Vito Puliafito

Combined Frequency-Amplitude Nonlinear Modulation: Theory and Applications

In this paper, we formulate a generalized theoretical model to describe the nonlinear dynamics observed in combined frequency-amplitude modulators whose characteristic parameters exhibit a nonlinear dependence on the input modulating signal. The derived analytical solution may give a satisfactory explanation of recent laboratory observations on magnetic spin-transfer oscillators and fully agrees with results of micromagnetic calculations. Since the theory has been developed independently of the mechanism causing the nonlinearities, it may encompass the description of modulation processes of any physical nature, a promising feature for potential applications in the field of communication systems.

Nanoscale spintronic oscillators based on the excitation of confined soliton modes

This paper demonstrates how to excite complex soliton modes in nanomagnets with perpendicular to plane magnetic anisotropy driven by the non-uniform injection of a spin-polarized current. We addressed the study toward two different scenarios, in the first the excitation of two rotating bubble/antibubble pairs is predicted, in the second one, by means of the topological density, we characterized the dissipative droplet recently measured as single constrained bubble/antibubble pair. Our results are important for the theoretical understanding of how to control the spatial structure of soliton modes for application in spintronics, magnonics, and domain wall devices.

Skyrmion based microwave detectors and harvesting

Magnetic skyrmions are topologically protected states that are very promising for the design of the next generation of ultra-low-power electronic devices. In this letter, we propose a magnetic tunnel junction based spin-transfer torque diode with a magnetic skyrmion as ground state and a perpendicular polarizer patterned as nano-contact for a local injection of the current. The key result is the possibility to achieve sensitivities (i.e., detection voltage over input microwave power) larger than 2000 V/W for optimized contact diameters. We also pointed out that large enough voltage controlled magnetocrystalline anisotropy could significantly improve the sensitivity. Our results can be very useful for the identification of a class of spin-torque diodes with a non-uniform ground state and to understand the fundamental physics of the skyrmion dynamical properties.

Switching Properties in Magnetic Tunnel Junctions With Interfacial Perpendicular Anisotropy: Micromagnetic Study

The role of universal memory can be successfully satisfied by magnetic tunnel junctions (MTJs) where the writing mechanism is based on spin-transfer torque (STT). An improvement in the switching properties (lower switching current density maintaining the thermal stability) has been achieved in MTJs with interfacial perpendicular anisotropy (IPA) at the interface between CoFeB and MgO. In this paper, micromagnetic simulations point out the influence of IPA and saturation magnetization (MS) on the properties of fast magnetization reversal achieved in 5, 10, and 20 ns. Both cases of in-plane and out-of-plane free layer are considered. In addition, the thermal effect is included for the in-plane switching at 20 ns and a complete analysis of energy dissipation during the switching is illustrated. This paper can provide useful information for the design of STT-based memories.

A generalized tool for accurate time-domain separation of excited modes in spin-torque oscillators

We propose and develop an advanced signal processing technique that, combined with micromagnetic simulations, is able to deeply describe the non-stationary behavior of spin-torque oscillators, both in terms of time domain and spatial distribution of the magnetization dynamics. The Hilbert-Huang Transform is used for the identification of the time traces of each oscillation in a multimode excitation and enhanced with masking signals and the Ensemble Empirical Mode Decomposition. We emphasize that the technique developed here is general and can be used for any physical non-linear system in the presence of multimode dynamical excitation or intermittence.

Self-Modulated Soliton Modes Excited in a Nanocontact Spin-Torque Oscillator

Self-modulated bubble-like solitons, namely droplets, recently observed in spin-torque nanooscillators with a perpendicular free layer, are promising for applications in spintronics, magnonics, and magnetic logic devices. This letter presents a micromagnetic analysis on soliton dynamics. Our results identify the role of physical parameters (i.e., external field, saturation magnetization, and exchange constant) in achieving experimental findings such as hysteretic, linear and nonlinear spin-wave excitations. The modes with different excitation frequencies have nonuniform spatial distributions of power, and the power of frequency sidebands is mostly located near the outer border of the nanocontact. At high currents, a wavelet-based analysis highlights that the magnetoresistive signal due to the sideband modes is not stationary, providing a possible origin of the asymmetric sidebands observed in spintronic self-modulators. We also identify the thermal field as the key ingredient for the excitation of linear modes in a subcritical regime.

