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HYPERFINE FIELDS AT Pt AND Au IN FERROMAGNETIC Pt3X ALLOYS : A TEST FOR THE PROPORTIONALITY BETWEEN CONDUCTION ELECTRON POLARIZATION AND MAGNETIC MOMENT

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HYPERFINE FIELDS AT Pt AND Au IN

FERROMAGNETIC Pt3X ALLOYS : A TEST FOR

THE PROPORTIONALITY BETWEEN

CONDUCTION ELECTRON POLARIZATION AND

MAGNETIC MOMENT

I. Vincze, F. Wagner, E. Baggio-Saitovitch, W. Koch

To cite this version:

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JOURNAL DE PHYSIQUE Colloque C6, supplkment au no 12, Tome 37, De'cembre 1976, page C6-423

HYPERFINE FIELDS AT Pt AND Au IN FERROMAGNETIC Pt3X ALLOYS

:

A

TEST FOR THE PROPORTIONALITY BETWEEN CONDUCTION ELECTRON

POLARIZATION AND MAGNETIC MOMENT

I. VINCZE (*), F. E. WAGNER, E. BAGGIO-SAITOVITCH (**) and W. KOCH Physik Department, Technische Universitat Miinchen, D-8046 Garching, Germany

Rksumk. - Les resonances Mossbauer de 195Pt (99 keV) et 197Au (77 keV) ont kt6 utilisees pour mesurer les champs hyperfins sur l'Au et sur le Pt dans les alliages cfc ferromagnetiques Pt3X oh X = Cr, Mn, Co. On a trouvk que ces champs hyperfins sont strictement proportionnels apx moments magnetiques des metaux 3d. Ceci constitue une preuve directe de l'hypothkse generale- ment faite que la polarisation des electrons de conduction depend uniquement de la distance et de l'intensite du moment magnetique voisin, mais ne depend pas explicitement de la nature de l'e16- ment de transition qu'occupe le site magnktique.

Abstract. - The Mossbauer resonances in 195Pt (99 keV) and 197Au (77 keV) have been used to measure the magnetic hyperfine fields at Pt and Au on the Pt sites in the ferromagnetic fcc Pt3X (X = Cr, Mn, Co) alloys. These hyperfine fields are found to be strictly proportional to the magnetic moment of the 3d'constituent. This constitutes direct evidence for the usual assumption that the conduction electron polarization depends only on the distance and magnitude of the neigh- bouring magnetic moments, but not explicitly on the specific transition element occupying the magnetic sites.

1. Introduction. -The hyperfine fields at noble

- -

TABLE I metal atoms in ferromagnetic hosts arise from conduc-

tion electron polarization (CEP). One usually assumes Properties of Pt3X alloys. The lattice parameter and that the CEP depends on the distance and magnitude of magnetic moment values are taken from Refs. [2-51. the neighbouring magnetic moments, but not explicitly The hyper-ne Jields at Au and Pt nuclei occupying on the specific transition element occupying the magne- the Pt sites in these alloys were obtained in the course tic sites. These assumptions are mainly based on of the present work.

indirect evidence [I] since in most systems a direct experimental test is barred by the occurrence of diffe- rent crystal structure for different magnetic consti- tuents, a typical example being the ferromagnetic transition metals Fe, Co and Ni.

The ferromagnetic Pt3X (X = Cr, Mn, Co) alloys are a favourable case in this respect because they all have the Cu3Au-type fcc structure when they are crys- tallographically ordered. Their lattice parameters and the individual magnetic moments derived from neutron diffraction measurements are compiled in Table I.

Using the 99 keV and 77 keV Mossbauer resonances in lgSPt and lg7Au, we have measured the hyperfine fields at Pt and Au nuclei in these alloys. The observed proportionality between these hyperfine fields and the magnetic moment of the 3d transition element X is direct evidenc- for the validity of the mentioned assumption.

(*) On leave from the Central Research Institute for Physics, Budapest, Hungary.

(**) On leave from the Centro Brasileiro de Pesquisas Fisicas, Rio de Janeiro, Brasil.

2. Experiments and results.

-

The Mossbauer mea- surements were carried out in a standard transmission spectrometer with sources and absorbers at 4.2 K. The Pt3Mn and Pt3Cr samples used in our experi- ments were identical with those used in previous investigations [4, 51. The Pt,Co sample was prepared by induction melting of constituents having a purity of better than 99.99

%

in an argon atmosphere and subsequent annealing in sealed quartz tubes at 600 OC for two weeks.

The Au hyperfine fields at the Pt sites were measured in source experiments with the 77 keV gammarays of lg7Au. The sources were made by neutron irradiation of about 50 mg of the respective alloy in a sealed quartz tube. After the irradiation the Pt3Mn and Pt3Cr samples were annealed for 6 h at 850 OC to heal

out the radiation-induced disorder incurred during the neutron activation. The Pt,Co was annealed for 8 h at

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C6-424 I. VINCZE, F. E. WAGNER, E. BAGGIO-SAITOVITCH AND W. KOCH

600 OC. The Mossbauer spectra indicate that Pt3Co always retains some disorder, whereas Pt3Cr and Pt3Mn give the symmetric hyperfine patterns expected for the ordered compounds.

The '"Au Mossbauer spectra were fitted with the eight-line patterns expected with 6 =

-

0.352 for the E 2/M 1 mixing ratio and gl/2/g3/2 = 8.72 for the ratio of the nuclear g-factors [6]. The hyperfine fields (Table I) were determined from the splittings of the 312' groundstate with p3/, =

+

0.14487 p,.

