Incommensurate Spin Ordering and Excitations in Multiferroic SrMnGe2O6

Anisotropic multiferroic properties of SrMnGe2O6 pyroxene single crystalswere systematically investigated by means of magnetization, heat capacity,pyroelectric current measurement and elastic and inelastic neutron scatteringexperiments. Single crystal neutron diffraction allows us to unambiguouslyreveal the presence of two incommensurate magnetic orderings: a non-polaramplitude-modulated collinear sinusoidal magnetic structure emerges atTN1=4.36(2)K followed by a polar elliptical cycloidal spin structure belowTN2=4.05(2)K. Pyroelectric current measurements on single crystal confirm theappearance of a spontaneous polarization within the (ac) plane below TN2associated with the latter magnetic symmetry through extendedDzyaloshinsky-Moriya mechanism. The magnetic phase diagram was calculatedconsidering the three isotropic exchange couplings relevant in this system. Themagnetic excitations spectra of SrMnGe2O6 measured by inelastic neutronscattering were successfully modeled using a set of exchange interactionsconsistent with this phase diagram.