Tuning the spontaneous exchange bias effect with Ba to Sr partial substitution in La${}_{1.5}$(Sr${}_{0.5-x}$Ba${}_x$)CoMnO${}_6$

The spontaneous exchange bias (SEB) effect is a remarkable phenomenon recently observed in some reentrant spin-glass materials. Here we investigate the SEB in La${}_{1.5}$(Sr${}_{0.5-x}$Ba${}_x$)CoMnO${}_6$ double-perovskites, a system with multifarious magnetic phases for which a notable increase in the exchange bias field is observed for intermediate Sr/Ba concentrations. The Ba to Sr substitution leads to the enhancement of the crystal lattice, which is accompanied by the raise of both the effective magnetic moment (${\mu }_{eff}$) and the antiferromagnetic (AFM) transition temperature that is observed below the ferromagnetic ordering. Such increases are likely related to the increased fraction of Co${}^{3+}$ in the high spin configuration, leading to the enhancement of Co${}^{3+}$–O–Mn${}^{4+}$ AFM phase and to the reduction in the uncompensation of the AFM coupling between Co and Mn. The combined effect of the increased ${\mu }_{eff}$ and AFM phase plausible explains the changes in the SEB effect.