Superspin Glass Mediated Giant Spontaneous Exchange Bias in a Nanocomposite of ${\mathrm{BiFeO}}_3\mathrm{\text{−}}{\mathrm{Bi}}_2{\mathrm{Fe}}_4{\mathbf{O}}_9$

We observe an enormous spontaneous exchange bias ($\sim 300–600\mathrm{Oe}$)—measured in an unmagnetized state following zero-field cooling—in a nanocomposite of ${\mathrm{BiFeO}}_3$ ($\sim 94\%$)-${\mathrm{Bi}}_2{\mathrm{Fe}}_4{\mathrm{O}}_9$ ($\sim 6\%$) over a temperature range 5–300 K. Depending on the path followed in tracing the hysteresis loop—positive ($p$) or negative ($n$)—as well as the maximum field applied, the exchange bias ($H_E$) varies significantly with $\mid -H_{Ep}\mid >\mid H_{En}\mid$. The temperature dependence of $H_E$ is nonmonotonic. It increases, initially, till $\sim 150\mathrm{K}$ and then decreases as the blocking temperature $T_B$ is approached. All these rich features appear to be originating from the spontaneous symmetry breaking and consequent onset of unidirectional anisotropy driven by “superinteraction bias coupling” between the ferromagnetic core of ${\mathrm{Bi}}_2{\mathrm{Fe}}_4{\mathrm{O}}_9$ (of average size $\sim 19\mathrm{nm}$) and the canted antiferromagnetic structure of ${\mathrm{BiFeO}}_3$ (of average size $\sim 112\mathrm{nm}$) via superspin glass moments at the shell.

Figure 1

(a) The hysteresis loop shift, signifying the SEB at different temperatures across 5–300 K, measured under 50 kOe following zero-field cooling; the region near the origin of the loop is blown up to show the extent of exchange bias clearly. (b) The CEB at different temperatures across 5–300 K measured under a field cooling with $+10\mathrm{kOe}$. (c) The switch in sign and change in magnitude of the loop shift at 5 K signaling asymmetry and tunability of the SEB depending on the sign of the starting field ($+50\mathrm{kOe}/-50\mathrm{kOe}$) of hysteresis loop measurement. (d) The switch in sign and change in magnitude of the CEB at 5 K measured following field cooling under $+50\mathrm{kOe}/-50\mathrm{kOe}$. (e) The variation of SEB and corresponding $H_C$ with temperature. (f) The variation of CEB—measured following field cooling under $+10\mathrm{kOe}$—and corresponding $H_C$ with temperature (lines are a guide to the eyes).

Figure 2

(a) The characteristic dip at $T_w\sim 21\mathrm{K}$ in the differential between two ZFC magnetization versus temperature patterns recorded under two different protocols— a simple ZFC and a ZFC with stop-and-wait approach; inset shows a similar dip even at 50 K. It appears to become sharper and more prominent with the increase in wait time. (b) The impact of training effect on CEB for sample $A$. The CEB and $H_C$ decreases with the increase in number of hysteresis cycles ($n$); inset shows a portion of the loop at first and twelfth cycle.

Figure 3

(a) The SEB for all the three samples with different volume fractions of the ${\mathrm{Bi}}_2{\mathrm{Fe}}_4{\mathrm{O}}_9$ phase. (b) The CEB and $H_C$ versus temperature plot for sample $B$. Large CEB (measured following field cooling under 50 kOe) could be observed at only below $T_B$. The zero-field cooled (ZFC), field cooled (FC), and remanent magnetization versus temperature plots for (c) sample $A$ and (d) sample $B$. The solid lines show the ZFC and FC magnetizations after subtraction of the contribution of the paramagnetic $\mathrm{C}/\mathrm{T}$ component in both the cases; $T_B$ turns out to be $>350\mathrm{K}$ for sample $A$ and $\sim 60\mathrm{K}$ for sample $B$.

Figure 4

Schematic of the ferromagnetic and antiferromagnetic spin interaction via superspin glass moments at the interface; left part shows the ferromagnetic core of finer ${\mathrm{Bi}}_2{\mathrm{Fe}}_4{\mathrm{O}}_9$ particle and superspin glass moments at the shell interacting with the local moments of spiral spin spin structure of bigger ${\mathrm{BiFeO}}_3$; right part shows the spin configuration and interaction energies.