Identification of microscopic spin-polarization coupling in the ferroelectric phase of magnetoelectric multiferroic CuFe1-x Alx O2

We have performed synchrotron radiation x-ray and neutron diffraction measurements on magnetoelectric multiferroic CuFe1-x Alx O2 (x=0.0155), which has a proper helical magnetic structure with incommensurate propagation wave vector in the ferroelectric phase. The present measurements revealed that the ferroelectric phase is accompanied by lattice modulation with a wave number 2q, where q is the magnetic modulation wave number. We have calculated the Fourier spectrum of the spatial modulations in the local electric polarization using a microscopic model proposed by T. Arima [J. Phys. Soc. Jpn. 76, 073702 (2007)]. Comparing the experimental results with the calculation results, we found that the origin of the 2q -lattice modulation is not the conventional magnetostriction but the variation in the metal-ligand hybridization between the magnetic Fe3+ ions and ligand O2- ions. Combining the present results with the results of a previous polarized neutron diffraction study, we conclude that the microscopic origin of the ferroelectricity in CuFe1-x Alx O2 is the variation in the metal-ligand hybridization with spin-orbit coupling.