Ferromagnetic order and the critical exponentγfor a Gd monolayer: An electron-spin-resonance study

A monolayer of Gd(0001) on W(110) is measured by electron-spin resonance from T=240 to 360 K in UHV. The ferromagnetic Curie temperature lies ≊20 K below the bulk Curie point. The measured temperature dependence of the static susceptibility—deduced from the ESR intensity—follows a power law χ∼${\mathrm{t}}^{\mathrm{-}\gamma }$ with γ≊1.8 for a monolayer and γ≊1.25 for an 80-Å film. This agrees well with the theoretical γ of 2D and 3D Ising systems. The experiment represents the first in situ UHV ESR with full surface analysis allowing the measurement of magnetic phase transitions on single-crystal surfaces.