Magnetic order-disorder phase transition in Cr70Fe30 thin films

Dynamic magnetic response of Cr70Fe30 thin films with thickness in the range 11 nm ≤ t ≤ 978 nm and composition above the critical concentration for ferromagnetism was determined by measuring linear (χ1) and nonlinear (χ2 - χ5) ac-magnetic susceptibilities over three decades of frequency (101 - 104 Hz) in the temperature range 2-300 K. An elaborate analysis of χ1(T), measured in the absence or presence of a superposed dc magnetic field, Hdc, reveals that the asymptotic critical behavior of the films with t ≥ 21 nm is that of a three-dimensional (3D) isotropic dipolar ferromagnet. Interplay between long-range dipole-dipole and short-range exchange interactions causes a crossover to the 3D isotropic Heisenberg critical behavior when the temperature is increased above the Curie temperature, TC, outside the asymptotic critical region (ACR). By contrast, in the ACR, the film with t = 11 nm behaves as a 3D Ising ferromagnet. TC decreases with film thickness in accordance with the finite-size scaling with the value for the spin-spin correlation length exponent that corresponds to the 3D isotropic dipolar universality class. In the films with t ≤ 42 nm, besides the macroscopic length scale of the ferromagnetic order in the static limit, there exists a length scale that corresponds to finite spin clusters, whose dynamics is spin glass-like.