We report a resonant x-ray magnetic scattering study of multiferroic SmMn2O5, for which the microscopic magnetic properties have not yet been revealed. Dielectric measurements indicated that ferroelectric phase transitions occur around TC1≃34 K and TC2≃28 K. Resonant x-ray magnetic scattering was observed at both the Sm LIII and Mn K edges in the ferroelectric phase below TC2. The Sm and Mn magnetic moments align antiferromagnetically with a magnetic propagation wave vector qM=(1/2,0,0), and the ferroelectric transition coincides with the antiferromagnetic transition. The azimuthal dependence of the resonant magnetic signals confirmed that the Sm and Mn magnetic moments have a dominant c-axis component, which is quite unique because Mn moments have a dominant a-axis component in other RMn2O5 (R=rare earth,Y,Bi) compounds. We propose a magnetic structure with the magnetic space group P2ab21m, which allows electric polarization along the b axis in the ferroelectric phase.
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