NUCLEAR ORIENTATION EFFECTS IN THE MÖSSBAUER SPECTRUM OF 125Te IN Pd2MnSb

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NUCLEAR ORIENTATION EFFECTS IN THE

MÖSSBAUER SPECTRUM OF 125Te IN Pd2MnSb

G. Langouche, B. Triplett, N. Dixon, S. Hanna, P. Boolchand

To cite this version:

JOURNAL DE PHYSIQUE Colloque C6, supplkment au no 12, Tome 37, Ddcembre 1976, page C6-653

NUCLEAR ORIENTATION

EFFECTS IN THE

MOSSBAUER

SPECTRUM OF lZ5Te

IN

Pd2MnSb

(*)

G. LANGOUCHE (**), B. B. TRIPLETT, N. S. DIXON, S. S. HANNA Department of Physics, Stanford University, Stanford, California 94305, U. S. A.

and

P. BOOLCHAND

Department of Physics, University of Cincinnati, Cincinnati, Ohio 45221, U. S. A.

R6sum6.

-

Nous avons CtudiC l'effet Mossbauer du lzsTe provenant d'une source de 125Sb dans le PdzMnSb refroidi h 139 mK. De lYasymm6trie observ6e dans ce spectre, nous avons dCduit que le moment magn6tique de I'Ctat fondamental du 125Sb est positif, et celui du niveau isomCrique

l25Tem n6gatif.

Abstract.

-

A source of 125Sb in PdzMnSb was cooled down to 139 mK and the 125Te Moss- bauer spectrum was recorded. From the asymmetry observed in this spectrum, a positive sign was established for the magnetic moment of the 125Sb ground state, and a negative sign for the 125Tem state moment.

There has been considerable interest recently [I-31 in the measurement and interpretation of magnetic hypefine fields in ferromagnetic Heusler alloys. In a series of investigations [4-101 unusually large hypedine fields were observed in 5sp elements substi- tuted for Sb in Pd2MnSb. The magnetic fields at Sn, Sb and Te in Pd2MnSb, obtained from Mossbauer spectroscopy on '"Sn, "Sb and l z 5 ~ e , respectively, were found to be H =

+

210 f 5 kOe at Sn (7), H =

+

706f 5 k O e a t S b ( 8 ) , a n d H = +857&9kOe at Te (10). These values are shown in figure 1 together with a curve based on a theoretical calculation by Jena [I], which has been renormalized using a larger scaling factor. The calculation is based on a modified

FIG. 1. -Internal field systematics for 5sp impurities in

PdtMnSb. (Ref. [41, 171, [8] and [lo]). The sign of the Cd field is

not known, but is assigned on the basis of the systematics. The

Cd measurement was done at liquid NZ temperature, while the

others were at liquid He temperature.

(*) Supported in part by the National Science Foundation.

(**) Present address : Leuven University, Belgium.

Daniel-Friedel model [ l l ] and predicts a maximum value of the hyperfine fields in Pd2MnSb near I. This is quite different from the behaviour of the hyperfine fields of these 5sp impurities in ferroma- gnetic Fe, where a maximum is reached at Xe.

The hyperfine fields measured at Sb and Te in PdzMnSb are the largest fields ever observed at these nuclei. The source 12%b in PdzMnSb in which lZ5Sb decays to lZ5Te can therefore be expected to be an excellent host for observing nuclear polarization effects in the magnetically split Mossbauer spectrum of 125Te at extremely low temperatures. Such an experiment is described in this paper and a more extensive report and experimental details will be given elsewhere [12].

