Giant exchange bias effect in Ruddlesden-Popper oxides SrLaFe0.25+xMn0.25Co0.5−xO4 ( x=0,0.25 ): Role of the cluster glass magnetic phase in a quasi-two-dimensional perovskite

Structural and magnetic studies on $\mathrm{SrLa}({\mathrm{Fe}}_0.25{\mathrm{Mn}}_0.25){\mathrm{Co}}_0.5{\mathrm{O}}₄$ (FMC1) and ${\mathrm{SrLaFe}}_0.5({\mathrm{Mn}}_0.25{\mathrm{Co}}_0.25){\mathrm{O}}₄$ (FMC2) reveal unusually large exchange bias behavior in these atomically disordered quasi-two-dimensional layered perovskites. Powder x-ray as well as neutron diffraction confirm tetragonal crystal structure with $I4/mmm$ space group for both the compounds. Magnetization measurements on FMC1 as well as FMC2 reveal short-range antiferromagnetic ordering around room temperature and frozen magnetic clusters at lower temperatures ($T<23\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ for FMC1 and $T<43\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ for FMC2). The random occupancy of mixed-valent magnetic ions ($\mathrm{F}{\mathrm{e}}³⁺/\mathrm{F}{\mathrm{e}}⁴⁺, \mathrm{M}{\mathrm{n}}³⁺/\mathrm{M}{\mathrm{n}}⁴⁺, \mathrm{C}{\mathrm{o}}²⁺/\mathrm{C}{\mathrm{o}}³⁺$) at the perovskite octahedral sites give rise to locally varying competing antiferromagnetic and ferromagnetic exchange interactions, resulting in low temperature frozen spin states. Giant exchange bias values of \ensuremath{\sim}6 and \ensuremath{\sim}9.5 kOe were observed for FMC1 and FMC2, respectively, at 2 K under an applied field of 50 kOe. In comparison, the Co-rich FMC1 exhibits higher magnetization and coercivity, whereas Fe-rich FMC2 possesses a higher exchange bias effect. Our experimental results reveal compositional tuning induced exotic magnetic behavior in quasi-two-dimensional layered oxides.