Magnetic properties of the reentrant ferromagnetic superconductor HoMo6S8

Neutron diffraction and ac susceptibility techniques have been used to study the magnetic and superconducting phase transitions in the ternary Chevrel phase system Ho${\mathrm{Mo}}_6$ ${\mathrm{S}}_8$. The material first becomes superconducting at $T_{c1\mathrm{}}=1.82\mathrm{}$ K. With further decrease of temperature ferromagnetic correlations develop, with a correlation range in real space which increases with decreasing temperature. On further cooling, a long-wavelength (∼230 Å) oscillatory magnetic state forms ($T_{\mathrm{os}}=0.71\mathrm{}$ K) in the superconducting phase, with a wavelength which increases with the application of a magnetic field. At still lower temperatures the superconductivity is destroyed ($T_{c2\mathrm{}}^{\mathrm{}\left(C\right)\mathrm{}}=0.612\mathrm{}$ K) as long-range ferromagnetic order sets in. The ferromagnetic phase at low temperatures is characterized by a saturated magnetic moment ${\mu }_z=9.06\mathrm{}\pm 0.3 {\mu }_B$, with the spins directed along the unique [111] trigonal axis. When the material is warmed from the ferromagnetic state, no oscillatory magnetic phase appears to form, with the system passing directly from ferromagnetism to superconductivity at $T_{c2\mathrm{}}^{\mathrm{}\left(W\right)\mathrm{}}=0.668\mathrm{}$ K.