From a Nuclear-Orientation-Mossbauer-Effect (NOME) experiment 1131 on a magnetically split source one can in general obtain the magnetic inter- action strength pH at the parent state. In contrast to experiments where the Nuclear Orientation is detected by observing the Angular Distribution (NOAD) of a nuclear transition, or where in addition the NMR technique is used (NONMR), the NOME technique is also sensitive to the sign of the magnetic interaction. That is, by looking at the sense of the asymmetry created in the ME spectrum, one can tell whether the signs of pH at the parent state and at the excited Mossbauer state are the same or opposite. The magnitude of the asymmetry, on the other hand, is proportional to p H of the parent state and to the temperature. The NOME experiment described here is unusual in the sense that two parent

C6-654 G. LANGOUCHE, B. B. TRIPLETI', N. S. DIXON, S. S. HANNA AND P. BOOLCHAND

E I n lZ5Sb ground state. On the other hand, the actual

,( keV) _{magnitude of the observed asymmetry determines} 125~b(T,,p2.77 y )

766 712 + whether the polarized lZ5Tem state produces an asymmetry in the same or in the opposite direction and thus determines the sign of p H in the lZ5Tem state. So, from a single NOME experiment on lZ5Sb in Pd,MnSb, we can expect to be able to derive simultaneously the hitherto unknown signs of two magnetic moments, while the magnitude of the observed effect should be consistent with the known magnitudes of these moments.

To provide an accurate temperature determination of the lZ5Sb in Pd,MnSb source, a NOME measu- rement was performed with a 57Co in Fe source and a 57Fe in stainless steel absorber. The source was kept at the same temperature as the lZ5Sb source, as described in detail elsewhere [12]. This spectrum and the spectrum of the actual NOME experiment on l Z 5 ~ e in Pd,MnSb are shown in figure 3. From

FIG. 2.

-

Part of the 125Sb decay scheme. The relative intensi- ties given (above the 671 keV level) are the number of transitions

per 100 decays of the 12sSb ground state.

states are present. Indeed, as shown in figure 2,

one of the intermediate levels in part of the decay of the lZ5Sb ground state to the 35 keV excited Mossbauer state is the 145 keV isomeric state of l Z 5 ~ e r n . Since most of the decaying lZ5Tern nuclei that contribute to the Mossbauer spectrum are already in this state when the source is cooled down, and since the other lZ5Tern nuclei formed during the experiment reorient completely because the lifetime is obviously much longer than the spin relaxation time, this state can be considered as an independent parent state, which has its own charac- teristic orientation independent of the Iz5Sb ground state orientation. As a consequence the l Z 5 ~ e ME spectrum will show the combined effect of the two parent-state orientations, so that it is impos- sible to determine both orientations independently from a single temperature NOME experiment. This difficulty is removed however as the absolute values of the magnetic moment of the lZ5Sb ground state

(I

p(lZ5Sb)

I

= 2.630

+

0.035 pN) and of the lZ5Tem isomeric state

(I

( = 0.93

+

0.05 pN) are known from a NONMR experiment [14] and a NOAD experiment [15], respectively. Using these values and the hyperfine fields as determined by Mossbauer spectroscopy, we can calculate the nuclear orientation of both parent states. As the orientation of the lZ5Sb ground state is much larger than that of the lZ5Tern state, the sense of the asymmetry in the Mossbauer spectrum gives the sign of p H of the

VELOCITY ( c m l s )

-

FIG. 3.

-

Mossbauer spectra (upper) of the 125Sb in PdzMnSb source and a 125Te in ZnTe absorber and (lower) of a 57C0 in Fe source and a 57Fe in stainless steel absorber. The sources were

kept at the same temperature.

NUCLEAR ORIENTATION EFFECTS IN THE MOSSBAUER SPECTRUM O F l2sTe I N PdzMnSb C6-655

that the asymmetries are opposite direction. Since the corresponding excited Mossbauer states of 57Fe and 1 2 ' ~ e have the same signs of pH, we conclude that the ground states of 57Co and lZ5Sb have opposite signs of pH. This leads to a positive sign for the moment of lZ5Sb. The same conclusion was reached by a more quantitive analysis [12]. In this analysis 1121 we also calculated the magnitude of the asymmetry for both a positive and a negative magnetic moment for l Z 5 ~ e m . The observed polarization, shown in figure 4, is close to the lower curve and establishes a negative moment for 1 2 5 ~ e m .

