NUCLEAR MAGNETIC RESONANCE AND RELAXATION IN U1-xPuxAl2

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NUCLEAR MAGNETIC RESONANCE AND RELAXATION IN U1-xPuxAl2

F. Fradin, M. Brodsky, A. Arko

To cite this version:

F. Fradin, M. Brodsky, A. Arko. NUCLEAR MAGNETIC RESONANCE AND RELAX- ATION IN U1-xPuxAl2. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-905-C1-906.

�10.1051/jphyscol:19711321�. �jpa-00214354�

JOURNAL

DE

Colloque C 1, suppliment au no 2-3, Tome 32, Fkvrier-Mars 1971, page C 1 - ⁹⁰⁵

NUCLEAR MAGNETIC RESONANCE AND RELAXATION IN U, -xPuxAI, (*)

By F. Y. FRADIN, M. B. BRODSKY, and A. J. A R K 0 Argonne National Laboratory, Argonne, Illinois

Rbum6.

Resonance et relaxation magnktique nuclkaire de 27A1 dans UI-~PuxAlz sont examinks en termesde localisation des ktats 5 f virtuels des ions Pu. On trouve la tempkrature de la transition magnktique quand X

0.3 et vers 5

Abstract. - 27Al nuclear magnetic resonance and relaxation in U I - ~ P U ~ A ~ Z is discussed in terms of the formation of virtual bound 5 f states on the Pu ions. Magnetic order is found for X

0.3 at about 5

The 5 f electron elements U, Np, Pu, etc., form an interesting class of alloys and intermetallic compounds. Whereas the pure metals do not appear to have localized magnetic moments nor magnetic ordering at low temperature [I], the intermetallic compounds of the actinides with the Group IVY V, and VI nontransition elements generally have localized moments and many types of magnetic ordering 121.

The different behavior is due to the fact that the spin- orbit interaction and the crystal-field interaction are of the same order of magnitude [3].

In order to study the transition of the 5 f electrons from band-like to localized in nature, we have measu- red the nuclear magnetic resonance (nmr) of 27A1 in the cubic Laves phase intermetallic coumpounds U, -xPuxA12, where 0 < X < 1. Preliminary measu- rements of the susceptibility and resistivity of U1-xPuxA12 have been reported by Arko et al. [4].

In the concentration range 0.3 5 X 5 1.0 the suscep- tibility follows a Curie-Weiss law (the paramagnetic moment per Pu atom dropping from 3.8 Bohr magne- tons at X

0.3 to 1.2 Bohr magnetons at X

1.0), whereas in the range 0 < ^X < 0.3 the susceptibility has a temperature dependence similar to that found by Gossard et al. [5] in UAl,.

Nmr measurements were made with a phase cohe- rent pulsed spectrometer capable of delivering 100 Oe radio-frequency field and a Varian electromagnet with Fieldial control. For UAl, and Uo~,Puo.,A12 we find sharp nmr spectra down to about 4 OK. The spec- tra clearly show first and second order quadrupole shifts but no magnetic broadening. The Uo~7Pu,.,Al, spectrum shows a rapid broadening followed by a vanishing signal on cooling from about 6 to 5 OK.

For 0.5 5 X d 1.0, the resonances broaden gradually on cooling over about a 10 OK temperature range.

The temperature of the initiation of the broadening varies in roughly a linear manner from about 16 OK at X

0.5 to about 32 OK at X = 1 .O. The extremely broad spectrum for the X

0.7 alloy vanishes shar- ply at the - 40K ordering temperature. The spin- spin relaxation time T,, measured by the 4 2 - z - ⁿ

Hahn echo pulse sequence, is found to be at least one order of magnitude larger than the reciprocal line

(*)

This work was

under

auspices of the U.

Atomic Energy Commission.

widths in the temperature regions of extreme broaden- ing, indicating that the broadening is due to an inhomogeneous magnetic interaction. The tempera- ture range of the, transition from resolved to broade- ned nmr spectra is in agreement with the temperature for anomalous kinks in the susceptibility and resis- tivity [4].

The Knight shift K versus susceptibility x is linear above the magnetic broadening temperature. In figure 1 the slope a of the K versus x curve as well as the

2 1 I I I I I 1 I I 0

.2 .4 .6 8 1.0

U

FIG.

- Composition dependence of

=

( K

O), and

for UI .xPuxA~z.

intercept x ^{(K =} 0) is given as a function of composi- tion. We have also plotted x,,,, the second order van Vleck temperature independent susceptibility, obtained in the manner of Gossard et al. [5]. It is clear that

x ^{(K = 0) and} x,,, have the same qualitative composi- tion dependence and thus the uncertainties in deter- mining x,,, ^form x (K

0) need not concern us here.

