C 60 fullerenes are interesting molecular semiconductors for spintronics since they exhibit weak spin-orbit and hyperfine interactions, which is a prerequisite for long spin lifetimes. We report spin-polarized transport in spin-valve-like structures containing ultrathin (<10 nm) C 60 layers, ferromagnetic (FM) epitaxial face-centered-cubic (fcc) Co (111) contacts, AlO x tunnel barriers, and nonmagnetic Al counter electrodes. Even though genuine spin-valve behavior cannot occur for only one FM contact, we find significant tunneling anisotropic magnetoresistance (TAMR) upon rotating the in-plane magnetization, originating from spin-orbit interaction (SOI) induced anisotropy of the fcc (111) Co bands. The uniaxial magnetocrystalline anisotropy of the Co electrodes results in a predominantly twofold symmetric in-plane TAMR effect. We investigated the TAMR effect in the direct tunneling regime (2 nm C 60 ), at the transition point to two-step tunneling (4 nm C 60 ), and in the multistep regime (8 nm C 60 ). A sizable TAMR of 4.5% is found at 5 K under application of a 500-mT in-plane magnetic field for C 60 layers of 2 nm, which is strongly suppressed at 8 nm thickness, indicating that TAMR may strongly contribute to the “spin-valve‿ signal for direct tunneling, but not for multistep tunneling. The TAMR effect is proposed to be due to a combination of SOI induced modulation of the tunneling DOS upon rotating the in-plane magnetization of the fcc epitaxial Co thin film, resonant tunneling processes involving interfacial states, and different Bychkov-Rashba SOI at the different interfaces.
|Number of pages||8|
|Journal||Physical review B: Condensed matter and materials physics|
|Publication status||Published - 16 May 2014|