Effect of V⁵⁺/V⁴⁺ substitution on structural and magnetic orderings of SrFeO_3-δ

SrFeO_3-δ is a structurally versatile material, showing cubic (C) symmetry at δ = 0, which transitions to tetragonal (T) and then orthorhombic (Or) with increasing oxygen vacancies. These structural changes alter magnetic behavior through variations in bond angles and lengths, enabling both antiferromagnetic (AFM) and ferromagnetic (FM) interactions between Fe⁴⁺/Fe³⁺ ions and oxygen (or vacancies). While such tunable magnetic phases offer rich physics and potential applications, achieving room-temperature (RT) FM remains challenging. This study explores V⁵⁺/V⁴⁺ doping in SrFeO_3-δ (SrFe_1-xV_xO_3-δ; 0 ≤ x ≤ 0.03) to induce FM persisting up to RT and investigates its structural-magnetic correlation. Structural analysis from X-ray diffraction (XRD) and Raman spectroscopy, supported by phonon mode calculations, reveals that SrFeO_3-δ is in a mixed T-Or phase. At the same time, V-doped samples exhibit an emerging C-phase in a dominant T-lattice. Magnetic hysteresis (M − H) loops show notable FM behavior within the AFM matrix at low temperature ∼10K, with a FM and paramagnetic phase at room temperature. Temperature-dependent magnetization measurements indicate a T-phase related Néel temperature (T_N) shift from 70K to 55K in the doped samples as compared to the pure one. An increased magnetization difference between the field-cooled (FC) and zero-field-cooled (ZFC) data with increasing V-content suggests an increasing magnetic frustration due to competing FM/AFM exchange interactions. X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge structure (XANES) analyses reveal a rise in Fe³⁺ and V⁵⁺ states, affecting oxygen vacancy distributions and corresponding structural shifts seen in XRD and Raman results. The multivalent Fe³⁺/Fe⁴⁺ and V⁴⁺/V⁵⁺ states enhance Double-Exchange (DE) interactions (Fe³⁺-O-Fe⁴⁺ and Fe³⁺-O-V⁵⁺), and V_O-mediated Fe³⁺-V_O-Fe³⁺ interaction, promoting ferromagnetism. Moreover, frequency-dependent magnetization studies display a subtle susceptibility peak shift, indicating spin-glass-like behavior in V-doped samples.