Origin of the asymmetric exchange bias in BiFeO_3/Bi_2Fe_4O_9 nanocomposite

摘要

We show from detailed magnetometry across 2-300 K that the BiFeO_3-Bi_2Fe_4O_9 nanocomposite offers a unique spin morphology where superspin glass (SSG) and dilute antiferromagnet in a field (DAFF) coexist at the interface between ferromagnetic Bi_2Fe_4O_9 and antiferromagnetic BiFeO_3. The coexisting SSG and DAFF combine to form a local spin texture, which gives rise to a path-dependent exchange bias below the spin freezing temperature (～29 K). The exchange bias varies depending on the protocol or path followed in tracing the hysteresis loop. The exchange bias has been observed below the blocking temperature (T_B) 60 K of the superparamagnetic Bi_2Fe_4O_9. The conventional exchange bias (CEB) increases nonmonotonically as temperature decreases. The magnitude of both exchange bias (H_E) and coercivity (H_C) increase with decrease in temperature and are found to be asymmetric below 20 K depending on the path followed in tracing the hysteresis loop and bias field. The local spin texture at the interface between ferromagnetic and antiferromagnetic particles generates a nonswitchable unidirectional anisotropy along the negative direction of the applied field. The influence of this texture also shows up in "asymmetric" jumps in the hysteresis loop at 2 K, which smears off at higher temperature. The role of the interface spin texture in yielding the path dependency of exchange bias is thus clearly delineated.