Magnetic Nanodisks Research Articles

Gyration eigenfrequencies of vertically coupled vortices in layered magnetic disks

The dynamic properties of vertically coupled vortices in two magnetic nanodisks were studied using numerical simulations and analytical calculations. If the core polarizations of the two vortices are parallel, there exist two distinct normal modes with two distinct eigenfrequencies corresponding to the apparent complex motions of two vortices. Conversely, only a degenerate mode with a single eigenfrequency exists when the cores have opposite polarization. We show that the gyration eigenfrequencies can be tuned by changing the coupling strength, i.e., the separation between the disks. The dependence of the normal modes and the eigenfrequencies on the relative vortex-state configuration can be well understood based on the analytic model.

Control of magnetic vortex chirality and polarization in indented and notched nanomagnetic disks

Magnetic vortex dynamics in nanoscale structures is currently a topic of intensive research not only from a fundamental physics point of view but also for their potential use in future generation spintronics and magnetic random access memories. We propose a method, where one can independently control the magnetic vortex polarization and chirality states by a combination of fine-tuning the applied magnetic field and breaking the geometrical symmetry of the magnetic nanostructure. Numerical simulations corroborate our proposal of achieving vortex switchability for the two different geometries we investigate: the indented disk and notched disk structures. Our results suggest that the notched disk structure offers more robust vortex dynamics and better switching characteristics, which makes this geometry ideal for use as a vortex-based magnetic memory device.

Magnetic nanodiscs fabricated from multilayered nanowires.

We report a simple, high throughput synthesis method of producing magnetic nanodiscs, in which the diameter and thickness are easily controlled. This method consists of two steps: (1) Electrodeposition for growing multilayered nanowires and (2) Selective etching of sacrificial layers. The electrodeposition step results in a bundle of multilayered nanowires. The nanowires consist of alternating layers of magnetic (e.g., Co) and sacrificial materials (e.g., Cu) inside the nanometer-sized pores of an anodized aluminum oxide (AAO) template. The diameter of each layer is determined by pore size, while the thickness is controlled by electrodeposition time. The selective wet etching step removes sacrificial layers, leaving the magnetic nanodiscs. Through this process, the magnetic nanodiscs are fabricated with aspect ratios ranging from 0.25 to 2.0.

Interaction between magnetic vortex cores in a pair of nonidentical nanodisks

The coupling of two nonidentical magnetic nanodisks, i.e., with different vortex gyrotropic frequencies, is studied. From the analytical approach, the interactions between the nanodisks along x and y directions (the coupling integrals) were obtained as a function of distance. From the numerical solution of Thiele's equation, we derived the eigenfrequencies of the vortex cores as a function of distance. The motion of the two vortex cores and, consequently, the time dependence of the total magnetization M(t) were derived both using Thiele's equation and by micromagnetic simulation. From M(t), a recently developed method, the magnetic vortex echoes, analogous to the Nuclear Magnetic Resonance spin echoes, was used to compute the distance dependence of the magnetic coupling strength. The results of the two approaches differ by approximately 10%; using one single term, a dependence with distance found is broadly in agreement with studies employing other techniques.

Dipolar ordering in a molecular nanomagnet detected using muon spin relaxation

Implanted muons have been used as a local probe to detect the magnetic ordering in the molecular magnetic nanodisk system Fe 19 . Two distinct groups of muon sites are identified from the relaxation data, reflecting sites near the magnetic core and sites distributed over the rest of the molecule. Dipole field calculations and Monte Carlo simulations confirm that the observed transition in Fe 19 is consistent with magnetic ordering driven by interactions between molecules that are predominantly dipolar in nature. The triclinic crystal structure of this system gives the dipolar field a significant component transverse to the easy spin axis and the parallel component provides a dipolar bias closely tuned to the first level crossing of the system. These factors enhance the quantum tunneling between levels, thus enabling the system to avoid spin freezing at low temperatures and efficiently reach the dipolar ordered state.

