Measurement Platform to Probe the Mechanism of Chiral-Induced Spin Selectivity through Direction-Dependent Magnetic Conductive Atomic Force Microscopy

This work introduces a magnetic conductive atomic force microscopy (mc-AFM) measurement platform for determining spin polarizations, arising from the chiral-induced spin selectivity (CISS) effect along different directions in helical conducting fibers. By using the principle that the spin preference for electron transport in a chiral material changes with the momentum of the electron, this method quantifies the spin polarization of chiral materials, which straddle a ferromagnetic electrode, i.e., by taking measurements in regions to the right and left of the electrode while it is magnetized in-plane. The working mechanism of the measurement is shown using chiral polyaniline (PANI) fibers, and they reveal that the longitudinal, along the fiber's helical axis, and transverse, perpendicular to the fiber axis, magnetoresistance differ by about a factor of 2. The observations imply that the spin polarization in PANI fibers is not consistent with models that attribute the spin selectivity (or magnetoresistance) solely to the spinterface or to spin-dependent charge injection barriers. In aggregate, this new platform offers a simplified approach for extending the mc-AFM method to resolving the spin-filtered charge currents along different directions in oriented samples.