Observation of Oscillatory Magnetic Order in the Antiferromagnetic Superconductor HoNi2B2C

The nature of the holmium order in the reentrant superconductor ($T_c=7.5\mathrm{} \mathrm{K}$) Ho${\mathrm{Ni}}_2$ ${\mathrm{B}}_2$C is revealed by neutron scattering. Upon cooling a transversely polarized oscillatory magnetic state is formed ($T_M\approx 8 \mathrm{K}$), characterized by a wave vector ($0,0,q_c$) where $q_c\approx 0.05$ ${\mathrm{\AA }}^{-1\mathrm{}}$ is only weakly dependent on temperature and field. At the reentrant superconducting transition (∼5 K) the amplitude of the oscillatory state abruptly decreases in favor of a commensurate antiferromagnet, whereby superconductivity is restored and coexists with antiferromagnetism at low temperatures.