Permalloy Nanodots Research Articles

Fabrication and Static Magnetic Properties of Novel One- and Two-Dimensional Bi-Component Magnonic Crystals

We have fabricated using advanced electron beam lithography, periodic arrays of lateral one-dimensional (1-D) and two-dimensional (2-D) bi-component exchange and magnetostatically coupled magnonic crystals (MC) consisting of alternating cobalt (Co) and Permalloy (Py) nanowires (NWs) and nanodots (NDs), lying side by side. For 1-D MCs, the width of the Co NWs is fixed at 160 nm while the width of the Py NWs is varied from 160 to 800 nm. We observed two distinct switching steps corresponding to the reversal of Py and Co NWs in the array, in contrast to single step switching of homogeneous NWs. For 2-D MCs, the diameter of both Co and Py dots were kept constant at 400 nm. We found that strong dipolar fields from neighboring Co dots strongly influence the reversal process of Py dots resulting in the tuneability of vortex nucleation and annihilations fields.

Extended longitudinal vector and Bragg magneto-optic Kerr effect for the determination of the chirality distribution in magnetic vortices

We have measured longitudinal magneto-optic Kerr effect (MOKE) for off-specular beams diffracted by a square array of Permalloy nanodots outside the plane of incidence and found that only the measurements performed outside the plane of incidence are sensitive to the distribution of rotational sense of vortices (vortex chirality) in the dot array. An asymmetry was introduced into the dot shape to ensure a uniform vortex chirality distribution in the dot array and to obtain a clear, well-defined chirality contrast in the MOKE signal. We also demonstrate an alternative method to measure the vortex chirality in dots of asymmetric shape without switching the chirality during magnetization reversal. In addition to the experiment, we have developed a general formalism that can be used to describe MOKE measured for off-specular beams inside as well as outside of the plane of incidence in all three Kerr geometries (polar, longitudinal, and transversal) with an arbitrary polarization state of incident light. Combined with micromagnetic simulation, this formalism reproduces the experimental results well.

Electric-current-driven vortex-core reversal in soft magnetic nanodots

The authors report on electric-current-driven vortex-core (VC) reversal (switching) and the accompanying spin-wave emission, driven by spin-polarized ac currents of different amplitudes and frequencies, investigated by micromagnetic calculations of the dynamic evolution of a magnetic vortex in Permalloy nanodots. The magnetization orientation of the VC is effectively switchable between its upward and downward bistates and controllable by applying current above its threshold density, but with sufficiently small magnitude at frequencies close to the vortex eigenfrequency. This VC reversal phenomenon occurs through the creation of a vortex-antivortex pair and the subsequent annihilation of the initial vortex and the created antivortex, when the velocity of the initial VC reaches its critical value of approximately 340±20m∕s for the given material and geometry. In the course of these serial processes and immediately after VC switching, strong spin waves are emitted. These results provide physical insights into how and when current-driven VC switching takes place, thereby offering a means to manipulate bistate VC orientations.

Soft spin waves and magnetization reversal in elliptical Permalloy nanodots: Experiments and dynamical matrix results

We present an experimental and theoretical investigation of magnetization reversal in Permalloy elliptical nanodots under the application of a varying (quasistatic) magnetic field directed along the hard (short) and easy (long) axes of the ellipses. The magnetic response of the particles is investigated with the magneto-optic Kerr effect (MOKE) and Brillouin scattering (BLS). Experimental MOKE results are reproduced by micromagnetic simulations. The spectrum of the normal modes, calculated with the dynamical matrix method, reproduces the BLS results. We find that when an abrupt magnetization switching occurs it is accompanied by a soft magnetic mode (different in the two cases), the symmetry of which determines the initial steps (onset) of the microscopic reversal path.

Elastic and Magnetic Dynamics of Nanomagnet-Ordered Arrays Impulsively Excited by Subpicosecond Laser Pulses

This Letter reports on the first observation of elastic and magnetic dynamics of ordered arrays of permalloy nanodots excited by low-intensity 120 fs light pulses. The first order of the diffraction pattern, generated by the probe beam in a pump-probe configuration, is used for time-resolved reflectivity and time-resolved magneto-optical Kerr effect measurements. The nonadiabatical absorption of the pump triggers an acoustic standing wave, detected by the reflected probe signal, with a frequency related to the array wave vector. Instead, the magneto-optical signal exhibits, on the nanosecond time scale, the signature of the heat-exchange diffusion processes. In addition, a clear oscillation of the magnetic signal, at a frequency close to the frequency of the acoustic wave, is unambiguously detected. Finally, the interplay between the elastic and magnetic dynamics is analyzed and interpreted.

