Spontaneous Magnetization in a Mixed-Valent Rare-Earth Compound

Clear experimental evidence is given for the first observation of a spontaneous magnetization in a mixed-valent rare-earth compound at low temperatures. The magnetic susceptibility, the magnetization, the magnetostriction, and the electrical conductivity of Tm${\mathrm{Se}}_{0.83}$ ${\mathrm{Te}}_{0.17}$ reveal a transition to a magnetically ordered state with a net magnetic moment below 5.5 K. This is in quantitative agreement with Varma's recent predictions based on a microscopic model for mixed-valent systems.