Synthesis and Characterization of Superparamagnetic CoPt Alloy Nanoparticles Exhibiting Magneto-Plasmonic Responses
Authors: Zhang Chen , Ishida Takuya , Lee Seung Hyuk , Tatsuma Tetsu 
Published: December 2024

Magnetic materials containing transition metals such as Fe, Co, and Ni have attracted considerable interest, in particular for their unique properties, which allow them to be applied to devices for magnetic control of optical communications and magneto-optical data storage. Magnetic control of catalysis and electrocatalysis is also a potential application of magnetic materials. Apparent catalytic activity can be controlled via a magnetohydrodynamic (MHD) effect, which affect mass transport rate in the vicinity of the magnetic material.[1] In addition, enantioselectivity can be given to the system by taking advantage of chiral-induced spin selectivity (CISS), which can be induced by magnetic materials.[2] If a magnetic material is broken down into nanoparticles, they could also obtain high catalytic activities. In addition, they would be superparamagnetic materials, which respond very rapidly to external magnetic fields without hysteresis. If the magnetic nanoparticles are metallic, they might exhibit magneto-optical responses such as magnetic circular dichroism (MCD), in association with localized surface plasmon resonance (LSPR).[3] LSPR would also allow the nanoparticles to be used for photocatalysis based on plasmon-induced charge separation (PICS).[4] These characteristics render magnetic nanomaterials suitable for catalysis, electrocatalysis, and photoelectrocatalysis with controllable activity and selectivity.

With these points in mind, we synthesize superparamagnetic CoPt nanoparticles through a convenient wet chemical process[5] and examine their magneto-optical properties, more specifically, magnetic circular dichroism (MCD), in the UV-visible range. MCD characteristics would be important indices for magnetic materials to assess their magnetic properties including CISS. CoPt nanoparticles are well known for their high electrocatalytic activities as well as high magnetism due to strong spin-orbit interaction.[6] CoPt superlattice nanoparticles has been reported to show MCD in the near-infrared range and applied to magnetic switching of plasmonic laser.[7] However, MCD in the UV-visible range has not yet been reported for CoPt materials to the best of our knowledge.

Co, Pt, and CoPt nanoparticles were synthesized via an oleylamine co-reduction method. Scanning electron microscopy (SEM) image shows that CoPt nanoparticles have a spherical shape with an average size of 6 nm (Fig. 1a). On the basis of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and superconducting quantum interference device (SQUID) analyses, we have confirmed that the synthesized CoPt nanoparticles have face-centered cubic (fcc) structure with a small amount of cobalt oxides at the particle surface of CoPt, and show superparamagnetism at room temperature.

The magneto-optical properties of the Co, Pt, and CoPt nanoparticles were evaluated by a dissymmetry factor, g
MCD-factor which was obtained by normalizing MCD with extinction. As shown in Fig. 1b, the Pt nanoparticles showed almost no MCD responses, and the Co nanoparticles exhibited very weak MCD responses, although the latter are magnetic. In contrast, the CoPt nanoparticles showed sufficiently strong MCD signals over the UV-visible range examined. The value of their g
MCD-factor was approximately 0.034, which was in the range of very high g
MCD-factor values.

The possible origin of the MCD properties is circular electron motion in the metallic nanoparticle induced by circularly polarized light. It could also be coupled with circular mode of LSPR. In an external magnetic field, a Lorentz force is applied to the rotating charges, and the resonant light energy depends on the direction of the rotation, resulting in lifted degeneracy (Fig. 1c). Although this type of MCD is also observed for non-magnetic Au nanoparticles, introduction of a magnetic component to the nanosystem enhances the local magnetic field and thereby MCD responses. Likewise, the magnetism of Co would be positive for the MCD responses in the present system. In addition, the spin-orbit coupling induced by Pt may enhance the MCD responses further.

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Figure 1