Finite-size shift of the Curie temperature of ferromagnetic lanthanum cobaltite thin films

The Curie temperature T_c of ferromagnetic La_0.7A_0.3CoO₃ (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(∞)∝d^−λ, with a constant critical shift exponent of λ≈1 over the entire thickness range was observed. This value of λ 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 λ.