NUCLEAR MAGNETIC RESONANCE IN DILUTE PALLADIUM AND PLATINUM ALLOYS

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NUCLEAR MAGNETIC RESONANCE IN DILUTE PALLADIUM AND PLATINUM ALLOYS

A. Narath, H. Weaver

To cite this version:

A. Narath, H. Weaver. NUCLEAR MAGNETIC RESONANCE IN DILUTE PALLADIUM AND PLATINUM ALLOYS. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-992-C1-994.

�10.1051/jphyscol:19711353�. �jpa-00214389�

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JOURNAL DE PHYSIQUE ColEoque C I, supplkment au no 2-3, Tome 32, Fkurier-Mars 1971, page C 1

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NUCLEAR MAGNETIC RESONANCE IN DILUTE PALLADIUM AND PLATINUM ALLOYS (*)

A. NARATH and H. T. WEAVER

Sandia Laboratories, Albuquerque, N. M., 87115, U. S. A.

Rksumk. - La rkponse magnktique locale de plusieurs impuretks dilukes dans Pd et Pt a kt6 examinee entre les temp6ratures de 1 OK et 4 ^OK,a rnoyen de technique R. M. N. pulske. Les dkplacements de Knight, ainsi que les temps de relaxation au spin-rkseau, furent mesurks dans des alliages pour lesquels la susceptibilitk d'impurete est non seulement.

plus grande [PdRh, E R h , E N i ] mais aussi plus petite [EPt, S A g , EAg, EdCu, PtCu] que la susceptibilite dela matrice.

Notre thkse Gintient que les faits et donnks concernant E N i ne peuvent &re coGpris correctement B partir de la theorie des fluctuations de spin localist5es7 en travaillant avec des alliages iso6Iectroniques.

Abstract. - The local magnetic response of several dilute impurities in Pd and Pt has been investigated in the temperature range 1-4 OK by means of pulsed NMR techniques. Resonance shifts and spin-lattice relaxation rates were measured in alloys for which the impurity susceptibility is both greater (LdRh, ERh, ENi) as well as smaller (GPt, PcJAg, EAg, PdCu, PtCu) than that of the host. It is argued that the E N i data cannot be understood on the basis of the localized - spin-fluauation model of isoelectronic alloys.

I. Introduction. -Because of their stronglyexchange- enhanced spin susceptibilities, palladium and platinum are useful hosts for impurity studies in metals.

The present work concerns a nuclear magnetic resonance (NMR) investigation of several dilute impurities in these metals, including examples for which the impurity susceptibility is greater (PdRh, PtRh, PtNi) as well as smaller (PdPt, - - PdAg, - K A ~ , -- ~ d C u , - KCU) than that of the host. All of these alloys are non- magnetic according to the hnderson-Wolff criteria.

The NMR technique allows the local magnetic response to be probed in an alloy. In particular, the observed relationship between the hyperfine-induced resonance shift ( K ) and spin-lattice relaxation rate (TI 7')-' gives a qualitative measure of the wavenumber dependence of the susceptibility enhancement which, in turn, is indicative of the relative localization of the impurity potential [I]. For the case of a dominant d-spin (core polarization) hyperfine interaction, the

(( Korringa product B is given by

where F, is the average reciprocal orbital degeneracy a t the Fermi level and K(a) corrects for differences between the exchange-enhancement of K and TI-'/".

The former is given by (1 - a)-' ; the latter is gene- rally much smaller because of the rapid decrease of the spin-fluctuation enhancement with increasing wavenumber (q) especially for large values of a. An exception occurs in the limit of strongly localized spin fluctuations in which case the q-dependence of the enhancement factor vanishes and K(a) x 1 independent of a.

11. Results and discussion. - The experiments were performed on

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300 mesh powders with a pulsed NMR spectrometer in the temperature range 1 - 4 OK

(*) This work was supported by the U. S . Atomic Energy Commission.

and external fields near 60 kOe. Alloys were prepared by arc melting appropriate quantitites of the elemental metals (nominal purity of 99.999 %). No annealing was attempted either before of after comminution. Our experimental results for the most dilute compositions are summarized in Table I. The absence of any compo- sition dependence was verifired in every case by studies of more concentrated alloys. The comparison given in the last column between some of the data and the Korringa relation is based on the assumption that the states at the Fermi level are principally oft,, symmetry (i. e., Fd = 3).

