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

The dc magnetic susceptibility of CoO·Al₂O₃·SiO₂ (14.3 at.% Co) below T_g=5.7 K decreases markedly with reduction in field for H<150 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.