Effects of interface microstructure on magnetotransport in organic spin valve structures*

Organic semiconductors hold promise for spintronics because of their potentially long spin diffusion length. We have studied Fe/Alq$_3$ (tris(8-hydroxyquinoline) aluminum)/Co multilayer films with Alq$_3$ thickness in the range 50 to 150 nm.[1] Similar to previously reported results, we found considerable variability in the magnetotransport properties for cross junctions made in nominally identical conditions. To explore the sources of these effects, we studied the microstructure of such multilayer films by X-ray reflectometry and polarized neutron reflectometry (PNR). We found that the films show well-defined layers with limited chemical intermixing (3-5 nm) at the Alq$_3$/ferromagnet (FM) interfaces. However, larger magnetoresistance (MR) is associated with sharper Alq$_3$/FM interfaces, and with a magnetically dead Fe-rich region at the Alq$_3$/Fe interface, which may potentially circumvent the resistivity mismatch problem. The PNR data also show that the Co layer on top of the Alq$_3$ can adopt a multi-domain magnetic structure at low field and a perfect anti-parallel state is not obtained. [1] Y. Liu et al., arXiv:0810.0289v1.