Neutron studies of the effect of $A$-site chemical disorder on the ferromagnetic transition and spin dynamics for the magnetoresistive perovskite ${\mathrm{L}\mathrm{a}}_{0.54}{\mathrm{B}\mathrm{a}}_{0.46}{\mathrm{M}\mathrm{n}\mathrm{O}}_3$ are reported. The low temperature spin waves reveal that disorder reduces exchange interactions by only 9%. The development of a quasielastic peak in the spectrum below $T_{\mathrm{C}}$ and long-time relaxation of the order parameter indicate that the transition is discontinuous in the disordered sample, while it appears continuous for the ordered sample. These results strongly suggest that chemical disorder lowers the energy for polaron formation in manganese perovskites, and is the origin of the dramatic 50 K reduction in $T_{\mathrm{C}}$.

• Received 9 January 2004

DOI:https://doi.org/10.1103/PhysRevLett.93.267204