Tuning Spin Current Injection at Ferromagnet-Nonmagnet Interfaces by Molecular Design*

Organic semiconductors have recently been found to have a comparably large spin diffusion time and length [1]. This makes them ideal candidates for spintronic devices. However, spin injection and transport properties in organic semiconductors have yet to be fully understood.
The efficiency of spin injection at ferromagnetic resonance (FMR) from a ferromagnetic material into an adjacent non-magnetic material is given by the spin mixing conductance g_eff. It can be quantified by measuring the linewidth broadening of the FMR absorption of the ferromagnet due to an increase in Gilbert damping caused by spin injection into the adjacent non-magnetic material. Here, we use this technique to systematically study spin injection from a metallic ferromagnet permalloy, Ni₈₀Fe_20, into dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT), one of the best performing small molecule organic semiconductors to date. The unique tunability of organic materials by molecular design allows us to study the impact of interfacial properties on the spin injection efficiency systematically. We show that both, spin injection efficiency at the interface as well as the spin diffusion length can be tuned sensitively by the interfacial molecular structure and side chain substitution of the molecule [2].
[1] S. Watanabe*, K. Ando* et al., Nature Physics, 10, 308−313 (2014)
[2] A. Wittmann et al., Physical Review Letters, 124, 027204 (2020)