Magnetic Dot Arrays Research Articles

Lithographically Ordered FePt L10 Dots with High Coercivity for Logic-Conditioned Magnetic Nanostructures

In order to develop the building blocks for future biosensing and spintronic applications, an engraving technique using electron beam lithography is employed in order to develop nanomagnetic pre-patterned structures with logic potential. The paper describes the realization and morphological and magnetic characterization of potentially logic-conditioned substrates, a building block to be further used as an integration platform upon which nanodevices, such as magnetic wires, or various geometrical shapes, circles, triangles, can be considered as pre-requisite for full integration into logic devices. As a proof of concept, regular arrays of FePt circles or magnetic dots were devised and structural characterization by X-ray diffraction and transmission electron microscopy proved the occurrence of the tetragonal L10 phase. Moreover, the magnetic characterization provided more insight into the potential of such arrays of magnetic devices as the hysteresis provided good values of magnetic coercivity, remanent and saturation magnetization. These findings show good potential for developing regular arrays of uniformly shaped magnetic entities with encouraging magnetic performances in view of potential applications in various applications.

Highly Coercive L10-Phase Dots Obtained through Low Temperature Annealing for Nano-Logic Magnetic Structures

Nano-logic magnetic structures are of great interest for spintronic applications. While the methods used for developing arrays of magnetic L10-phase dots are, in most cases, based on deposition followed by annealing at high temperatures, usually around 700 °C, we demonstrate here a technique where a much lower annealing temperature (i.e., 400 °C) is needed in order to promote fully the disorder–order phase transformation and achievement of highly coercive L10-phase dots. In order to develop building blocks based on arrays of L10-phase FePt dots for further spintronic applications, an engraving technique using electron beam lithography is employed. This paper describes the fabrication, as well as the morphological and magnetic characterization, of regularly placed FePt dots of various shapes, as pre-requisites for integration into nano-logic devices. As a proof of concept, regular arrays of FePt circular dots were devised and their structural characterization, using X-ray diffraction (XRD) and transmission electron microscopy (TEM), was performed. It has been shown that annealing at only 400 °C for 30 min proved the occurrence of the tetragonal L10 phase. Moreover, structural characterization showed that the disorder–order phase transformation was complete with only the L10 phase detected in high resolution TEM. The magnetic characterization provided more insight into the potential of such arrays of magnetic devices with convenient values of magnetic coercivity, remanent and saturation magnetization. These findings show good potential for developing regular arrays of uniformly shaped magnetic entities with encouraging magnetic performances in view of various applications.

Влияние массива субмикронных магнитных точек на намагниченность, критический ток и структуру вихревых конфигураций в ВТСП

A vortex lattice in a high-temperature superconductor (HTS) with an array of submicron magnetic dots on the surface has been studied by the Monte Carlo method within the framework of a three-dimensional model of a layered high-temperature superconductor. The adjustment of the vortex lattice to an array of magnetic points was observed — ordered states arising in the process of remagnetization — configurations of one, two, three, or more vortex filaments fixed to a single magnetic point. The occurrence of these configurations was accompanied by peaks on the magnetization curve. The influence of the HTS anisotropy on the adjustment of the vortex lattice is analyzed. The ordered configurations of the vortex lattice are also associated with the non-monotonic nature of the dependences of the superconductor critical current on the magnetic field. The influence of temperature, magnetic moment of points, and film thickness on the critical current is investigated. With an increase in temperature and a decrease in the magnetization of magnetic points, the maximum of the critical current shifts towards a lower field. The structure of vortex filaments in the inhomogeneous field of a magnetic point is analyzed in detail. The mechanism of the influence of ordered vortex configurations on magnetization and critical current is discussed.

Detection of extremely large magnetoresistance in a ring-shaped array of magnetic quantum dots with very high performance and controllable parameters.

