Tuning multifunctional properties of Co80La20 thin films through thickness control and thermal annealing on polymer substrates

Amorphous cobalt–lanthanum (Co₈₀La₂₀) thin films with thicknesses of 10 nm to 50 nm were successfully deposited on flexible polyethylene terephthalate (PET) and polymethyl methacrylate (PMMA) substrates via direct-current (DC) magnetron sputtering and subsequently studied under as-deposited conditions as well as mild annealing at 40 °C and 80 °C. X-ray diffraction (XRD) confirmed that all films remained amorphous, demonstrating exceptional structural stability attributed to the high glass-forming ability of Co–La alloys. Atomic force microscopy (AFM) and contact angle measurements revealed that surface roughness, hydrophobicity, and surface energy increased with annealing, with PET-supported films showing the most pronounced morphological evolution. Notably, 40 °C annealing produced dense, ordered surfaces with the highest surface energy and maximum water contact angle. Optical measurements exhibited a strong thickness-dependent transmittance decrease, with PET films slightly less transparent than PMMA due to enhanced scattering, while annealing had minimal effect, confirming optical stability. Electrical characterization revealed that sheet resistance decreased from 650 Ω/sq to 82 Ω/sq on PET and from 655 Ω/sq to 68 Ω/sq on PMMA as film thickness increased, while mild annealing significantly enhanced carrier mobility, reaching 494 cm²/V·s and 721 cm²/V·s on PET and PMMA substrates, respectively. Mechanical testing revealed maximum hardness and Young's modulus of 5.1 GPa and 26.7 GPa on PET, while magnetic analysis demonstrated optimal saturation magnetization (Ms) 1212 emu/cm³ and remanent magnetization (Mr) 860 emu/cm³ at 40 °C annealing. These findings indicate that precise control of substrate, thickness, and mild thermal treatment enables tuning of electrical, mechanical, and magnetic properties, positioning Co₈₀La₂₀ thin films as promising candidates for flexible electronics, wearable sensors, and multifunctional device applications.