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(theta) 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 less than or equal to T less than or equal to 99 K. The validity of our approach is demonstrated by the perfect agreement of magnetic specific heat data as calculated from g(theta) 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.