Hybrid Metallic Nanowires: Tailoring Magnetic and Plasmonic Properties by Pulsed Laser Deposition

Abstract

Hybrid nanomaterials, combining ferromagnetic and plasmonic metals, are potential model systems for the study of emergent magnetoplasmonic phenomena, which motivate the development of new nanoarchitectures through top-down and bottom-up approaches. Vertically aligned nanocomposites (VANs) consist of two functional phases embedded in a matrix as illustrated on Fig. 1. Such epitaxial systems can be grown by pulsed laser deposition (PLD); the embedded nanowires (NWs) are formed by self-assembly during the growth. In this contribution, we report on the growth and study of the structural and physical properties of new VANs consisting of hybrid NWs with ferromagnetic and plasmonic parts embedded in an epitaxial oxide thin film.Cobalt and gold, compounds that show low bulk miscibility, are a typical material choice for the growth of hybrid NWs [1]. However, at room temperature pure Co has a hexagonal close-packed structure, which is detrimental to the uniaxial anisotropy of the NWs, and thus would not permit to obtain a bistable magnetic system. In order to get hard uniaxial magnetic properties, the ferromagnetic NWs were made of a CoNi alloy. A compromise of 80% of cobalt and 20% of nickel was chosen to retain an optimal anisotropy with the face-centered cubic phase (fcc) [2]. The plasmonic NWs were made of Au, known to be able to sustain localised plasmon resonances, collective oscillations of the free electrons, in the visible-near infrared range.Au/Co0.8Ni0.2 nanowires embedded in a SrTiO3 matrix were grown on a SrTiO3(100) substrate at a substrate temperature of 600 °C and high vacuum (< 10-5 mbar) by sequential pulsed laser deposition, leveraging self-assembly mechanisms driven by metal-oxide phase segregation. Sequential PLD, based on the repetition of a basic sequence, is well suited to get a precise nanoarchitecture; as it offers control over the structure (morphology, epitaxy) and the composition. Such control is crucial for future magnetoplasmonic studies, given that both magnetic and plasmonic properties are significantly affected by the structural properties.The structural properties of the NWs were investigated by transmission electron microscopy (TEM) and x-ray diffraction (XRD) using synchrotron radiation. TEM images (Fig. 1) in plan view show circular sections with a diameter of ≈ 4 nm, whereas scanning transmission microscopy images in high-angle annular dark field mode in cross-sectional view reveal elongated segregated ribbons, thus proving the growth of VANs displaying phase segregation in oxide matrix. Further TEM investigations show the proneness of gold NWs to grow onto cobalt-nickel NWs with a proportion of ≈ 80%. XRD indicates a fcc structure with a predominant cube-on-cube epitaxy of Au, Co0.8Ni0.2 and SrTiO3 lattices; in addition, an axial strain up to 5% is measured, pointing a stretch of the Co0.8Ni0.2 NWs along the growth axis.The magnetic hysteresis of hybrid NWs were recorded by vibrating sample magnetometry. As shown on Fig. 2, the response is anisotropic and the easy axis of magnetisation is along the long axis of the NW, with a coercive field Hc ≈ 1 T at 10 K; such significant anisotropy stems from the elongated shape of the NWs (shape anisotropy) and the axial strain induced by the epitaxy with the STO matrix (magnetoelastic anisotropy). Furthermore, spectroscopic ellipsometry measurements display two absorption bands at visible and near-infrared wavelengths, attributed to the transverse and longitudinal localised plasmon resonances (LPRs). Such LPRs can be shifted to a suitable wavelength by control of the morphology of the Au NWs. Due to the Joule effect, the plasmon excitation would provoke the heating of the Au NWs, which would act as nanosources of heat for future heat-assisted magnetisation reversal experiments.The separate control of the structure of Au and CoNi NWs enables the tuning of highly anisotropic magnetic and optical properties. This establishes hybrid metallic NWs embedded in an oxide matrix grown by PLD as promising systems for future magnetoplasmonic studies. **