Microscopic spin interactions in the CMR manganites

Using inelastic neutron scattering we measured the microscopic magnetic coupling associated with the ferromagnetic (FM) clusters of the “colossal magnetoresistance” (CMR) compound Pr0.70Ca0.30MnO3. 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 indicates that the probed regions undergo a firstorder transition from an insulating to a metallic state, and that there are no FM metallic domains in the insulating region. We argue that the I-M transition in the CMR manganites is more complex than the simple percolation of large FM metallic clusters.