Finite-size shift of the Curie temperature of ferromagnetic lanthanum cobaltite thin films -: art. no. 092406

The Curie temperature T-c of ferromagnetic La(0.7)A(0.3)CoO(3) (A=Ca, Sr, Ba) thin films was studied as a function of the film thickness d for 400 nm>d >= 2.6 nm. A finite-size shift of the critical temperature relative to the bulk value, 1-T-c(d)/T-c(infinity) proportional to d(-lambda), with a constant critical shift exponent of lambda approximate to 1 over the entire thickness range was observed. This value of lambda is rather unusual for thick films (d > 100 nm) and normally expected for two-dimensional magnetic structures (i.e., ultrathin magnetic films with a thickness of a few monolayers) or spin-glass systems. Measurements of the magnetoresistivity reveal spin-dependent variable range hopping in the ultrathin films, which is strongly suppressed with increasing film thickness by percolation. The occurrence of charge-carrier localization and spin-misorientation hints to an exchange interaction between isovalent Co ions and thus to a phase-separation of ferromagnetic and antiferromagnetic regions, which may be the reason for the observed unusual critical shift exponent lambda.