High-resolution neutron powder diffraction, synchrotron x-ray diffraction, and magnetometry were used to investigate the structural and magnetic properties of ${\mathrm{Bi}}_{1-x}{\mathrm{Sm}}_x{\mathrm{Fe}}_{0.94}{\mathrm{Ti}}_{0.06}{\mathrm{O}}_3$ ($x=0.1,0.12$) perovskites near the morphotropic phase boundary. Both compositions exhibit coexisting rhombohedral ($R3c$) and antipolar orthorhombic ($Pbam$) phases with nearly constant phase fractions in the temperature range of 1.5 to 300 K. Anomalous behavior of low-temperature magnetic properties, including the change in the weak ferromagnetic state and exchange-bias effects, cannot be explained by tiny variations in the phase content. The magnetic response correlates with temperature-dependent local distortions of the ${\mathrm{FeO}}_6$ octahedra, particularly tilting, rotation, and bond-angle variations occurred in the orthorhombic Pbam phase. The distortion parameter $\mathrm{\Delta }$ shows a sharp increase at low temperatures, correlating with the evolution of magnetization. The local structural distortions strengthen Dzyaloshinskii-Moriya interactions, promoting spin canting and weak ferromagnetism. The results underscore the decisive role of nanoscale lattice distortions over the macroscopic phase transitions in controlling the magnetic ground state of ${\mathrm{Sm}}^{3+}$- and ${\mathrm{Ti}}^{4+}$-co-doped ${\mathrm{BiFeO}}_3$ multiferroics.

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• Dzyaloshinskii-Moriya interaction
• Exchange bias
• Magnetism
• Multiferroics