Effect of Chiral Molecules on the Electron’s Spin Wavefunction at Interfaces

Kelvin-probe measurements on ferromagnetic thin film electrodes coated with self-assembled monolayers of chiral molecules reveal that the electron penetration from the metal electrode into the chiral molecules depends on the ferromagnet’s magnetization direction and the molecules’ chirality. Electrostatic potential differences as large as 100 mV are observed. These changes arise from the applied oscillating electric field, which drives spin-dependent charge penetration from the ferromagnetic substrate to the chiral molecules. The enantiospecificity of the response is studied as a function of the magnetization strength, the magnetization direction, and the handedness and length of the chiral molecules. These new phenomena are rationalized in terms of the chiral-induced spin selectivity (CISS) effect, in which one spin orientation of electrons from the ferromagnet penetrates more easily into a chiral molecule than does the other orientation. The large potential changes (>kT at room temperature) manifested here imply that this phenomenon is important for spin transport in chiral spintronic devices and for magneto-electrochemistry of chiral molecules.

in the main text, ITO substrates were coated with 3 nm of Ti buffer layer, followed by 10 nm of polycrystalline Ni, and 10 nm of Au using a Plassys evaporator. For the experiments describe in Fig. 4, E-beam evaporation of Ti/Ni/Au-10nm/120nm/10nm on Si-100 was used as the substrate.
Epitaxial thin films with a cobalt ferromagnetic layer, were used in Figures 2 and 3. The films were grown using PREVAC molecular beam epitaxy system (MBE) with a base pressure of 10 -10 Torr, according to the following configuration Al2O3 (0001)/Pt (5 nm)/Au (20 nm)/Co (1.5nm)/Au (5 nm). A Pt buffer layer was deposited at 650°C to ensure an atomically flat buffer layer surface allowing the epitaxial growth of consecutive layers. Sharp streaks were visible in the in-situ obtained RHEED images for each growth step. The crystallographic orientation of the epitaxially grown layers was as follows: Pt(111), Au(111), Co(0001), and Au(111). The Pt buffer and Co layers were deposited using an electron gun; Au layers were deposited from high temperature effusion cells. The sample with the flat uniform Co thickness of 1.5 nm has a perpendicular anisotropy and features a rectangular hysteresis loop for a magnetic field direction perpendicular to the sample plane, which is characteristic of the out-of-plane easy axis. Wedge type samples were also studied and they were deposited using a movable shutter controlled by a step motor. Two types of wedge samples were also grown: an Au-wedge Al2O3 (0001)/Pt (5 nm)/Au (20 nm)/Co (1.5nm)/Au wedge (2-10 nm) and a Co-wedge: For a Co layer thickness above 2 nm a spin reorientation transition occurs, and the magnetization easy axis changes from perpendicular to in-plane direction, resulting in the inplane anisotropy. Hysteresis loops were measured by the polar magneto-optic Kerr effect (P-MOKE) along the Co wedge direction and by SQUID, for the sample with flat Co layer (see figure S1).
The standard, phenomenological relationship based on Neel predictions S3 K1eff (d) = -2π Ms 2 s + K1v + 2K1s/d where d denotes thin film thickness and Ms saturation magnetization, 1 is used for describing the anisotropy in the thin films, containing the demagnetization term (shape anisotropy) and two anisotropy terms: volume K1v and surface term K1s originating from the interfaces with buffer layer and the overlayer. As the K1s is inversely proportional to the thin film thickness d, for a higher value of d a spin reorientation transition takes place and the easy axis direction rotates from perpendicular to in-plane direction. Figure S1. Coercivity dependence on the thickness of the Co layer for epitaxial Au/Co/Au substrates Cobalt was deposited as a wedge using a computer controlled movable shutter. measurements were performed at a 20 nm distance from the surface. Because of the dielectric nature of the monolayers, the sample was not grounded. For the magnetic field experiments, a permanent magnet was placed underneath the substrate such that ~ 200 mT magnetic field (measured using a Gauss meter) was applied to the substrate. Because the measurements are susceptible to environmental conditions and dust the acquired data are only used for measurements with reproducible KPFM scans. Figure S2 shows an example of a topography and KPFM image for one of the samples.

Macroscopic CPD measurements
The CPD of the surfaces was determined using a commercial Kelvin probe instrument (Delta Phi Besocke, Jülich, Germany) within a Faraday cage at atmospheric pressure. The reference probe consisted of a gold grid. The CPD signal was allowed to stabilize before recording, where ∆ is the difference in the direction of magnetic field.
Substrate preparation and cleaning: Substrates used for the CPD measurement are grown by electron-beam (ODEM evaporator) where Si-100 wafer used as substrate to grow Ti/Ni/Au-