arXiv subjects

Matteo Pancaldi

Publications and source records attributed to Matteo Pancaldi.

29 records · Page 2Linked to original sources

arXiv2021

Magnetization switching in the inertial regime

We have numerically solved the Landau-Lifshitz-Gilbert (LLG) equation in its standard and inertial forms to study the magnetization switching dynamics in a $3d$ thin film ferromagnet. The dynamics is triggered by ultrashort magnetic field pulses of varying width and amplitude in the picosecond and Tesla range. We have compared the solutions of the two equations in terms of switching characteristic, speed and energy analysis. Both equations return qualitatively similar switching dynamics, characterized by regions of slower precessional behavior and faster ballistic motion. In case of inertial dynamics, ballistic switching is found in a 25 % wider region in the parameter space given by the magnetic field amplitude and width. The energy analysis of the dynamics is qualitatively different for the standard and inertial LLG equations. In the latter case, an extra energy channel, interpreted as the kinetic energy of the system, is available. Such extra channel is responsible for a resonant energy absorption at THz frequencies, consistent with the occurence of spin nutation.

cond-mat.mes-hall↗

arXiv2021

Observation of magnetic helicoidal dichroism with extreme ultraviolet light vortices

We report on the experimental evidence of magnetic helicoidal dichroism, observed in the interaction of an extreme ultraviolet vortex beam carrying orbital angular momentum with a magnetic vortex. Numerical simulations based on classical electromagnetic theory show that this dichroism is based on the interference of light modes with different orbital angular momenta, which are populated after the interaction between the light phase chirality and the magnetic topology. This observation gives insight into the interplay between orbital angular momentum and magnetism, and sets the framework for the development of new analytical tools to investigate ultrafast magnetization dynamics.

physics.optics↗

arXiv2020

Chiral switching and dynamic barrier reductions in artificial square ice

Collective dynamics in lithographically-defined artificial spin ices offer profound insights into emergent correlations and phase transitions of geometrically-frustrated Ising spin systems. Their temporal and spatial evolution are often simulated using kinetic Monte Carlo simulations, which rely on the precise knowledge of the switching barriers to obtain predictive results in agreement with experimental observations. In many cases, however, the barriers are derived from simplified assumptions only, and do not take into account the full physical picture of nanomagnetic switching. Here we describe how the immediate magnetic environment of a nanomagnet reversing via quasi-coherent rotation can induce clockwise and counter-clockwise switching channels with different barrier energies. This barrier splitting for chiral reversal channels can be sizeable and, as string-method micromagnetic simulations show, is relevant for artificial spin ice systems made of both exchange -- as well as magnetostatically --dominated units. Due to the barrier splitting (and further reductions due to non-uniform reversal) transition rates can be exponentially enhanced by several orders of magnitude compared to mean-field predictions, especially in the limit of rare switching events where thermal excitation is less likely. This leads to significantly faster relaxation time scales and modified spatial correlations. Our findings are thus of integral importance to achieve realistic kinetic Monte Carlo simulations of emergent correlations in artificial spin systems, magnonic crystals, or the evolution of nanomagnetic logic circuits.

cond-mat.mes-hall↗

arXiv2020

Dependence of energy barrier reduction on collective excitations in square artificial spin ice: A comprehensive comparison of simulation techniques

We perform micromagnetic simulations to study the switching barriers in square artificial spin ice systems consisting of elongated single domain magnetic islands arranged on a square lattice. By considering a double vertex composed of one central island and six nearest neighbor islands, we calculate the energy barriers between two types of double vertices by applying the string method. We investigate by means of micromagnetic simulations the consequences of the neighboring islands, the inhomogeneities in the magnetization of the islands and the reversal mechanisms on the energy barrier by comparing three different approaches with increasing complexity. The micromagnetic models, where the string method is applied, are compared to the currently common method, the mean barrier approximation. Our investigations indicate that a proper micromagnetic modeling of the switching process leads to significantly lower energy barriers, by up to 35% compared to the mean-barrier approximation, so decreasing the expected average life time up to seven orders of magnitude. Hereby, we investigate the influence of parallel switching channels and the conceptional approach of using a mean-barrier to calculate the corresponding rates.

physics.comp-ph↗

arXiv2019

Tunable axion plasma haloscopes

We propose a new strategy to search for dark matter axions using tunable cryogenic plasmas. Unlike current experiments, which repair the mismatch between axion and photon masses by breaking translational invariance (cavity and dielectric haloscopes), a plasma haloscope enables resonant conversion by matching the axion mass to a plasma frequency. A key advantage is that the plasma frequency is unrelated to the physical size of the device, allowing large conversion volumes. We identify wire metamaterials as a promising candidate plasma, wherein the plasma frequency can be tuned by varying the interwire spacing. For realistic experimental sizes we estimate competitive sensitivity for axion masses $35-400\,μ$eV, at least.

hep-ph↗

arXiv2019

Nonlinear magnetization dynamics driven by strong terahertz fields

We present a comprehensive experimental and numerical study of magnetization dynamics triggered in a thin metallic film by single-cycle terahertz pulses of $\sim20$ MV/m electric field amplitude and $\sim1$ ps duration. The experimental dynamics is probed using the femtosecond magneto-optical Kerr effect (MOKE), and it is reproduced numerically using macrospin simulations. The magnetization dynamics can be decomposed in three distinct processes: a coherent precession of the magnetization around the terahertz magnetic field, an ultrafast demagnetization that suddenly changes the anisotropy of the film, and a uniform precession around the equilibrium effective field that is relaxed on the nanosecond time scale, consistent with a Gilbert damping process. Macrospin simulations quantitatively reproduce the observed dynamics, and allow us to predict that novel nonlinear magnetization dynamics regimes can be attained with existing table-top terahertz sources.

