Density of Zeros on the Lee-Yang Circle Obtained from Magnetization Data of a Two-Dimensional Ising Ferromagnet

In order to provide experimental access to the statistical theory of Lee and Yang [Phys. Rev. 87, 410 (1952)] the density function g(θ) of zeros on the Lee-Yang circle has been determined for the first time by analyzing isothermal magnetization data $m(H)$ of the Ising ferromagnet FeCl₂ in axial magnetic fields H at temperatures 34≤T≤99K. The validity of our approach is demonstrated by the perfect agreement of magnetic specific heat data as calculated from g(θ) and $m(H)$ via Maxwell's relation. Moreover, the correct in-plane exchange constant of FeCl₂ emerges from the series expansion of $m(H)$ involved in the analysis.