Stabilization of ferromagnetism in BiFeO$_3$:Ho at hydrostatic pressure

The isothermal magnetization of the Bi$_{1-x}$Ho$_x$FeO$_3$ ($x=0 - 0.2$) multiferroic has been studied at a hydrostatic pressure up to $9$ GPa in the range of room temperatures. A new anomaly at $P_C\approx3.81$ GPa related to intermediate phases between the structural transition $R3c\to Pnma$ has been found against the background of the pressure-induced antiferromagnetic ordering in BiFeO$_3$ (BFO) at $P\approx2.59$ GPa. It is established that the ferromagnetic behavior under pressure depends on the Ho impurity concentration: $P_C$ decreases at $0.05 \leqslant x\leqslant 0.1$ because of the decrease in $R3c$ bond lengths in the structure, and the stabilization of ferromagnetism is implemented at $0.1\leqslant x\leqslant 0.2$ probably because of the coexistence of the $R3c$ and $Pnma$ phases. The results of studies indicate that, in Bi$_{1-x}$Ho$_x$FeO$_3$ with $x=0.2$, the transition pressure $P_C=3.7$ GPa exceeds the values for BFO doped with other $4f$ elements (Eu, Y, Sm) in the region $R3c\to Pnma$ of the transition.