Investigating Evidence for a Kosterlitz-Thouless Transition in Fe/W(001) Ultrathin Films

Abstract

The magnetic susceptibility of 3-4ML ultrathin Fe/W(001) films was measured in situ under ultrahigh vacuum using the surface magneto-optic Kerr effect (SMOKE). Susceptibility measurements indicate that Fe/W(001) is a 2DXY system, and therefore undergoes a finite-size Kosterlitz-Thouless (KT) transition at the critical temperature T_KT. The films were grown using molecular beam epitaxy (MBE) and were characterized using Auger electron spectroscopy (AES) and low-energy electron diffraction (LEED). Three distinct categories of susceptibility signals were observed, and are referred to as Type I, II, and III. The primary difference between these signals is the size of the imaginary susceptibility, which likely corresponds to dissipative effects such as domain wall motion. The critical behaviour of the susceptibility in the paramagnetic region is described in the theory by χ(T) ~exp⁡〖〖(B/(T/T_KT-1) 〗^a)〗. A least-squares fit to this paramagnetic region from many independently grown films gives values of a=0.50±0.03 and B=3.48±0.16, which are in quantitative agreement with the KT theory. In comparison to 2nd order phase transitions, a power law fit to the paramagnetic region of the susceptibility yields an effective critical exponent of γ_eff≈3.7±0.7, which does not correspond to any known universality class.