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Néel skyrmions are generally realized in asymmetric multilayers made of heavy metals (HMs) and ultrathin ferromagnets possessing strong interfacial Dzyaloshinskii-Moriya interactions (iDMIs). Depending on the relative strengths of iDMIs at the interfaces, various types of Néel skyrmions have been suggested, which are typified with characteristically different topological properties and current-driven dynamics. This suggests the importance of a precise quantification of their spin chiralities. In this paper, we explore the possibility of realizing Néel skyrmions in magnetic multilayers without the direct usage of standard HMs. Specifically, through depositing a thin layer of ferrimagnetic (FIM) CoTb layer on top of an antiferromagnetic (AFM) quantum material of composition Mn3Sn, the AFM exchange interaction at the asymmetric interface provides an equivalent iDMI for stabilizing FIM Néel skyrmions. Secondly, through using advanced four-dimensional Lorentz scanning transmission electron microscopy (4D LSTEM), in combination with x-ray magnetic circular dichroism photoemission electron microscopy (XMCD-PEEM), we can directly determine the spin chirality of FIM Néel skyrmions. The present findings not only broaden the phase space for chiral interfacial magnetism but also provide a possibility for future applications of heavy-metal-free skyrmionic devices.

Novel computational paradigms may provide the blueprint to help solving the time and energy limitations that we face with our modern computers, and provide solutions to complex problems more efficiently (with reduced time, power consumption and/or less device footprint) than is currently possible with standard approaches. Spintronics offers a promising basis for the development of efficient devices and unconventional operations for at least three main reasons: (i) the low-power requirements of spin-based devices, i.e., requiring no standby power for operation and the possibility to write information with small dynamic energy dissipation, (ii) the strong nonlinearity, time nonlocality, and/or stochasticity that spintronic devices can exhibit, and (iii) their compatibility with CMOS logic manufacturing processes. At the same time, the high endurance and speed of spintronic devices means that they can be rewritten or reconfigured frequently over the lifetime of a circuit, a feature that is essential in many emerging computing concepts. In this perspective, we will discuss how spintronics may aid in the realization of efficient devices, primarily focusing on magnetic tunnel junctions. We then provide a perspective on how these devices can impact the development of three unconventional computing paradigms, namely, reservoir computing, probabilistic computing and memcomputing. These paradigms may be used to address some limitations of modern computers, providing a realistic path to intelligent hybrid CMOS-spintronic systems.

The efficient generation, manipulation and detection of magnetic skyrmions are important for the development of future spintronic devices. One approach is to use electric-current-induced spin torques. Recently, thermally induced skyrmion motion has also been observed, but wider experimental evidence and its capabilities remain limited. Here we report the thermal generation, manipulation and thermoelectric detection of nanoscale skyrmions in microstructured metallic multilayers integrated with on-chip heaters. The local application of heat can facilitate a domain morphological transition and the formation of skyrmions at the device edge, where a low energy barrier exists. We observe the unidirectional diffusion of skyrmions from hot regions to cold regions, which is due to the interplay among the repulsive forces between skyrmions, thermal spin–orbit torques, entropic forces and magnonic spin torques. The thermally generated skyrmions can also be electrically detected via the Nernst voltage.

Finding new energy-efficient methods for exciting magnetization dynamics is one of the key challenges in magnonics. In this work, we present an approach to excite the gyrotropic dynamics of magnetic vortices through the phenomenon of inverse magnetostriction, also known as the Villari effect. We develop an analytical model based on the Thiele formalism that describes the gyrotropic motion of the vortex core including the energy contributions due to inverse magnetostriction. Based on this model, we predict excitations of the vortex core resonances by surface acoustic waves whose frequency is resonant with the frequency of the vortex core. We verify the model's prediction using micromagnetic simulations and show the dependence of the vortex core's oscillation radius on the surface acoustic wave amplitude and the static bias field. Our study contributes to the advancement of energy-efficient magnetic excitations by relying on voltage-induced driven dynamics, which is an alternative to conventional current-induced excitations. [ABSTRACT FROM AUTHOR]

This paper introduces the concept of spin–orbit-torque-magnetic random access memory (SOT-MRAM) based physical unclonable function (PUF). The secret of the PUF is stored into a random state of a matrix of perpendicular SOT-MRAMs. Here, we show experimentally and with micromagnetic simulations that this random state is driven by the intrinsic nonlinear dynamics of the free layer of the memory excited by the SOT. In detail, a large enough current drives the magnetization along an in-plane direction. Once the current is removed, the in-plane magnetic state becomes unstable evolving toward one of the two perpendicular stable configurations randomly. In addition, we propose a hybrid CMOS/spintronics model to simulate a PUF realized by an array of 16 × 16 SOT-MRAM cells and evaluate the electrical characteristics. Hardware authentication based on this PUF scheme has several characteristics, such as CMOS-compatibility, non-volatility (no power consumption in standby mode), reconfigurability (the secret can be reprogrammed), and scalability, which can move a step forward the design of spintronic devices for application in security. [ABSTRACT FROM AUTHOR]

