Stability constraint for spin equation of state

A generalized Frenkel condition is proposed for use in spin hydrodynamics to relate the spin density and spin polarization (or spin chemical potential) tensors. It allows for independent treatment of electric- and magnetic-like components of the spin density tensor, which helps to fulfill the stability conditions recently derived in the literature. The generalized Frenkel condition extrapolates between the original Frenkel condition, where only the magnetic-like part of the spin tensor is present, and the case where the spin density tensor is directly proportional to the spin polarization tensor. We also demonstrate that our approach is supported by the result of a microscopic calculation.