MÖSSBAUER EFFECT STUDIES IN HEUSLER ALLOYS

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MÖSSBAUER EFFECT STUDIES IN HEUSLER ALLOYS

R. Segnan, W. Ferrando, D. Sweger, P. Webster

To cite this version:

R. Segnan, W. Ferrando, D. Sweger, P. Webster. MÖSSBAUER EFFECT STUDIES IN HEUSLER ALLOYS. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-792-C1-793.

�10.1051/jphyscol:19711278�. �jpa-00214114�

JOURNAL DE PHYSIQUE

Colloque C 1, supplkment au no 2-3, Tome 32, Fkvrier-Mars 1971, page C 1 - 792

MOSSBAUER EFFECT STUDIES IN HEUSLER ALLOYS (*) R. SEGNAN, W. A. FERRANDO (**), and D. SWEGER (**)

American University, Washington, D. C., 20016 and P. J. WEBSTER

University of Salford, Salford, England

R6sum6. - Le champ hyperfin au niveau du noyau Sn dans NizMnSn, CuzMnSn et ZnzMnSn a kt6 mesure par effet Mossbauer. Nous avons trouve que ce champ est de + 53 kOe dans NizMnSn de moins de 10 kOe.dans CuzMnSn et de - 30 kOe dans ZnzMnSn. Ces rksultats sont compares a ceux que I'on deduit de la thkorie de Caroh-Blandm.

Abstract. - The hyperfine field at tin nuclei in NizMnSn, CuzMnSn and ZnzMnSn has been measured by using the Mossbauer effect technique. We find that the field is

53 kOe in NinMnSn, less than 10 kOe in Cu2MnSn7 and - 30 kOe in ZnzMnSn. These results are compared to the theoretical values deduced from the Caroli-Blandin theory.

I. Introduction. - The Heusler alloys, with compo- sitions close to Cu2MnX (where X = Al, In, Sn, ...),

have been the subject of many investigations because, although the component elements are paramagnetic or diamagnetic, the alloys themselves are ferromagne- tic. Ferromagnetism is associated with the P-phase, which may be described as body-centered cubic with Cu at the cube corners and Mn and X at alternate body centers. It is usually assumed that the magnetic moment of about 4 ^p, (Bohr magneton) is concentrated on the manganese atoms [l-31. This assumption and the large interatomic distance between the Mn atoms (- 4.3 A) would indicate that the ferromagnetism of these alloys is due to an indirect exchange interac- tion via the conduction electrons. However, one can- not rule out a superexchange mechanism through non-magnetic atoms. According to a theoretical pic- ture proposed by B. Caroli and A. Blandin 141, the magnetization of these alloys arises from exchange spin-split Friedel-Anderson virtual d bound states on the Mn atoms interacting through a free-electron-like conduction band. From their calculations Caroli and Blandin deduced a hyperfine field of about - 199 kOe, - 295 kOe, - 213 kOe, in Cu2MnAl, Cu,MnSn, Cu2MnIn respectively, at the Cu nuclei. These values are in good agreement with nuclear magnetic reso- nance measurements [3, 51.

11. Experiment. - The present experiment repre- sents an attempt to determine the value and sign of the hyperfine field at the 'I9sn nuclei in Cu2MnSn, and the Heusler-type alloys Ni2MnSn and Zn,MnSn by using the Mossbauer effect technique. We assume here that the hyperfine field at the tin nuclei arises from the spin polarization produced in the conduction electrons by the manganese moments.

The source used in the experiments was in the form of BaSnO,. The Cu,MnSn absorbers consisted of three different samples made both by induction melt- ing and furnace melting in vacuum (low7 torr). The X-ray diffraction powder analysis indicated the pre-

(*) Work supported by National Aeronautics and Space Administration Grant

09-003-014.

(**) AEC Graduate Laboratory Fellows.

sence of the P-phase only. Mossbauer spectra taken on these samples at room temperature in zero applied magnetic field (Fig. 1) consisted of an unresolved

FIG. 1. - Mossbauer spectra of CuzMnSn at room temperature.

The pure Sn spectrum is given for comparison.

narrow line. From the line width we deduce a value for the hyperfine field at the Sn nuclei of less than 10 kOe at room temperature. Spectra taken at 77 OK and 18 OK also appeared unsplit and similar to figure 1.

The 77 OK hyperfine field value of - 20 kOe is about 1/10 of that measured by Chekin et al. [6]. Little addi-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711278

MOSSBAUER EFFECT STUDIES IN HEUSLER ALLOYS C 1 - ⁷⁹³

tional broadening was evident at 18

We must Ni,MnSn alloy yielded the value of 53 kOe for the further emphasize that no great increase in hyperfine hyperfihe field at tin nuclei. A transverse external field field is expected between 18 OK and 4

since the of 10 kOe split further the absorption lines (Fig. 3) Curie temperature of the P-phase alloy is about 500 OC.

Measurements in a 2.5 kOe field applied normal to I , m = ~ ‘?mi3 ~ ~

m ^{~ r l}

m- the y-ray direction yielded little change in line width 1 I

(Fig. I).

Measurements were performed on two Zn,MnSn samples prepared by arc melting in an argon atmos-

phere and by vacuum melting ( l o w 7 torr) in quartz _o,s tubes. A hyperfine field of about 30 kOe was deduced

from the splitting of the lines in zero applied magnetic

field. The application of a transverse magnetic field of 10 kOe modified the Mossbauer pattern. By visually

analyzing the pattern (Fig. 2) we deduce that the hyper-

MLaC1T.f ~ W S )

FIG. 2. - Mossbauer spectra of ZnzMnSn at room temperature.

fine field in Zn2MnSn is negative. We would like to point out however that this sign determination is by no means conclusive. The measurements on the

FIG. 3. - Mossbauer spectra of NizMnSn at room temperature.

giving a hyperfine field of 58 kOe. From these values we deduce that the demagnetizing field is about 5 kOe.

The positive hyperfine field of 53 kOe at tin nuclei in Ni2MnSn is to be compared with the value of 107 kOe in CozMnSn [7]. The theoretical values of the hyperfine field at tin nuclei in the alloys, asdeduced from Caroli and Blandin theory, are about + ^{30 kOe}

(Ni2MnSn), and about - 230 kOe for Cu,MnSn. In conclusion, the agreement with the theory of Caroli and Blandin is not very good especially in the case of Cu,MnSn. Possibly a better agreement could be achieved by refining the virtual bound state picture, for instance by introducing the periodicity of Mn atoms as suggested by E. Daniel [8].

References

[I] OXLEY (D. P.), TEBBLE (R. S.) and WILLIAMS (K. C.) [S] SUGIBUCHI (K.), and ENDO (K.), J. Phys. Chem.

J. Appl. Phys., 1963, 34, 1362. solids, 1964, 25, 1217.

[2] FELCHER (G. P.), CABLE (J. W.), and WILK~NSON (M. K.), [6] CHEKIN (V. V.), DANILENKO (L. E.) and KAP~LENKO J. Phys. Chein. Solids, 1963, 24, 1663. (A. I.), Sov. Phys.-JETP, 1967,24,472-4.

[3] SHINOHARA (T.) and WATANABE (H.), J. Phys. Soc [7] WILLIAMS (J. M.), J. Phys. C (Solid State Physics)

Japan, 1966, 21, 1658. 1969, 2, 2037.

[4] CAROLI (B.) and BLANDIN (A.), J. Phys. Chem. Solids, [8] DANIEL (E.), private communication.

1966, 27, 503.