Imaging Topological Defects in a Noncollinear Antiferromagnet

Onset of Multiferroicity in Prototypical Single-Spin Cycloid BiFeO3 Thin Films

“…5 As the BiFeO 3 film is grown under an isotropic in-plane compressive strain of −0.67 ± 0.03% on SrTiO 3 (111) (Figure 1c,d), the degeneracy among the three k vectors is not lifted (Figure 2a), giving rise to this multi-k magnetic pattern (Figure 2c, left). This complex magnetic lamellar pattern exhibits a resemblance to the magnetic landscape observed at the surface of BiFeO 3 single crystals, 6 here with smaller antiferromagnetic domains. We propose that breaking the isotropic in-plane strain in the (111) plane will lift the degeneracy in the k vector propagation while maintaining a single ferroelectric domain state.…”

“…This suggests that the cycloid propagation is not fully locked to the ⟨−110⟩ high-symmetry crystal axes (Note S1 and Figure S3), as recently observed at the surface of BiFeO 3 single crystals. 6 In contrast, a very clear pair of intense spots at +q and −q, symmetric with respect to the specular beam, is observed on the REXS pattern of BiFeO 3 grown on DyScO 3 (011) o (Figure S4), as expected for a periodic antiferromagnetic structure with a single propagation direction all over the probed area. In addition, the spots show a very strong magnetic circular dichroism (Figure 2d, right), which corroborates the presence of a unique chiral object such as the antiferromagnetic spin cycloid of BiFeO 3 .…”

“…3 It is also a G-type antiferromagnet with an additional antisymmetric magnetoelectric interaction that stabilizes an incommensurate spin cycloid, with propagation vectors along the ⟨−110⟩ directions, 4,5 so that each ferroelectric domain can harbor three different energetically degenerate antiferromagnetic cycloidal domains. 6 In thin-film form, while the influence of epitaxial strain on ferroelectric domain engineering has attracted a great deal of attention, 7−9 the investigation of the antiferromagnetic order only recently boomed with the development of ultrasensitive neutron 10−13 or synchrotron 14−17 experiments as well as scanning magnetometry 18−20 techniques. The substrateinduced epitaxial strain can trigger either a noncollinear cycloidal spin texture or a pseudocollinear canted G-type antiferromagnetic ordering for low or high strain values, respectively.…”

“…In bulk, BiFeO 3 exhibits rhombohedral distortion ( R 3 c space group), giving rise to four ferroelastic variants corresponding to eight possible ferroelectric domain states, with polarization lying along ⟨111⟩ (pseudocubic notation used by default) . It is also a G-type antiferromagnet with an additional antisymmetric magnetoelectric interaction that stabilizes an incommensurate spin cycloid, with propagation vectors along the ⟨−110⟩ directions, , so that each ferroelectric domain can harbor three different energetically degenerate antiferromagnetic cycloidal domains …”

Thermally Activated Transitions Between Micromagnetic States

“…14b shows that a typical configuration in this regime contains intricate labyrinthic paths with almost constant widths. In this regime, magnetic singularities [98][99][100] prevent the system from relaxing to the cycloidal state with a constant direction of the helical axis. These defects have many similarities with disclinations in lamellar phases, and can be classified accordingly; they are shown in Fig.…”