Antisite-disorder driven tuning of magnetic properties and exchange-bias in Nd₂₋ₓSrₓCoMnO₆₋δ (0 ≤ x ≤ 1) (δ 0.5) double perovskites

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We demonstrate precise control of exchange bias (EB) in the Nd₂₋ₓSrₓCoMnO₆₋δ (0 ≤ x ≤ 1) double-perovskite series through Sr²⁺ induced hole doping, unveiling a remarkable transition between normal and inverse EB states. Employing neutron powder diffraction and X-ray absorption spectroscopy, we reveal a structural evolution from a B-site-ordered monoclinic (P2₁/n) phase to a disordered rhombohedral (R3c) phase with increasing x, accompanied by a shift in the effective Co valence from +2 toward +3, while the Mn valence remains essentially unchanged. DC magnetization measurements indicate a gradual suppression of ferromagnetism with hole doping, whereas AC susceptibility measurements at x = 0.75 reveal pronounced cluster-glass behavior and the highest EB field of 4 kOe at 8 K under a 6 T cooling field. After correcting for minor-loop effects, we identify robust inverse EB at x = 0.75, persisting even under a cooling field of 6 T. We attribute this phenomenon to competing ferromagnetic--antiferromagnetic and ferromagnetic--glassy interfaces, governed by strong magnetic frustration and the magnetocrystalline anisotropy associated with rare-earth 4f electrons. These findings elucidate the pivotal role of doping-induced structural and magnetic competition in tailoring EB behavior in rare-earth double perovskites, providing new insights for the design of advanced magnetic materials.