Using a Green's function method, we study the magnetoresistance (MR) effect in a ring-shaped array of magnetic quantum dots (QDs), with or without magnetic leads, while the magnetic QDs play the role of magnetic layers in conventional multilayer MR devices. Due to the multiple electronic interferences in this proposed ring-shaped structure, it exhibits extremely large MR values, typically up to four million percent (3.9 × 106% in an array that includes 14 QDs), as it tends to be infinite for a system with more dots. Our results show that when the magnetic moment of all QDs is parallel (antiparallel), the charge current through the outgoing lead is maximal (minimal). In addition, the Rashba spin-orbit interaction and the bias voltage are two significant factors that can control the MR values. Besides, by adjusting the magnitude of the magnetic moment of the QDs, the MR effect can be optimized. Finally, it is revealed that the onsite energy of the QDs is an effective parameter for modifying the value and sign of the MR percentage. Consequently, this ring-shaped structure, with controllable parameters and impressive MR percentages, can be an efficient preferred alternative for ordinary layered MR-based devices, for design of the next generation of nanoscale spintronic systems.

Exciton-Polariton Topological Insulator with an Array of Magnetic Dots

Recently there have been several proposals on exciton-polariton topological insulators, most requiring strong external magnetic fields induced by bulky superconducting coils. We propose an alternate design for a polariton topological insulator, where excitons are in a proximity of an additional layer of a ferromagnetic material with a predefined magnetic moment located between the pillars of the cavity. Our design supports a variety of topological phases and transitions between Chern numbers $\pm 2$ and $\pm 1$ by varying either the magnetic moment of the ferromagnetic material or the spin-orbit coupling between different spin projections, thus enabling compact polariton devices harnessing switchable topological edge modes.

Transient magnetization dynamics of spin-torque oscillator and magnetic dot coupled by magnetic dipolar interaction: Reading of magnetization direction using magnetic resonance

We study the magnetization dynamics of a spin-torque oscillator (STO) and a magnetic dot coupled by a magnetic dipolar field using micromagnetic simulation with the aim of developing a read method in magnetic recording that uses magnetic resonance. We propose an STO with a perpendicularly magnetized free layer and an in-plane-magnetized fixed layer as a suitable STO for this resonance read method. When the oscillation frequency of the STO is near the ferromagnetic resonance (FMR) frequency of the magnetic dot, the oscillation amplitude of the STO decreases because FMR excited in the magnetic dot causes additional dissipation. To estimate the read rate of the resonance read method, we study the transient magnetization dynamics to the coupled oscillation state from an initial state where the STO is in a free-running state and the magnetic dot is in a stationary stable state. The STO shows transient dynamics within a time scale of 1 ns, which means that the STO can perform resonance reading with a response time within this time scale. This response time is shorter when the separation length between the STO and the magnetic dot is shorter, which indicates that the response speed can become faster by increasing the strength of the interaction between the STO and the magnetic dot. Successive reads are demonstrated by moving the STO over an array of magnetic dots.

Nanostructured L10-CoPt dot arrays with perpendicular magnetic anisotropy

In this work, we have focused on the elaboration and characterization of nanostructured L10-CoPt magnetic dot arrays prepared by depositing Co50Pt50 nanoparticles preformed in the gas phase on Si substrates patterned by electron beam lithography (EBL). The MFM observations have revealed an out-of-plane single domain state. The VSM measurements have indicated a correlated super-spin glass state (CSSG) collective behavior. Consequently, the magnetic properties are mainly governed by a correlated super-spin glass state with strong inter-dot dipolar interactions.

Magnetic transition from dot to antidot regime in large area Co/Pd nanopatterned arrays with perpendicular magnetization

We have studied the transition between two different magnetization reversal mechanisms for thin Co/Pd multilayers with perpendicular magnetic anisotropy, appearing in magnetic dot and antidot arrays, which were prepared by nanosphere lithography. Various ordered arrays of nanostuctures, both magnetic dots and antidots, were created by varying size and distance between the nanospheres employing RF-plasma etching. We have shown that the coercivity values reach a maximum for the array of antidots with a separation length close to the domain wall width. In this case, each area between three adjacent holes corresponds to a single domain configuration, which can be switched individually. On the contrary, small hole sizes and large volume of material between them results in domain wall propagation throughout the system accompanied by strong domain wall pinning at the holes. We have also shown the impact of edge effects on the magnetic anisotropy energy.

