Superconductivity and ferromagnetism are antagonistic types of ordering, and their mutual effects give rise to several interesting phenomena which have recently been studied in rare earth compounds. A theoretical analysis shows that while a ferromagnetic superconductor is a type II superconductor near the superconducting transition point $T_\mathrm{c1}$, it becomes a type I superconductor near the ferromagnetic transition point $T_\mathrm{M}$. A new theory derived for the case $T_\mathrm{M}\ll T_\mathrm{c1}$ predicts the formation of a transverse domain-like magnetic structure near $T_\mathrm{M}$. In clean superconductors the electron spectrum is gapless. A change in the behavior from type II to type I upon cooling to TM has been observed experimentally in ErRh$_4$B$_4$. Experimental data on ErRh$_4$B$_4$, HoMo$_6$Se$_8$ and HoMo$_6$Se$_8$ prove the existence of superconductivity and a magnetic ordering below $T_\mathrm{M}$.
@article{Buzdin:1984,author = {A. I. Buzdin and L. N. Bulaevskii and M. L. Kulich and S. V. Panyukov},title = {Magnetic superconductors},publisher = {Physics-Uspekhi},year = {1984},journal = {Phys. Usp.},volume = {27},number = {12},pages = {927-953},url = {https://ufn.ru/en/articles/1984/12/b/},doi = {10.1070/PU1984v027n12ABEH004085}}