Onset of antiferromagnetism in heavy-fermion metals

There are two main theoretical descriptions of antiferromagnets. The first arises from atomic physics, which predicts that atoms with unpaired electro ns develop magnetic moments. In a solid, the coupling between moments on ne arby ions then yields antiferromagnetic order at low temperatures(1). The s econd description, based on the physics of electron fluids or 'Fermi liquid s', states that Coulomb interactions can drive the fluid to adopt a more st able configuration by developing a spin density wave(2,3). It is at present unknown which view is appropriate at a 'quantum critical point', where the antiferromagnetic transition temperature vanishes(4-7). Here we report neu tron scattering and bulk magnetometry measurements of the metal CeCu6-xAux, which allow us to discriminate between the two models. We rnd evidence for an atomically local contribution to the magnetic correlations which develo ps at the critical gold concentration (x(c) = 0.1), corresponding to a magn etic ordering temperature of zero. This contribution implies that a Fermi-l iquid-destroying spin-localizing transition, unanticipated from the spin de nsity wave description, coincides with the antiferromagnetic quantum critic al point.