Evidence for Nuclear Antiferromagnetism in Copper

We have studied, in a two-stage nuclear demagnetization cryostat, the NMR properties of copper nuclei in magnetic fields from 0 to 15 mT and at polarizations up to 0.9. In zero field the spin entropy decreases less rapidly with temperature than expected for an interacting paramagnet with a constant local field; at 50 nK the entropy is still 45% of $R\ln 4$. The temperature dependence of magnetic susceptibility points to nuclear antiferromagnetism in copper.