  It is still under intensive discussion, how magnetoelectric coupling actually
occurs at the atomic scale in multiferroic BiFeO3. Nuclear solid-state
techniques monitor local fields at the atomic scale. Using such an approach, we
show that, contrary to our own expectation, ferroelectric and magnetic ordering
in bismuth ferrite (BiFeO3 or BFO) decouple at the unit-cell level. Time
differential perturbed angular correlation (TDPAC) data at temperatures below,
close, and above the magnetic N\'eel temperature show that the coupling of the
ferroelectric order to magnetization is completely absent at the bismuth site.
It is common understanding that the antiferromagnetic order and the cycloidal
ordering due to the Dzyaloshinskii-Moriya interaction generate a net zero
magnetization of the sample cancelling any magnetoelectric effect at the
macroscopic level. Our previous data show that a very large coupling of
magnetic moment and electrical distortions arises on the magnetic sub-lattice
(Fe-site). The oxygen octahedra around the iron site experience a large tilt
due to the onset of magnetic ordering. Nevertheless, the Bi-containing
complementary sub-lattice carrying the ferroelectric order is practically
unaffected by this large structural change in its direct vicinity. The
magnetoelectric coupling thus vanishes already at the unit cell level. These
experimental results agree well with an ab-initio density functional theory
(DFT) calculation.