Intrinsic synchronization of an array of spin-torque oscillators driven by the spin-Hall effect

This paper micromagnetically studies the magnetization dynamics driven by the spin-Hall effect in a Platinum/Permalloy bi-layer. For a certain field and current range, the excitation of a uniform mode, characterized by a power with a spatial distribution in the whole ferromagnetic cross section, is observed. We suggest to use the ferromagnet of the bi-layer as basis for the realization of an array of spin-torque oscillators (STOs): the Permalloy ferromagnet will act as shared free layer, whereas the spacers and the polarizers are built on top of it. Following this strategy, the frequency of the uniform mode will be the same for the whole device, creating an intrinsic synchronization. The synchronization of an array of parallely connected STOs will allow to increase the output power, as necessary for technological applications.

Micromagnetic Modeling of Nanocontact Spin-Torque Oscillators With Perpendicular Anisotropy at Zero Bias Field

In this paper, we present a numerical study to determine the feasibility of exciting and sustaining stable magnetization oscillations in magnetic nanocontact devices subjected to the combined action of a spin-polarized current and a perpendicular anisotropy field when no external field is applied. A systematic numerical analysis of the properties exhibited by such spintronic oscillators is carried out by means of a micromagnetic framework. The study reveals a nonlinear behavior of the excited frequency as the anisotropy field strength is varied. More noteworthy, full-scale investigations result in a hysteretic dependence of the excited frequency on the applied current together with the existence of two kinds of precessional spin-wave modes: an anisotropic radially propagating mode and a gyrotropic motion of a magnetic vortex-core.

Hysteretic Synchronization in Spin-Torque Nanocontact Oscillators: A Micromagnetic Study

Several experiments report the presence of finite jumps in the properties of spin-torque oscillators at room temperature, such as oscillation frequency and power as functions of current or field. On the basis of micromagnetic simulations, this paper links those experimental discontinuities to the changes in the curve slope numerically observed in the absence of thermal effects. Our numerical results show the key ingredient triggering this behavior is the presence of abrupt changes in the oscillation axis of the magnetization precession. We also predict that by fixing the bias point of the oscillator near those critical regions, it is possible to observe hysteretic synchronization. This result should be a key point in the design of nanoscale on-chip phase-locked loop receivers with improved sensitivity.

Spin-Hall nano-oscillator with oblique magnetization and Dzyaloshinskii-Moriya interaction as generator of skyrmions and nonreciprocal spin-waves

Spin-Hall oscillators (SHO) are promising sources of spin-wave signals for magnonics applications, and can serve as building blocks for magnonic logic in ultralow power computation devices. Thin magnetic layers used as “free” layers in SHO are in contact with heavy metals having large spin-orbital interaction, and, therefore, could be subject to the spin-Hall effect (SHE) and the interfacial Dzyaloshinskii-Moriya interaction (i-DMI), which may lead to the nonreciprocity of the excited spin waves and other unusual effects. Here, we analytically and micromagnetically study magnetization dynamics excited in an SHO with oblique magnetization when the SHE and i-DMI act simultaneously. Our key results are: (i) excitation of nonreciprocal spin-waves propagating perpendicularly to the in-plane projection of the static magnetization; (ii) skyrmions generation by pure spin-current; (iii) excitation of a new spin-wave mode with a spiral spatial profile originating from a gyrotropic rotation of a dynamical skyrmion. These results demonstrate that SHOs can be used as generators of magnetic skyrmions and different types of propagating spin-waves for magnetic data storage and signal processing applications.