Pressure-induced tricritical point and critical behavior near the points of phase-diagram wings in a CoS$_2$ ferromagnet

A pressure-induced change in the order of magnetic phase transition observed for CoS$_2$ in [1] is explained using the exchange-striction ferromagnet model allowing for the first- and second-order magnetoelastic interactions. It is shown that this model offers a satisfactory quantitative explanation of the majority of experimental facts observed in CoS$_2$. The magnetic phase diagram of CoS$_2$ is calculated in the vicinity of a tricritical point in the temperature ($T$), pressure ($P$), and magnetic-field ($H$) variables. The critical behavior of the thermodynamic quantities near the tricritical points of phase-diagram wings is discussed.