The magnetization of a quasisingle-crystalline ${\mathrm{La}}_{0.75}{\mathrm{Sm}}_{0.25}{\mathrm{Mn}}_2{\mathrm{Si}}_2$ sample was measured under high pressures to $P=1.2\mathrm{GPa}$. The compound exhibits four different spontaneous magnetic phase transitions: the ordering of Sm sublattice at a critical temperature $T_{\mathrm{Sm}}=14\mathrm{K}$, the ferro-antiferromagnetic phase transition at $T_{\mathrm{AF}}=150\mathrm{K}$, the Curie temperature of the ferromagnetic phase at $T_{\mathrm{C}}=305\mathrm{K}$, and finally, the transition to the paramagnetic state at $T_{\mathrm{P}}=403\mathrm{K}$. The changes of the critical temperatures of the spontaneous magnetic phase transitions under pressure are $-0.6$, $+203$, $-12$, and $\sim -60\mathrm{K}∕\mathrm{GPa}$, respectively. The temperature range of ferromagnetic phase shrinks with increasing pressure, and at $P⩾0.6\mathrm{GPa}$ no ferromagnetic ordering of Mn magnetic moments is observed. For the antiferromagnetic phase, application of a relatively low magnetic field produces a sharp first-order field-induced metamagnetic transition. The critical field of the metamagnetic transition depends on the direction of applied field and increases with increasing pressure. The nature of various magnetic phase transitions in $R{\mathrm{Mn}}_2{X}_2$ compounds is discussed using a thermodynamic analysis of the obtained results and a phenomenological model of the step change of $\mathrm{Mn}-\mathrm{Mn}$ interlayer exchange interaction at the transition.

• Received 17 May 2005

DOI:https://doi.org/10.1103/PhysRevB.72.064446