Manipulating the morphology of the nano oxide domain in AuCu-iron oxide dumbbell-like nanocomposites as a tool to modify magnetic properties

Abstract

We report the colloidal synthesis of hybrid dumbbell-like nanocrystals (NCs) which feature a plasmonic metal domain (M) attached to a morphologically-tunable magnetic oxide domain (MOx). We highlight how the modulation of the amount of oleic acid (OlAc) in the synthesis mixture influences the final composition of the M domain, the morphology of the MOx domain and, consequently, the magnetic properties of the hetero-structures. In the presence of high amounts of OlAc, a crystalline, magnetite MOx is mainly formed, coupled with a partial dealloying between Au and Cu in the M domain. Decreasing the amount of OlAc preserved the AuCu alloy and resulted in the formation of core-shell structures in the MOx. Here, a disordered, poorly crystalline, glass-like maghemite shell was coupled with a highly disordered iron rich core. An investigation into the magnetic properties revealed that the disordered phase was likely responsible for the observed exchange bias, rather than the interfacial stress between the M and MOx.

Scheme 1. Schematic illustration of the synthesis procedure. Indeed, all conditions were similar except for the content of OlAc in the reaction mixture.

Fig. 1. HAADF-STEM image of the as-prepared AuCu@FeOx colloidal NCs; scale bars = 20 nm. Some of the NCs which had a core/shell morphology in the MOx domains are marked with red arrows, while the homogeneous oxide domains are highlighted by the green arrows.

Fig. 2. Zero-loss filtered BF-TEM image and corresponding elemental maps for Cu Cyan), Fe (green) and O (red), showing an Fe-rich core in the metal oxide domain. The data was collected from D3-F sample.

Fig. 3. X-ray diffraction (XRD) pattern of the as-synthesized AuCu@FeOx dumbbell NCs. Experimental data are compared with the Inorganic Crystal Structure Database (ICSD) patterns.

Fig. 4. The magnitude of the Fourier transforms of the k²χ(k) for the fresh NCs at different edges of (a) Au-L₃, (b) Cu–K and (c) Fe–K. The scattered points are the experimental data while the solid lines represent the fitted profiles.

Fig. 5. Mössbauer absorption spectra taken at different temperature for samples D1-F (a) and D3-F (b).

Fig. 6. (a) Field-cooled (FC) magnetization hysteresis loops of the samples measured at 10 K and (b) at different temperatures for D3-F. (c) and (d) show the magnified loops at small fields for (a) and (b), respectively.

Scheme 2. Schematic summary of effect of changing the OlAc content in the synthesis.