arXiv subjects

Francesco Foggetti

Publications and source records attributed to Francesco Foggetti.

8 recordsLinked to original sources

arXiv2026

Optimizing spin-based terahertz emission from magnetic heterostructures

Terahertz radiation pulses can be generated efficiently through femtosecond laser excitation of a ferromagnetic/nonmagnetic heterostructure, wherein an ultrafast laser-induced spin current results in an electromagnetic THz pulse due to spin-charge conversion. It is, however, still poorly understood how the THz emission amplitude and its bandwidth can be optimized. Here, we perform a systematic analysis of the THz emission from various magnetic heterostructures. The dynamics of the spin current is described by the semiclassical, superdiffusive spin-transport model and the energy dependence of the spin Hall effect of hot electrons is taken into account, leading to emission profiles for Co(2 nm)/Pt(4 nm) bilayer in good agreement with experiment. To identify the optimal {conditions} for THz emission, {we study} the properties of the emitted THz wave profile by systematically varying the layer thicknesses of metallic bilayers, their interfacial spin-current transmission properties, their materials' dependence, and influence of the pump laser-pulse width, allowing us to give optimization guidelines. We find that thin nonmagnetic layer thicknesses of 5-6 nm provide the largest bandwidth in the case of Co/Pt and that the peak frequency of the THz emission depends only on the geometry of the emitter and not on the laser pulse width. The THz bandwidth {is conversely found to} depend on several factors such as exciting laser pulse width, layers' thicknesses, and interface transmission-reflection properties, with the limitation that an increase in the bandwidth by tuning the interface properties comes with a trade-off in the energy efficiency of the emitter. Lastly, we propose a double pulse excitation protocol of a trilayer system that could provide broadband THz emission with a large bandwidth. {Our results contribute to establishing guidelines for optimizing spintronic THz generation.

cond-mat.mtrl-sci↗

arXiv2026

Tunnel-Barrier-Engineered Ultrafast Demagnetization and Spin Transport in Graphene-Based Heterostructures

Heterostructures combining graphene with 3d transition metal ferromagnets (FMs) enable various spin-based phenomena at ultrafast timescales. However, challenges such as the interfacial impedance mismatch, FM deposition-induced defect generation, and interface modification by interfacial coupling or hybridization can impede their functionalization for spin-orbitronics. In this work, we utilize insulating TiOx barrier layers (BLs) to modify the interfacial spin conductance structurally, disentangle spin pumping and magnetic proximity effects (MPE), and establish external control over ultrafast magnetization dynamics in single-layer graphene/TiOx/Co systems. All-optical time-resolved magneto-optical Kerr effect measurements of femtosecond to nanosecond spin dynamics reveal systematic tunability of ultrafast magnetic parameters via barrier engineering. The thickness-dependent damping modulation in Co indicates strong spin pumping, with interfacial spin transparency close to half its physical limit in the presence of an ultrathin BL, where MPE is eliminated. Our results show that appropriately chosen ultrathin BLs can prevent interfacial alterations from ferromagnetic metals, facilitating efficient spin detection in graphene and enhancing control over spin angular momentum dissipation in graphene/FM interfaces.

cond-mat.mes-hall↗

arXiv2024

Quantitative modeling of spintronic terahertz emission due to ultrafast spin transport

In spintronic terahertz emitters, THz radiation is generated by exciting an ultrafast spin current through femtosecond laser excitation of a ferromagnetic-nonmagnetic metallic heterostructure. Although an extensive phenomenological knowledge has been built up during the last decade, a solid theoretical modeling that connects the generated THz signal to the laser induced-spin current is still incomplete. Here, starting from general solutions to Maxwell's equations, we model the electric field generated by a superdiffusive spin current in spintronic emitters, taking Co/Pt as a typical example. We explicitly include the detector shape which is shown to significantly influence the detected THz radiation. Additionally, the electron energy dependence of the spin Hall effect is taken into account, as well as the duration of the exciting laser pulse and thickness of the detector crystal. Our modeling leads to realistic emission profiles and highlights the role of the detection method for distinguishing key features of the spintronic THz emission.

cond-mat.mtrl-sci↗

arXiv2024

Electric field-induced domain wall motion in spin spiral multiferroics

Switching in magnetic materials gives rise to rich physical phenomena and lies at the heart of their technological applications. Although domain wall motion in ferro- and antiferromagnets has been studied, in spiral magnets it is still poorly understood despite 20 years of active research since the discovery of spiral multiferroics. The problem of the domain wall motion in a spiral magnet is a compelling one, the more so the magnetic domain walls in cycloidal spiral phase are also ferroelectric, thus enabling electric control of magnetism, i.e. domain wall motion under the action of an external electric field. Phase transition to a spiral phase leads to a formation of chiral domains with opposite spin rotation senses, that are separated by chiral domain walls. Spiral order breaks inversion symmetry and induces a ferroelectric polarization, whose sign is determined by the chirality of the domain. Thus the spiral order allows for the manipulation of spins via an external electric field. Here we study domain wall motion in magnets with spiral ground state, that are the most basic non-collinear magnets. We formulate a simplified variational model and derive the equation of motion for the domain wall driven by an external electric field. The results are corroborated with atomistic spin dynamics simulations. The results suggest a linear dependence of the wall speed on the external electric field, and a peculiar dependence on the system geometry and domain structure.

