Microscopic spin interactions in colossal magnetoresistance manganites

Using inelastic neutron scattering we measured the microscopic magnetic coupling associated with the ferromagnetic clusters of the “colossal magnetoresistance” compound ${\mathrm{Pr}}_{0.70}{\mathrm{Ca}}_{0.30}{\mathrm{MnO}}_3$. When the insulating-to-metal (I-M) transition is induced by an external magnetic field there is a discontinuous change in the spin-wave stiffness constant. This result implies that the I-M transition is not achieved by the simple percolation of micron-sized metallic clusters as currently believed, but involves a first-order transformation.