Magnetic and thermodynamic properties of nanodisks by Monte Carlo simulations

The idea that the properties of nano materials depend on their crystalline structure and the form and geometric sizes of them is discussed in this paper. The progress in the growth area of nano magnetic devices attracts the interest from both experimental and theoretical study of the nano structures for potential applications in industry. In various fields of electronics there is need to have nano materials with a wide range of magnetic properties. In this case theoretical research could provide a way to get the nano compounds with the required characteristics. In this paper we present the results of Monte Carlo simulations of magnetic nanodisks, which are based on simple cubic and body-centered cubic unit cell. The magnetization of spin, magnetic susceptibility and specific heat are investigated for nanodisks with the different diameters, thicknesses and for different values of the ratio of the correlation constants. The combination of dipolar and Heisenberg model interaction are considered for ferromagnetic case and are show that the magnetic and thermodynamic properties of the nanostructures are strongly dependent on their geometry. The structures with body-centered cubic unit cell demonstrate stronger dependences on the thickness of the disks and also higher critical temperature and wider hysteresis loop.

Functionalization of high-moment magnetic nanodisks for cell manipulation and separation

Synthetic antiferromagnetic (SAF) nanoparticles are layer-structured particles with high single-particle magnetic moments. In order to covalently bind these nanoparticles to cells, they were coated with a silica shell followed by conjugation with streptavidin. The silica coating generates both SAF@SiO2 core-shell nanoparticles and silica core-free nanoparticles. Using a simple magnetic separation, silica nanoparticles were removed and SAF@SiO2 nanoparticles were purified. After streptavidin conjugation, these particles were used to stain lung cancer cells, making them highly magnetically responsive. The stained cells can rotate in response to an external magnetic field and can be captured when a blood sample containing these cells flows through the sifter. Open image in new window

Skyrmion ground state and gyration of skyrmions in magnetic nanodisks without the Dzyaloshinsky-Moriya interaction

We show by micromagnetic simulations that a spontaneous skyrmion ground state can exist in Co/Ru/Co nanodisks without the Dzyaloshinsky-Moriya interaction, which can remain stable in the applied magnetic field along the $+$$z$ direction even up to 0.44 T. The guiding center (${R}_{x}$,${R}_{y}$) of skyrmion defined by the moments of the topological density presents a gyration with a star-like trajectory in a pulsed magnetic field and a hexagonal trajectory after the field is switched off, which is different from that of a vortex or bubble. One of the coupled skyrmions could move without an external magnetic field, just induced by the motion of the other one due to strong interlayer magnetostatic interactions. This work sheds light on how skyrmions can be discovered in various (not limited to magnetic) systems with competing energies and contributes to the understanding of the dynamical properties of skyrmions.

Some Magnetic Properties of the Different Nanodisks Obtained by Monte Carlo Method

The prediction of the magnetic properties of new nano compounds is potentially important for applications in electronics, which require materials with a wide range of magnetic properties. We present the results of the Monte Carlo simulations of magnetic nanodisks, which are based on the simple cubic, body-centered cubic and face-centered cubic structures. The spin configurations, thermal equilibrium magnetization, magnetic susceptibility and the specific heat are investigated for the nanodisks of different geometrical sizes. The combination of dipolar and Heisenberg model interactions are considered for the ferromagnetic case. The general conclusion of the work is that the magnetic and thermodynamic properties of the nanostructures are strongly dependent on their geometry. The dependencies of magnetic and thermodynamic properties on temperature and on the external magnetic field are quite similar for the structures with simple cubic and face-centered cubic unit cells. On the other hand, the structures with body-centered cubic unit cells demonstrate stronger dependences on the thickness of the disks and also higher critical temperature and wider hysteresis loop.

Study of Dipolar Neighbor Interaction on Magnetization States of Nano-Magnetic Disks

We investigate the effect of magnetic neighbor interaction on the state behavior of nano-magnetic disks for data storage and computation applications. We have observed and verified that a nano-magnetic disk, with certain dimension, can exist either in the single domain state or in the vortex state depending on the edge-to-edge spacing between the nano-magnetic disks. The experiments were conducted by varying the diameters and thicknesses with respect to edge-to-edge spacing. The dimensions were based on the phase diagram between the single domain state and the vortex state. We have observed nano-magnetic disks spaced far apart from its neighbor, settled in the vortex state and coupled nano-magnetic disks with less spacing settled in the single domain state. This phenomenon was observed for nano-magnetic disks with thickness between 8 nm to 20 nm and diameters between 80 nm to 140 nm. We validated the simulation by fabricating pairs of nano-magnetic disks with a thickness of 10 nm and diameter of 110 nm along edge-to-edge spacing from 20 nm to 260 nm in steps of 20 nm.

