A Mystery at Cryogenic Temperatures
Scientists have discovered a fascinating secret about praseodymium aluminide. When PrAl2 is cooled, its crystal structure changes from high symmetry cubic to low symmetry tetragonal below -400 °F (32 K). However, when the cooling is done in a high magnetic field, the material retains the cubic structure. This change is not observed in other rare-earth aluminides. Furthermore, PrAl2 has an anomalous heat capacity per unit mass at low temperatures. It is 10x higher than pure praseodymium. (Heat capacity is the amount of heat needed to raise the temperature of a material by one degree.) First principle calculations confirm the mysterious behaviors originate from an unexpectedly strong competition between the interactions of magnetic (4f) moments mediated by the sea of conduction electrons in PrAl2 and the splitting of the energy levels of the 4f electrons of the rare-earth atoms in both the cubic and tetragonal lattices. The structure’s flexibility may yield practical applications, including magnetostrictive transducers and magnetoresistive sensors, and may serve as a foundation for energy efficient and environmentally friendly magnetic cooling for consumer use.