Thickness and strain effects on electronic transport and Curie temperature in La0.67Ca0.33MnO3 films

La0.67Ca0.33MnO3 films with thickness between 25 and 400nm were fabricated on (001) SrTiO3 and (001) LaAlO3 substrates, using pulsed laser deposition. The magnetization and resistivity p of the films were studied as functions of thickness. The reduction of t causes the decrease of the Curie temperature T-C in addition to the increase of p of the films. Compared with the compressive strain in the La0.67Ca0.33MnO3/LaAlO3 films I the tensile strain in the La0.67Ca0.33MnO3/SrTiO3 films causes a faster reduction of T-C with t, and a larger value of rho(0). The low-temperature rho is found to follow well rho = rho(0) + Bomega(s)/sinh(2) (homega(s)/2k(B)T) + CTn, with rho(0) being the residual resistivity. The second term arises from the scattering by a soft optical phonon mode, and the third term denotes the other possible transport mechanisms. B, omega(s) ( the frequency of a soft optical mode) and C are fitting parameters. At temperatures higher than T-C, rho of all the films show activated conduction with rho = DTexp(E-a/k(B) T). Here, E-a is the activation energy. Based on the investigation of rho(0), omega(s) and E-a,E- the thickness and strain effects in the films are discussed, separately. Further study shows that the variation of T-C is mainly associated with E-a and omega(a), confirming the crucial role of the electron-phonon coupling in the strain effects on T-C.