Thickness-dependent Curie temperatures of ultrathin magnetic films: effect of the range of spin-spin interactions.

We present a simple model of spin-spin coupling which provides insight into the nature of the rapid decrease in the Curie temperature with decreasing thin film thickness n (number of monolayers). The shift of Curie temperature t(n) = 1-T(c)(n)/T(c)(infinity) follows the usual power law t(n) approximately n(-lambda) in thin films crossing over to linear behavior t(n) approximately n in the ultrathin film thickness limit. Experimental results for ferromagnetic thin films are compared, and shown to follow curves of t(n) with lambda values dependent on the nature of the spin-spin interactions.