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The realistic modeling of spin-transfer torque (STT)-magnetoresistive random access memory (MRAM) for the simulations of hybrid CMOS/Spintronics devices in comprehensive simulation environments requires a full description of stochastic switching processes in the state-of-the-art STT-MRAMs. Here, we compare micromagnetic simulations with an analytical formulation that takes into account the spin-torque asymmetry of the spin-polarization function by considering the mean, standard deviation, skewness, and kurtosis. We find that, while the first and second statistical moments exhibit a very similar behavior, skewness and kurtosis are substantially different and must be taken into account in order to provide an accurate prediction of the switching performance. In fact, a reasonable fit of the probability density function (PDF) of the switching time is given by a Pearson Type IV PDF. The main achievements of this paper underline the need of data-driven design of STT-MRAM that uses a full micromagnetic simulation framework for the statistical proprieties PDF of switching processes. [ABSTRACT FROM PUBLISHER]

The technological implementation of STT-magnetic random-access memory (MRAM) in real devices needs a complete description of the statistical switching behavior at room temperature. In this paper, we investigate, by means of a full micromagnetic model, the effect of the interfacial Dzyaloshinskii–Moriya interaction (IDMI) on the critical current density and probability density function (PDF) of the switching time in perpendicular STT-MRAMs. We show that for large enough values of the symmetric exchange interaction, the negative effect of the IDMI on the critical current is strongly reduced. In addition, a comprehensive analysis of the four main statistical moments (mean, standard deviation, skewness, and kurtosis) points out that to fit the switching time PDF in order to maintain a quadratic error at least one order of magnitude smaller than skew normal PDF, a Pearson type IV PDF has to be used also in presence of the IDMI. [ABSTRACT FROM AUTHOR]

The magnetization dynamics of a ferromagnet can be influenced by the second-order anisotropy contribution when the magnetocrystalline perpendicular anisotropy is of the same order of the out-of-plane demagnetizing field (compensation point). This condition can be fulfilled in magnetic tunnel junctions (MTJs) where the induced interfacial perpendicular anisotropy can be controlled by the thickness of the free layer. This paper describes, within a both micromagnetic and analytical/numerical framework, how switching and self-oscillation of the magnetization are affected by the second order magnetocrystalline anisotropy near the compensation point. We have derived analytical expressions for the switching critical current and the oscillation frequencies that are well reproduced by numerical simulations. Our results can support the design of MTJs for application either in storage and nanoscale signal generators. [ABSTRACT FROM PUBLISHER]

A strategy for a scalable synchronization of an array of spin-Hall oscillators (SHOs) is illustrated. In detail, we present the micromagnetic simulations of two and five SHOs realized by means of couples of triangular golden contacts on the top of a Pt/CoFeB/Ta trilayer. The results highlight that the synchronization occurs for the whole current region that gives rise to the excitation of self-oscillations. This is linked to the role of the magnetodipolar coupling, which is the phenomenon driving the synchronization when the distance between oscillators is not too large. Synchronization also turns out to be robust against geometrical differences of the contacts, simulated by considering variable distances between the tips ranging from 100nm to 200nm. Besides, it entails an enlargement of the radiation pattern that can be useful for the generation of spin-waves in magnonics applications. Simulations performed to study the effect of the interfacial Dzyaloshinskii-Moriya interaction show nonreciprocity in spatial propagation of the synchronized spin-wave. The simplicity of the geometry and the robustness of the achieved synchronization make this design of array of SHOs scalable for a larger number of synchronized oscillators. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Current-driven domain wall dynamics in long Permalloy strips with square cross section, taking into account the influence of the Oersted field and the nonadiabatic spin-torque term, are studied in this work. Depending on the size of the square cross section two types of domain wall configurations can be nucleated, transverse or Bloch-point walls. The transverse wall dynamics showed a linear relation between the wall velocity and the applied current density. Also seen for this type of wall is that its dynamic is practically unaffected by either the presence of the Oersted field or the value of the nonadiabatic parameter, and its behavior resembles the one seen in cylindrical wires as it also does not have a Walker breakdown. In the case of the Bloch-point wall there is a threshold current density in order to promote the wall movement and it also depends on the value of the nonadiabatic parameter. The effect of the Oersted field for some values of the nonadiabatic parameter is the increase of the wall's velocity and reduction of its width. For values above 10^13\ A\cdot m^-2 the wall transforms into complex unstable structures. [ABSTRACT FROM AUTHOR]

Narrow linewidth spin transfer driven vortex self-oscillations in Py/Cu/Py spin valves of elliptical cross section (160 nm\,\times\,75 nm) are investigated by means of micromagnetic simulations. “GPmagnet”, a high performance GPU based micromagnetic package, is used for this purpose, allowing for 100 KHz spectral resolution at reasonable computational time. The variation of the linewidth with applied current (10\times 10^7 A/cm^2\ < J < 20\times 10^7 A/cm^2) is analyzed in the temperature range 0 K < T < 300 K. Linewidths around 1 MHz are found at T = 0 K. At T = 300 K, linewidths remain below 2 MHz for J < 16\times 10^7 A/cm^2, whereas for larger currents the presence of two oscillation modes lead to a further linewidth increase. [ABSTRACT FROM AUTHOR]

We report enhanced spin-torque oscillator results obtained in spin-valve nanopillars. When biased within the optimal range of a moderate, ≤600 Oe, hard axis field, the spin-torque-driven oscillations exhibit a sharp increase in power and a sharply narrowed linewidth, ≤10 MHz, which, based on micromagnetic simulations, we ascribe to a transition from incoherent to coherent dynamics. The simulations indicate that the coherent dynamics are enabled by the combination of strong coupling between the two oscillator end modes of the magnetic free layer and strong non-linear damping arising from a non-uniform magnetization that leads to a spatially varying anti-damping spin torque. [ABSTRACT FROM AUTHOR]

In this work we present a physical-related model of vector hysteresis for magnetic simulation of nanometric structures. The mathematical formulation, based on a generalization of the Preisach and Stoner-Wohlfarth models, is very simple and elegant. The numerical implementation is very easy and the computation time required for the simulations is extremely fast. However the accuracy is not comparable with the codes based on the micromagnetic theory, but it is sufficient for the use as preprocessing of more accurate and more complex methods, in order to speed up the convergence. In this paper the model identification is performed for a nanoscale spin valve using the data generated by a 3D micromagnetic code. [ABSTRACT FROM AUTHOR]

The nonautonomous dynamics of spin-torque oscillators in presence of both microwave current and field has been numerically studied in nanostructured devices. When both microwave current and field are applied at the same frequency, integer phase locking at different locking ratio is found. In the locking region, a study of the intrinsic phase shift between the locking force (current or field) and the giant magnetoresistive signal as a function of the bias current is also exploited. [ABSTRACT FROM AUTHOR]

Abstract: The influence of the magnetostatic coupling (MC) on magnetization reversal processes driven by spin-polarized current has been 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 MC plays a crucial role and it speeds the magnetization switching process. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]