Spin dynamics in oriented lithium phthalocyanine thin films investigated by pulsed electron spin resonance

Abstract

Thin films of monolithium phthalocyanine (PcLi) were prepared by vacuum deposition on cleaved mica. The highly anisotropic spin diffusion in these films was studied by electron spin resonance (ESR). Using averaging techniques, we succeeded in the detection of films as thin as 100 nm using pulsed ESR and 2 nm with cw ESR. The electronic relaxation is shown to depend on the substrate temperatures Ts during film deposition which determine the morphology of the films. We find relaxation rates of films which are faster by a factor of 20 (T1e−1) to 30 (T2e−1) than those of single crystals. At ambient temperature, a low-dimensional spin exchange mechanism is proposed for the single crystal. At low temperatures, in single crystals the electronic relaxation rates T1e−1 and T2e−1 hint at a reduced exchange with increased dimensionality d≳1. This holds for the films at all temperatures and is explained by different crystal structures of single crystals and films. A generally applicable relaxation model is developed which describes the transition from one-dimensional to three-dimensional spin propagation.