Soft spin waves in the low-temperature thermodynamics of Pro_0.7Ca_0.3MnO₃

We present a detailed magnetothermal study of Pr_0.7Ca_0.3MnO₃, a perovskite manganite in which an insulator-metal transition can be driven by magnetic field, but also by pressure, visible light, x rays, or high currents. We find that the field-induced transition is associated with an enormous release of energy which accounts for its strong irreversibility. In the ferromagnetic metallic state, specific heat and magnetization measurements indicate a much smaller spin-wave stiffness than that seen in any other manganite, which we attribute to spin waves among the ferromagnetically ordered Pr moments. The coupling between the Pr and Mn spins may also provide a basis for understanding the low-temperature phase diagram of this most unusual manganite.