Nuclear magnetic resonance in NiCu, Zn and Ga alloys

Solid State Communications,

Vol. 9, pp. 1889—1892, 1971. Pergarnon Press.

Printed in Great Britain

NUCLEAR MAGNETIC RESONANCE IN Ni—Cu, —Zn AND —Ga ALLOYS* Shizuko Ogawa and Jan Smit Departments of Electrical Engineering and Materials Science, University of Southern California, Los Angeles, California 90007 (Received 15 July 1971 by N.B. h’annay)

61Ni, 63Cu, 65Cu, ~Ga, and 71Ga has been observed in ferroNMR of alloys of nickel with up to 10 at. % of Cu, Zn or Ga. From magnetic an analysis of the structure of the complex spectra it is concluded that the excess electrons stay localized around the solute atoms. The hyperfine field of Ga at 4.2°K is 23koe.

IN ALLOYS of nickel with metals such as Cu, Zn, Al, and Si, the spontaneous magnetization changes linearly with the concentration of the solute atoms and with their excess number AZ of valence electrons per atom. This is usually held as evidence for the rigid band model, in which it is assumed that the extra (AZ-na) conduction electrons of the solute atom enter the d band of the Ni atoms. Here n~is the number of conduction electrons per atom; for nickel = (2/g)n~ = 0.55 for g = 2.19. It is to be expected, however, that the extra nuclear charge AZ of the solute atom (Cu, etc.) will attract the electrons, causing them to reside mainly in the d shells of nearest neighbor Ni atoms, thereby reducing their moments.

Ni—Cu, —Zn, and —Ga alloy powders were prepared by a reduction method1 and the cornpositions were checked by chemical analysis. The grain size of the powder is about 100g. NMR was studied by means of the spin echo method with the same apparatus as used for measurements on a Heusler alloy.2 Only the 6~Ni isotope with 1.2% natural abundance has a nuclear spin (I = 3/2). Most of the present experiments were carried out at 4.2°K, to ensure an undistorted resonance line.3 The sample was immersed directly into the liquid helium in order to avoid strong spurious resonance arising from powder vibrations excited by the rf magnetic field.

We present a nuclear magnetic resonance study of alloys of Ni with up to 10% Cu, Zn or Ga which supports this hypothesis; it appears that the hyperfine fields of Ni in all these alloys lie on one straight line when plotted as a function

In most cases the spectrum consists of a number of partly resolved lines. Figure la and b show the frequency spectra of the Ni and Cu nuclei respectively for a series of Ni—Cu alloys. At low concentrations, the resonances of the isotopes 63Cu(31%) and 6~Cu(69%)are observed separately because of their different gyromagnetic

of the average moment of the nearest neighbor magnetic atoms calculated under the above assumption.

ratios. Data for Ni—Zn alloys are shown in Fig. lc; resonance of the 4% abundant 67Zn was undetected. Figure ld shows the spectra of Ni—Ga alloys, which are complicated because of the

*

overlap of the resonances of Ni and Ga; the latter element occurs as 69Ga(60%) and 71Ga(40%). The hyperfine field of Ga is 23 koe.

This work was supported by the Joint Services Electronics Program (U.S. Army, U.S. Navy, and U.S. Air Force) under Grant No. AFOSR—69— 1622A. 1889

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NMR IN Ni—Cu, —Zn AND —Ga ALLOYS

Vol. 9, No. 22

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61Ni (a c, d), u (b) and FIG 1. Spin echo amplitude as a function of frequency in Ni-rich alloys of ~ 71Ga (d). The solid curve is the sum of Gaussians with widths — and strengths calculated for different numbers of Ni nearest neighbors (12, 11, . . .) as indicated by the vertical lines (a) 6tNi and (b) 63Cu (—) and 65Cu (---) in Ni—Cu alloys; (c) 61Ni in Ni—Zn alloys; (d) 61Ni (—), 69Ga (- --) and 71Ga (— — —) in Ni—Ga alloys.

