We investigate magnetically-induced ferroelectricity in Cu2MnSnS4 by means ofLandau theory of phase transitions and of ab initio density functional theory.As expected from the Landau approach, ab initio calculations show that anon-zero ferroelectric polarization P along the y direction is induced by thepeculiar antiferromagnetic configuration of Mn spins occurring in Cu2MnSnS4.The comparison between P, calculated either via density-functional-theory oraccording to Landau approach, clearly shows that ferroelectricity is mainlydriven by Heisenberg-exchange terms and only to a minor extent by relativisticterms. At variance with previous examples of collinear antiferromagnets withmagnetically-induced ferroelectricity (such as AFM-E HoMnO3), the ionicdisplacements occurring upon magnetic ordering are very small, so that theexchange-striction mechanism (i.e. displacement of ions so as to minimize themagnetic coupling energy) is not effective here. Rather, the microscopicmechanism at the basis of polarization has mostly an electronic origin. In thisframework, we propose the small magnetic moment at Cu sites induced byneighboring Mn magnetic moments to play a relevant role in inducing P. Finally,we investigate the effect of the anion by comparing Cu2MnSnSe4 and Cu2MnSnS4:Se-4p states, more delocalized compared to S-3p states, are able to bettermediate the Mn-Mn interaction, in turn leading to a higher ferroelectricpolarization in the Se-based compound.
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