Effects of Structural Strain and Rare-Earth Moments on Spin Cycloidal-Plane in Multiferroic Rare-Earth Manganites

A series of orthorhombic multiferroic manganites (o-ReMnO₃) thin films with deliberately designed strain states and different rare-earth elements were prepared to unveil the roles played by the strain and 4f -electron moments in determining the magnetic ordering and associated polarizations. The results obtained from temperature-dependent magnetization, dielectric and ferroelectric measurements revealed that the orientations of magnetic reordering and polarization varied with the specific rare-earth element involved. Namely, with the increase of total angular momentum J of the 4f -electrons of rare-earth element the orientation of magnetic reordering was changed from the b-axis (sinusoidal state) for Yb³⁺ to the a-axis (ab-cycloidal) for Tm³⁺ and, eventually to the c-axis (bc-cycloidal) for Er³⁺ and Ho³⁺. These results indicate that the Re-Mn magnetic exchange interaction plays a more important role than the external strain does in determining the magnetic and hence polarization states in multiferroic o-ReMnO₃ manganites. Our results also confirmed the coexistence of different magnetic spin states in o-ReMnO₃ thin films.