Polarized small-angle neutron scattering studies of single-crystalline multiferroic BiFeO(3) reveal a long-wavelength spin density wave generated by ∼1° 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 [110], to several microns along [111], confirming a long-standing theoretical prediction. The average local magnetization is 0.06 μ(B)/Fe. Our results provide an indication of the intrinsic macroscopic magnetization to be expected in ferroelectric BiFeO(3) thin films under strain, where the magnetic cycloid is suppressed.

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The recovery of a modulated magnetic structure in epitaxial BiFeO 3 thin fi lms as revealed by neutron diffraction is reported. The magnetic structure in thin fi lms is found to strongly depend on substrate orientation. The substrate orientation causes different strain-relaxation processes resulting in different thin-fi lm crystal structures. The (110) oriented fi lm with a monoclinic structural phase has a single-domain modulated magnetic structure where the magnetic moment lies in the HHL plane. For the (111) oriented fi lm that has a rhombohedral structure, a modulated structure superimposed on the G-type antiferromagnetic order is found. These results indicate that slight structural modifi cations in the BiFeO 3 thin fi lm cause drastic changes in the magnetic structure.

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