PHYSICAL REVIEW LETTERS, cilt.107, sa.20, 2011 (SCI-Expanded)
Polarized small-angle neutron scattering studies of single-crystalline multiferroic BiFeO3 reveal a long-wavelength spin density wave generated by similar to 1 degrees spin canting of the spins out of the rotation plane of the antiferromagnetic cycloidal order. This signifies weak ferromagnetism within mesoscopic regions of dimension 0.03 microns along [1 (1) over bar0], to several microns along [111], confirming a long-standing theoretical prediction. The average local magnetization is 0.06 mu(B)/Fe. Our results provide an indication of the intrinsic macroscopic magnetization to be expected in ferroelectric BiFeO3 thin films under strain, where the magnetic cycloid is suppressed.