Exchange bias effect inAu-Fe3O4dumbbell nanoparticles induced by the charge transfer from gold

Abstract

We have studied the origin of the exchange bias effect in the Au-Fe 3 O 4 dumbbell nanoparticles in two samples with different sizes of the Au seed nanoparticles (4.1 and 2.7 nm) and same size of Fe 3 O 4 nanoparticles (9.8 nm). The magnetization, small-angle neutron scattering, synchrotron x-ray diffraction and scanning transmission electron microscope measurements determined the antiferromagnetic FeO wüstite phase within Fe 3 O 4 nanoparticles, originating at the interface with the Au nanoparticles. The interface between antiferromagnetic FeO and ferrimagnetic Fe 3 O 4 is giving rise to the exchange bias effect. The strength of the exchange bias fields depends on the interfacial area and lattice mismatch between both phases. We propose that the charge transfer from the Au nanoparticles is responsible for a partial reduction of the Fe 3 O 4 into FeO phase at the interface with Au nanoparticles. The Au-O bonds are formed, presumably across the interface to accommodate an excess of oxygen released during the reduction of magnetite.2

Exchange Bias Effects in Iron Oxide-Based Nanoparticle Systems

“…This is in contrast with the case of Au/γ-Fe 2 O 3 nanocrystals, in which γ-Fe 2 O 3 has been shown to disfavor such a spin polarization transfer process [94]. A later study of Feygenson et al [38] has indeed confirmed that the charge transfer from the Au nanoparticles is responsible for a partial reduction of the Fe 3 O 4 into the FeO phase at the interface with Au nanoparticles, and that the coupling between the Fe 3 O 4 and FeO induces an EB effect. Since FeO is formed at the interface between Fe 3 O 4 and Au [94], in the case of Au/γ-Fe 2 O 3 core/shell MNPs [93] the absence of FeO suggests that the EB effect is attributed to the presence of γ-Fe 2 O 3 hollow morphology.…”

“…An issue of potential interest is if the surface spin alignment in nanoparticles could be influenced by forming interfaces with other non-magnetic materials [37,38,91,92,93,94,95,96]. If this is indeed possible, then it would provide an excellent mechanism to control the exchange coupling between the surface and core spins in individual MNPs leading to novel magnetic properties.…”

“…Since FeO is formed at the interface between Fe 3 O 4 and Au [94], in the case of Au/γ-Fe 2 O 3 core/shell MNPs [93] the absence of FeO suggests that the EB effect is attributed to the presence of γ-Fe 2 O 3 hollow morphology. Figure 10a,b shows TEM images of two types of MNPs with different interfaces [94], through which the different EB mechanisms, as recently proposed for these nanostructures and others [38,93], can be understood.…”

“…Recently, iron/iron oxide core/shell nanoparticle systems have been exploited for such applications because the combination of the high magnetization of the core (Fe) and the chemical stability and biocompatibility of the shell (Fe 3 O 4 or γ-Fe 2 O 3 ) lead to more suitable overall properties than either material alone [24,25,26]. 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].…”

Citrate Stabilized Au‐FexOy Nanocomposites for Variable Exchange Bias, Catalytic Properties and Reversible Interaction with Doxorubicin

“…In recent times, among gold nanocomposites, EB effect has been reported in Au‐Fe 3 O 4 core‐shell and nano‐dumbbell . Presence of FeO phase is quite common for such Au‐Fe 3 O 4 nanomaterials, where the EB arises due to the interface of the antiferromagnetic FeO and ferromagnetic Fe 3 O 4 . This FeO phase is generally obtained as a reduced product of Fe 3 O 4 .…”