Equation-of-state analysis of the effects of an induced staggered field on the phase transition ofCoF2

We have determined the field-scaling parameters of the magnetic specific heat $C_h$ of the three-dimensional (3D) Ising antiferromagnet ${\mathrm{CoF}}_2$ near $T_N$, in an induced staggered magnetic field $H_{\mathrm{st}}$. The measurements were made in external fields 0≤H≤1.92 T using a capacitance technique. The H=0 results yield values of α=0.109(6) and $A_{+}$/$A_{\mathrm{-}}$=0.538(6) for the critical exponent and amplitude ratio, respectively, in excellent agreement with renormalization-group (RG) predictions for the 3D Ising model. From the angular dependence and magnitude of the rounding of the phase transition with H, we find $H_{\mathrm{st}\mathrm{\propto }{H}_x{H}_y{H}_z}$, as was earlier predicted by Alben et al. The $C_h$ data for H>0 were analyzed using the so-called linear parametric model (LPM) for the equation of state, which has been shown to be correct to order ${\epsilon }^2$ in the RG theory by Wallace, but only for Ising systems. An excellent fit of the data to the LPM was obtained. From the field scaling of the data, a value of δ=4.94(13) is obtained for the critical isotherm exponent, in good agreement with the RG result δ=4.82(2). This is the first time δ has been experimentally determined for an antiferromagnet. For the optimum orientation of the magnetic field with respect to the crystalline axes, we obtained $H_{\mathrm{st}=1.0\mathrm{}\left(3\right)\mathrm{}}$ mT at H=1.92 T. A similar study in ${\mathrm{FeF}}_2$ shows that a maximum staggered field of only 0.024(13) mT is induced in this system by the application of the same uniform field.