Magnetic and Optical Properties of Au-Co Solid Solution and Phase-Separated Thin Films and Nanoparticles

David A Garfinkel; Nan Tang; Grace Pakeltis; Reece Emery; Ilia N Ivanov; Dustin A Gilbert; Philip D Rack

doi:10.1021/acsami.2c02028

Magnetic and Optical Properties of Au-Co Solid Solution and Phase-Separated Thin Films and Nanoparticles

ACS Appl Mater Interfaces. 2022 Apr 6;14(13):15047-15058. doi: 10.1021/acsami.2c02028. Epub 2022 Mar 25.

Authors

David A Garfinkel¹, Nan Tang¹, Grace Pakeltis¹, Reece Emery¹, Ilia N Ivanov², Dustin A Gilbert¹, Philip D Rack¹

Affiliations

¹ Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996, United States.
² Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States.

PMID: 35333040
DOI: 10.1021/acsami.2c02028

Abstract

The chemical composition and morphology of Au_xCo_1-x thin films and nanoparticles are controlled via a combination of cosputtering, pulsed laser-induced dewetting (PLiD), and annealing, leading to tunable magnetic and optical properties. Regardless of chemical composition, the as-deposited thin films and as-PLiD nanoparticles are found to possess a face-centered cubic (FCC) Au_xCo_1-x solid-solution crystal structure. Annealing results in large phase-separated grains of Au and Co in both the thin films and nanostructures for all chemical compositions. The magnetic and optical properties are characterized via vibrating sample magnetometry (VSM), ellipsometry, optical transmission spectroscopy, and electron energy loss spectroscopy (EELS). Despite the exceptionally high magnetic anisotropy inherent to Co, the presence of sufficient Au (72 atom %) in the Au_xCo_1-x solid solution results in superparamagnetic thin films. Among the as-PLiD nanoparticle samples, an increased Co composition leads to a departure from traditional ferromagnetism in favor of wasp-waisted hysteresis caused by magnetic vortices. Phase separation resulting from annealing leads to ferromagnetism for all compositions in both the thin films and nanoparticles. The optical properties of Au_xCo_1-x nanostructures are also largely influenced by the chemical morphology, where the Au_xCo_1-x intermixed solid solution has significantly damped plasmonic performance relative to pure Au and comparable to pure Co. Phase separation greatly enhances the quality factor, optical absorption, and electron energy loss spectroscopy (EELS) signatures. The enhancement of the localized surface plasmon resonances (LSPRs) scales with the reduction in Co composition, despite EELS evidence that excitation of the Co portions of a nanoparticle can provide a similar, and in some instances enhanced, LSPR resonance compared to Au. This behavior, however, is seemingly limited to the LSPR dipole mode, while higher-order modes are greatly damped by a Co aloof position. This observed magneto-plasmonic functionality and tunability could be applicable in biomedicine, namely, cancer therapeutics.

Keywords: magnetic; nanoparticles; optical; phase separation; plasmonic; solid solution.