Morin-type spin-reorientation transition below the Néel transition in the monoclinic compositions of(1−x)BiFeO3-xPbTiO3(x=0.25and 0.27): A combined dc magnetization and x-ray and neutron powder diffraction study

The dc magnetization $M$($T$) studies on monoclinic compositions of ($1\ensuremath{-}x$)BiFeO${}_{3}$-$x$PbTiO${}_{3}$ (BF-$x$PT), $x$ $=$ 0.25 and 0.27, reveal another anomaly at spin-reorientation phase transition (${T}_{\mathrm{OPT}}$) below the N\'eel transition temperature (${T}_{N}$). From a Rietveld refinement of the magnetic structure using neutron powder diffraction data, it is shown that the anomaly at ${T}_{\mathrm{OPT}}$ is due to a spin-reorientation transition from a long-range magnetically ordered phase (${\mathbit{G}}_{\mathbf{y}}$, ${\mathbit{F}}_{\mathbit{x}z}$) stable at ${T}_{\mathrm{OPT}}lTl{T}_{N}$ to another long-range ordered phase (${\mathbit{G}}_{\mathbit{xz}}$, ${\mathbit{F}}_{\mathbf{y}}$) stable below T${}_{\mathrm{OPT}}$, wherein the ferromagnetic component of the noncollinear magnetic structure undergoes a spin flop. The spin-reorientation transition is not linked with any structural phase transition as confirmed by x-ray diffraction studies. Further, this transition is not accompanied with any magnetoelastic coupling. Unlike the N\'eel transition, the spin-reorientation transition is purely of magnetic origin. The spin-reorientation transition in BF-$x$PT is similar to the Morin transition in hematite and differs from the spin-reorientation transition in orthoferrites that is driven by the coupling of two magnetic sublattices.