Different approaches to generate matching effects using arrays in contact with superconducting films.

Superconducting films in contact with non-superconducting regular arrays can exhibit commensurability effects between the vortex lattice and the unit cell of the pinning array. These matching effects yield a slowdown of the vortex flow and the corresponding dissipation decrease. The superconducting samples are Nb films grown on Si substrates. We have studied these matching effects with the array on top, embedded or threading the Nb superconducting films and using different materials (Si, Cu, Ni, Py dots and dots fabricated with Co/Pd multilayers). These hybrids allow for studying the contribution of different pinning potentials to the matching effects. The main findings are: (i) Periodic roughness induced in the superconducting film is enough to generate resistivity minima; (ii) A minor effect is achieved by magnetic pinning from periodic magnetic field potentials obtained by dots with out of plane magnetization grown on top of the superconducting film, (iii) In the case of array of magnetic dots embedded in the films, vortex flow probes the magnetic state; i.e. magnetoresistance measurements detect the magnetic state of very small nanomagnets. In addition, we have studied the role played by the local order in the commensurability effects. This was attained using an array that mimics a smectic crystal. We have found that preserving the local order is crucial. If the local order is not retained the magnetoresistance minima vanish.

All-electrical magnetic vortex array sensing

Vortex sensing magnetometers based on arrays of soft magnetic dots are good candidates for high-resolution and accurate spatial magnetic-field estimation. When the arrays are laid out along different spatial directions they can perform tensor gradiometry allowing the measurement of field components and their spatial derivatives as a function of orientation. Detection is based on using spin-polarized currents to counteract vortex displacements or to excite vortex oscillation modes triggered by magnetic-field application. Sensor linearization, field detection range and conditions to obtain large sensitivity electronic compatibility and scalability are discussed.

Magnon Dirac materials

We demonstrate how a Dirac-like magnon spectrum is generated for localized magnetic moments forming a two-dimensional honeycomb lattice. The Dirac crossing point is proven to be robust against magnon-magnon interactions, as these only shift the spectrum. Local defects induce impurity resonances near the Dirac point, as well as magnon Friedel oscillations. The energy of the Dirac point is controlled by the exchange coupling, and thus a two-dimensional array of magnetic dots is an experimentally feasible realization of Dirac magnons with tunable dispersion.

Bias-free spin-wave phase shifter for magnonic logic

A design of a magnonic phase shifter operating without an external bias magnetic field is proposed. The phase shifter uses a localized collective spin wave mode propagating along a domain wall “waveguide” in a dipolarly-coupled magnetic dot array with a chessboard antiferromagnetic (CAFM) ground state. It is demonstrated numerically that the remagnetization of a single magnetic dot adjacent to the domain wall waveguide introduces a controllable phase shift in the propagating spin wave mode without significant change to the mode amplitude. It is also demonstrated that a logic XOR gate can be realized in the same system.

Magnetization reversal in magnetic dot arrays: Nearest-neighbor interactions and global configurational anisotropy

Various proposals for future magnetic memories, data processing devices, and sensors rely on a precise control of the magnetization ground state and magnetization reversal process in periodically patterned media. In finite dot arrays, such control is hampered by the magnetostatic interactions between the nanomagnets, leading to the non-uniform magnetization state distributions throughout the sample while reversing. In this paper, we evidence how during reversal typical geometric arrangements of dots in an identical magnetization state appear that originate in the dominance of either Global Configurational Anisotropy or Nearest-Neighbor Magnetostatic interactions, which depends on the fields at which the magnetization reversal sets in. Based on our findings, we propose design rules to obtain the uniform magnetization state distributions throughout the array, and also suggest future research directions to achieve non-uniform state distributions of interest, e.g., when aiming at guiding spin wave edge-modes through dot arrays. Our insights are based on the Magneto-Optical Kerr Effect and Magnetic Force Microscopy measurements as well as the extensive micromagnetic simulations.

