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ORBITAL CONTRIBUTION TO THE MAGNETIC HYPERFINE FIELD OF ISOLATED Ni IMPURITIES IN Pd

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Submitted on 1 Jan 1988

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ORBITAL CONTRIBUTION TO THE MAGNETIC

HYPERFINE FIELD OF ISOLATED Ni IMPURITIES

IN Pd

W. Müller, H. Bertschat, H. Haas, B. Spellmeyer, W.-D. Zeitz

To cite this version:

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JOURNAL DE PHYSIQUE

Colloque C8, Supplement au no 12, Tome 49, dkcembre 1988

ORBITAL CONTRIBUTION TO THE MAGNETIC HYPERFINE FIELD OF

ISOLATED

Ni

IMPURITIES IN

Pd

W. Miiller, H. H. Bertschat, H. Haas, B. Spellrneyer and W.-D. Zeitz

Hahn-Meitner-Institut Berlin GmbH, Bereieh Kern- und Strahlenphysik, 0-1000 Berlin 39, F.R.G.

Abstract. - The temperature, magnetic field and concentration dependence of the magnetic hyperfine field of Ni impuri- ties in Pd and PtPd alloys were determined by measuring the Larmor frequency shift of isomeric states in the 6 3 ~ i isotope using the PAD method. The recoil implanted Ni nuclei, products of heavy ion reactions, were distributed in practically infinite dilution in the hosts. The magnetic hyperfine field for Ni in Pd was compared with its susceptibility contribution obtained from extrapolated susceptibility measurements in dilute NiPd alloys. Within a simple molecular field model the different contributions to the hyperfine field could be estimated. The negative core polarization field of the impurity spin moment is compensated by a transferred hyperfine field correlated with the host polarization in the neighbourhood of the impurity. The observed positive hyperfine field is due to an orbital moment of 0.3 p~ at the impurity site. The values obtained for the different contributions agree with extrapolated results from KKR-CPA calculations for concentrated NiPd alloys.

Magnetic hyperfine fields of 3d element impurities in the Pd host show a remarkable behaviour. While the hyperfine fields in ferromagnetic FePd alloys are nega- tive in the whole concentration range [I], the fields in CoPd and NiPd alloys grow up t o large positive values with increasing Pd content [2-41. A transferred hy- perfine field was suggested by various authors as the origin of the positive field a t the impurity site [2, 3,

51. On the other hand, there are theoretical as well as experimental hints for orbital contributions t o the magnetic moments in these systems leading t o posi- tive fields. Theoretical predictions for the hyperfine field contributions in ferromagnetic NiPd alloys were based on bandstructure calculations using the KKR- CPA method [6]. The change of sign of the hyperfine field was deduced from the concentration dependence of a transferred field. However, t o get an agreement of the predicted overall saturation hyperfine field with experimental results an additional orbital contribution had t o be assumed. Expressing this contribution as an electronic g-factor shift agreement with g-factor mea- surements from FMR experiments was obtained [7]. Experimental hints for orbital magnetism were also ex- tracted from the observation of broad hyperfine field distributions in NMR experiments [3, 81.

In order t o get further insight into the magnetic behaviour and the magnetic hyperfine field contribu- tions for Ni impurities in P d and PtPd alloys, we per- formed hyperfine field measurements over a wide range of temperature, magnetic field and P t concentration. We used the PAD (perturbed angular distribution) method [9] in combination with the heavy ion reaction 4 8 ~ a (180,3n) 63 Ni to produce and recoil implant the

aligned and excited probe nuclei 6 3 ~ i - into the hosts. From the Larmor precession of the nuclear moments of the (9/2)+ and (512)- isomeric states around the

local magnetic field - applied external field Bext and

induced internal magnetic hyperfine field (Bhf) - we extract the frequency shift K = (Bhf) /Bext. The im- purity concentration was well below 1 ppm during the measurement.