It is interesting to note the small disagreement between the calculated and the observed asymmetries. In this connection we note that the value used for p(lZ5Tem) was obtained from a NOAD experiment [I 51,

in which a unique hyperfine field was assumed for all the 1 2 5 ~ e m nuclei implanted in Fe. Recent ME experiments [16, 171, however, indicate that upon implantation in Fe a large number of Te ions come to rest at lower field sites. In all room temperature implantation experiments [17] it was found that only about 65

%

of the ions occupy substitutional lattice sites. This would indicate that the real

value is larger than the reported one. The effect would raise the upper curve in figure 4 and lower

the lower one, thereby improving the agreement with our experiment.

FIG. 4.

-

Calculated and observed polarizations in the NOME spectrum of lz5Te in PdZMnSb. The polarization is deiined as

(I+ - I-)/(I+

+

I-), I+ and I- being the intensities of the outer peaks in the 125Te ME spectrum at positive and negative veloci-

ties respectively.

References

[I] JENA, P., GELDART, D. J. W., Solid State Commun. 15

(1974) 139. . ,

[2] CAMPBELL, C. C. M., J. Phys. F : Metal Phys. 5 (1975) 1931.

131 CAMPBELL, I. A., BLANDIN, A., J. Magn. and Magn. Mat.

1 (1975) 1.

[4] WALU~, W., GOSS-ZUREK, R., STYCZEN, B., SZYTULA, A., STYCZEN, J., in International Conference on Hyperfine

Interactions Studied in Nuclear Reactions and Decay, Contributed Papers (Uppsala, June 1974), p. 181. [5] CAMPBELL, C. C. M., LEIPER, W., in A. I. P. Conference

on Magnetism and Magnetic Materials (Boston 1973)

p. 319.

[6] ~WARTZENDRUBER, L. J., EVANS, B. J., Phys. Lett. 3SA

(1972) 511.

[7] BOOLCHAND, P., TENHOVER, M., JHA, S., LANGOUCHE, G., TRIPLETT, B. B. and HANNA, S. S., to be published.

[8] LANGOUCHE, G., DIXON, N. S., MAHMUD, Y., TRI-

PLETT, B. B., HANNA, S. S., BOOLCHAND, P., P h y ~ .

Rev. C 13 (1976) 2589.

[9] MACKAY, G. R., BWAUW, C., LEIPER, W., J. Phys. F :

Metal Phys. 5 (1975) L 166.

[lo] BOOLCHAND, P., TENHOVER, M., JHA, S., LANGOUCHE, G., TRIPLETT, B. B., HANNA, S. S., JENA, P., Phys. Lett. 54A (1975) 293.

[ l l ] DANIEL, E., FRIEDEL, J., J. Phys. Chem. Solids 24 (1963) 1601.

[I21 LANGOUCHE, G., TRIPLETT, B. B., DIXON, N. S., HANNA, S. S., BOOLCHAND, P., to be published in

Phys. Rev. C.

[13] DASH, J. G., TAYLOR, R. D., NAGLE, D. E., CRAIG, P. P., VISSCHER, W. M., Phys. Rev. 122 (1961) 1116. [14] CALLAGHAN, P. T., SHOTT, M., STONE, N. J., Nucl. Phys.

A 221 (1974) 1.

[15] SILVERANS, R. E., COUSSEMENT, R., DUMONT, G., PATTYN, H., VANNESTE, L., Nucl. Phys. A 193 (1972) 367.

[16] REINTSEMA, S. R., DE WAARD, H., DRENTJE, S. A., in

Proceedings of the International Conference on Miiss- bauer Spectroscopy (Cracow, August 1975), Vol. 1,

p. 525.