Elastic properties of thin h-BN films investigated by Brillouin light scattering

Hexagonal BN films have been deposited by rf-magnetron sputtering with simultaneous ion plating. The elastic properties of the films grown on silicon substrates under identical coating conditions have been de-termined by Brillouin light scattering from thermally excited surface phonons. Four of the five independent elastic constants of the deposited material are found to be c11 = 65 GPa, c13 = 7 GPa, c33 = 92 GPa and c44 = 53 GPa exhibiting an elastic anisotropy c11/c33 of 0.7. The Young's modulus determined with load indenta-tion is distinctly larger than the corresponding value taken from Brillouin light scattering. This discrepancy is attributed to the specific morphology of the material with nanocrystallites embedded in an amorphous matrix

All-Optical Excitation and Detection of Picosecond Dynamics of Ordered Arrays of Nanomagnets with Varying Areal Density

We have demonstrated optical excitation and detection of collective precessional dynamics in arrays of coupled Ni80Fe20 (permalloy) nanoelements with systematically varying areal density by an all-optical time-resolved Kerr microscope. We have applied this technique to precisely determine three different collective regimes in these arrays. At very high areal density, a single uniform collective mode is observed where the edge modes of the constituent elements are suppressed. At intermediate areal densities, three nonuniform collective modes appear and at very low areal density, we observe noncollective dynamics and only the centre and edge modes of the constituent elements appear.Comment: 12 pages, 4 figure

Spin dynamics characterization in magnetic dots

The spin structure in a magnetic dot, which is an example of a quantum few-body system, is studied as a function of exchange coupling strength and dot size with in the semiclassical approximation on a discrete lattice. As the exchange coupli ng is decreased or the size is increased, the ground state undergoes a phase cha nge from a single domain ferromagnet to a spin vortex. The line separating these two phases has been calculated numerically for small system sizes. %, and analytically for larger dots. The dipolar interaction has been fully included in our calculations. Magnon frequencies in such a dot have also been calculated in both phases by the linearized equation of motion method. These results have also been reproduced f rom the Fourier transform of the spin autocorrelation function. From the magnon Density Of States (DOS), it is possible to identify the magnetic phase of the dot. Furthermore, the magnon modes have been characterized for both the ferromagnetic and the vortex phase, and the magnon instability mechanism leading to the vortex-ferro transition has also been identified. The results can also be used to compute finite temperature magnetization or vort icity of magnetic dots.Comment: 11 figures (12 figure files + 1 tex file

Imaging of Spin Dynamics in Closure Domain and Vortex Structures

Time-resolved Kerr microscopy is used to study the excitations of individual micron- scale ferromagnetic thin film elements in their remnant state. Thin (18 nm) square elements with edge dimensions between 1 and 10 $\mu$m form closure domain structures with 90 degree Neel walls between domains. We identify two classes of excitations in these systems. The first corresponds to precession of the magnetization about the local demagnetizing field in each quadrant, while the second excitation is localized in the domain walls. Two modes are also identified in ferromagnetic disks with thicknesses of 60 nm and diameters from 2 $\mu$m down to 500 nm. The equilibrium state of each disk is a vortex with a singularity at the center. As in the squares, the higher frequency mode is due to precession about the internal field, but in this case the lower frequency mode corresponds to gyrotropic motion of the entire vortex. These results demonstrate clearly the existence of well-defined excitations in inhomogeneously magnetized microstructures.Comment: PDF File (Figures at reduced resolution

Film Edge Nonlocal Spin Valves

Spintronics is a new paradigm for integrated digital electronics. Recently established as a niche for nonvolatile magnetic random access memory (MRAM), it offers new functionality while demonstrating low power and high speed performance. However, to reach high density spintronic technology must make a transition to the nanometer scale. Prototype devices are presently made using a planar geometry and have an area determined by the lithographic feature size, currently about 100 nm. Here we present a new nonplanar geometry in which one lateral dimension is given by a film thickness, the order of 10 nm. With this new approach, cell sizes can shrink by an order of magnitude. The geometry is demonstrated with a nonlocal spin valve, where we study devices with an injector/detector separation much less than the spin diffusion length.Comment: 10 pages, 3 figure