Metamaterials can be engineered to interact with waves in entirely new ways, finding application on the nanoscale in various fields such as optics and acoustics. In addition, acoustic metamaterials can be used in large-scale experiments for filtering and manipulating seismic waves (seismic metamaterials). Here, we propose seismic isolation based on a device that combines some properties of seismic metamaterials (e.g., periodic mass-in-mass systems) with that of a standard foundation positioned right below the building for isolation purposes. The concepts on which this solution is based are the local resonance and a dual-stiffness structure that preserves large (small) rigidity for compression (shear) effects. In other words, this paper introduces a different approach to seismic isolation by using certain principles of seismic metamaterials. The experimental demonstrator tested on the laboratory scale exhibits a spectral bandgap that begins at 4.5 Hz. Within the bandgap, it filters more than 50% of the seismic energy via an internal dissipation process. Our results open a path toward the seismic resilience of buildings and a critical infrastructure to shear seismic waves, achieving higher efficiency compared to traditional seismic insulators and passive energy-dissipation systems. [ABSTRACT FROM AUTHOR]

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. Index Terms--Nonlinear oscillators, perpendicular magnetic anisotropy, spin-waves, vortex dynamics.

The results of micromagnetic simulations of magnetic tunnel junctions (MTJs) patterned from thin films consisting of CoFe(8 nm)/[Al.sub.2][O.sub.3](0.8 nm)/Py(4 nm) with an elliptical cross section (90 * 35 n[m.sup.2]), where the CoFe and the Py are the fixed layer (PL) and the free layer (FL), respectively is presented. The results reveal that the switching mechanism does not change qualitatively by introducing a misalignment while the switching mechanism changes qualitatively when a nonideal shape is introduced.

Micromagnetic computational spectral mapping is used to analyze the magnetic oscillation modes excited by either a microwave circularly polarized magnetic field or a spin polarized current flowing through Permalloy (Py) spin valves. Micromagnetic spectra mapping method is an adequate tool for describing the temporal evolution of the magnetization spatial patterns in nanostructures.

This paper introduces an identification procedure of the classical and modified scalar Preisach models based on genetic algorithms. It can be used when a reduced set of experimental data is known. The main advantage of this approach is the numerical stability of the solution. The results show that the coupling between our genetic identification procedure and the modified scalar Preisach model is able to reproduce the experimental behavior of different magnetic soft steels. Index Terms--Genetic algorithms, identification, magnetic hysteresis, numerical stability, Preisach model, soft magnetic materials.

The paper shows the equivalence between algorithms that use different experimental data in order to identify the Preisach probability function. Furthermore, it is proposed an identification procedure which increases the computation accuracy of the numerical Preisach probability function of magnetic materials and reduces the necessary experimental data to solve the identification problem. The proposed approach uses a set of symmetric minor loops and a new discrimination function of the Preisach plane. Index Terms--Hysteresis, numerical optimization, Preisach model, soft magnetic materials.

In this paper, we discuss in general the numerical problem of the identification of the Preisach function in the Modified Scalar Preisach Model (MSPM) for soft materials. We show that the solution of the inverse problem using a bilinear algebraic system approach can introduce numerical instabilities when operating with measured data: these instabilities can be avoided, or greatly reduced, using a recursive technique explained in the paper, or alternately a complete two-dimensional inversion procedure. We show the results obtained for laminated magnetic steels, and in particular the shape of the product functions and the Preisach probability functions computed. Index Terms--Magnetic hysteresis, numerical identification, soft magnetic material.

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. [ABSTRACT FROM AUTHOR]

We employed micro-focused Brillouin light scattering to study the amplification of the thermal spin wave eigenmodes by means of a pure spin current, generated by the spin-Hall effect, in a transversely magnetized Pt(4 nm)/NiFe(4 nm)/SiO2(5 nm) layered nanowire with lateral dimensions 500×2750 nm2. The frequency and the cross section of both the center (fundamental) and the edge spin wave modes have been measured as a function of the intensity of the injected dc electric current. The frequency of both modes exhibits a clear redshift while their cross section is greatly enhanced on increasing the intensity of the injected dc. A threshold-like behavior is observed for a value of the injected dc of 2.8 mA. Interestingly, an additional mode, localized in the central part of the nanowire, appears at higher frequency on increasing the intensity of the injected dc above the threshold value. Micromagnetic simulations were used to quantitatively reproduce the experimental results and to investigate the complex non-linear dynamics induced by the spin-Hall effect, including the modification of the spatial profile of the spin wave modes and the appearance of the extra mode above the threshold. [ABSTRACT FROM AUTHOR]

Spintronic diodes are emerging as disruptive candidates for impacting several technological applications ranging from the Internet of things to artificial intelligence. Here, an overview of the recent achievements on spintronic diodes is briefly presented, underlying the major breakthroughs that have led these devices to have the largest sensitivity measured to date for a diode. For each class of spintronic diodes (passive, active, resonant, nonresonant), we indicate the remaining developments to improve the performances as well as the future directions. We also devoted the last part of this Perspective to ideas for developing spintronic diodes in multiphysics systems by combining two-dimensional materials and antiferromagnets. [ABSTRACT FROM AUTHOR]

