Magnetic field-induced superconductivity in the ferromagnetic state of HoMo6S8

The Chevrel-phase HoMo/sub 6/S/sub 8/ is one of the few known ferromagnetic reentrant superconductors, having an upper superconductive critical temperature T/sub c1/ = 1.78 K, a Curie temperature T/sub m/ = 0.75 K, and reentering into a normal ferromagnetic state below T/sub c2/ = 0.64 K. After a brief survey of the characteristic properties of HoMo/sub 6/S/sub 8/, they present new results obtained by simultaneous resistance and magnetization measurements on a single crystal of HoMo/sub 6/S/sub 8/, performed in the whole temperature range 35mK less than or equal to T less than or equal to 4.2 K and for several orientations of the easy magnetization axis with respect to the applied magnetic field. Magnetic field-induced superconductive transitions are observed well below T/sub c2/ in the ferromagnetic state. Those transitions can be understood as the first experimental observation of a purely electromagnetic effect earlier suggested by Ginzburg: if the internal field is antiparallel to the spontaneous magnetization, the magnetic induction may be reduced below its critical value and a superconductive transition may take place. It is possible if exchange interactions are negligible; their results concerning the anisotropy of the internal upper critical field H/sub c2/ are consistent with that requirement. Theymore » also discuss the possible origins for the low-resistance state in zero field: it might be due to domain wall superconductivity.« less