Magnetic properties of the actinide elements and their metallic compounds

Abstract

A review is given of the magnetic properties of metallic actinides, and of the relationship between their properties and their electronic structures. It is shown that the 5f electrons of light actinides often form into bands, rather than exist as localised electrons as is usually the case for the 4f electrons of metallic rare earths. The 5f bands form due to 5f-5f wavefunction overlap or to hybridisation with s, p or d electrons. As a result of the band formation, a wide variety of magnetic phenomena may occur. These range from non-magnetic transition-metal behaviour, to spin fluctuations and itinerant magnetism, as well as localised 5f electron magnetism.Whereas the non-self-consistent properties of the pure metals are difficult to understand, a number of actinide alloys and intermetallic compounds have properties which are representative of well-characterised types of magnetic behaviour. These are most useful in explaining the behaviour of other systems by extrapolation.Speoial properties of the actinides are discussed which have limited quantitative understanding of actinide magnetism. Corresponding theoretical difficulties are discussed also. Emphasis is given to the properties of simple compounds as their systematic study has aided in the development of useful relationships to theoretical ideas.

Density functional study of the actinide nitrides

“…For non-interacting electrons, the electronic specific heat coefficient γ is proportional to the total density of states N(E F ) at the Fermi level and is given by But for UN and PuN, we observe a large discrepancy between our estimated values and experimental data. To match theoretical electronic specific heat coefficient γ to experimental values, it should be corrected as γ(corrected) = γ (band)*(1 + λ) where the parameter λ takes into account, the electron-phonon interactions and many body effects, and γ(band) is our computed value [37]. We intend to pursue such studies in the future.…”

Nature of the5fstates in actinide metals

“…Finally, spin fluctuations have been proposed as the reason for the anomalous low-temperature resistivity behavior of Pu that shows no magnetism in the metal (Nellis et al, 1970;Arko et al, 1972). Spin fluctuations can be thought of as spin alignments that have lifetimes less than ~10 -14 seconds and are therefore too short to see via specific heat, susceptibility, or nuclear magnetic resonance (Brodsky 1978).…”

“…In the case of the localized rare-earth metals, the atoms are usually in a unique ground state; however, this is not the case for the light actinides where delocalization and d-f mixing causes mixed-valence to occur. Indeed, as far back as the late 1970's this was understood, as evidenced by the following quote by Brodsky (1978): "In the case of the actinides (not unlike nickel) the ground state is nearly always a mixture of configurations, and it is only on rare occasions that the nearly equal energies of the 5f, 6d and 7s (also 7p) become separated and permit a single configuration to be the ground state." From Am on, the actinides exhibit a unique ground state due to localization of the 5f states.…”

Self-interaction corrected local spin density calculations of actinides

“…The actual electronic structure of these compounds is thus more complex than can be reproduced by either the localized or itinerant f limit of the SIC-LSD single Slater determinant wavefunction. If the true ground-state is fluctuating between a given set of configurations, as is the case for PuC (where the f 3 and f 4 configurations are close to being energetically equivalent), some intermediate degree of localization may be expected. The fact that f 3 gives the global energy minimum, and that localizing an additional f-electron is only slightly less favorable, might also indicate an intermediate scenario with three localized f-electrons and a strongly renormalized itinerant f-peak.…”

“…The electronic and magnetic properties of U compounds for example are very different from those of the corresponding Bk compounds [3]. Even for a given actinide element, its various alloys and compounds can display a wide spectrum of behavior, from localized to itinerant, due to the large effect that small changes in external or chemical pressure have on the f-electron contribution to the chemical bonding.…”