6 3 C ~ , '09Ag :

The present data [2] together with earlier results on NiCu [2], PdCu [4], PtCu [4], and PdAg [5] support

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the commonly accepted view that the impurity d-spin susceptibility is essentially zero in these alloys due to screening effects (i. e., the d levels of the noble element impurity lie below the Fermi level). That the negative shifts are not the result of a local d-spin interaction is clearly demonstrated by the exceptionally long relaxation times 161. Whereas the shifts are approximately in the ratio of the host susceptibilities, the relaxation rates are essentially the same for either host.

The '09Ag TIys are approximately twice as long as in silver metal. This is consistent with the expected decrease in the effective density of ^<(s-states >> at the Fermi level with decreasing electron density. (A comparison of the 6 3 C ~ TI3s with the pure copper value, TI T = 1.2 s OK, is less meaningful because copper is much less free-electron like than silver.) The observed shifts are thus dominated by s-d polarization effects, while the relaxation rates are determined principally by the direct s-contact interaction. The ineffectiveness of the indirect interaction as a relaxation mechanism can be related to the long range of the s-d polarization.

Thus, only long-wavelength disturbances of the host d-spin magnetization produce significant spin-fluctuation amplitudes at the impurity. Since the nuclear relaxation rate measures a wavenumber average, K(a) for the d-spin mechanism may be very small.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711353

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NUCLEAR MAGNETIC RESONANCE I N DILUTE PALLADIUM AND PLATINUM ALLOYS C 1 - 993 l o 3 ~ h :

The PdRh data have been discussed in detail else- where [7]. The '03Rh results show that the local susceptibility at the rhodium sites, although

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^{3.3 times}

greater than the pure palladium susceptibility, accounts for only -- 12 % of the total impurity susceptibility.

The extended nature of the impurity potential explains the very small K(a) values. Thus, the dynamic susceptibility at the impurity site resembles more nearly that of a uniform exchange-enhancement model than that of a localized spin-fluctuation model [I]. This is not unexpected since Gh x a,,, where Ui represents the combined effects of the effective intra-atomic Coulomb (U) and exchange (J) potentials. Hence, the charge contrast AZ = 1 constitutes the only major perturbation. It is interesting to note that the lo3Rh relaxation rate in PtRh, for which i?&, >

up,,

^is

significantly faster than in a PdRh alloy ( -- 23 at % Rh) having the same shift value.

6 1 ~ i : -

- Since nickel and platinum are isoelectronic with UNi > Gpt it was expected that the localized spin- fluctuation model would have quantitative significance for PtNi [8]. For an isoelectronic alloy this model assumes that AU represents the only impurity perturbation. The local one-electron density-of-states func- tion in this approximation is the same as that of the host. Hence, the model predicts [8] an impurity K(a) equal to the host value which for Pt may be estimated to be -- 0.5. The experimental magnitude of

is seen to be in striking disagreement with this result.

It is immediately obvious that an additional hyperfine interaction is required which yields a rapid relaxation and/or positive shift contribution. The direct s-contact mechanism can be eliminated since the negative sign of the observed shift shows that core-polarization is the dominant spin-dependent hyperfine mechanism.

Furthermore, neither in palladium nor platinum does the s-contact interaction play a dominant role. An explanation for the anomalous behavior of PtNi must therefore be found in the orbital hyperfine mechanism.

(The orbital shift and relaxation rate in a band model are not connected by a Korringa-like relation.) In the

first approximation the localized spin-fluctuation model predicts an orbital shift and relaxation rate which can be derived from the host values by adjusting for any differences in the respective orbital hyperfine coupling constants. Taking atomic hyperfine fields, I;,,, = $, and a bare density of states N(0) = 6.0 x 1011 (erg-atom)-' we obtain for the Ni impurity

KO,, x + ^0.2^% and (TI T),,, x 2.4 s OK.

Even if allowance is made for possible exchange- enhancement effects [for the orbital interaction a K (U - J ) instead of (U +(F~-' - 1) J ) as in the case of the d-spin (core-polarization) interaction]

the magnitude of KO,, and (TI T),: are much too small. We must conclude that the model is not applicable in detail. Most likely (at least those oft,, symmetry) the Ni d-states are relatively localized, perhaps due to differences between the Pt-Ni and Pt-Pt transfer integrals. Such localization would enhance x,,, and hence KO,, [9]. In the limit that the orbital rate remains unimportant we may estimate Kd from Eq. (1) by substituting the experimental rate.

