The exchange bias behavior of BiFeO3 nanoparticles with natural core-shell structure

Abstract

The surface and interface effects of small antiferromagnetic nanostructures are important on the modulation of their magnetic properties. In this report, temperature and particle size dependent magnetic exchange bias effect was investigated in BiFeO3 (BFO) nanoparticles that possess natural core-shell structure. Nonmonotonic variation of exchange bias field, interesting surface spin-glass state and improved exchange bias training effect are only obtained in 18 nm BFO particles. Based on comparative experiments on particles with different sizes, we found that the surface spins and the interaction among them show great effect on the interfacial exchange coupling of the core-shell structure, and thus are responsible for the peculiar exchange bias behavior in small BFO nanoparticles. Our work provides the effect of surface spin state on the magnetic characteristics of nanomaterials and will favor their applications on spintronic devices.

Control of Multiferroic properties in BiFeO3 nanoparticles

“…This effect is observed at low temperatures close 2 K. In our NPs, such effect could be associated with a core-shell structure of NPs with an antiferromagnetic core and a ferromagnetic shell. This effect have been observed in BFO NPs 29,30 . EB field has been calculated with the left ( H C 1 ) and right ( H C 2 ) coercive fields as .…”

Spin-glass transition in porous spheres BiFeO3

“…On the contrary, these moments are arranged parallel to the applied field during FC measurement, which leads to a large divergence of FC and ZFC curves below the freezing temperature. Similar changes in FC and ZFC are observed in BFO nanopowders [7][8][9]. A distinctive feature is that the maximum on the ZFC curve in nanoparticles is observed as a rule at a temperature of about 50 K.…”

“…The coercive force values at room temperature are also similar. The temperature dependence H C (T ) exhibits a minimum near 100 K. An analogous H C (T ) dependence is observed in nanocrystals [9] with a minimum at 50 -60 K. The authors of work [8] attribute the H C (T ) minimum to the transition of iron ions on the surface of nanoparticles to spin cluster glass state, which is confirmed by the exchange displacement of hysteresis loops. It is interesting to study the exchange displacement in porous spheres BFO.…”

“…Recently, the study of magnetic properties was focused on BFO films and nanopowders. Different experiments, including direct magnetic measurements, suggest several magnetic transitions in the temperature range from 5 to 250 K [7][8][9]. The nature of these transitions is not entirely understood.…”

“…At the same time, some questions remain undetermined. Unusual effects of exchange displacement of hysteresis loop above transition temperature to spin glass state and nonmonotonic variation of coercive force require further research [9,13]. Park et al [3] inferred that the anomalous magnetization behavior in BFO nanoparticles arises from a complex interplay between the finite size effects, interparticle interaction, and a random distribution of anisotropy axis in nanoparticle assemblies.…”