In comparison to 3d or 4f metals, magnetism in actinides remains poorly understood due to experimental complications and the exotic behavior of the 5f states. In particular, plutonium metal is most especially vexing. Over the last five decades theories proposed the presence of either ordered or disordered local moments at low temperatures. However, experiments such as magnetic susceptibility, electrical resistivity, nuclear magnetic resonance, specific heat, and elastic and inelastic neutron scattering show no evidence for ordered or disordered magnetic moments in any of the six phases of plutonium. Beyond plutonium, the magnetic structure of other actinides is an active area of research given that temperature, pressure, and chemistry can quickly alter the magnetic structure of the 5f states. For instance, curium metal has an exceedingly large spin polarization that results in a total moment of about 8 Bohr magneton/atom, which influences the phase stability of the metal. Insight in the actinide ground state can be obtained from core-level x-ray absorption spectroscopy (XAS) and electron energy-loss spectroscopy (EELS). A sum rule relates the branching ratio of the core-level spectra measured by XAS or EELS to the expectation value of the angular part of the spin-orbit interaction.
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