Strong local structural distortion and magnetic order suppression in spin‐driven multiferroic BaYFeO ₄ upon Co doping

Understanding the interplay between structural, electronic, and magnetic properties is essential for unraveling the unique magnetic phenomena and spin‐driven ferroelectricity in BaYFeO 4 and related compounds. Herein, we present a comprehensive study on the effects of cobalt doping on the structural, electronic, and magnetic properties of BaYFe 1− x Co x O 4 polycrystals ( x = 0–0.15), using a combination of X‐ray and neutron powder diffraction, Raman spectroscopy, X‐ray photoemission spectroscopy, and magnetic measurements. The initial orthorhombic Pnma crystal structure remains stable upon Co doping, though local structural changes arise due to the complex spin states of Co 3+ ions. These ions adopt low‐spin and high‐spin configurations depending on their coordination, respectively, while Fe 3+ ions retain a high‐spin state. All samples exhibit an incommensurate collinear spin‐density wave antiferromagnetic order at the Néel temperature T N1 , which, evolves into a cycloidal antiferromagnetic order at a lower T N2 . These long‐range magnetic orders are gradually suppressed with increasing Co content. Additionally, the onset temperature of short‐range magnetic correlations—emerging well above T N1 —decreases with increased doping. These findings highlight the significant influence of Co doping on both intrachain and interchain magnetic interactions, governing the overall magnetic behavior of BaYFe 1− x Co x O 4 . The underlying mechanisms are discussed in detail.