Surface spin disorder and exchange-bias in hollow maghemite nanoparticles

Abstract

We report a comparative study of the magnetic properties of polycrystalline hollow γ-Fe2O3 nanoparticles with two distinctly different average sizes of 9.2 ± 1.1 nm and 18.7 ± 1.5 nm. High-resolution transmission electron microscopy images reveal the presence of a shell with thickness of 2 nm and 4.5 nm for the 9.2 nm and 18.7 nm nanoparticles, respectively. The field-cooled hysteresis loops show interesting features of enhanced coercivity and horizontal and vertical shifts associated with the polarity of the cooling field for both types of nanoparticles. While the anomalously large horizontal shifts and open hysteresis loop in a field as high as 9 T observed for the 9.2 nm nanoparticles corresponds to a “minor loop” of the hysteresis loop, the loop shift observed for the 18.7 nm nanoparticles manifests an intrinsic “exchange bias” (EB). Relative to the 18.5 ± 3.2 nm solid nanoparticles, a much stronger EB effect is achieved in the 18.7 nm hollow nanoparticles. Our studies point to the importance of inner and outer surface spin disorder giving rise to surface anisotropy and EB and reveal a perspective of tuning EB in hollow magnetic nanoparticle systems.