Finite-size Kosterlitz-Thouless transition in Fe/W(001) ultrathin films

The Kosterlitz-Thouless (KT) transition involves the unbinding of topological excitations (vortex-antivortex pairs) in an infinite, isotropic, 2DXY system. In real 2D ferromagnetic films, it is not clear that such a transition occurs in the presence of crystalline anisotropy and finite size effects. We report MOKE magnetic susceptibility measurements χ(T) of 4-fold in-plane ultrathin Fe/W(001) films, where a prominent peak is observed. Above the peak temperature, the paramagnetic susceptibility is in excellent agreement with the form expected for a vortex-antivortex gas: χ(T) = χ₀ exp[B/(T/T_KT -1)^a] , with a = 0.50±0.03 and B = 3.48±0.16 . In accord with finite size KT theory, the KT transition temperature,T_KT, is tens of K below the susceptibility peak, giving a “finite size” consistent with μm magnetic domains. Fitting instead to a power law, as would be consistent with a 2^nd order transition, gives unphysical parameters γ_eff = 3.7±0.7 and a Curie temperature T_γ far below the peak temperature. Below the peak, the measured susceptibility has a complicated behaviour qualitatively consistent with the re-emergence of 4-fold anisotropy and magnetic domains.