Evidence for exchange bias coupling at the perovskite/brownmillerite interface in spontaneously stabilized $\mathrm{SrCo}{\mathrm{O}}_{3-\delta }/{\mathrm{SrCoO}}_{2.5}$ bilayers

Interface effect in complex oxide thin-film heterostructures lies at the vanguard of current research to design technologically relevant functionality and explore emergent physical phenomena. While most of the previous works focus on the perovskite/perovskite heterostructures, the study of perovskite/brownmillerite interfaces remains in its infancy. Here, we investigate spontaneously stabilized perovskite-ferromagnet ($\mathrm{SrCo}{\mathrm{O}}_{3-\delta }$)/brownmillerite-antiferromagnet $\left(\mathrm{SrCo}{\mathrm{O}}_{2.5}\right)$ bilayer with $T_{\mathrm{N}}>T_{\mathrm{C}}$ and discover an unconventional interfacial magnetic exchange bias effect. From magnetometry investigations, it is rationalized that the observed effect stems from the interfacial ferromagnet/antiferromagnet coupling. The possibility for coupled ferromagnet/spin-glass interface engendering such effect is ruled out. Strikingly, a finite coercive field persists in the paramagnetic state of $\mathrm{SrCo}{\mathrm{O}}_{3-\delta }$, whereas the exchange bias field vanishes at $T_{\mathrm{C}}$. We conjecture the observed effect to be due to the effective external quenched staggered field provided by the antiferromagnetic layer for the ferromagnetic spins at the interface. Our results not only unveil a paradigm to tailor the interfacial magnetic properties in oxide heterostructures without altering the cations at the interface, but also provide a purview to delve into the fundamental aspects of exchange bias in such unusual systems, paving a big step forward in thin-film magnetism.

Figure 1

Left panel: (a) schematic representation of magnetic ordering temperature for ferromagnetic $\mathrm{SrCo}{\mathrm{O}}_3$ and antiferromagnetic $\mathrm{SrCo}{\mathrm{O}}_{2.5}$; (b) layout of the designed $\left[\mathrm{SC}{\mathrm{O}}_{\mathrm{PC}}\left(\sim 24\mathrm{nm}\right)/\mathrm{SC}{\mathrm{O}}_{\mathrm{BM}}\left(\sim 2\mathrm{nm}\right)\right]$ bilayer on STO; (c) schematic MHL representing MEBE under positive field cooling. Right panel: (d) $\theta \text{−}2\theta$ x-ray diffraction pattern; (e) ϕ scan along the asymmetric planes of STO(103), $\mathrm{SC}{\mathrm{O}}_{\mathrm{PC}}$(103), $\mathrm{SC}{\mathrm{O}}_{\mathrm{BM}}\left(11\underline{12}\right)$; (f) measured and fitted x-ray reflectivity, and (g) off-specular reciprocal space mapping around STO(103) of $\left[\mathrm{SC}{\mathrm{O}}_{\mathrm{PC}}\left(\sim 24\mathrm{nm}\right)/\mathrm{SC}{\mathrm{O}}_{\mathrm{BM}}\left(\sim 2\mathrm{nm}\right)\right]$ bilayer.

Figure 2

(a) Temperature-dependent zero-field-cooled and field-cooled magnetization. (b) $M$($H$) loop at 5 K after zero-field cooling from room temperature [the inset shows the enlarged view of $M$($H$) loop indicating symmetric coercive field on positive and negative field axis]. (c) $M$($H$) loops at 5 K after field cooling from 350 K in a $+3\text{−}\mathrm{T}$ field (dark yellow circles) and in a −3-T field (orange circles). (d) Schematic representation of two possible growth structures, and (e) the thermoremnant magnetization of $\left[\mathrm{SC}{\mathrm{O}}_{\mathrm{PC}}\left(\sim 24\mathrm{nm}\right)/\mathrm{SC}{\mathrm{O}}_{\mathrm{BM}}\left(\sim 2\mathrm{nm}\right)\right]$ bilayer.

Figure 3

Temperature dependence of magnetic exchange bias effect: (a)–(i) enlarged view of the original and inverted $M$($H$) loops of the $\left[\mathrm{SC}{\mathrm{O}}_{\mathrm{PC}}\left(\sim 24\mathrm{nm}\right)/\mathrm{SC}{\mathrm{O}}_{\mathrm{BM}}\left(\sim 2\mathrm{nm}\right)\right]$ bilayer. (j), (k) The estimated $H_{\mathrm{EB}}$ and $H_C$ as a function of temperature, respectively.

Figure 4

$M$($H$) loops at 5  K after field cooling (at various fields) from 350 K (top panel). $H_{\mathrm{EB}}$ as a function of field (bottom panel).