4f spin driven ferroelectric-ferromagnetic multiferroicity in PrMn2O5 under a magnetic field

In contrast to all other members of the $R{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$ family with nonzero $4f$ electrons ($R$ = Nd to Lu), ${\mathrm{PrMn}}_{2}{\mathrm{O}}_{5}$ does not show any spin driven ferroelectricity in the magnetically ordered phase. By means of high-field electric polarization measurements up to 45 T, we have found that this exceptional candidate undergoes a spin driven multiferroic phase under magnetic field. X-ray magnetic circular dichroism studies up to 30 T at the Pr ${L}_{2}$ edge show that this ferroelectricity originates from and directly couples to the ferromagnetic component of the ${\mathrm{Pr}}^{3+}$ spins. Experimental observations along with our generalized gradient-approximation $+\phantom{\rule{4pt}{0ex}}U$ calculations reveal that this exotic ferroelectric-ferromagnetic combination stabilizes through the exchange-striction mechanism solely driven by a $3d\ensuremath{-}4f$-type coupling, as opposed to the other $R{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$ members with $3d\ensuremath{-}3d$ driven ferroelectric-antiferromagnetic-type conventional type-II multiferroicity.