Nuclear ferromagnetism and superconductivity at negative nuclear temperatures

Quantitative theory of the effect of nuclear ferromagnetism on the superconductivity of metals is proposed taking into account the electron-nuclear spin-spin interactions. At negative nuclear temperatures, when the nuclear magnetization is in opposition to an external magnetic field, nuclear ferromagnetism is favorable to superconductivity rather than suppressing it. The critical magnetic field in Be and TiH$_{2.07}$ hydrate metals may exceed the critical field of a nonmagnetic superconductor by an order of magnitude.