Magnetic State Estimator to Characterize the Magnetic States of Nano-Magnetic Disks

We present an image processing system-magnetic state estimator (MSE) to estimate the magnetization states of lithographically patterned single domain nano-magnets. The tool requires digital images of a magnetic force micrograph, an atomic force micrograph and a computer-aided design layout of the magnetic nanostructures to estimate their magnetization states. The tool also yields a confidence value of the estimated magnetization state. We have tested the MSE on randomly patterned Permalloy nano-magnetic disks on a Silicon wafer. The tool was able to correctly identify 403 single domain states out of 499 and 256 vortex states out of 297, producing an accuracy rate of 83%.

Molecular dynamics simulation of Lorentz force microscopy in magnetic nano-disks

In this paper, we present a molecular dynamics simulation to model the Lorentz force microscopy experiment. Experimentally, this technique consists in the scattering of electrons by magnetic structures in surfaces and gases. Here, we will explore the behavior of electrons colliding with nano-magnetic disks. The computational molecular dynamics experiment allows us to follow the trajectory of individual electrons all along the experiment. In order to compare our results with the experimental one reported in literature, we model the experimental electron detectors in a simplified way: a photo-sensitive screen is simulated in such way that it counts the number of electrons that collide at a certain position. The information is organized to give in grey scale the image information about the magnetic properties of the structure in the target. Computationally, the sensor is modeled as a square matrix in which we count how many electrons collide at each specific point after being scattered by the magnetic structure. We have used several configurations of the magnetic nano-disks to understand the behavior of the scattered electrons, changing the orientation direction of the magnetic moments in the nano-disk in several ways. Our results match very well with the experiments, showing that this simulation can become a powerful technique to help to interpret experimental results.

Dynamic switching of the spin circulation in tapered magnetic nanodisks

Magnetic vortices are characterized by the sense of in-plane magnetization circulation and by the polarity of the vortex core. With each having two possible states, there are four possible stable magnetization configurations that can be utilized for a multibit memory cell. Dynamic control of vortex core polarity has been demonstrated using both alternating and pulsed magnetic fields and currents. Here, we show controlled dynamic switching of spin circulation in vortices using nanosecond field pulses by imaging the process with full-field soft X-ray transmission microscopy. The dynamic reversal process is controlled by far-from-equilibrium gyrotropic precession of the vortex core, and the reversal is achieved at significantly reduced field amplitudes when compared with static switching. We further show that both the field pulse amplitude and duration required for efficient circulation reversal can be controlled by appropriate selection of the disk geometry.

Resonant amplification of vortex-core oscillations by coherent magnetic-field pulses

Vortex structures in soft magnetic nanodisks are highly attractive due to their scientific beauty and potential technological applications. Here, we experimentally demonstrated the resonant amplification of vortex oscillations by application of simple coherent field pulses tuned to optimal width and time intervals. In order to investigate vortex excitations on the sub-ns time scale, we employed state-of-the-art time-resolved full-field soft X-ray microscopy of 70 ps temporal and 25 nm lateral resolution. We found that, due to the resonant enhancement of the vortex gyration motion, the signal input power can be significantly reduced to ~ 1 Oe in field strength, while increasing signal gains, by increasing the number of the optimal field pulses. We identified the origin of this behavior as the forced resonant amplification of vortex gyration. This work represents an important milestone towards the potential implementation of vortex oscillations in future magnetic vortex devices.