The behaviour of the Ni impurities in P d and P t P d alloys as a function of temperature and P t concen- tration is plotted in figures 1 and 2. In Pd a large positive shift K occurs with a Curie-Weiss like tem- perature dependence. At low temperatures the shift deviates from the Curie-Weiss law and a linear depen- dence of the induced hyperfine field on the external field is observed (insert, Fig. 1). In contrast t o mag- netocrystalline anisotropies in ferromagnetic crystals of the alloys [lo] no anisotropy of the hyperfine field (above a limit of 2 % ) was found for the paramagnetic probe nuclei in a Pd single crystal.

I I I I I

0 200 400 600 800

T l K l

Fig. 1.

-

Larmor frequency shift at various temperatures and the dependence of the induced hyperfine field on the external field for Ni impurities in Pd at 15 K.

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C8

-

130 JOURNAL DE PHYSIQUE In the PtPd alloy, the local moment like behaviour

of the isolated Ni impurity vanishes with increasing Pt content, but even above 30 at% P t a still large positive shift remains. The dependence on the P t content fol- lows the concentration dependence of the macroscopic susceptibility of the PtPd alloy (Fig. 2 ) , taken from reference [11]

.

Fig. 2. - Variation of the frequency shift of Ni impurities in PtPd alloys (0) and the alloy susceptibility (a) with the Pt content.

Following the model of Takahashi and Shimizu [12]

we expressed the temperature dependence of the fre- quency shift K ( T ) by the susceptibility of the Ni atom in the Pd host AX ( T ) [13] and the host sus- ceptibility ~ p(T) d

.

Thereby we allowed for the fol-

lowing hyperfine field contributions: a core polariza- tion field is caused by the Fermi contact interaction of the polarized inner shell s-electrons. An orbital field results from the orbital contribution to the local mo- ment expressed through a pfactor shift Ag. Finally, a hyperfine field Bt, is transferred per p~ magneti- zation localized in the nearest neighbour Pd shell of the Ni impurity. A detailed discussion will be given in a forthcoming paper. If we take Ag = 0.8(1) and

B n = +7.4(1) T / ~ B the temperature dependence of the Larmor frequency shift is parameterized in accor- dance with our data over the whole temperature range (solid line in Fig. 1 ) . With the above values the cor- responding saturation hyperfine field contributions are easily calculated. In figure 3 the results are plotted t~ gether with the hyperfine field contributions given by

Akai [6]. The saturation hyperfine field is determined by the orbital field corresponding to a weak orbital moment of 0.3 p ~ .

Fig. 3. - Contributions to the saturation hyperfine field derived for the isolated Ni impurity in Pd (closed symbols) and from band structure calculations [6] for ferromagnetic

NiPd alloys (open symbols). Orbital field (V), transferred field (A), core polarization field ( 0 ), total field ( 0 ) .

[ I ] Craig, P. P. et al., Phys. Rev. Lett. 14 (1965) 896.

[2] Erich, U . et al., Phys. Rev. Lett. A 31 (1970) 492.

[3] Tansil, J . E. et al., Phys. Rev. B 6 (1972) 2769. [4] Katayama, M. et al.

,

J. Phys. Soc. Jpn 40 (1976)

429.

[5] Fink, J . , Czjzek, G., Schmidt, H. and Obenshain,

F. E., preprint (1986).

[6] Akai, H., J. Phys. Soc. Jpn 51 (1982) 468. [7] Fischer, G . et al., J. Appl. Phys. 39 (1968) 545. [8] Le Dang Khoe et al., J. Phys. F. 6 (1976) L197. [9] Raghavan, P. and Raghavan, R. S., Hyperfine In-

teractions 24-26 (1985) 855.

[ l o ] Bagguley, D. M. S. and Robertson, J. A., J. Phys.

F 4 (1974) 2282.

[ l l ] Treutmann, W., 2. Angew. Phys. 30 (1970) 5 . [12] Takahashi, T. and Shimizu, M., J. Phys. Soc. Jpn

20 (1965) 26.

[13] Chouteau, G., thesis, University of Grenoble

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