Moreover, the peak at T w becomes sharper and more prominent when the waiting time increases from 1 h to 4 h (Fig. The exchange bias field is calculated based on H E = ( H c1 + H c2), where \()\) appears exactly at T w, which is the signature of memory effect. Moreover, the magnetic hysteresis loops exhibit a small shift along the magnetic field axes, implying the presence of an exchange bias effect in 18 nm BFO particles even in ZFC condition. 1(a), which is measured under zero-field cooling (ZFC) condition, the magnetic behavior of the sample, such as coercivity ( H c), remanent magnetization ( M r), etc. Figure 1 shows the magnetic measurement results of the 18 nm BFO particles. The prepared nanoparticles present single-phase perovskite structure, and the diameter of the smallest particles is about 18 ± 5 nm 10. Such interesting behavior is considered to be closely to its enhanced surface anisotropy and the presence of surface spin-glass state. Temperature-dependent nonmontonic variation of H E along with improved training effect is observed only in 18 nm BFO particles. In this paper, the exchange bias effect of BFO nanoparticles with different sizes is comparatively investigated. While the origin of such complex sample-dependent behavior is still unclear and needs further investigation. Recently, exchange bias effect was observed in BFO and its doped particles, and the results obtained by different research groups reveal that both the magnitude of the exchange bias field ( H E) and its temperature-dependent behavior show obvious sample dependence 12, 19, 20, 21, 22. The exchange coupling at the AFM/FM interface of a core-shell structure often leads to a phenomenon called “exchange bias”, which draws significant interests in recent years due to its important potential technological applications in various magnetic devices 13, 14, 15, 16, 17, 18. Therefore, BFO nanoparticles can be modeled by a superposition of an antiferromagnetic (AFM) core and a ferromagnetic (FM) surface 8, 10, 11, 12. While with the decrease of particle size, surface effect usually leads to a breaking of the sublattice pairing in antiferromagnet and results in a net surface magnetic moment. The combined action of exchange and spin-orbit interactions produces spin canting away from perfect antiferromagnetic ordering, resulting in a spiral spin arrangement with a wavelength of about 62 nm 7, 8, 9, 10. The magnetic structure of BFO is of G-type antiferromagnetic order with Fe magnetic moments aligned ferromagnetically within pseudocubic (111) planes and antiferromagnetically between adjacent (111) planes. BiFeO 3 (BFO) has received considerable attention as one of the most important single-phase multiferroics because of its high ferroelectric Curie temperature ( T c) of about 1103 K and antiferromagnetic Néel temperature ( T N) of about 643 K 1, 2, 3, 4, 5, 6.
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