Chirality selection in the vortex state of magnetic nanodisks with a screw dislocation

Structural defects in magnetic crystalline materials may locally change magnetic properties and can significantly influence the behavior of magnetic nanostructures. E.g., surface-induced Dzyaloshinskii-Moriya in- teractions can strongly a ect vortex structures in magnetic nanodisks causing a chirality selection. Near lattice defects, the spin-orbit interactions induce local antisymmetric Dzyaloshinskii-Moriya exchange and cause eec- tive anisotropies, which can result in spin canting. Broken inversion symmetry near a defect leads to locally chiral exchange. We present a phenomenological approach for dislocation-induced Dzyaloshinskii-Moriya couplings. As an example we investigate eects of a screw dislocation at the center of a magnetic nanodisk with a vortex state. By numerical calculations on vortex profiles we analyze equilibrium parameters of the vortex as functions of applied magnetic field and the material and geometrical parameters. It is proposed that magnetic nanodisks with defects provide a suitable experimental setting to study induced chirality by spin-orbit eects.

Dynamics of the vortex core in magnetic nanodisks with a ring of magnetic impurities

In this work, we used numerical simulations to study the effect of a ring of magnetic impurities on the vortex core dynamics in nanodisks of Permalloy. The presence of the ring not only allowed us to modulate the gyrotropic frequency but also provided us a way to confine the vortex core. We observed that the gyrotropic frequency depends on the ring parameters. Moreover, we have noticed that the switching of the vortex core polarity can be obtained from the vortex core-impurity interaction under peculiar conditions, in particular, when the ring works for pinning the vortex core.

Measurement of the Dynamical Dipolar Coupling in a Pair of Magnetic Nanodisks Using a Ferromagnetic Resonance Force Microscope

We perform a spectroscopic study of the collective spin-wave dynamics occurring in a pair of magnetic nanodisks coupled by the magnetodipolar interaction. We take advantage of the stray field gradient produced by the magnetic tip of a ferromagnetic resonance force microscope to continuously tune and detune the relative resonance frequencies between two adjacent nano-objects. This reveals the anticrossing and hybridization of the spin-wave modes in the pair. At the exact tuning, the measured frequency splitting between the binding and antibinding modes corresponds to the strength of the dynamical dipolar coupling Ω. This accurate ferromagnetic resonance force microscope determination of Ω is measured versus the separation between the nanodisks. It agrees quantitatively with calculations of the expected dynamical magnetodipolar interaction in our sample.

MAGNETIC VORTEX BEHAVIOR IN NANO-STRUCTURES

The existence of a vortex in the ground state of magnetic nano-disks has open a wide range of possibilities for constructing new ultra-compact devices. In this work we study the dynamical behavior of a vortex in a magnetic nano-particle. First, we introduce magnetic impurities in the system. It is observed that depending on the strength of the interaction the impurities can behave both as a pinning (attractive) or scattering (repulsive). By using the known values of the parameters for Permalloy-79 we have calculated the interaction energy of the vortex core with a single defect. We estimated the interaction range as approximately 10 nm. Both results agree quite well with experimental measurements. As a second point we discuss how the vortex dynamics in nano-disks can be used for building a nano-spin valve.

Low Temperature FMR in the System of Non-Interacting Magnetic Nanodisks

Magnetodynamical properties of systems of magnetic nanodisks at the microwave range and low temperatures were studied experimentally. Several periodic square arrays of Permalloy nanodisks deposited onto a glass substrate have been studied. The interdisk center-to-center distance in each array was equal to double disk diameter . The magnetoresonance studies were performed at 70-80 GHz frequency band and at the temperature of 4.2 K. The static external magnetic field was applied perpendicularly to the disks array. As a result the magnetoresonance absorption spectra have been obtained, containing both fundamental mode and spin wave modes. A comparative analysis of the results is performed with the X-band results obtained at T = 300K.

The influence of magnetic impurities in the vortex core dynamics in magnetic nano-disks

In this work we have used spin dynamics simulations to study the gyrotropic frequency behavior in nano-disks of Permalloy with magnetic impurities. We consider the effect of attractive impurity and repulsive impurity placed near the vortex core gyrotropic trajectory. We observed that the gyrotropic frequency is affected by the presence of impurity. The gyrotropic frequency shift depends on the relative position between the impurity and the vortex core gyrotropic trajectory and if impurity is attractive or repulsive. Our results agree with the analytical model and with experimental behavior for the gyrotropic frequency shown in the literature.