Surface spin disorder and exchange-bias in hollow maghemite nanoparticles

Abstract

We report a comparative study of the magnetic properties of polycrystalline hollow c-Fe 2 O 3 nanoparticles with two distinctly different average sizes of 9.2 6 1.1 nm and 18.7 6 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 6 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. V

Silicon/Hollow γ-Fe2O3 Nanoparticles as Efficient Anodes for Li-Ion Batteries

“…The hysteresis loop of Sample 2 (hollow particles) was asymmetric due to large magnetic hysteresis as already reported for nonsupported γ-Fe 2 O 3 hollow nanoparticles. 19 Temperature dependent measurement at low field (5.0 mT) shows a blocking temperature higher than 300 K for the three samples, consistent with the existence of a ferromagnetic phase and the particle size (>11 nm) obtained for the three samples ( Figure 4B). The M-T curves do not show any paramagnetic phase as evidenced by the low temperature behavior of the FC.…”

Exchange bias effect in Au-Fe3O4nanocomposites

“…While no EB is observed for the Fe 3 O 4 nanoparticles, both the dimers and the clusters are found to exhibit substantial exchange fields (H E ) which were calculated using the formula: the cooling field creating a vertical asymmetry. 26 The EB results suggest that there is a pinning layer present somewhere in the Au-Fe 3 O 4 particles (which is not present in the spherical Fe 3 O 4 nanoparticles) and that this pinning is much stronger in the clusters than in the dimers.…”

Exchange Bias Effects in Iron Oxide-Based Nanoparticle Systems

“…Hollow magnetic nanoparticles have been especially attractive for the study of EB due to the formation of an additional inner surface [40,41,44]. The increased surface area can give rise to an enhanced spin disorder and hence a higher anisotropy.…”

“…By contrast, SPM susceptibility contributes 97% to the total magnetic moment and only a 3% contribution comes from the PM susceptibility for the case of the 18 nm hollow MNPs. Since a highly linear contribution to the magnetization results mainly from the uncompensated spins at the shell surfaces, these results reveal a larger number of disordered surface spins present in the 9 nm hollow particles than in the 18 nm hollow particles [41], thus explaining the non-saturation feature of magnetization and the smaller value of magnetization for the 9 nm hollow MNPs (Figure 8c). This can be reconciled with our recent study that shows that the magnetic relaxation in the γ-Fe 2 O 3 hollow nanoparticle ensembles is best described by a non-interacting particle model, as the dominant role of disordered surface spins and severely reduced particle magnetization renders the influence of dipolar interactions negligible in determining the low-temperature magnetic behavior [42,43].…”

“…These systems also provide excellent models for probing the roles played by interface and surface spins and their impacts on EB in exchange-coupled nanostructures, inspiring a large body of work on core/shell Fe/γ-Fe 2 O 3 [27,28,29], Fe/Fe 3 O 4 [30,31], FeO/Fe 3 O 4 [32,33,34], Fe 3 O 4 /γ-Fe 2 O 3 [35], γ-Fe 2 O 3 /CoO [36], and Au/Fe 3 O 4 particles [37,38,39]. Oxidization-driven migration of metal atoms from the core to the shell of iron/iron oxide nanoparticle systems has been shown to occur via the Kirkendall effect, leading to a morphological transformation into hollow nanoparticles, where the additional (inner) surface area strongly affects magnetic properties including EB [40,41,42,43,44]. …”