cond-mat.mes-hall↗

arXiv2019

Magnetoplasmonics in nanocavities: Dark plasmons enhance magneto-optics beyond the intrinsic limit of magnetoplasmonic nanoantennas

Enhancing magneto-optical effects is crucial for size reduction of key photonic devices based on non-reciprocal propagation of light and to enable active nanophotonics. We disclose a so far unexplored approach that exploits dark plasmons to produce an unprecedented amplification of magneto-optical activity. We designed and fabricated non-concentric magnetoplasmonic-disk/plasmonic-ring-resonator nanocavities supporting multipolar dark modes. The broken geometrical symmetry of the design enables coupling with free-space light and hybridization of dark modes of the ring nanoresonator with the dipolar localized plasmon resonance of the magnetoplasmonic disk. Such hybridization generates a multipolar resonance that amplifies the magneto-optical response of the nanocavity by ~1-order of magnitude with respect to the maximum enhancement achievable by localized plasmons in bare magnetoplasmonic nanoantennas. This large amplification results from the peculiar and enhanced electrodynamic response of the nanocavity, yielding an intense magnetically-activated radiant magneto-optical dipole driven by the low-radiant multipolar resonance. The concept proposed is general and, therefore, our results open a new path that can revitalize research and applications of magnetoplasmonics to active nanophotonics and flat optics.

physics.optics↗

arXiv2019

Selective and Fast Plasmon-Assisted Photo-Heating of Nanomagnets; A New Route for Opto-Activated Nanomagnetic Logic and Artificial Spin Systems

Thermal relaxation of nanoscale magnetic islands, mimicking Ising macrospins, is indispensable for studies of geometrically frustrated artificial spin systems and low-energy nanomagnetic computation. Currently-used heating schemes based on contact to a thermal reservoir, however, lack the speed and spatial selectivity required for the implementation in technological applications. Applying a hybrid approach by combining a plasmonic nanoheater with a magnetic element, in this work we establish the robust and reliable control of local temperatures in nanomagnetic arrays by contactless optical means. Plasmon-assisted photo-heating allows for temperature increases of up to several hundred Kelvins, which lead to thermally-activated moment reversals and a pronounced reduction of the magnetic coercive field. Furthermore, the polarization-dependent absorption cross section of elongated plasmonic elements enables sublattice-specific heating on sub-nanosecond time scales. Using optical degrees of freedom, i.e. focal position, polarization, power, and pulse length, thermoplasmonic heating of nanomagnets offers itself for the use in flexible, fast, spatially-, and element-selective thermalization for functional magnetic metamaterials.

cond-mat.mes-hall↗

arXiv2018

Magnetic Force Microscopy Imaging Using Geometrically Constrained Nano-Domain Walls

Domain wall probes (DW-probes) were custom-made by modifying standard commercial magnetic force microscopy (MFM) probes using focused ion beam lithography. Excess of magnetic coating from the probes was milled out, leaving a V-shaped nanostructure on one face of the probe apex. Owing to the nanostructure's shape anisotropy, such probe has four possible magnetic states depending on the direction of the magnetization along each arm of the V-shape. Two states of opposite polarity are characterised by the presence of a geometrically constrained DW, pinned at the corner of the V-shape nanostructure. In the other two states, the magnetization curls around the corner with opposite chirality. Electron holography studies, supported by numerical simulations, demonstrate that a strong stray field emanates from the pinned DW, whilst a much weaker stray field is generated by the curling configurations. Using in situ MFM, we show that the magnetization states of the DW-probe can be easily controlled by applying an external magnetic field, thereby demonstrating that this type of probe can be used as a switchable tool with a low or high stray field intensity. We demonstrate that DW-probes enable acquiring magnetic images with a negligible interference with the sample magnetization, similar to that of commercial low moment probes, but with a higher magnetic contrast. In addition, the DW-probe in the curl state provides complementary information about the in-plane component of the sample's magnetization, which is not achievable by standard methods and provides additional information about the stray fields, e.g. as when imaging DWs.

physics.ins-det↗

arXiv2017

THz-driven demagnetization with Perpendicular Magnetic Anisotropy: Towards ultrafast ballistic switching

We study THz-driven spin dynamics in thin CoPt films with perpendicular magnetic anisotropy. Femtosecond magneto-optical Kerr effect measurements show that demagnetization amplitude of about $1\%$ can be achieved with a peak THz electric field of $300$~kV/cm, and a corresponding peak magnetic field of $0.1$~T. The effect is more than an order of magnitude larger than observed in samples with easy-plane anisotropy irradiated with the same field strength. We also utilize finite-element simulations to design a meta-material structure that can enhance the THz magnetic field by more than an order of magnitude, over an area of several tens of square micrometers. Magnetic fields exceeding $1$~Tesla, generated in such meta-materials with the available laser-based THz sources, are expected to produce full magnetization reversal via ultrafast ballistic precession driven by the THz radiation. Our results demonstrate the possibility of table-top ultrafast magnetization reversal induced by THz radiation.

cond-mat.mes-hall↗

arXiv2017

Local Terahertz Field Enhancement for Time-Resolved X-ray Diffraction

We report local field strength enhancement of single-cycle terahertz (THz) pulses in an ultrafast time-resolved x-ray diffraction experiment. We show that patterning the sample with gold microstructures increases the THz field without changing the THz pulse shape or drastically affecting the quality of the x-ray diffraction pattern. We find a five-fold increase in THz-induced x-ray diffraction intensity change in the presence of microstructures on a SrTiO3 thin-film sample.

physics.ins-det↗