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 temperatures T = 280 pK and ''up'' to T = -750 pK . At T > 0, the static susceptibility follows the antiferromagnetic Cu rie-Weiss law with theta = -1.8 +/- 0.3 nK. At T < 0, the expected fer romagnetic behavior in the vicinity of T = -0 changes into antiferroma gnetic tendency around -6 nK. If only nearest and nest nearest neighbo r interactions are assumed, our data yield J(nn)/h = -17 +/- 3 Hz and J(nnn)/h = 10 +/- 3 Hz for the exchange interaction coefficients. We h ave also investigated the field and polarization dependence of the spi n-lattice relaxation time tau(1) at positive and negative nanokelvin t emperatures; surprisingly tau(1) is longer when T < 0. In fields below 1 mT, the Korringa constant shows a strong decrease, which becomes mo re pronounced as the conduction electron temperature decreases. Observ ed behavior is consistent with conduction electrons scattering from ma gnetic impurities, provided that the impurity-induced contribution to spin-lattice relaxation is proportional to the inverse of the nuclear spin temperature in small fields.