Evidence for a New State in Aluminosilicate Spin-Glasses in Very Weak Magnetic Fields

The dc magnetic susceptibility of CoO·${\mathrm{Al}}_2$ ${\mathrm{O}}_3$·Si${\mathrm{O}}_2$ (14.3 at.% Co) below $T_g=5.7\mathrm{}$ K decreases markedly with reduction in field for $H<\mathrm{}150\mathrm{}$ mOe. The observation of this field sensitivity, equilibrium behavior, and a sharp cusp at $T_g$ in this very weak field range is evidence for a cooperative transition at $T_g$ below which correlations extend over many spins. This state is characterized by an energy scale per spin comparable to that of magnetic dipolar coupling between nearest-neighbor spins.