Revealing the morphology and the magnetic properties of single buried cobalt-ZnTPP hybrid interfaces by ferromagnetic nuclear resonance spectroscopy

Abstract

The deeply buried, yet most important part of any spintronic device is the interface. This is even more interesting and much more complex when soft, light materials like organic molecules are in contact with an inorganic metallic electrode. Hence, exceptional care is required to better understand the phenomena driven by this type of organic/inorganic interfaces. To this end, ferromagnetic nuclear resonance (FNR) spectroscopy studies were performed to investigate the morphology and the magnetic properties of the hybrid organic-inorganic interfaces when zinc tetra-phenyl porphyrin (ZnTPP) molecules are at the vicinity of ferromagnetic metallic cobalt (Co) layers. The FNR experimental results show that when ZnTPP is deposited on top of Co the resulting interface is smoother and sharper compared to the more extended interface obtained when Co is deposited on top of ZnTPP. The shape of the spectra suggests that no chemical bonds take place between the interfacial Co atoms and the ZnTPP molecules and that interactions at the interfaces are governed by weak van der Waals forces. Finally, FNR also showed that the magnetic anisotropy at the Co-ZnTPP hybrid interfaces is reduced compared to the magnetic anisotropy of the Co atoms inside the Co films.

Nuclear magnetic resonance in ferromagnets: Ferromagnetic nuclear resonance; a very broadband approach

“…While the role of interfaces is important when building inorganic systems, their influence becomes even stronger when one of the components is an organic material. In this framework we have recently studied Co/zinc tetra-phenyl porphyrin (Zn/TPP) heterostructures [49,50]. When studying Co thin films, the basic investigation method consists in studying a series of samples as a function of the Co thickness.…”

In‐Depth NMR Investigation of the Magnetic Hardening in Co Thin Films Induced by the Interface with Molecular Layers

“…[26] More recently, it has also been successfully employed for the investigation of the interface (spinterface) quality in hybrid spintronic devices with organic molecules placed between two ferromagnetic electrodes. [27,28] In this work, we investigated the 59 Co FNR response of 7 nm polycrystalline cobalt films interfaced with two different molecules: Buckminster-fullerene (C 60 ) and Tris(8-hydroxyquinolinato)gallium (Gaq 3 ). The surface morphology of the starting cobalt layer was probed by ex-situ AFM microscopy, while its crystallinity and the quality of the Co/molecule interface were studied by cross-section transmission electron microscopy (TEM).…”

Exchange bias at the organic/ferromagnet interface may not be a spinterface effect

“…However, in a previous report we show that there is no magnetic hardening occurring at the Co/ZnTPP interfaces as the hybrid interfaces are magnetically softer than the bulk part of the Co film. 29 Finally, and more trivially, although we show that 10nm of Au capping layer is thick enough to protect the reference sample it is possible that the same Au(10nm) capping layer is not thick enough to protect the bilayer samples leading to their contamination due to exposure to air. This exposure to air might lead to the partial oxidation of the Co film resulting in an exchange coupling between the remaining metallic Co layer and newly formed cobalt oxide (CoO).…”

“…ZnTPP molecules are diamagnetic with Zn(II) closed shell central-ion and they have weak van der Waals interactions with the metallic surfaces. 29 Therefore, ZnTPP molecules are more likely to be insensitive to the proximity of the Co layer. Before measuring the magnetization loop of the full stack hybrid samples, the behavior of the Co film in the Au(25nm)/Co(6nm)/Au(10nm) stack is used as a reference for comparison with the Co/MTPP and Co/CoPc hybrid samples.…”

“…This work also demonstrates that new experimental approaches have to be developed to probe the stacking quality of such organic/inorganic hybrid heterostructures. 29,40…”