Lockin to Weak Ferromagnetism in TbNi2B2C and ErNi2B2C
  This article describes a model in which ferromagnetism necessarily accompanies a spin-density-wave lockin transition in the borocarbide structure provided the commensurate phase wave vector satisfies Q = (m/n)a* with m even and n odd. The results account for the magnetic properties of TbNi2B2C, and are also possibly relevant also for those of ErNi2B2C. 
M. B. Walker and C. Detlefs