Analysis of ferromagnetic resonance response of square arrays of permalloy nanodots

We present a detailed comparison between the results of ferromagnetic resonance (FMR) measurements on a square array of permalloy nanodots and a numerical simulation based on the eigenvalues of the linearized Landau–Lifshitz equation. Of particular importance is the implementation of a recently developed technique for numerically calculating the absorption spectrum from the associated eigenvectors as a function of the orientation, frequency, and position dependence of the applied RF field. The method is also used to predict results of some possible future experiments.

Magnetization reversal process in elliptical Permalloy nanodots

Using the Vectorial Magneto-Optic Kerr Effect (V-MOKE) and numerical simulations, we have investigated the magnetization reversal process in arrays of 15-nm-thick Permalloy nanometer-scale dots, having elliptical shape and eccentricity, varying from 1 to 2.5. V-MOKE hysteresis loops revealed that the magnetization reversal is incoherent for elements with eccentricity of 1 and 1.5, while it becomes an almost perfect coherent magnetization rotation for elements with eccentricity equal to 2 and 2.5. In the latter case, the V-MOKE loops agree well with those predicted by the Stoner–Wohlfarth model for particles with uniaxial magnetic anisotropy. We were able to reproduce the V-MOKE results with micromagnetic simulations, gaining a deeper insight into the magnetic configurations that develop during the reversal process.

ダマシン法によるＹ字型Ｎｉ８０Ｆｅ２０構造規則配列の作製と磁気的評価

Regularly aligned arrays (linear and honeycomb arrangements) of Y-shaped Ni80Fe20 (permalloy) nanodots buried in silicon wafers were fabricated using the damascene technique, and electron beam lithography (EB). The distributions of surface magnetization were observed by means of magnetic force microscopy (MFM), and the internal spin distributions were analyzed by a micromagnetic simulation using the Landau-Lifshitz-Gilbert equations (LLG eq.). Spin distributions are determined by the effects of geometric shapes and magnetostatic interactions working between adjacent dots.

Fabrication and magnetic force microscopy (MFM) observation of nano scale ferromagnetic nanodot arrays

A novel electron beam lithography technique was utilized to fabricate nano scale magnetic dots array with high periodicity. C60 film (fullerine) was used as an electron beam sensitive resist for electron beam patterning carried out under a high accelerating voltage of 200 kV in a transmission electron microscope with a scanning mode. 50 nm sized permalloy (Py) nanodot arrays were successfully fabricated using the method followed by ion-milling. An array of Co nanodots having a size of 100×70 nm was alternatively prepared by liftoff instead of ion milling so as to prevent surface damage by ion milling. Lift-off does not yield any surface roughness, which is important for device fabrication. Magnetic force microscopy (MFM) observations were carried out on the fabricated Co nanodot arrays. A single domain state could be realized in patterned Co nanodots. MFM tip induced magnetization effects were also clearly demonstrated in the array of Co nanodots.

Micromagnetic simulations of vortex-state excitations in soft magnetic nanostructures

The dynamic susceptibility spectra of Permalloy nanodots, supporting a vortex-type magnetic configuration, are studied within the frequency range $0.2--20\phantom{\rule{0.3em}{0ex}}\mathrm{GHz}$ as a function of dot thickness $(10\phantom{\rule{0.3em}{0ex}}\mathrm{nm}\ensuremath{\leqslant}{L}_{z}\ensuremath{\leqslant}80\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$ by means of three-dimensional dynamic micromagnetic simulations. In addition to the low-frequency vortex translation mode (gyrotropic motion of the vortex core), a second vortex core mode is revealed at a higher frequency for thicker nanodots (in-plane pumping field). This mode whose resonance frequency decreases rapidly with increasing dot thickness originates from the nonuniform vortex structure along the dot normal axis. Higher frequency modes are also observed for both in-plane and perpendicular pumping field orientations and correspond mostly to spin excitations localized outside the vortex core. The possible detection of the two vortex core modes within an individual nanodot using resonance experiments is discussed on the basis of the dispersion relation frequency versus perpendicular static magnetic field.

Scalable parallel micromagnetic solvers for magnetic nanostructures

A parallel finite element micromagnetics package has been implemented, that is highly scalable, easily portable and combines different solvers for the micromagnetic equations. The implementation is based on the standard Galerkin discretization on tetrahedral meshes with linear basis functions. A static energy minimization, a dynamic time integration, and the nudged elastic band method have been implemented. The details of the implementation and some aspects of the optimization are discussed and timing and speedup results are given. Nucleation and magnetization reversal processes in permalloy nanodots are investigated with this micromagnetics package.