EPR determination of the critical exponent of ${\mathrm{Mn}}^{2+}$-doped (${\mathrm{NH}}_4$${)}_2$${\mathrm{SO}}_4$ below the ferroelectric phase transition

The ${\mathrm{Mn}}^{2+}$ EPR line splitting in (${\mathrm{NH}}_4$ ${)}_2$ ${\mathrm{SO}}_4$ has been exploited to determine the critical exponent β for the electric polarization below the ferroelectric phase-transition temperature. The estimated value of β=0.5 is consistent with the exactly solvable spherical model for spatial dimension d=3.