Magnetic Properties of Uranium Tetrafluoride

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Abstract
The magnetic susceptibility of a number of UF4 samples has been measured in the temperature range 1.3°—20.4°K, using the mutual inductance method of McKim and Wolf. Although some of the samples were highly purified, initial measurements indicated appreciable differences between their susceptibilities below 4.2°K. These differences were eventually overcome by subjecting the specimens to the same heat treatment just before susceptibility measurement. The effect of the heat treatment is not entirely understood, but a number of possibilities have been investigated. Theoretically, the main features of the susceptibility variation have been explained on a simple crystal field model of noninteracting ions, and this also accounts for the recent specific heat measurements of Burns, Osborne, and Westrum. An even better theoretical fit has been obtained by introducing a small, adjustable interaction between neighboring uranium ions. Below 2°K the measured susceptibility was anomalous: Samples which at higher temperatures were very similar (±1%), showed considerable differences (∼25%), the susceptibility in some cases increasing rapidly with decreasing temperature, and becoming dependent on measuring field. This behavior is unexplained but it is probably not a property of ``pure'' UF4.