Observations of large spin mixing conductance in poly(3-methylthiophene) polymer brushes/NiFe heterostructures*

Observations of large inverse spin Hall effect (ISHE) in a fullerene (C₆₀) thin film – usually only appearing in metals and semiconductors with heavy elements - indicate that spin-orbit coupling in terms of spin Hall angles would be significantly influenced by chemical structure parameters, such as stacking and curvatures. Here, we report studies on the effective spin mixing conductance in ‘spuncast’ poly(3-hexylthiophene) (P3HT) and ‘polymer brush’ poly(3-methylthiophene) (P3MT) thin films using ferromagnetic resonance (FMR) and spin pumping techniques. P3HT and P3MT films were spuncast and self-assembly grown, respectively, onto ITO/glass substrates followed by NiFe. Frequency-dependent FMR and ISHE measurements were carried out on both films as a function of film thickness, from which the effective spin mixing conductances were obtained. We find that the P3MT films exhibit a large increase in the effective spin mixing conductance, in contrast to that in P3HT films. The large spin mixing conductance in the ‘polymer brush’ P3MT thin films is attributed to morphological differences as compared with the spuncast films.