Effect of Co–Mn Ordering on Defect-Induced Modulation of Complex Magnetic, Metamagnetic, Griffiths Phase, and Exchange Bias-like Behavior of Eu2CoMnO6

Abstract

The double perovskite Eu₂CoMnO₆ (ECMO), known for its complicated metamagnetic behavior, was studied in this report to examine how postannealing synthesis affects its crystal structure and magnetic and electronic behavior. The slow-cooling, Argon treatment, and quenching procedure during the sample synthesis indicated the important significance of antisite disorder (ASD) in influencing the material’s magnetic response. The magnetic study revealed diverse transitions, including two low-temperature antiferromagnetic (AFM)-like transitions at ∼51 and ∼10 K in the slow-cooled sample and vibronic ferromagnetic (FM) superexchange interactions at ∼105 K in the argon (Ar)-treated sample, while the quenched sample displayed an AFM behavior at low temperatures. The XPS analysis indicated the presence of diverse concentrations of Co and Mn in multiple valence states, specifically (2⁺, 3⁺) and (3⁺, 4⁺) respectively, across the samples subjected to different annealing processes. The Griffiths phase was particularly noticeable in the quenched sample, highlighting the role of disorder with Griffith’s disorder exponent (λ) = 0.81. The M(H) data at 2.5 K under zero-field-cooled mode revealed that the Ar-treated sample had a smooth, saturating-like loop, while the quenched sample had the hysteresis loop shifted toward the positive field axis with a reduced magnetic moment of 2.2 μ_B/f.u., and the slow-cooled sample exhibited sharp metamagnetic jumps with an unsaturated magnetic moment of 3.3 μ_B/f.u. While M(H) data recorded under a field-cooled protocol altered the position of critical fields (H_C) for the slow-cooled sample, the evolution of an extra magnetization jump was noticed in the case of the Ar-annealed sample, and the quenched sample showed the loop shifting completely toward the negative field axis. The loop shift and varying H_C values were explained in terms of an exchange bias-like spin-pinning mechanism. Additionally, DFT calculations corroborate the experimental results, revealing an increased likelihood of antisite disorders in the presence of oxygen vacancies, as well as altered behavior of Co and Mn spin states in relation to the disorders and oxygen vacancies. The effect of the disorder and oxygen vacancies on the electrical and magnetic ground states was also investigated, and the results were complemented with the experimentally observed magnetic behavior. This study demonstrates how postannealing conditions may be carefully controlled to regulate the disorder and, thus, magnetic behavior, including valence states, Griffiths phase, and metamagnetic behavior of ECMO, opening up new avenues for developing materials with desired functional properties.