The improvement of ferromagnetic properties is critical for the practical application of multiferroic materials, to be exact, BiFeO₃ (BFO). Herein, we have investigated the evolution in the structure and morphology of Ho or/and Mn-doped thin films and the related diversification in ferromagnetic behavior. BFO, Bi_0.95Ho_0.05FeO₃ (BHFO), BiFe_0.95Mn_0.05O₃ (BFMO) and Bi_0.95Ho_0.05Fe_0.95Mn_0.05O₃ (BHFMO) thin films are synthesized via the conventional sol-gel method. Density, size and phase structure are crucial to optimize the ferromagnetic properties. Specifically, under the applied magnetic field of 10 kOe, BHFO and BFMO thin films can produce obvious magnetic properties during magnetization and, additionally, doping with Ho and Mn (BHFMO) can achieve better magnetic properties. This enhancement is attributed to the lattice distortions caused by the ionic sizes difference between the doping agent and the host, the generation of the new exchange interactions and the inhibition of the antiferromagnetic spiral modulated spin structure. This study provides key insights of understanding the tunable ferromagnetic properties of co-doped BFO.

Co³⁺ doping in BiFeO₃ is expected to be an effective method for improving its magnetic properties. In this work, pristine BiFeO₃ (BFO) and doped BiFe_1-xCo_xO₃ (BFC_xO, x = 0.01, 0.03, 0.05, 0.07 and 0.10) composite thin films were successfully synthesized by a sol–gel technique. XRD and Raman spectra indicate that the Co³⁺ ions are substituted for the Fe³⁺ ion sites in the BFO rhombohedral lattice. Raman vibration of oxygen octahedron is obviously weakened due to the lattice distortion induced by the size mismatch between two B-site cations (Fe³⁺ and Co³⁺ ions), which has an impact on the magnetic properties of BFC_xO. SEM images reveal a denser agglomeration in Co-doped samples. TEM results indicate that the average size of grains is reduced due to the Co³⁺ substitution. XPS measurements illustrate that the replacement of Fe³⁺ with Co³⁺ effectively suppresses the generation of oxygen defects and increases the concentration of Fe³⁺ ions at the B-site of perovskite lattice. Vibrating sample magnetometer (VSM) measurements show that the remanent magnetization (Mr) of BFC_0.07O (3.6 emu/cm³) and the saturation magnetization (Ms) of BFC_0.10O (48.84 emu/cm³) thin film both increase by approximately two times at room temperature, compared with that of the pure BFO counterpart.