Magnetization precession due to a spin polarized current in a thin nanoelement: numerical simulation study

In this paper a detailed numerical study (in frames of the Slonczewski formalism) of magnetization oscillations driven by a spin-polarized current through a thin elliptical nanoelement is presented. We show that a sophisticated micromagnetic model, where a polycrystalline structure of a nanoelement is taken into account, can explain qualitatively all most important features of the magnetization oscillation spectra recently observed experimentally (S.I. Kiselev et al., Nature, vol. 425, p. 380 (2003), namely: existence of several equidistant spectral bands, sharp onset and abrupt disappearance of magnetization oscillations with increasing current, absence of the out-of-plane regime predicted by a macrospin model and the relation between frequencies of so called small-angle and quasichaotic oscillations. However, a quantitative agreement with experimental results (especially concerning the frequency of quasichaotic oscillations) could not be achieved in the region of reasonable parameter values, indicating that further model refinement is necessary for a complete understanding of the spin-driven magnetization precession even in this relatively simple experimental situation.Comment: Submitted to Phys. Rev. B; In this revised version figure positions on the page have been changed to ensure correct placements of the figure caption

Transition from the macrospin to chaotic behaviour by a spin-torque driven magnetization precession of a square nanoelement

We demonstrate (using full-scale micromagnetic simulations) that the spin injection driven steady-state precession of a thin magnetic nanoelement exhibit a complicate transition from the quasi-macrospin to the chaotic behaviour with the increasing element size. For nanoelement parameters typical for those used experimentally we have found that the macrospin approximation becomes invalid already for very small nanoelement sizes (~ 30 nm), in contrast to the previously reported results (Li and Zhang, Phys. Rev. B, vol. B68, 024404-1 (2003))Comment: Submitted to Phys. Rev.

Micromagnetic simulations of the magnetization precession induced by a spin polarized current in a point contact geometry

This paper is devoted to numerical simulations of the magnetization dynamics driven by a spin-polarized current in extended ferromagnetic multilayers when a point-contact setup is used. We present (i) detailed analysis of methodological problems arising by such simulations and (ii) physical results obtained on a system similar to that studied in Rippard et al., Phys. Rev. Lett., v. 92, 027201 (2004). We demonstrate that the usage of a standard Slonczewski formalism for the phenomenological treatment of a spin-induced torque leads to a qualitative disagreement between simulation results and experimental observations and discuss possible reasons for this discrepancy.Comment: Invited paper on MMM2005 (San Jose); accepted for publication in J. Applied Physic

Synchronization of spin-torque driven nanooscillators for point contacts on a quasi-1D nanowire: Micromagnetic simulations

In this paper we present detailed numerical simulation studies on the synchronization of two spin-torque nanooscillators (STNO) in the quasi-1D geometry: magnetization oscillations are induced in a thin NiFe nanostripe by a spin polarized current injected via square-shaped CoFe nanomagnets on the top of this stripe. In a sufficiently large out-of-plane field, a propagating oscillation mode appears in such a system. Due to the absence of the geometrically caused wave decay in 1D systems, this mode is expected to enable a long-distance synchronization between STNOs. Indeed, our simulations predict that synchronization of two STNOs on a nanowire is possible up to the intercontact distance 3 mkm (for the nanowire width 50 nm). However, we have also found several qualitatively new features of the synchronization behaviour for this system, which make the achievement of a stable synchronization in this geometry to a highly non-trivial task. In particular, there exist a minimal distance between the nanocontacts, below which a synchronization of STNOs can not be achieved. Further, when the current value in the first contact is kept constant, the amplitude of synchronized oscillations depends non-monotonously on the current value in the second contact. Finally, for one and the same currents values through the contacts there might exist several synchronized states (with different frequencies), depending on the initial conditions.Comment: 13 pages with 4 figurews, recently submitted to PR

An Exploratory Study of the Effects of Mild Cognitive Impairment on Elderly Internet Users

