Low-Temperature Thermal Expansion ofγ-Irradiated Ruby

Measurements of the low-temperature thermal expansion of $\gamma$-irradiated rubies have been made with use of a three-terminal capacitance dilatometer. The data exhibit a positive Schottky-type anomaly at ∼3.9 K, not present in the data obtained for a pure ${\mathrm{Al}}_2$ ${\mathrm{O}}_3$ sample, and this contribution to the thermal expansion has been tentatively attributed to the presence of ${\mathrm{Cr}}^{2+}$ (produced by $\gamma$ irradiation) exhibiting a large positive magnetic Grüneisen coefficient as predicted by a dynamic Jahn-Teller model.