In figure 2, we have plotted the product of the electric field gradient q times the cube of the lattice parameter a, at 300 OK as a function of composition.

(The aluminum atoms have trigonal point symmetry.) Also shown in figure 2 is the quantityzR = (TI T)-I which is the spin-lattice relaxation rate times the reci- procal temperature. The data shown were obtained at 770K. Preliminary results at lower temperature indicates that R increases as T decreases, especially for X > ^0.3.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711321

C

-

F. Y. FRADIN, M. B. BRODSKY AND A.

A R K 0

A q a: AT 300PK

(T, TI-' AT 77*K OLD SAMPLES (T, T)" AT 77'K NEW SAMPLES

U Atg PU A12

FIG.

- Composition dependence of qai at 300

and R = ^{(TI T)-1} at 77 OK for Ul . x P ~ x A l a .

Watson et al. [6] have theoretically shown that q/qlattice is proportional to the bare density of states at the Fermi level. Here qlattic, is the electric field gradient as calculated from, a point charge model.

Thus, since qlatti,, is proportional to a i 3 , the composi- tion dependence of qa; should indicate in a qualita- tive sense the composition dependence of the bare density of states. Following Gossard et al. [5], one expects that x,,, is roughly proportional to the inverse band-width of the 5 f electron states. Both a and R have complicated dependencies on the spin density and spin fluctuations, respectively, at the aluminum sites. Both a and R are expected [7] to increase as the enhanced density of states at the Fermi level increases and as the mixing between the Sf-states and the other conduction band states increases.

A model is now presented to qualitatively explain the nmr results in Ul-xP~xA12. A band model with a peaked 5 f band at the Fermi level has been used by Refer [I] Ross (J. W.) and LAM (D. J.), Phys. Rev., 1968, 165,

617 ; DUNLAP ^(B. D.), BRODSKY (M. B.), SHE-

NOY

(G. K.) and KALVIUS (G. M.), Phys. Rev., 1970,1,44 ; FRADIN (F. Y.) and BRODSKY (M. B.), International Journal of Magnetism, to be published.

[2] FRADIN (F. Y.), Proc. of the IV International Confe- rence on Plutonium and Other Actinides, Santa Fe, New Mexico, 1970, 264.

[3] CHAN (S. K.) and LAM (D. J.), Proc. of the IV Inter-

Gossard et al. [5] to explain the temperature depen- dence of the Knight shift and the susceptibility in UAl,. If we use a rigid band model for UA1, with small plutonium additions of up to X = 0.1, the additional two electrons of the dissolved plutonium will move the Fermi level to higher energy. Thus, the Fermi level is shifted off the peak in the density of states of UA1, and the 5 f band is effectively broadened. At about X

0.3, a narrow spin split virtual 5 f state appears on the plutonium atoms. One spin state is full and the other empty. This narrow state gives rise to the large

x,,, and since the state does not intersect the Fermi

level the density of states can continue to drop.

Correlation effects in this narrow state on the Pu can yield the Curie-Weiss behavior 181, magnetic broaden- ing of the nmr line, and the antiferromagnetic transi- tion at low temperature (where the coupling between localized states is greater than the thermal energy).

From X

0.3 to X

0.7 the virtual 5 f state broadens somewhat due to 5 3 5 f as well as Sf-conduction electron interactions. For X > 0.7, the 5 f state broa- dens sharply decreasing x,,,. The 5 f state now inter- sects the Fermi level increasing qa; and a = AKlAx.

At X = 1.0, the spin split 5 f states are largely over- lapping, causing a peak in the density of states and a minimum in x,,,.

A comparison of qa!, x ( K

O), and a for UAl, and PuAl, indicates that the former compound has a larger bare density of states at the Fermi level and the latter compound appears to have a greater mixing of the 5 f-states with the plane wave states. The most dramatic difference is the large magnetic broadening of the nmr line in PuA1, below about 30 OK which is absent in UA1,.

Acknowledgement.

The authors would like to thank Mr. John Downey for experimental assistance.

ences

national Conference on Plutonium and Other Actinides, Santa Fe, New Mexico, 1970, 219.

[4] ARKO (A. J.), BRODSKY (M. B.) and NELLIS (W. J.), Bull. Am. Phys. Soc., 1970, 15, 293.

[5] GOSSARD (A. C.), JACCARINO (V.) and WERNICK (J. H.), Phys. Rev., 1962, ^128, 1038.

[6] WATSON (R. E.), GOSSARD (A. C.) and YAFET (Y.), Phys. Rev., 1965, 140, A375.

[7] FRADIN (F. Y.), J. Phys. Chem. Solids, to be published.

[8] LEVINE (M.) and SUHL (H.), Phys. Rev., 1968,171, 567.