arXiv subjects

Luca Gnoli

Publications and source records attributed to Luca Gnoli.

3 recordsLinked to original sources

arXiv2025

Collapse of the standard ferromagnetic domain structure in hybrid Co/Molecule bilayers

We show that, upon the chemisorption of organic molecules, Co thin films display a number of unique magnetic properties, including the giant magnetic hardening and the violation of the Rayleigh law in magnetization reversal. These novel properties originate from the modification of the surface magnetic anisotropy induced by the molecule/film interaction: the {\pi}-d molecule/metal hybridization modifies the orbital population of the associated cobalt atoms and induces an additional and randomly oriented local anisotropy. Strong effects arise when the induced surface anisotropy is correlated over scales of a few molecules, and particularly when the correlation length of the random anisotropy field is comparable to the characteristic exchange length. This leads to the collapse of the standard domain structure and to the emergency of a glassy-type ferromagnetic state, defined by blurred pseudo-domains intertwined by diffuse and irregular domain walls. The magnetization reversal in such state was predicted to include topological vortex-like structures, successfully measured in this research by magnetic-force microscopy. Our work shows how the strong electronic interaction of standard components, Co thin films and readily available molecules, can generate structures with remarkable new magnetic properties, and thus opens a new avenue for the design of tailored-on-demand magnetic composites.

cond-mat.mtrl-sci↗

arXiv2025

Electrons and phonons in pentacene, insights from comparison between experiment and simulations

We have performed a comprehensive computational study of the vibrational properties and electron-phonon couplings in the three known polymorphs of pentacene. Vibrational patterns and electron-phonon interactions were calculated at several q-points of the Brillouin zone, allowing for a detailed mapping of the phonon landscape and the associated coupling mechanisms relevant to charge transport. Using a pool of post-processing tools, we analyze the different phonon dispersions. Thus, we shed light on how low-frequency phonons modulate the transport differently in the polymorphs via their distinct electron-phonon coupling (EPC) signatures in reciprocal space. In fact, we show that distinct phonons dominate in high-temperature and thin-film polymorphs with respect to the high mobility low-temperature polymorph, and that these lead to different decoherence/localization trends. We describe the microscopic origin of the mobility in bulk polymorph, demonstrating that polymorphism not only modulates the transfer integral but also the phonon pattern. For the first time we show that different EPC patterns lead to very different mobility values even in similar structures. Also, we explain how phonon confinement is responsible for the increased mobility observed in 2D phase. Finally, we address the problem of possible coexistence of multiple polymorphs within a single specimen, frequently encountered in organic crystals due to their subtle energy landscape and processing conditions. In this context, we consider the implications of polymorph intergrowth, structural defects and disorder.

cond-mat.mtrl-sci↗

arXiv2021

Controlling Domain-Wall Nucleation in Ta/CoFeB/MgO Nanomagnets via Local Ga+ Ion Irradiation

Comprehensive control of the domain wall nucleation process is crucial for spin-based emerging technologies ranging from random-access and storage-class memories over domain-wall logic concepts to nanomagnetic logic. In this work, focused Ga+ ion-irradiation is investigated as an effective means to control domain-wall nucleation in Ta/CoFeB/MgO nanostructures. We show that analogously to He+ irradiation, it is not only possible to reduce the perpendicular magnetic anisotropy but also to increase it significantly, enabling new, bidirectional manipulation schemes. First, the irradiation effects are assessed on film level, sketching an overview of the dose-dependent changes in the magnetic energy landscape. Subsequent time-domain nucleation characteristics of irradiated nanostructures reveal substantial increases in the anisotropy fields but surprisingly small effects on the measured energy barriers, indicating shrinking nucleation volumes. Spatial control of the domain wall nucleation point is achieved by employing focused irradiation of pre-irradiated magnets, with the diameter of the introduced circular defect controlling the coercivity. Special attention is given to the nucleation mechanisms, changing from a Stoner-Wohlfarth particle's coherent rotation to depinning from an anisotropy gradient. Dynamic micromagnetic simulations and related measurements are used in addition to model and analyze this depinning-dominated magnetization reversal.

cond-mat.mes-hall↗