Revealing the key role of molecular packing on interface spin polarization at two-dimensional limit in spintronic devices

Understanding spinterfaces between magnetic metals and organic semiconductors is essential to unlock the great potentials that organic materials host for spintronic applications. Although plenty of efforts have been devoted to studying organic spintronic devices, exploring the role of metal/molecule spinterfaces at two-dimensional limit remains challenging because of excessive disorders and traps at the interfaces. Here, we demonstrate atomically smooth metal/molecule interfaces through nondestructively transferring magnetic electrodes on epitaxial grown single-crystalline layered organic films. Using such high-quality interfaces, we investigate spin injection of spin-valve devices based on organic films of different layers, in which molecules are packed in different manners. We find that the measured magnetoresistance and the estimated spin polarization increase markedly for bilayer devices compared with their monolayer counterparts. These observations reveal the key role of molecular packing on spin polarization, which is supported by density functional theory calculations. Our findings provide promising routes toward designing spinterfaces for organic spintronic devices.