Uranium dioxide is one of the most studied actinide materials because of its widespread use as a nuclear fuel in reactors, especially in the high temperature regime at which reactors operate.
Interestingly, the first low-temperature measurements on UO₂ were performed under the cloak of the Manhattan Project, revealing a sharp anomaly in the specific heat around 30 K, indicating a phase transition which was suggested to be antiferromagnetic in nature.
Antiferromagnetism was first explained by Louis Néel in 1936. He described it as a type of magnetic order that prefers antiparallel alignment of spins or magnetic moments, as opposed to ferromagnetism in which the alignment is parallel. It took several decades and significant experimental and theoretical efforts to elucidate the exact spin structure of UO₂ and the type of antiferromagnetic order present.
UO₂ crystallizes in the face-centered cubic (fcc) calcium fluoride-type structure, which can be visualized as uranium atoms in a type of cubic lattice with an oxygen cage in the center (Fig. 1a). Above 30 K, UO₂ is a paramagnetic Mott insulator. The antiferromagnetic order of the uranium magnetic moments below 30 K can be described by three wave vectors (3k-type) with four spin-sub-lattices pointing along the body diagonals. This 3k antiferromagnetic order is accompanied by a Jahn-Teller distortion, a type of structural change which reduces symmetry and lowers the overall energy of the material. This directly impacts its properties as a material: the structural distortion is believed to be a substantial contributing factor to UO₂’s extraordinary poor thermal conductivity, which is more than fifty times lower than its isostructural, nonmagnetic analog ThO₂ at low temperatures.
Piezomagnetism in UO₂
The 3k magnetic order breaks time-reversal symmetry enabling a phenomenon called piezomagnetism—namely, magnetization that appears when a material is subjected to external pressure (piezo is the Greek word for “push” or pressure). Conversely, linear strain arises when the material is magnetized. This effect happens due to the tilting of the antiferromagnetic sublattices or a change in their relative magnitudes of magnetization. It is analogous to piezoelectricity where applied stress leads to an electric polarization and vice-versa.
Piezomagnetism in UO₂ was discovered in 2017 by magnetostriction measurements in pulsed magnetic fields up to 60 T at 2.5 K, performed at the National High Magnetic Field Laboratory (NHMFL), a pulsed field facility at Los Alamos National Laboratory (LANL). These types of measurements detect a change in sample length when a material is exposed to a magnetic field. Non-destructive resistive pulse magnets were used with a characteristic pulse duration of several tens of milliseconds. The characteristic features observed were linear magnetostriction (a change in sample length) and coercitivity (here, the ability of a material to withstand
Figure 1. (a) Calcium fluoride crystal structure of UO₂ with the U atoms (grey spheres) forming a cubic (fcc) lattice containing the O cage (red bonds/spheres) in the center. The arrows visualize the alignment of magnetic moments forming the 3k antiferromagnetic structure with the four magnetic sublattices in different colors. (b) Piezomagnetic butterfly-shaped hysteresis loop of UO₂ measured at 1.7 K in pulsed magnetic fields. The red and blue curves indicate opposite antiferromagnetic ordering vector L₀. Sharp transitions are observed at the coercive magnetic fields of ±18 T.
a magnetic field without inducing a flip of magnetic domains). These features led to a sharp reorientation of magnetic domains, creating a characteristic butterfly-shaped hysteresis loop (Fig. 1b). The width of the hysteresis extends to ±18 T, making UO₂ the strongest known piezomagnet.
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