Assuming K(a) = 1 and Fd = +we obtain Kd = - 2.7 %.

Hence, by difference, KO,, = + ^1.3% which corres- ponds to x,,, ^x1.0 x l o p 4 emulg-atom. (It is impor- tant to note that an orbital shift of

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⁺^2.7^%^has

been clearly identified for 59Co in the related alloy RhCo [lo]. Finally, we note that the d-spin shift given above yields xd ⁼15 x emulg-atom (for

(d) = - 100 kOe/pB) which is close to the B N i

HhfS

impurity susceptibility x ⁼²⁰x emu/g-atom [l 11. This conclusion is consistent with the relatively small host resonance shifts measured by Launois [12].

These data are unfortunately difficult to interpret since the relative magnitudes of the s-band )) [13]

and ⁽⁽d-band ^>)polarizations are unknown.

Although PdPt is an isoelectronic alloy like PtNi [14], it differs in that the impurity susceptibility issmaller than the host susceptibility (&, < a,,). The resonance shifts (K = - 2.4 (2) %) are independent of platinum concentration below 2.5 at %; the relaxation rates, however, double in this range with decreasing concentration, reaching values which exceed the Kor- ringa prediction (for K(a) = 1) by a factor of -- 4.

Summary of NMR data. Numbers in parentheses indicate the estimated uncertainty in the preceding digit

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C 1 - 994 A. NARATH AND H. T. WEAVER

Thus, as in PtNi, the impurity state appears to localize, yielding an enhanced xorb. Since G, ^<Gi the spin and orbital enhancement factors are, of course, much smaller.

111. Conclusions. - The results of the present NMR study support earlier conclusions regarding the magnetic properties of Cu, Ag, and Rh impurities in Pd and Pt. Our PtNi ^- and PdPt data on the other

hand are difficult to reconcile on the basis of existing theories such as the localized spin-fluctuation model.

Instead, our results suggest the formation of nearly- filled resonant-bound d-states in both alloys. Such localization would presumably have no effect on bulk properties such as the susceptibility, specific heat. and electrical resistivity since the general form of the local spin-fluctuation spectrum should be qualitatively the same as in the localized exchange-enhancement model.

References NARATH (A.), J. Appl. Phys., 1970, 41, 1122. P I Resonance shifts were calculated with respect to the ¹⁹¹

following v/H references in kHz/Oe [Nuclear Data Tables, 1969 A 5 , 4331 ; 63Cu : 1.1285, [lo]

109Ag : 0.19809, 103Rh : 0.13380, 61Ni : 0.38048,

195Pt : 0.9089. [I11

BANCROFT (M. N.), Phys. Rev., 1970, B2, 182.

KOBAYASHI (S.), ASAYAMA (K.) and ITOH (J.), J. Phys. SOC. Japan, 1963, 18, 1735 ; ITOH (J.), ASAYAMA (K.) and KOBAYASHI (S.), Procee- dings of the International Conference on Magne- tism, Nottingham, 1964, p. 162.

NARATH (A.), J. Appl. Phys., 1968, 39, 553.

Although the Cu shifts given in ref. 4 are .in good agreement with our results, the relaxation rates are much faster and appear to be in error.

NARATH (A.) and WEAVER (H. T.), Phys. Rev. (to be published).

LEDERER (P.). Solid State Commun. 1969. 7 . 209.

DWORIN and NARATH (A.), phys. Re;. ~ k t t e r s , 1970, 25,1287.

WALSTEDT (R. E.), SHERWOOD (R. C.) and WER-

NICK (J. H.), J . Appl. P h y ~ . , 1968,39, 555.

For a listing of references see. for exam~le. MACK-

LIET (c- A.), SCHINDLER (A. I.) and ^GILLESPIE (D. J.), Phys. Rev., 1970, B 1, 3238.

LAUNOIS (H), Theses (Universite de Paris, Orsay,

1969).

WALSTE~T (R. E.) and WERNICK (J. K.), Phys. Rev.

Letters, 1968. 20, 856.

Earlier NMR measurements by KOBAYASHI (S.), LAUNOIS (H.), LEDERER (P.), FROIDEVAUX (C.), TREUTMAN (W.), and VOGT (E.), Solid State Commun., 1968, 6, 265 and NARATH (A.) and WEAVER (H. T.), Solid State Commun., 1968, 6, 413, were restricted to higher impurity concen- trations.