Three-stage Phase Transitions and Field-induced Phases in CeCoSi:A Landau Theory

We investigate both the nonmagnetic and magnetic ordered phases of CeCoSi using Landau theory. Our analysis predicts three successive phase transitions at zero magnetic field. A quadrupole order parameter that emerges below $T_0=12$ K acts as a weak symmetry-breaking field on the antiferromagnetic ordering below $T_{\rm N}=9.4$ K, leading to two-stage magnetic transitions. In the higher-temperature antiferromagnetic phase within the range $T_{\rm s2}=8~{\rm K}<T<T_{\rm N}$, an out-of-plane component of the antiferromagnetic moment may or may not be present. If present, magnetic fields applied along the [100] and [110] directions induce additional magnetic phases.