Femtosecond Laser Ablation-Based Interfacial Engineering of Magneto-Plasmonic Janus Au–Ni and Au–Co Nanoparticles for Multifunctional Applications

Femtosecond laser ablation-based synthesis offers unique opportunities to tailor nanoscale interfaces in magneto-plasmonic nanomaterials, yet directing alloying versus oxidation under nonequilibrium conditions remains challenging. Here, a two-step femtosecond pulsed laser ablation in liquid (PLAL) strategy is used to synthesize magneto-plasmonic Janus Au–Ni and Au–Co nanoparticles, where the ablation sequence (AuinNi, NiinAu, AuinCo, and CoinAu) acts as a kinetic design parameter for interfacial architecture. High-resolution transmission electron microscopy (HRTEM), together with X-ray diffraction, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy, reveals sequence-dependent nanoscale alloy–oxide heterostructures with distinct fractions of single-contrast, multiphase spherical, and Janus-like particles; qualitative HRTEM statistics show that Janus-like morphologies constitute 16–44% of the sub-15 nm population. Despite the thermodynamic preference for fully oxidized core–shell structures, femtosecond PLAL stabilizes metastable configurations in which AuNi or AuCo regions coexist with NiO/Ni(OH) 2 or CoO/Co 3 O 4 within individual nanoparticles. These sequence-engineered interfaces produce markedly different low-temperature magnetic responses. In particular, AuinNi exhibits enhanced coercivity of 2130 Oe and exchange bias field of −881 Oe at 5 K, exceeding values for Ni and Co oxide references and comparable Au–Ni systems. Optical extinction spectra show sequence-dependent damping and red-shifts of the Au localized surface plasmon resonance arising from alloy formation, oxide environment, and interparticle coupling. As a functional illustration, the nanocomposites also support analyte-dependent Raman responses of methylene blue under identical 532 nm excitation; given the limited data set and preresonant conditions, these measurements are treated as preliminary comparative trends rather than quantitative surface-enhanced Raman scattering benchmarking. Overall, this work demonstrates a femtosecond laser ablation-based route for interfacial engineering of magneto-plasmonic Janus Au–Ni and Au–Co nanoparticles, yielding magnetically hard and plasmonically addressable nanomaterials with potential relevance for magnetic manipulation, optical readout, and comparative Raman-based analyte response.