Direct observation of a coupling between spin, lattice and electric dipole moment in multiferroicYMnO3

${\mathrm{YMnO}}_{3}$ has an antiferromagnetic transition at 76 K with a ferroelectric transition at much higher temperature, making it a rare example of systems having both ferroelectric and magnetic transitions and thus a so-called multiferroic compound. Through high-resolution neutron diffraction studies, we have demonstrated here that at the antiferromagnetic transition of ${\mathrm{YMnO}}_{3}$ there is a strong coupling between the spin and lattice degrees of freedom splitting two Mn--O(3) and Mn--O(4) bond distances on a basal plane. This coupling then induces an unmistakable change in the electric dipole moments, i.e., a coupling between the magnetic and electric dipole moments. We discuss how this rare phenomenon can occur within a Ginzburg-Landau theory.