Antisite-disorder driven tuning of magnetic properties and exchange bias in Nd_2−xSr_xCoMnO_6−δ (0 ⩽ x ⩽ 1) (δ ∼ 0.5) double perovskites

Abstract

We demonstrate precise control of exchange bias (EB) in the Nd_2−xSr_xCoMnO_6−δ (0 ⩽ x ⩽ 1) double-perovskite series through Sr²⁺-induced hole doping, revealing a remarkable transition between normal and inverse EB. Using neutron powder diffraction and x-ray absorption spectroscopy, we uncover a structural evolution from a B-site-ordered monoclinic (P2₁/n) to a disordered rhombohedral (R3c) phase with increasing x. This transition is accompanied by a shift in the effective Co valence from +2 toward +3, while the Mn valence remains close to +3.6 throughout the series. DC magnetization measurements indicate a gradual suppression of ferromagnetism with hole doping, whereas ac magnetization measurements for x = 0.75 reveal pronounced cluster-glass behavior. Notably, x = 0.75 also exhibits the highest EB field of ∼4 kOe at 8 K under a 20 kOe cooling field. After correcting for minor-loop effects, we identify robust inverse EB at x = 0.75, persisting even under a 60 kOe cooling field. The coexistence of pronounced glassiness with large EB fields and robust inverse EB suggests that interfacial interactions involving frustrated spins play a central role in stabilizing these exotic EB states. Furthermore, the apparent nonmonotonic evolution of the measured EB with doping is found to be largely influenced by minor-loop effects. After correcting for these effects, the intrinsic EB exhibits an overall reduction with increasing doping and antisite disorder. Interestingly, the intrinsic EB displays a local maximum when the ferromagnetic and nonferromagnetic phase fractions become comparable, despite the persistence of strong glassy dynamics across the series. These findings establish rare-earth double perovskites as a promising platform for engineering tunable EB functionalities through controlled structural disorder and magnetic phase competition, opening new avenues for advanced spintronic and magnetic memory applications.