We shall assume that the structure and broadening of the spectra of the alloys are caused by the differences in hyperfine fields of atoms having various numbers of nickel atoms as nearest neighbors.

neighbors is

The crystal structure of Ni alloys is f.c.c., in which each atom has twelve nearest neighbors. In an alloy with concentration x of solute atoms, the probability that an atom has n Ni nearest

intensities proportional to P~(x)and Gaussian line broadenings as indicated. The center frequency of each configuration is taken to be linear in n. The calculated spectra were obtained

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Each spectrum in Fig. la—d is simulated by a superposition of a set of discrete lines with

Vol. 9, No.22

NMR IN Ni—Cu, —Zn AND —~aALLOYS

by a Du Pont 310 gaussian curve resolver and are shown by solid lines. The accuracy in position of the lines of nuclei with 12 Ni co-

1891

608

at all concentrations. The intensity of the Ga ordination is better than ±0.2 MHz or ±0.5 koe resonance is much weaker than that of Ni; this may be caused by the large nuclear quadrupole moment of Ga in which case its relative intensity

kOe

would 5befine 0.4.structure In a previous due tostudy different on Ni—Cu isotopes alloys

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and nearest reported. As mentioned neighbor above, configurations we shall assume was notthat at low concentrations each impurity atom distri-

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butes its (AZ-n 3) excess electrons over the d shells of the 12(1 — x) nearest Ni neighbors. For the twelve Ni coordination the spin magnetic moment of nearest neighbor Ni atoms varies with x because of substitution of more distant Ni atoms by solute atoms. A Ni atoiv in the nearest neighbor shell has four nearest neighbors in that same shell which are certainly Ni; it has therefore only seven atoms as nearest neighbors which can be replaced at random b solute atoms. The number of Bohr magnetons per nearest neighbor Ni atom is then T(x) = n~(0) — —s-— (AZ-ri~). (1) 1 — x n~ In Fig. 2 the hyperfine fields of Ni and of Cu in twelve Ni coordination are plotted as a function of n~(x)together with the known hyperfine fields6 of these atoms dissolved in Co and Fe; all these points lie on two parallel straight lines (for n~ ~> 0.55 both the abcissa and the ordinate are reduced by a factor 30). For Co with g = 2.22~~= 1.55, resulting inns = 0.55. It is known that in iron alloys the magnetic moment per Fe atom does not change and there-

FIG. 2. Hyperfirie fields at the nickel nucleus (HN6) and at the copper nucleus (H~~) as a function of the average Bohr-magneton number n’ of the nearest neighbor atoms for Ni—Cu (o), Ni—Zn (.), Ni—Ga (:) alloys and cobalt and iron doped with nickel or copper (@). For nW~> 0.55 both the horizontal and vertical scales are reduced b~a factor 30. fore its value ~

=

1.98 (g-2. 15) is left Un-

corrected. In the 2% Ni—Cu alloy the reduction in the hyperfine field of Cu (48koe) by one nearest neighbor Cu atom is 3.6koe; this is about 1/12th. This observation confirms that the hyperfine field of Cu is mainly determined by the nearest neighbor Ni atoms. For the eleven Ni coordination, the reduction in hyperfine field of Ni is 2.5, 5.6, and 8.Okoe for 2% alloys of Cu, Zn, and Ga respec. tively, which is approximately proportional to AZ. The corresponding value observed in a Ni—Al alloy7 is 8.5koe which is about the same as that in the Ni—Ga alloy; both solute atoms have AZ = 3.

REFERENCES 1.

BEST R.J. and RUSSEL W.W., J. .4,n. Chem. Soc. 76, 828 (1954).

2.

OGAWA S. and SMIT J., J. Phys. Chem. Solids, 30, 657 (1969).

3.

STEARNS M.B., Phys. Rev. 162, 496 (1967).

4.

STAUSS G.H. and RUBINSTEIN M., J. app1. Phys. 37, 1238 (1966).

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NMR IN Ni—Cu, —Zn AND —Ga ALLOYS

Vol. 9, No.22

5.

ASAYAMA K., J. Phys. Soc. Japan, 18, 1727 (1963).

6.

KONTANI M. and ITOH J., J. Phys. Soc. Japan, 22, 345 (1967); KOl Y., TSUJIMURA A., HIHARA T. and KUSHIDA 1., J. Phys. Soc. Japan, 17, Suppi. B—I, 96 (1962).

7.

STREEVER R.L. and URIANO A.G., Phys. Rev. 149, 295 (1966).

Die Kern-Spinresonanz in Legierungen of Ni mit Cu, Zn oder Ga wurde gemesseri. Die Analyse der komplexen Spektren ergab em

Verbleiben der Uberschusselektronen in der Nãhe der Fremdatome.