cond-mat.str-el↗

arXiv2023

Néel-Vector Switching and THz Spin-Wave Excitation in Mn$_2$Au due to Femtosecond Spin-Transfer Torques

Efficient and fast manipulation of antiferromagnets has to date remained a challenging task, hindering their application in spintronic devices. For ultrafast operation of such devices, it is highly desirable to be able to control the antiferromagnetic order within picoseconds - a timescale that is difficult to achieve with electrical circuits. Here, we demonstrate that bursts of spin-polarized hot-electron currents emerging due to laser-induced ultrafast demagnetization are able to efficiently excite spin dynamics in antiferromagnetic Mn$_2$Au by exerting a spin-transfer torque on femtosecond timescales. We combine quantitative superdiffusive transport and atomistic spin-model calculations to describe a spin-valve-type trilayer consisting of Fe$|$Cu$|$Mn$_2$Au. Our results demonstrate that femtosecond spin-transfer torques can switch the Mn$_2$Au layer within a few picoseconds. In addition, we find that spin waves with high frequencies up to several THz can be excited in Mn$_2$Au.

cond-mat.mtrl-sci↗

arXiv2022

Ferroelectricity and topological vortices from molecular ordering in metal-organic frameworks

Metal-organic frameworks comprehend a wide class of hybrid organic-inorganic materials with general structure A$_m$BX$_n$, with $A$ and $X$ being organic molecules and B a metal cation. This often results in enhanced structural flexibility and new functionalities. Hybrid perovskites ABX$_3$ are a well-known example.} In an Iron-based perovskites, (DMA)Fe^{II-III}(COOH)_3, dimethylammonium (DMA) molecules are organized in a hexagonal structure. They are orientationally disordered at high temperatures, but order at around $T=100$~K in a peculiar toroidal pattern. Recent experimental and theoretical study suggest the appearance of ferroelectric polarization in this phase, although the measured polarization is small, and the mechanism of ferroelectricity is still debated. We formulate a Landau-type theory that clarifies the connection between the electric polarization, molecular pattern, and distortive modes of the inorganic lattice. We find a remarkable mechanism of improper ferroelectricity, analogue to the trimerization process in inorganic hexagonal ferrites and manganites, but here driven by the ordering of organic molecules in a metal-organic framework. Our study reveals an extremely rich phase diagram with the prediction of topological domain walls, where the ferroelectricity arise from tripling the unit cells due to molecular ordering. Wide domain walls with inner structure are predicted.

cond-mat.mtrl-sci↗

arXiv2020

Soft magnon contributions to dielectric constant in spiral magnets with domain walls

Competing magnetic exchange interactions often result in non-collinear magnetic states, such as spin spirals, which break the inversion symmetry and induce ferroelectric polarization. The resulting strong interactions between magnetic and dielectric degrees of freedom lead to a technologically important possibility to control magnetic order by electric fields and to electromagnons, magnetic excitations that can be excited by an electric dipole of the electromagnetic field. Here we study the effects of chiral domain walls on magnetoelectric properties of spiral magnets. We use a quasi-1D model Hamiltonian with competing Heisenberg exchange interactions, leading to a spin spiral, and Dzyaloshinskii-Moriya interactions, that couple spins and electric dipoles and mix magnon and phonon excitations. The results suggest that low frequency dielectric anomalies in spiral magnets, such as TbMnO3 and MnWO4, may originate from hybrid magnon - polar phonon excitations associated with domain walls.

cond-mat.str-el↗

arXiv2019

Magnetic monopoles and toroidal moments in LuFeO$_3$ and related compounds

Magnetic monopoles and toroidal order are compelling features that have long been theorized but remain elusive in real materials. Multiferroic hexagonal ferrites are an interesting realization of frustrated triangular lattice, where magnetic order is coupled to ferroelectricity and trimerization. Here we propose a mechanism, through which magnetic monopolar and toroidal orders emerge from the combination of 120$^\circ$ antiferromagnetism and trimerization, present in hexagonal manganites and ferrites. The experimentally observable signatures of magnetic monopolar and toroidal orders are identified in the inelastic neutron scattering cross section, simulated from a microscopic model of LuFeO3. The non-reciprocal magnon propagation is demonstrated.

cond-mat.str-el↗