Effects of single versus triple B-site substitution on multiferroic properties of rare-earth-modified BiFeO3

The effects of single B-site substitution (Co, Ti, Ta) and triple co-substitution (Co 1/3 Ti 1/3 Ta 1/3 , CTT) on the multiferroic properties of Bi 0.9 (La 1/4 Nd 1/4 Sm 1/4 Gd 1/4 ) 0.1 FeO 3 were systematically compared. PUND measurements reveal switching-current peaks for both the Co- and CTT-sub samples, confirming switchable ferroelectric polarization. The Co-substituted sample exhibits well-resolved switching peaks, supporting the reliability of its remanent polarization ( P r  = 26.9 μ C/cm 2 ). The CTT-sub sample shows a larger PUND-supported P r (45.1 μ C/cm 2 ); however, its switching peaks are strongly merged, so the P r extracted directly from the merged peaks is less precise than that of the Co-sub sample. For the Ti- and Ta-sub samples, no PUND switching-current peaks are observed, indicating that their P r values are not reliable indicators of intrinsic ferroelectric properties. More importantly, Co substitution yields the highest remanent magnetization ( M r  = 73.39 emu/mol), which is attributed to effective suppression of the helical spin structure via enhanced superexchange interactions. In contrast, the CTT-sub sample, despite its larger P ᵣ, exhibits only a moderate M r of 51.93 emu/mol. The relatively lower magnetic performance of triple co-substitution is due to dilution of magnetic Co ions by non-magnetic Ti 4+ /Ta 5+ and local lattice disorder from competitive B-site occupancy. After DC poling, the Co-sub sample exhibits a 52.5% increase in M r (to 111.91 emu/mol). Overall, Co substitution provides the most reliable ferroelectric switching and the best overall multiferroic performance, especially magnetization, whereas CTT substitution gives the largest PUND-supported polarization but with less well-resolved switching.