An absorber of 100 mg/cm2 of metallic gold was used in all experiments. The width of the individual lines of the partly resolved magnetic hyperfine patterns obtained in this way was typically 2.8 mm/s, i. e. somewhat larger than the line-width of 2.36 (2) mm/s obtained with the same absorber but a single-line source of Pt metal. This broadening may be due to a small electric quadrupole interaction with different angles between the electric field gradient and the hyperfine field on different lattice sites, or to a small degree of disorder in the samples.

The Pt hyperfine fields were measured with the alloys as absorbers and a source of lg5"Pt in Pt metal made by reactor irradiation of enriched lg4Pt. The Mossbauer patterns were fitted with the six-line patterns expected for this pure M 1 transition and g,/,/g,,, =

-

0.34 [7] for the ratio of the g-factors. The typical linewidth obtained in this way with absorbers containing about 100 mg/cm2 of the alloys is 20 mm/s, i. e. about 20

%

larger than the natural linewidth. Since the magnetic hyperfine patterns are not resolved, the influence of the absorber thickness on the hyperfine fields obtained by fitting superposi- tions of Lorentzian lines was studied in experiments with Pt3Cr absorbers of different thicknesses between 50 mg/cm2 and 200 mg/cm2. The values obtained in this way for the hyperfine field turned out to be inde- pendent of the absorber thickness. Within their limits of error, the Pt hyperfine fields given in Table I should therefore not be influenced by the effects of finite absorber thickness.

3. Discussion.

-

In the fcc structure of the ordered Pt,X alloys, the X atoms are located at the corners of the unit cell, while the Pt atoms occupy the face-centered positions. Consequently, the nearest neighbours of Pt are four X and eight Pt atoms, and the next-nearest neighbours are six Pt atoms.

The conduction electron polarization at the Au and Pt sites should have contributions from the magnetic moments of the 3d element and from the platinum moments. The Pt hyperfine fields are expected to be especially sensitive to the Pt magnetic moment because of the core polarization (CP) contribution to the hyperfine field, which is expected to be given by

H,, = appt, where the proportionality constant is approximately a w 1 000 kOe/p, [8]. Thus the neutron diffraction data suggest that between Pt3Cr and Pt3Co

there is a difference in the CP contribution of about 500 kOe due to the opposite sign of Pt moments. Figure 1 shows, however, that the hyperfine fields at both the Pt and Au nuclei are, within the limits of error, proportional to the magnetic moment of the 3d ele- ment only. This suggests that the CP contribution arising from the magnetic moment on Pt in these alloys is at least one order of magnitude smaller than the values resulting from the neutron diffraction values for PP,.

The discrepancy between the Mossbauer results, which yielded no indication of a CP contribution to the Pt hyperfine fields, and the substantial magnetic moments for Pt in the Pt,X alloys suggested by the neutron diffraction experiments, may well arise from a partial disorder of the samples used for the neutron diffraction work. The nominal magnetic moments carried by the Pt in these systems may then be due to 3d elements sitting on Pt sites. In the case of Pt3Cr this assumption would correspond to an antiferroma- gnetic coupling of the magnetic moments on Cr occupying Pt sites. This is also suggested by magneti- zation measurements [5] yielding a mean atomic moment of 0.56 p,, as compared to 0.38 pB deduced from the neutron diffraction measurements. This dis- crepancy strongly indicates that the neutron diffrac- tion experiments were performed with a partially disordered. sample. The value of the Cr magnetic moment obtained in the neutron diffraction work, however, does not seem to be affected by this disorder : with the mean magnetic moment resulting from the magnetization measurements, and assuming that the Pt moment is zero, one obtains 1*,,=4 x 0.56=2.24 pB for the Cr moment. This agrees well with the neutron diffraction value of p,, = 2.33 (10) ,LA, [2].

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HYPERFINE FIELDS AT Pt AND AU IN FERROMAGNETIC Pt3X ALLOYS C6-425

close neighbours in the periodic system the ratios of the atomic hyperfine coupling constants describe the

FIG. 1.

-

AU (m) and Pt

(n)

hyperfine fields in PtsX as a func- tion of the magnetic moment of the iron group element.

situation in metallic systems quite well, even though the individual values for A(Z) in free atoms and in metals may be quite different.

Finally, since the lattice parameters are virtually the same for all three alloys, the linear correlations of Fig. 1 provk in a very direct way that the CEP depends only on the magnitude of the magnetic moment, but not on the detailed electronic structure of the 3d constituent.

Acknowledgement. - We would like to thank Prof. G. M. Kalvius, Dr. I. A. Campbell and Dr. G. Wortmann for stimulating discussions. We are indebted to Dr. M. 3. Besnus and Dr. G. Konczos for the loan of their samples. Two of us (I. V. and E. B.-S.) wish to thank Prof. G. M. Kalvius for his hospitality at the Technical University of Munich, and to acknow- ledge financial support from the TUM (I. V.) and the KFA (3iilich)-CNPq (Brasil) (E. B.-S.).

References

[I] CAMPBELL, I. A. and BLANDIN, A., J. Magn. Magn. Mat. 1 [5] BESNUS, M. J. and MEYER, A. J. P., Phys. Stat. Sol. (b) 55

(1975) 1. (1973) 521 ; 58 (1973) 533.

[Z] PICKART, S. J. and NATHANS, R., J. Appl. Phys. 33 (1962) [6] PROSSER, H., WAGNER, F. E., WORTMANN, G., KALVTUS, G. 1336 ; 34 (1963) 1203. M. and W~PPLING, R., Hyperf. Int. 1 (1975) 25. D] MENZINGER, F. and PAOLETTI, A., Phys. Rev. 143 (1966) 365. [7] AGRESTI, D., KANKELEIT, E. and PERSSON, B., Phys. Rev. 155

[4] KREN, E., KADAR, G., PAL, L., SOLYOM, J., SZABO, P. and (1967) 1339.

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