Mario Carpentieri

Micromagnetic simulations using Graphics Processing Units

The methodology for adapting a standard micromagnetic code to run on graphics processing units (GPUs) and exploit the potential for parallel calculations of this platform is discussed. GPMagnet, a general purpose finite-difference GPU-based micromagnetic tool, is used as an example. Speed-up factors of two orders of magnitude can be achieved with GPMagnet with respect to a serial code. This allows for running extensive simulations, nearly inaccessible with a standard micromagnetic solver, at reasonable computational times.

A strategy for the design of skyrmion racetrack memories.

Magnetic storage based on racetrack memory is very promising for the design of ultra-dense, low-cost and low-power storage technology. Information can be coded in a magnetic region between two domain walls or, as predicted recently, in topological magnetic objects known as skyrmions. Here, we show the technological advantages and limitations of using Bloch and Néel skyrmions manipulated by spin current generated within the ferromagnet or via the spin-Hall effect arising from a non-magnetic heavy metal underlayer. We found that the Néel skyrmion moved by the spin-Hall effect is a very promising strategy for technological implementation of the next generation of skyrmion racetrack memories (zero field, high thermal stability, and ultra-dense storage). We employed micromagnetics reinforced with an analytical formulation of skyrmion dynamics that we developed from the Thiele equation. We identified that the excitation, at high currents, of a breathing mode of the skyrmion limits the maximal velocity of the memory.

Neural-ICA and wavelet transform for artifacts removal in surface EMG

Recent works have shown that artifacts removal in biomedical signals, like electromyographic (EMG) or electroencephalographic (EEG) recordings, can be performed by using discrete wavelet transform (DWT) or independent component analysis (ICA). Often, the removal of some artifacts is very hard because they are superimposed on the recordings and they corrupt biomedical signals also in frequency domain. In these cases DWT and ICA methods cannot perform artifacts cancellation. We present a method based on the joint use of wavelet transform and independent component analysis. We show the obtained results and the comparisons among the proposed method, DWT and ICA techniques. In this preliminary study, a user interface is needed to identify the artifact.

Switching of a single ferromagnetic layer driven by spin Hall effect

The magnetization switching of a thin ferromagnetic layer placed on top of a heavy metal (such as Pt, Ta, or W) driven by an in-plane current has been observed in recent experiments. The magnetization dynamics of these processes is studied in a full micromagnetic framework which takes into account the transfer-torque from spin Hall effect due to the spin-orbit coupling. Simulations indicate that the reversal occurs via nucleation of complex magnetization patterns. In particular, magnetic bubbles appear during the reversal of the magnetization in the perpendicular configuration while for the in-plane configuration, nucleation of vortexes is observed.

Effect of the classical ampere field in micromagnetic computations of spin polarized current-driven magnetization processes

Magnetization reversal by spin polarized current flowing perpendicular to pillar nanostructures with different geometries and materials is studied by means of a micromagnetic model. The spin transfer torque is included as an additional term in the Gilbert equation following previous theoretical calculations by Slonczewski. The ampere field (HAmp) due to the current and the dipolar antiferromagnetic coupling between the ferromagnetic layers are also taken into account. The HAmp plays a crucial role; in fact it can speed, retard or even inhibit the magnetization switching process depending on the kind of structure under test. The studied nanostructures are circular and elliptic nanopillars of Co∕Cu∕Co and permalloy∕Cu∕permalloy.

Spin-transfer-torque resonant switching and injection locking in the presence of a weak external microwave field for spin valves with perpendicular materials

The effects of a weak microwave field in the magnetization dynamics driven by spin-transfer-torque in spin-valves with perpendicular materials have been systematically studied by means of full micromagnetic simulations. In the system we studied, depending on the working point (bias field and current) in the dynamical stability diagram, we observe either resonant switching and injection locking. The resonant switching, observed in the switching region, occurs when the field frequency is approaching the frequency of the main pre-switching mode giving rise to an asymmetric power distribution of that mode in the sectional area of the free layer. At the resonant frequency, the switching time is weakly dependent on the relative phase between the instant when the current pulse is applied and the microwave field. The injection locking, observed in the dynamical region, is characterized by the following properties: (i) a locking bandwidth which is linearly dependent on the force locking, and (ii) a locking for integer harmonics of the self-oscillation frequency. We compare our numerical results with analytical theory for non-autonomous non-linear system obtaining a good agreement in the current region where the oscillation frequency and output power are characterized from a linear relationship.

Giant spin-torque diode sensitivity in the absence of bias magnetic field.

Microwave detectors based on the spin-torque diode effect are among the key emerging spintronic devices. By utilizing the spin of electrons in addition to charge, they have the potential to overcome the theoretical performance limits of their semiconductor (Schottky) counterparts. However, so far, practical implementations of spin-diode microwave detectors have been limited by the necessity to apply a magnetic field. Here, we demonstrate nanoscale magnetic tunnel junction microwave detectors, exhibiting high-detection sensitivity of 75,400 mV mW−1 at room temperature without any external bias fields, and for low-input power (micro-Watts or lower). This sensitivity is significantly larger than both state-of-the-art Schottky diode detectors and existing spintronic diodes. Micromagnetic simulations and measurements reveal the essential role of injection locking to achieve this sensitivity performance. This mechanism may provide a pathway to enable further performance improvement of spin-torque diode microwave detectors.

Semi-implicit integration scheme for Landau–Lifshitz–Gilbert-Slonczewski equation

This paper shows how to implement a semi-implicit algorithm based on the Adams-Bashforth algorithm as a predictor, and a second order Adams-Moulton procedure as a corrector in the Landau–Lifshitz–Gilbert-Slonczewski equation. We compare the results with a Runge-Kutta scheme of the 5th order, while for the standard problem #4 (and, in general, for the LLG equation) the computational speeds are of the same order, and we found better performance when the thermal fluctuations or the spin-polarized currents are taken into account.

High frequency spin-torque-oscillators with reduced perpendicular torque effect based on asymmetric vortex polarizer

In this paper it is demonstrated micromagnetically the possibility to have high frequency and high power microwave emission at zero field in magnetic tunnel junctions with an off-centered vortex polarizer for current densities smaller than 107 A/cm2. We found power spectra characterized by two harmonics (around 6 GHz and 12 GHz) where the second one has the larger power. Due to the non-uniform configuration of the polarizer, the effect of the perpendicular torque is partially compensated by the Oersted field. Our predictions give rise to the design of a more compact and easily embedded spin-torque oscillators for all-on-chip applications.

Spin-Hall nano-oscillator: A micromagnetic study

This Letter studies the dynamical behavior of spin-Hall nanoscillators from a micromagnetic point of view. The model parameters have been identified by reproducing recent experimental data quantitatively. Our results indicate that a strongly localized mode is observed for in-plane bias fields such as in the experiments, while predict the excitation of an asymmetric propagating mode for large enough out-of plane bias field similarly to what observed in spin-torque nanocontact oscillators. Our findings show that spin-Hall nanoscillators can find application as spin-wave emitters for magnonic applications where spin waves are used for transmission and processing information on nanoscale.