Modulating Spin Polarization in Paramagnetic Alq₃-Co Hybrid Clusters
Li-Chung Yu1*, Yu-Ling Lai1, Ming-Wei Lin1, Hung-Wei Shiu1, Jiu-Hua Lin1,2, Yao-Jane Hsu1,3
1National Synchrotron Radiation Research Center, Hsinchu, Taiwan
2Master Degree Program for Science and Technology of Accelerator Light Sources, National Chiao Tung University, Hsinchu, Taiwan
3Department of Photonics, National Cheng Kung University, Tainan, Taiwan
* Presenter:Li-Chung Yu,
Tris(8-hydroxyquinoline)aluminium (Alq₃) with high electron-transport properties and promising emitting layer in organic light emitting diode has been successfully sandwiched by two ferromagnetic electrodes as a buffer layer in organic spin valves (OSVs). The negative magnetoresistance (MR) change was found in vertical LSMO/Alq₃/Co OSV devices. However sign and values of MR is strongly influenced by the spin polarization at LSMO/Alq₃/Co interface. According to literatures, the formation of Co-Alq₃ hybrid structure caused by the charge transfer from Co to Alq₃ at the interface of Alq₃ and Co plays a key role to affect the interfacial spin polarization. The N atoms in Co-Alq₃ hybrid structure is spin polarized in external magnetic field, proving Alq₃ a unique and promising organic material for spin filtering in OSV. To manipulate the spin polarization of hybrid N atoms, we co-evaporated Alq₃ and Co to modulate the degree of hybridization between Co and Alq₃ in experiments. Three paramagnetic Alq₃-Co thin films with different volume ratios of Alq₃ to Co were fabricated by co-evaporation processes. The results of X-ray photoelectron spectra (XPS) demonstrate the quantities of hybrid N correlates with the deposition amount of Co in three paramagnetic thin films. The coercivity (Hc) of all paramagnetic thin films increases with the growth of Co concentration. Noticeably, the dichroic (DR) ratio of Co L-edge XMCD in co-evaporated film with same volume of Alq₃ and Co is larger than pure metallic Co, meaning that the magnetic hardening is induced by the hybrid structure which polarize the Co 3d unpaired electrons. In conclusion, Alq₃ not only successfully mixed with Co to form hybrid structure but also induce magnetic hardening upon hybridization which expresses efficient spin filtering.

Keywords: spin polarization, hybridization, magnetic coupling, co-evaporation, spin-filter