Observation of cooperative nuclear magnetic order in PrCu2 below 54 mK

Magnetic-susceptibility and specific-heat measurements in Van Vleck paramagnetic PrCu₂ down to 30 mK reveal that the Pr nuclei enter an antiferromagnetically ordered state below 54 mK. This high nuclear ordering temperature results from magnetic exchange interactions between the Pr ions, which must be close to the critical value necessary for spontaneous electronic magnetic order in this compound.