Divergences of the Magnetic Properties of CrBr3 near the Critical Point

We have used the Faraday effect to study the magnetic properties of chromium tribromide along several isotherms near the critical point. Using external fields up to 1000 G, we have measured the reduced magnetization σ from 0<σ<0.25 along 21 isotherms from 400 mdeg K below Tc to 2.2 deg above Tc. For T>Tc the susceptibility along the critical isochore (σ=0) diverges like (T/Tc-1)−γ with γ=1.215±0.02. Along the critical isotherm σ∼H1/δ with δ=4.28±0.1, and along the coexistence curve, σ goes to zero as (1-T/Tc)β with β=0.368±0.005. The value of δ agrees with the scaling law prediction δ=γ/β+1. Our data are consistent with an equation of state of the form proposed by Widom: viz. H=σΦ[(T/Tc-1), σ1/β], where Φ is a homogeneous function of degree γ of its arguments. This experiment is the first confirmation of the scaling laws for an insulating ferromagnet.