Magnetization Reversal in BiFe_0.9Co_0.1O₃ Thin Films 1- Spin Structure and Magnetization Reversal by Electric Field at Room Temperature in Co-Substituted Bismuth Ferrite Thin Film

Electric-field control of magnetism in multiferroic materials is a significant challenging issue on the development of novel low-energy-consuming nonvolatile memory devices. Perovskite bismuth ferrite BiFeO₃ is widely studied multiferroic material because of strong magnetoelectric coupling between the electrical polarization and antiferromagnetic magnetic ordering above room temperature, but BiFeO₃ does not show such spontaneous magnetization due to the presence of cycloidal spin modulation superimposed on G-type antiferromagnetism. Cobalt-substitution is one of the powerful ways to modify the electric and magnetic properties of BiFeO₃. Partial substitution of Co for Fe stabilizes the collinear phase with weak ferromagnetism preserving the <i>R</i>3<i>c</i> crystal structure. Neutron powder diffraction and Mössbauer spectroscopy measurements revealed a change in the spin structure from a cycloidal one at low temperature to a collinear one at room temperature. However, because the magnetic structure of BiFeO₃ is quite sensitive to lattice strain, the choice of single crystal substrate for thin film growth is of great importance.<br/>Here we grew a single-phase ferromagnetic ferroelectric BiFe_0.9Co_0.1O₃ thin film grown on GdScO₃ substrate. The conversion electron Mössbauer spectroscopy on a nearly 100% ⁵⁷Fe-enriched thin film sample revealed the collinear spin structure, whereas the spectrum of the BFO thin film on the same substrate showed a cycloidal spin structure. The value of quadrupole shift was close to that of bulk BFCO (−0.20 mm/s) and that of (111)pc-oriented BFCO thin films (−0.217 mm/s), strongly suggesting that the spins are aligned perpendicular to the electric field gradient (<i>V_zz</i> &gt; 0), which was along the polar axis. Further analysis of Mössbauer spectrum revealed that the possible directions of antiferromagnetic spin direction are limited to four, which is quite close to four of six 〈121〉 directions in the easy plane perpendicular to the polarization along the [111] direction. Each ferroelectric domain has four possible magnetization directions, all of which have an out-of-plane ferromagnetic component. The spontaneous magnetization is in four 〈110〉 directions, perpendicular to both antiferromagnetic spin direction and polarization. Magnetization reversal due to 71° ferroelectric switching is expected to occur if the octahedral rotation is accompanied by a change in the antiferromagnetic spin direction.<br/>The correlation between the ferroelectric and ferromagnetic domains was investigated by performing piezoelectric force microscopy (PFM) and MFM on the as-grown BFCO thin film. Both PFM and MFM images exhibi A striped domain configuration with four polarization down domains can be seen, indicating the both ferroeectric and ferromagnetic domains are strongly coupled with each other. Magnetization reversal by polarization switching was demonstrated by tip bias of −7 V. In the area where pure out-of-plane 71° domain switching occurred, the magnetic domain structure was also preserved but the contrast of MFM phasel was reversed, indicating that the out-of-plane magnetization switched from downward to upward and vice versa in this area. This was the first direct observation of magnetization reversal by the electric field at room temperature. On the other hand, in the area where the striped domain structure is heavily modified, MFM image is also changed largely by the poling, though the contrast is correlated with the ferroelectric domain structure after the poling. These results indicate that an out-of-plane magnetization reversal can be achieved by 71° polarization switching without reconstruction of the ferroelectric domain.