Micromagnetic simulations of spinel ferrite particles

This paper presents the results of simulations of the magnetization field {\it ac} response (at $2$ to $12$ GHz) of various submicron ferrite particles (cylindrical dots). The ferrites in the present simulations have the spinel structure, expressed here by M$_{1-n}$Zn$_{n}$Fe$_2$O$_4$ (where M stands for a divalent metal), and the parameters chosen were the following: (a) for $n=0$: M = \{ Fe, Mn, Co, Ni, Mg, Cu \}; (b) for $n=0.1$: M = \{ Fe, Mg \} (mixed ferrites). These runs represent full 3D micromagnetic (one-particle) ferrite simulations. We find evidences of confined spin waves in all simulations, as well as a complex behavior nearby the main resonance peak in the case of the M = \{ Mg, Cu \} ferrites. A comparison of the $n=0$ and $n=0.1$ cases for fixed M reveals a significant change in the spectra in M = Mg ferrites, but only a minor change in the M = Fe case. An additional larger scale simulation of a $3$ by $3$ particle array was performed using similar conditions of the Fe$_3$O$_4$ (magnetite; $n=0$, M = Fe) one-particle simulation. We find that the main resonance peak of the Fe$_3$O$_4$ one-particle simulation is disfigured in the corresponding 3 by 3 particle simulation, indicating the extent to which dipolar interactions are able to affect the main resonance peak in that magnetic compound.Comment: 35 pages, 11 figures, Journal of Magnetism and Magnetic Materials, in press

On the rotational dynamics of the Rattleback

The Rattleback is a very popular science toy shown to students all over the world to demonstrate the non-triviality of rotational motion. When spun on a horizontal table, this boat-shaped object behaves in a peculiar way. Although the object appears symmetric, the dynamics of its motion seem very asymmetric. When spun in the preferred direction, it spins smoothly, whereas in the other direction it starts to oscillate wildly. The oscillation soon dies out and the rattleback starts to spin in the preferred way. We will construct and go through an analytical model capable of explaining this behaviour in a simple and intelligible way. Although we aim at a semi-pedagogical treatise, we will study the details only when they are necessary to understand the calculation. After presenting the calculations we will discuss the physical validity of our assumptions and take a look at more sophisticated models requiring numerical analysis. We will then improve our model by assuming a simple friction force.Comment: 17 pages and 2 figures, typos corrected, some minor additions and rewording

Stochastic dynamic simulations of fast remagnetization processes: recent advances and applications

Numerical simulations of fast remagnetization processes using the stochastic dynamics are widely used to study various magnetic systems. In this paper we first address several crucial methodological problems of such simulations: (i) the influence of the finite-element discretization on the simulated dynamics, (ii) choice between Ito and Stratonovich stochastic calculi by the solution of micromagnetic stochastic equations of motion and (iii) non-trivial correlation properties of the random (thermal) field. Next we discuss several examples to demonstrate the great potential of the Langevin dynamics for studying fast remagnetization processes in technically relevant applications: we present numerical analysis of equilibrium magnon spectra in patterned structures, study thermal noise effects on the magnetization dynamics of nanoelements in pulsed fields and show some results for a remagnetization dynamics induced by a spin-polarized current.Comment: Invited paper submittedto JEMS'04 (Dresden, Germany

Nano-structured magnetic metamaterial with enhanced nonlinear properties

Nano-structuring can significantly modify the properties of materials. We demonstrate that size-dependent modification of the spin-wave spectra in magnetic nano-particles can affect not only linear, but also nonlinear magnetic response. The discretization of the spectrum removes the frequency degeneracy between the main excitation mode of a nano-particle and the higher spin-wave modes, having the lowest magnetic damping, and reduces the strength of multi-magnon relaxation processes. This reduction of magnon-magnon relaxation for the main excitation mode leads to a dramatic increase of its lifetime and amplitude, resulting in the intensification of all the nonlinear processes involving this mode. We demonstrate this experimentally on a two-dimensional array of permalloy nano-dots for the example of parametric generation of a sub-harmonic of an external microwave signal. The characteristic lifetime of this sub-harmonic is increased by two orders of magnitude compared to the case of a continuous magnetic film, where magnon-magnon relaxation limits the lifetime