Critical spin fluctuations and Curie temperatures of ultrathin Ni(111)/W(110): A magnetic-resonance study in ultrahigh vacuum

Ferro- and paramagnetic resonance is performed in situ in ultrahigh vacuum on 15- to 40-{} thin Ni(111) films from 300 to 630 K. As a function of temperature a huge peaklike broadening of the linewidth {Δ}Hₚₚ is observed. From the thickness dependence of this peak we deduce the Curie temperatures Tc(d) of the respective films with thickness d, e.g., Tc(15 {})=512 K=0.81Tc(bulk). The shift exponent {λ} of the thickness dependence of Tc is determined to be 1.42±{}0.3. The temperature dependence of {Δ}Hₚₚ below Tc is analyzed in terms of {Δ}Hₚₚ{∝}(1-T/Tc{)}^{{{-}}{{β}}}$, with {β} the critical exponent of the spontaneous magnetization. {β} ranges from 0.38 for bulk Ni to 0.29 for 15-{} film.