Polarized Nuclei in Normal and Superconducting Rhodium

We have performed SQUID-NMR measurements on rhodium nuclei at ultralow temperatures. With initial polarizations of up to p = 0.95, the susceptibility data showed clear indications of an antiferromagnetic tendency, but nevertheless no signs of nuclear magnetic ordering. The lowest actually measured nuclear temperature was 280 pK, whereas the lowest nuclear temperatures achieved were below 100 pK. Double and triple-spin-flip resonance lines were detected, yielding direct information of the interactions between the nuclear spins. The superconducting transition was observed with the critical values, Tc = 210 μK and Bc(0) = 3.4 μT. Measurements with polarized nuclei were performed also in the superconducting state, where the effect of superconductivity on the spin-lattice relaxation rate was studied. The spin-lattice relaxation time was longer in the superconducting state at all temperatures.