This study focuses on the effects of Mild Cognitive Impairment and other minor memory impairments on a person's ability to successfully use the Internet. Participants over sixty-five years of age were recruited from retirement communities and were selected based on self-reports of Mild Cognitive Impairment or other cognitive difficulties when using the Internet. Interviews with the participants focused on their abilities to use Email, Chat/Instant Messenger and the World Wide Web. Participants were then asked to step through several Internet-related tasks in order to further identify problem areas. Seven participants were interviewed, and six of them completed the optional observation session. The data collected from the interviews and observation sessions were then broken down into different categories, based on the type of difficulties experienced during Internet use. Finally, recommendations were made for good Web design practices intended to overcome the difficulties identified during the study

Thermal fluctuations and longitudinal relaxation of single-domain magnetic particles at elevated temperatures

We present numerical and analytical results for the swiching times of magnetic nanoparticles with uniaxial anisotropy at elevated temperatures, including the vicinity of T_c. The consideration is based in the Landau-Lifshitz-Bloch equation that includes the relaxation of the magnetization magnitude M. The resulting switching times are shorter than those following from the naive Landau-Lifshitz equation due to (i) additional barrier lowering because of the reduction of M at the barrier and (ii) critical divergence of the damping parameters.Comment: 4 PR pages, 1 figur

Irreversible relaxation behaviour of a general class of magnetic systems

Abstract. It is shown that magnetic systems after magnetization in a weak external field for a finite time t mag exhibit a universal time-dependent relaxation behaviour. The normalized magnetization decay after switching off an external field does not depend on any sample parameters and follows a universal law m(t) ∼ log(1 + t mag /t). This universal time dependence is confirmed by magnetic relaxation measurements performed on different powders of small barium hexaferrite magnetic particles at room temperature. The measurements were performed using the PTB SQUID magnetometer in the Berlin Magnetically Shielded Room

Magnetodipolar interlayer interaction effect on the magnetization dynamics of a trilayer square element with the Landau domain structure

We present a detailed numerical simulation study of the effects caused by the magnetodipolar interaction between ferromagnetic ͑FM͒ layers of a trilayer magnetic nanoelement on its magnetization dynamics. As an example, we use a Co/ Cu/ Ni 80 Fe 20 element with a square lateral shape where the magnetization of FM layers forms a closed Landau-like domain pattern. First, we show that when the thickness of the nonmagnetic ͑NM͒ spacer is in the technology relevant region h ϳ 10 nm, magnetodipolar interaction between 90°Neel domain walls in FM layers qualitatively changes the equilibrium magnetization state of these layers. In the main part of the paper, we compare the magnetization dynamics induced by a sub-nsec field pulse in a single-layer Ni 80 Fe 20 ͑Py͒ element and in the Co/ Cu/ Py trilayer element. Here, we show that ͑i͒ due to the spontaneous symmetry breaking of the Landau state in the FM/NM/FM trilayer, its domains and domain walls oscillate with different frequencies and have different spatial oscillation patterns; ͑ii͒ magnetization oscillations of the trilayer domains are strongly suppressed due to different oscillation frequencies of domains in Co and Py; ͑iii͒ magnetization dynamics qualitatively depends on the relative rotation sense of magnetization states in Co and Py layers and on the magnetocrystalline anisotropy kind of Co crystallites. Finally, we discuss the relation of our findings with experimental observations of magnetization dynamics in magnetic trilayers, performed using the element-specific time-resolved x-ray microscopy

Magnetization Switching in Nanowires: Monte Carlo Study with Fast Fourier Transformation for Dipolar Fields

For the investigations of thermally activated magnetization reversal in systems of classical magnetic moments numerical methods are desirable. We present numerical studies which base on time quantified Monte Carlo methods where the long-range dipole-dipole interaction is calculated with the aid of fast Fourier transformation. As an example, we study models for ferromagnetic nanowires comparing our numerical results for the characteristic time of the reversal process also with numerical data from Langevin dynamics simulations where the fast Fourier transformation method is well established. Depending on the system geometry different reversal mechanism occur like coherent rotation, nucleation, and curling.Comment: 7 pages, 5 figures, submitted to J. Magn. Magn. Ma