Spinterfaces in Alq3 and Crq3
Figure 1. Left panel : Spin polarized tunneling spectra of single Alq3 molecules that show a modifucation of the surface state near 0 V compared to the clean Cr(001) state; Reight panel: similar spectra for Crq3 where the surface state is split into unoccupied (positive bias) and occupied (negative bias) bands with opposite spin polarization. The lower half of the image shows the DFT calculation of the difference in bonding geometry of the two molecules that leans to such dramatic variation in spinterface electronic structure.
The first observation of interesting spin-dependent charge trasnsport phenomena in organic semiconductor films was for diodes employing the green emitter tris-(8-hydroxyquinolate)-aluminum ("Alq3"). relatively thick films of this material sandwiched between magnetic electrodes showed giant magnetoresistance values [1] but mechanistic understanding was generally poor since strong conductivity mismatch effects were expected. Over the years magnetoresistance effects for thick films [2] and tunneling magnetoresitance [3] effects for nanostructures were reported for this material by different groups but there were also significant negative results [4]. The tunneling studies in particular suggested that complex and diverse interfacial electronic structure modifications could be responsible for much of the phenomenology in Alq3-based spintronic devices.
A major goal in our research group was to identify relevant model interface where we could study the Alq3 spinterface with molecular precision. We developed a spin polarized scanning tunneling microscope (SPSTM) as part of a DOE-funded project (DE-SC0010324) that could measure the spin-dependent electronic structure of single molecules on magnetic surfaces. A particularly important case was Alq3 adsorbed onto the Cr(001) surface, which hosts a spin polarized surface state typical of many bcc magnetic metals. Our SPSTM experiments (Figure 1) showed that single Alq3 molecules significantly hybridize with the surface state and enhance its spin polarization [5]. Moreover, we found that when the molecule is slightly changed by substituting Cr for Al to make Crq3, the hybridization is dramatically altered to establish a gap in the surface state energy region with states above and below the gap having opposite spin polarization. These effects could be modeled using Density Functional Theory calculation by collaborators at the University of Utah (in the group of Prof. Feng Liu) [5]. Importantly it is clear that the properties of hybrid molecule-metal spinterface states are very sensitive to details of molecular electronic structure and this sensitivity must come be controlled in spintronic devices.
References
[1] Xiong et al., Nature 427, 821 (2004).
[2] Sun et al., Phys. rev. Lett, 104, 236602 (2010).
[3] Barraud et al., Nature Physics 6, 615 (2011).
[4] Riminucci et al., Appl. Phys. Lett. 102, 092407 (2013).
[5] Wang et al., Phys. Rev. B 95, 241410R (2017).