Finite-size scaling in band ferromagnets with non-universal critical behavior

Abstract

The ultra-high sensitivity of the ferromagnetic resonance (FMR) technique has been fully exploited to study the finite-size effects in the critical region near the ferromagnetic to paramagnetic phase transition in Cr_75-xFe_25+x(x = 0, 5) thin films of high structural and magnetic quality. Conclusive experimental evidence is provided for the validity of finite-size scaling. Irrespective of the film thickness and composition, the critical exponents β, γ and ν for spontaneous magnetization, initial magnetic susceptibility and spin-spin correlation length retain their bulk values so that no dimensionality crossover occurs within the film thickness range covered in the FMR experiments. The present results indicate that (i) like Cr_75-xFe_25+x, the previously studied Fe, Co, Ni, and CoNi₃ thin films behave as itinerant-electron (band) ferromagnets in which the isotropic long-range interactions between spins decay as J(r)~r^-(d+σ) (σ>0), and (ii) the lattice dimensionality d, spin dimensionality m, and range of spin-spin interactions (via the material-specific parameter s) decide the (non-universal) values of the critical exponents.