Critical rare-earth Yb3+ solubility, magnetic tuning and enhanced low-field magnetostriction in sintered CoFe2-xYbxO4 ceramics

This work establishes the critical solubility limit and magnetomechanical consequences of Yb 3+ substitution in sintered cobalt ferrite ceramics, CoFe 2-x Yb x O 4 (0 ≤ x ≤ 0.05), prepared by a tartrate-assisted sol–gel route followed by high-temperature sintering. A key outcome of the study is the identification of x = 0.025 as the maximum Yb3+ content that can be accommodated in the CFO spinel lattice without detectable secondary phase formation. X-ray diffraction and Raman spectroscopy confirm retention of the single-phase spinel structure up to x = 0.025, whereas higher Yb contents lead to the emergence of orthorhombic YbFeO 3 , revealing the structural solubility threshold. The results further show that low-level Yb 3+ incorporation induces lattice expansion, local distortion, and magnetic tuning, while preserving phase purity and magnetic softness. Among all compositions, CoFe 1.975 Yb 0.025 O 4 exhibits the most favorable low-field magnetomechanical response, with enhanced magnetostriction and strain sensitivity compared with unsubstituted CoFe 2 O 4 , making it particularly attractive for sensor-oriented applications. In addition, particle-size-controlled studies on this optimum composition reveal a strong microstructure–magnetism correlation, highlighting the role of crystallite size in governing magnetic behavior. These findings provide a clear composition–structure–property framework for Yb-substituted cobalt ferrites and identify CoFe 1.975 Yb 0.025 O 4 as a promising rare-earth-modified magnetostrictive ceramic for low-field torque sensor and actuator applications.