Superspin glass mediated giant spontaneous exchange bias in a nanocomposite of BiFeO₃-Bi₂Fe₄O₉

We observe an enormous spontaneous exchange bias (∼300-600 Oe) - measured in an unmagnetized state following zero-field cooling - in a nanocomposite of BiFeO₃ (∼94%)-Bi₂Fe₄O ₉ (∼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 -H_Ep>H_En. The temperature dependence of H_E is nonmonotonic. It increases, initially, till ∼150 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 Bi₂Fe₄O₉ (of average size ∼19 nm) and the canted antiferromagnetic structure of BiFeO₃ (of average size ∼112 nm) via superspin glass moments at the shell.