L. Lopez-Diaz

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.

Micromagnetic tailoring of periodic antidot permalloy arrays for high density storage

A micromagnetic study on antidot permalloy arrays is carried out in order to delimit their possibilities for high density storage. The dependence of the recorded bit quality on antidot size, separation distance, and film thickness is reported as well as hysteresis loops and coercivities. The simulations show a limit for the antidot size around 80 nm leading to maximum areal storage densities of about 10 Gbits/in2. The results are interpreted in terms of demagnetizing, anisotropy, and exchange energies’ balance.

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.

Reversible and irreversible current induced domain wall motion in CoFeB based spin valves stripes

The authors present results on current induced domain wall motion in Co∕Cu∕CoFeB trilayered stripes. The threshold current densities are around 106A∕cm2 at zero field, i.e., about two orders of magnitude smaller than in single NiFe stripes. The domain wall motion is assisted when the field torque acts in the same direction as the spin torque. When the field torque is opposed to the spin transfer one and above a threshold field, the authors observe a reversible displacement of the domain wall (peak in the dV∕dI measurements). This can be ascribed to the onset of domain wall fluctuations, which is confirmed by micromagnetic simulations.

Micromagnetic dynamic computations including eddy currents

In this paper we present a 3D dynamic micromagnetic model which includes the effect of eddy currents and its application to magnetization reversal processes in permalloy nanocubes.

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.

Micromagnetic simulations of nanosecond magnetization reversal processes in magnetic nanopillar

In this paper we study by means of the spin torque model the fast switching behavior of the Co(20nm)∕Cu(5nm)∕Co(2.5nm) magnetic multilayers of two different cross sections: ellipse (130×70nm2) and ellipse (130×40nm2). Simulations have been performed at zero and room (300K) temperatures, these point out that the magnetization inversion occurs by nucleation processes in three main steps, for both structures. In particular, for zero temperature the third step of the switching depends on the value of the spin-polarized current. Furthermore, for all of the simulated currents the switching processes are thermally activated and smoother with respect to zero temperature.

Magnetization dynamics driven by the combined action of ac magnetic field and dc spin-polarized current

The spin-polarized current flowing through a ferromagnet can apply a torque able to drive magnetization reversal or to excite persistent dynamical states of magnetization. In the present work we simulate the dynamic behavior of nanomagnets in nanopillar spin valve structures due to the combined action of ac magnetic fields and dc spin-polarized current. The simulations are performed using a micromagnetic model which includes the effect of the spin-polarized current. We also discuss possible experimental realizations of the ac-field-assisted current-induced switching. Two particular cases of the ac-field polarization are studied: circular and linear. We find that in both cases ac field accelerates the switching process.

Coupling of spin-transfer torque to microwave magnetic field: A micromagnetic modal analysis

Micromagnetic computational spectral mapping technique is applied 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. A complete study has been carried out on multilayers Py(10 nm)/Cu(5 nm)/Py(2.5 nm) with rectangular cross section (60×20 nm2). The magnetic normal modes obtained agree with recent analytical spin wave models in patterned nanostructures. When both excitations, microwave field and spin polarized current, are applied at the same time a complex coupling process is observed. The detailed micromagnetic analysis of the coupling shows three different stages: (i) The initial stage in which the magnetic normal modes are dominant, (ii) an intermediate stage showing an incoherent behavior, and (iii) the final stage where a persistent domain wall oscillation is present. Micromagnetic spectral mapping technique is shown to be an adequate tool for describing the temporal evoluti...

Boundary Conditions for Spin-Wave Absorption Based on Different Site-Dependent Damping Functions

Finite-difference time-domain techniques applied to nonconfined geometries impose the numerical implementation of computational areas smaller than the physical ones, so that it is necessary to develop a method for the waves absorption at the artificial boundaries. Due to the impossibility to implement some sort of analytical Higdon-type operators within a micromagnetic framework for point-contact geometries, two different absorbing boundary conditions are proposed. They are based on different site-dependent damping functions, whose value rises either abruptly or smoothly close to the computational boundaries. In this paper, it is first tested their robustness and then pointed out their effectiveness in reducing the rate of wave reflection of about three orders of magnitude in some cases