Equation-of-state analysis of the effects of an induced staggered field on the phase transition of CoF2.

Abstract

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\ensuremath{\le}H\ensuremath{\le}1.92 T using a capacitance technique. The H=0 results yield values of \ensuremath{\alpha}=0.109(6) and ${A}_{+}$/${A}_{\mathrm{\ensuremath{-}}}$=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 ${H}_{\mathrm{st}\mathrm{\ensuremath{\propto}}{H}_{x}{H}_{y}{H}_{z}}$, 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 ${\ensuremath{\epsilon}}^{2}$ 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 \ensuremath{\delta}=4.94(13) is obtained for the critical isotherm exponent, in good agreement with the RG result \ensuremath{\delta}=4.82(2). This is the first time \ensuremath{\delta} has been experimentally determined for an antiferromagnet. For the optimum orientation of the magnetic field with respect to the crystalline axes, we obtained ${H}_{\mathrm{st}=1.0(3)}$ 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.