ = 56 kOe (30 25 20 15 ^, 10 ^I 5 ^, 0 - 5 -10 ^, -15 ^, -20 ^, -25 ^I - 3 0 * ^,

at 4.2 OK. Relative Velocity

therefore, obvious that He, is POSITIVE and has the value

+

260 kOe at 4.2 OK. Previously measured fields have all been in fairly good agreement with the predictions based on the theory [4] [5] in which they are attributed to oscillations in the conduction electron spin polarization induced by Mn moments.

Such a calculation of the field at Sn in Cu,MnSn yields a value of approximately - 300 kOe. Shin- hara [6] in a similar calculation based on the R. K.

K. Y. interaction obtains a value of - 157 kOe.

FIG. 3. - Predicted polarization spectra calculated by assu- ming (a) Hefp positive-solid line, and (b) Hefp negative-broken

line.

It is thus clear that such models are not accurate and the good agreement with experiment must in some cases be fortuitous. This is not unexpected as the calculations involve several uncertain parameters, in addition to assuming the asymptotic form of the spin density oscillations even at near neighbour dis- tances.

References

[I] HEUSLER (O.), Ann. Phys., 1934, 19, 155. [4] CAROLI (B.) and BLANDIN (A.), ^J.Phys. Chem. Solids, [2] FLETCHER (G. P.), CABLE (J. W.) and WILKINSON (M. 1966, 27, 503.

K.), J. Phys. Chem. Solidr, 1963, 24, 1663. [5] WILLIAMS (J. M.), J. Phys. C., 1969, 2, 2037.

[3] TAKATA (H.), J. Phys. SOC. Jap., 1965, 20, 1743. [6] SHINOHARA (T.), J. Phys. Soc. Jap., 1969, 27, 1127.