Susceptibility and relaxation measurements on rhodium metal at positive and negative spin temperatures in the nanokelvin range

We have measured the susceptibility of polycrystalline rhodium foils, down to nuclear spin temperaturesT = 280 pK and "up" toT = -750 pK. AtT > 0, the static susceptibility follows the antiferromagnetic Curie-Weiss law with θ = -1.8±0.3 nK. AtT < 0, the expected ferromagnetic behavior in the vicinity ofT = -0 changes into antiferromgnetic tendency around -6 nK. If only nearest and next nearest neighbor interactions are assumed, our data yieldJ_n/h = -17±3 Hz andJ_nnn/h = 10±3 Hz for the exchange interaction coefficients. We have also investigated the field and polarization dependence of the spin-lattice relaxation time τ₁ at positive and negative nanokelvin temperatures; surprisingly τ₁ is longer whenT< 0. Infields below 1 mT, the Korringa constant shows a strong decrease, which becomes more pronounced as the conduction electron temperature decreases. Observed behavior is consistent with conduction electrons scattering from magnetic impurities, provided that the impurity-induced contribution to spin-lattice relaxation is proportional to the inverse of the nuclear spin temperature in small fields.