Excess magnetic susceptibility arising from self damage in the

correlated electron systems a- and d-Pu

Scott McCall, Michael J. Fluss*, Brandon Chung, George Chapline,
Michael McElfresh, Damon Jackson

Lawrence Livermore National Laboratory, Livermore CA 94552 USA

*corresponding author: fluss1@llnl.gov

The f-electrons of Pu are delicately poised on the edge between localized and itinerant behavior. This transition is happening within the Pu phase diagram itself as evident by the large volume change from α-Pu to δ-Pu. In the case of a- and d-Pu, the electrons are nearly localized in a narrow f-band and the Pauli magnetic susceptibility is the largest of any element although there is no evidence of local moments or collective magnetism.

As a consequence of the unusual nature of plutonium’s electronic structure, point- and extended-defects are expected to, and do exhibit extraordinary properties. Low temperature magnetic susceptibility measurements on a- and d-Pu show that the magnetic susceptibility increases as a function of time, yet upon annealing the specimen returns to its initial magnetic susceptibility value. This suggests that the excess magnetic susceptibility (EMS), χ_xs(t,T), arises from the a-decay and U recoil damage cascades which produce vacancy and interstitials as point and extended defects. The time dependence of χ_xs is well described by χ_xs~χ_T(1-exp(-t/τ)) where χ_T and τ are both functions of temperature. This behavior leads naturally to a universal, time independent description of the excess susceptibility in terms of a growing aggregated region, or volume of influence, somewhat like a bubble, surrounding each damage cascade that eventually affects the entire specimen.

We have measured the magnetic susceptibility radiation-damage isochronal-annealing-curve for α- and δ-phase Pu 5<T<350K. For α-Pu, (Figure 1), specific annealing phases are seen that agree with earlier isochronal annealing resistivity data of Wigley [1]. Similar results will be reported for δ-Pu. In general, these annealing studies demonstrate that this excess magnetic susceptibility (χ_xs) is the result of the accumulated radiation damage. The location of the interstitial migration temperature also provides us with the temperature limits by which to characterize the time and temperature evolution of the accumulating damage cascades.

Figure 2 shows several isotherms for the accumulating EMS, χ_xs, in the α-Pu specimen. The time dependence of χ_xs is well described by a saturation-like behavior χ_xs ~ χ_T(1-exp(-t/τ)) where χ_T and τ are both functions of T that allows us to deduce a universal time independent susceptibility for the aggregated affected volume / region, χ_U.

This saturation picture also leads directly to a determination of the microscopic volume of the specimen that is affected by the frozen-in damage cascade. For our measurements in α-Pu we calculate from the data at 2K a diameter of ~200Å per damage cascade. This should be compared with an estimated volume that encloses the damage cascade (determined from molecular dynamics) of ~100Å. Hence the ratio of these volumes is ~8.

The observed anomalous behavior is a consequence of the highly correlated nature of the electrons. We are speculating that this may be a quantum Griffith’s effect and possible evidence for a nearby quantum criticality. We note that an even larger influenced volume is determined for the affected region in δ-Pu. Similarities with defects in hole-doped superconductors (F. Rullier-Albenque [2]) suggest a general phenomenon in strongly correlated electron systems, of which Pu may be a particularly unusual or special example.

It is also possible to anneal the accumulated radiation damage to produce defect populations with vacancy and small vacancy clusters as the dominant defect structure. We have done that at three successively higher temperatures and we have observed the temperature dependence of the χ_xs due to vacancy structures alone as compared to complex cascades. Additionally, examination in earlier work of the temperature dependence of the resistivity of vacancies in fcc delta phase Pu(3.3 at% Ga) is characterized by a ‑ln(T), behavior suggesting a Kondo-like impurity, local moments with a low critical temperature [3]. Again, this is similar in several ways to the notion of vacancies inducing a Kondo-like impurity region, described in the work reported by Rullier-Albenque et. al. [2] involving electron irradiation induced vacancy defects in hole doped superconductors.

This work was performed under the auspices of the U. S. DOE by Lawrence Livermore National Laboratory, under contract W-7405-Eng-48.

[1] D.A. Wigley, Proc. R. Soc. A 284 (1964) 344.

[1] F. Rullier-Albenque, H. Alloul, and R. Tourbot, Phys. Rev Lett., 91(4), 047001, 2003

[3] M.J. Fluss, B.D. Wirth, M. Wall, T.E. Felter, M.J. Caturla, A. Kubota, T. Diaz de la Rubia, Journal of Alloys and Compounds 368 (2004) 62–74