In this paper we discuss about the possibility to reproduce the behavior of a magnetic material at micromagnetic scale with a phenomenological vector model of hysteresis based on the definition of a vector hysteron. We compare data computed by means of two different static magnetic models. The former is based on the solution of Brown's equation (micromagnetic scale). The latter is a vector generalization of the Classical Scalar Preisach Model (macromagnetic scale). We show that the magnetization versus applied field curves computed by both models are in good agreement, and that the macromagnetic-scale model is able to reproduce the physical behavior of the magnetic materials at micromagnetic scale. Index Terms--Magnetic hysteresis, micromagnetic modeling, Preisach vector modeling.

Antiferromagnets (AFMs) can extend the working frequency of spintronics to terahertz range while using similar geometries and concepts of gigahertz ferromagnet-based spintronic devices. Here, we investigate micromagnetically the dynamics of spin-Hall oscillators driven by the simultaneous action of a dc and an ac current. Our findings include the clear occurrence of three types of dynamics, depending on the frequency of the ac with respect to the self-oscillation frequency excited by the dc: 1) frequency modulation; 2) frequency pulling; and 3) injection locking. These results confirm the suitability of AFMs for the development of a terahertz technology which can impact the field of telecommunications. [ABSTRACT FROM AUTHOR]

In this paper, we will discuss the use of the Lorentzian function as a possible candidate for the accurate approximation of the Preisach function in the modeling of scalar hysteresis for nonoriented grain steels. In particular we discuss here the identification procedure of the parameters of the function from measured data. Index Terms--Lorentzian function, nonoriented grain steels, Preisach function, scalar magnetic hysteresis modeling.

We measure and simulate micromagnetically a framework based upon a nano-contact spin torque oscillator where two distinct localized evanescent spin-wave modes can be detected. The resulting frequency spectrum is composed by two peaks, corresponding to the excited modes, which lie below the ferromagnetic resonance frequency, and a low-frequency tail, which we attribute to the non-stationary switching between these modes. By using Fourier, wavelet, and Hilbert-Huang transforms, we investigate the properties of these modes in time and spatial domains, together with their spatial distribution. The existence of an additional localized mode (which was neither predicted by theory nor by previous numerical and experimental findings) has to be attributed to the large influence of the current-induced Oersted field strength which, in the present setup, is of the same order of magnitude as the external field. As a further consequence, the excited spin-waves, contrarily to what usually assumed, do not possess cylindrical symmetry: the Oersted field induces these modes to be excited at the two opposite sides of the region beneath the nano-contact. [ABSTRACT FROM AUTHOR]

A concept of an electrically tunable resonance detector of THz-frequency signals based on an antiferromagnetic/heavy metal (AFM/HM) heterostructure is proposed. The conversion of a THz-frequency input signal into DC voltage is done using the inverse spin Hall effect in an (AFM/HM) bilayer. An additional bias DC in the HM layer can be used to vary the effective anisotropy of the AFM and, therefore, to tune the antiferromagnetic resonance (AFMR) frequency. The proposed AFM/HM heterostructure works as a resonance-type quadratic detector, which can be tuned by the bias current in the range of at least 10% of the AFMR frequency, and our estimations show that the sensitivity of this detector could be comparable to that of modern detectors based on the Schottky, Gunn, or graphene-based diodes. [ABSTRACT FROM AUTHOR]

Periodic or quasi-periodic arrangements of artificial structures can be used to design a class of materials, i.e., metamaterials, with intriguing properties. Recently, it has been proposed to use periodic systems with internal resonances for the attenuation of acoustic/seismic waves. However, large input displacements due to seismic waves can drive the working point of these systems in a nonlinear regime. Here, we have studied the nonlinear dynamics of periodic chain of mass-in-mass systems, which can be used to model composite foundations, where the external spring is characterized by an anharmonic potential. The main finding of this work is the identification of two attenuation mechanisms, one is characterized by an exponential amplitude decay and is observed in the strongly anharmonic regime, whereas the other has a linear decay pattern and characterizes the weak anharmonic dynamics. This result has a direct impact in the design of low frequency seismic metamaterials. [ABSTRACT FROM AUTHOR]

This paper describes a full micromagnetic characterization of the magnetization dynamics driven by spin-polarized current in anisotropic spin-torque oscillators (STOs). For field angles approaching the hard in-plane axis, the excited mode is uniform and a super-critical Hopf-bifurcation takes place at the critical current density JC. For field angles close to the easy axis of the free layer, the excited mode is localized (non-uniform) and a sub-critical Hopf-bifurcation occurs at JC. In this latter region, a hysteretic behaviour is, therefore, found. We demonstrate numerically that the non-linearities of the STO are strongly reduced when the oscillation frequency at the critical current is near the ferromagnetic resonance (FMR) frequency computed at zero bias current, and in particular, this condition corresponds to the field orientation at which a minimum in the FMR-frequency is achieved. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]