Correlation between the mobility of domain wall and polarization flop direction in a slanted magnetic field in the helimagnetic ferroelectrics Tb _1-xDy_xMnO₃

It has been pointed out that domain walls (DW's) in spin-induced ferroelectrics play an important role in enhancing their magnetoelectric coupling, similar to the large magnetic permeability at around the coercive force in a ferromagnet. Here we report a systematic variation of the effect of 90^° DW's on the magnetocapacitance in a series of helimagnetic multiferroic cocrystals Tb_1-xDy_xMnO₃, which separate two regions with both the spin rotating planes and the ferroelectric polarization differing by 90^°. We have also found a systematic switch of the rotation directions of the ferroelectric polarization in a canted magnetic field. The results are well explained by taking account of the thickness-dependent mobility of DW's.