Influence of topology on the phase transition of a ferromagnetic metal

The topological ferromagnet CoS₂ exhibits an anhysteretic, weakly first-order transition at the Curie temperature of 119.8 K with a tricritical point µ₀H_tcp at 0.034 T. Magnetic symmetry and the mixing of majority and minority spin e_g bands at a subband crossing just above the Fermi level produce a topological component of the magnetization that leads to a negative M³ term in the Landau free energy. The position of the Fermi level relative to the subband crossing is critical for controlling the order of the transition. Hole doping in Co_0.89Fe_0.11S₂ drains the minority-spin e_g pocket and results in a normal second-order phase transition. Electron doping in Co_0.94Ni_0.06S₂ raises the Fermi level toward the subband gap, producing a strongly first-order transition with 15 K hysteresis. Our results demonstrate a relation between topological electronic structure and thermal hysteresis at the Curie point, which may help in the search for magnetocaloric materials.