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-Å thin Ni(111) films from 300 to 630 K. As a function of temperature a huge peaklike broadening of the linewidth Δ${\mathit{H}}_{\mathit{p}\mathit{p}}$ is observed. From the thickness dependence of this peak we deduce the Curie temperatures ${\mathit{T}}_{\mathit{c}}$(d) of the respective films with thickness d, e.g., ${\mathit{T}}_{\mathit{c}}$(15 Å)=512 K=0.81${\mathit{T}}_{\mathit{c}}$(bulk). The shift exponent λ of the thickness dependence of ${\mathit{T}}_{\mathit{c}}$ is determined to be 1.42±0.3. The temperature dependence of Δ${\mathit{H}}_{\mathit{p}\mathit{p}}$ below ${\mathit{T}}_{\mathit{c}}$ is analyzed in terms of Δ${\mathit{H}}_{\mathit{p}\mathit{p}}$∝(1-T/${\mathit{T}}_{\mathit{c}}$ ${)}^{\mathrm{-}\mathrm{\beta }}$, with β the critical exponent of the spontaneous magnetization. β ranges from 0.38 for bulk Ni to 0.29 for 15-Å film.