Polydopamine Assisted Electroless Deposition of Strongly Adhesive NiFe Films for Flexible Spintronics

Cost-effective techniques for depositing durable magnetic thin films are essential for realizing flexible spintronic devices in wearable and soft robotics applications. Here, we introduce a highly scalable electroless deposition technique for coating ferromagnetic Ni₈₀Fe₂₀ thin films onto flexible polyimide and polyethylene terephthalate substrates via a polydopamine intermediate layer. The resultant films demonstrated very good adhesion strength, especially for those on polyimide substrates, which attained the highest ASTM 3359 rating of 5B and exhibited a tensile pull-off strength greater than 5.82 MPa (ASTM D4541). With the assistance of XPS and TEM results, this unusual adhesive strength of NiFe on the polymers can be traced to the formation of primary bonds across the entire coating: (i) covalent bonding between the polymer substrate and polydopamine, (ii) coordinate covalent bonding between catechol groups in polydopamine and palladium ions, and (iii) metallic bonding between palladium and NiFe. By annealing the NiFe-on-polyimide samples at 300 °C-400 °C, the crystallinity of the material was found to improve, which increased the saturation magnetization of NiFe to 1350 emu/cm³ and decreased the coercivity to 20 Oe. Ferromagnetic resonance also showed ∼31% improvement in the film's Gilbert damping constant after annealing at 300 °C. Application of a bending strain increased the squareness ratio by 22% but decreased the saturation magnetization of the annealed film by 18%. These results suggest that the highly adhesive NiFe films on flexible polyimide substrates were of sufficient quality for spintronic devices, although their properties could be improved if the surface roughness could be decreased to that of physically deposited films.