Disentangling Magnetic Hardening and Molecular Spin Chain Contributions to Exchange Bias in Ferromagnet/Molecule Bilayers

Abstract

We performed ferromagnetic resonance and magnetometry experiments to clarify the relationship between two reported magnetic exchange effects arising from interfacial spin-polarized charge transfer in ferromagnetic metal (FM)/molecule bilayers: the magnetic hardening effect and spinterface-stabilized molecular spin chains. To disentangle these effects, we tuned the metal phthalocyanine molecule central site's magnetic moment to enhance or suppress the formation of spin chains in the molecular film. We find that both effects are distinct, and additive. In the process, we extend the list of FM/molecule candidate pairs that are known to generate magnetic exchange effects, experimentally confirm the predicted increase in anisotropy upon molecular adsorption, and show that spin chains within the molecular film can enhance magnetic exchange. Our results confirm, as an echo to progress regarding inorganic spintronic tunnelling, that spintronic tunnelling across structurally ordered organic barriers has been reached through previous magnetotransport experiments.

Probing the Growth of Organic Molecular Films Embedded between Cobalt and Iron Electrodes: Ferromagnetic Nuclear Resonance Approach

“…Finally, even if the magnetic measurements were performed at 2 K after a field cooling procedure, all the magnetization curves were perfectly symmetric. No exchange bias, as in other metal/organic systems, [ 1,2 ] was observed in these samples at least when they were measured freshly after deposition. However, after some time of exposure to air, the onset of exchange bias appears.…”

“…One of the main difficulties in studying metal/organic heterostructures is the control of the organic layers that originates from the soft, light‐weight character of the organic molecules. The complex behavior of organic molecules on metallic surfaces can also lead to artefacts like exchange bias attributed to the molecular films [ 1,2 ] that was also observed in the samples under investigation here, but revealed to be due to the partial oxidation of the cobalt film. Similarly, devices based on organic tunnel junctions revealed tunnel magnetoresistance (MR) effect with, surprisingly, effective barrier thicknesses that are an order of magnitude thinner than the deposited organic layer.…”

“…One of the main difficulties in studying metal/organic heterostructures is the control of the organic layers that originates from the soft, light-weight character of the organic molecules. The complex behavior of organic molecules on metallic surfaces can also lead to artefacts like exchange bias attributed to the molecular films [1,2] that was also observed in the samples under investigation here, applied to study organic films, namely, molecular layers of zinc tetraphenyl porphyrin (ZnTPP). Compared to the widely studied phthalocyanines, tetraphenyl porphyrins are nonplanar molecules.…”

Emergence of Multispinterface and Antiferromagnetic Molecular Exchange Bias via Molecular Stacking on a Ferromagnetic Film

“…One of the remarkable phenomena in molecular spinterfaces is the activation of an antiferromagnetic molecular spin chain from paramagnetic molecules via planar stacking on a ferromagnet . Here, the magnetic interaction at the surface of a ferromagnetic film leads to a cascade intermolecular exchange coupling on the successive molecular stack.…”

“…Here, the magnetic interaction at the surface of a ferromagnetic film leads to a cascade intermolecular exchange coupling on the successive molecular stack. This magnetic ordering has shown its inherent feature of exchange bias to the underlying ferromagnet, which can be tunable upon various molecular interactions . Most of these heterostructures develop the ferromagnetic exchange coupling at the interface .…”