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```math \boldsymbol{\sigma}(\mathbf{F}) = \frac{J^{-1}\mu J_m {\mathbf{B}^* }'}{J_m - \mathrm{tr}(\mathbf{B}^* ) + 3} + \frac{\kappa}{2}\left(J - \frac{1}{J}\right)\mathbf{1} ```