Effect of surface phonons on the properties of the granular manganese oxideLa0.67Ca0.33MnO3

The effect of grain size on the colossal magnetoresistance mechanism is analyzed. While the metal-insulator transition temperature $T_{R\max }$ is markedly affected by the grain size, the magnetic transition temperature $T_c$ is not sensitive to the grain size. It is found that the surface phonon plays an important role in $T_{R\max }$ for samples with a small grain size. The coupling between the surface phonon and electron determines the kinetic energy of spin-polarized tunneling electrons which influences the ferromagnetic correlation between neighbor grains. The large discrepancy between $T_{R\max }$ and $T_c$ is explained by considering the different natures of magnetization and resistive measurements. Infrared spectra show clear experimental evidence of the surface phonon and reflect the variation of the electron-phonon coupling with different grain sizes.