Magnetic-field-induced spin flop transition and magnetoelectric effect in Ca$_{2}$Fe$_{\mathrm{2-x}}$Al$_{\mathrm{x}}$O$_{5}$

Ca$_{2}$Fe$_{\mathrm{2-x}}$Al$_{\mathrm{x}}$O$_{5}$ compounds with x \textgreater\ 0.5 have the same crystal structure as brownmillerite, where (Fe,Al)O$_{6}$ octahedron layers and (Fe,Al)O$_{4}$ tetrahedron layers alternately stacks. The space group is orthorhombic Ibm2, which allows the presence of spontaneous polarization along the c-axis. These materials also exhibit the antiferromagnetic transition at the 350K $\sim$ 570K. We have investigated the magnetoelectric effect of single crystals. In a magnetic field applied along the spin easy axis, a metamagnetic transition is observed to accompany an anomaly of the electric polarization and the dielectric constant. The anomalies can be ascribed to a noncollinear spin arrangement in the domain walls between two magnetic phases and/or the spin direction dependent modulation of the metal-ligand hybridization.