Effect of ordered array of magnetic dots on the dynamics of Josephson vortices in stacked SNS Josephson junctions under DC and AC current

We use the anisotropic time-dependent Ginzburg-Landau theory to investigate the effect of a square array of out-of-plane magnetic dots on the dynamics of Josephson vortices (fluxons) in artificial stacks of superconducting-normal-superconducting (SNS) Josephson junctions in the presence of external DC and AC currents. Periodic pinning due to the magnetic dots distorts the triangular lattice of fluxons and results in the appearance of commensurability features in the current-voltage characteristics of the system. For the larger values of the magnetization, additional peaks appear in the voltage-time characteristics of the system due to the creation and annihilation of vortex-antivortex pairs. Peculiar changes in the response of the system to the applied current is found resulting in a “superradiant” vortex-flow state at large current values, where a rectangular lattice of moving vortices is formed. Synchronizing the motion of fluxons by adding a small ac component to the biasing dc current is realized. However, we found that synchronization becomes difficult for large magnetization of the dots due to the formation of vortex-antivortex pairs.

Discrete breathers in an one-dimensional array of magnetic dots

The dynamics of the one-dimensional array of magnetic particles (dots) with the easy-plane anisotropy is investigated. The particles interact with each other via the magnetic dipole interaction and the whole system is governed by the set of Landau–Lifshitz equations. The spatially localized and time-periodic solutions known as discrete breathers (or intrinsic localized modes) are identified. These solutions have no analogue in the continuum limit and consist of the core where the magnetization vectors precess around the hard axis and the tails where the magnetization vectors oscillate around the equilibrium position.

Magnetization reversal using excitation of collective modes in nanodot matrices.

The large arrays of magnetic dots are the building blocks of magnonic crystals and the emerging bit patterned media for future recording technology. In order to fully utilize the functionalities of high density magnetic nanodots, a method for the selective reversal of a single nanodot in a matrix of dots is desired. We have proposed a method for magnetization reversal of a single nanodot with microwave excitation in a matrix of magneto-statically interacting dots. The method is based on the excitation of collective modes and the spatial anomaly in the microwave power absorption. We perform numerical simulations to demonstrate the possibility of switching a single dot from any initial state of a 3 by 3 matrix of dots, and develop a theoretical model for the phenomena. We discuss the applicability of the proposed method for introducing defect modes in magnonic crystals as well as for future magnetic recording.

Influence of shape anisotropy on reversal process in coupled ellipsoidal magnetic dot array

The magnetization reversal of two configurations of linear magnetic ellipsoidal arrays has been studied by using the micromagnetic simulations. The effect of magnetostatic interaction on the switching field is investigated by altering aspect ratio of ellipsoids while boundary edge spacing is kept fixed. The simulation results showed that, when the field is parallel to the ellipsoid easy axis, the shifted switching field is more than the case when the field is parallel to the ellipsoid hard axis. However, in both configurations the shifted switching field accompany with the more relaxed magnetic pattern.

Anomalies in vortex lattice dynamics driven by induced ac currents in superconducting films with magnetic arrays of two-fold symmetry

We study the dynamics of the vortex lattice driven by ac induced currents in the critical state regime, for T > 0.70 TC. The samples are superconducting films grown on top of two-fold symmetry array of magnetic dots. In these heterostructures, the induced ac currents flow parallel to the short and to the long side of the pinning array in different areas of the samples simultaneously. This behavior produces remarkable effects in the vortex lattice dynamics. First of all, periodic features are observed in the ac susceptibility versus applied magnetic field measurements which are related to matching effects between the vortex lattices and the magnetic array. However, the vortex lattice reconfiguration observed in magnetotransport experiments is absent. Some of these features are revealed as maxima instead of being minima, indicating higher mobility at certain matching fields. Competing unstable vortex configurations could lead to increase vortex mobility precluding the reconfiguration transition. At high temperatures, where the matching effects show up, the magnetic permeability of the dots is the mechanism that governs the JC(T) behavior. Moreover, the temperature dependence of the pinning force FP(T) shows a temperature crossover related to an unexpected enhancement in vortex mobility. Vortex–vortex interaction and the interplay between trapped and interstitial vortices are a hint to explain these phenomena.

How finite sample dimensions affect the reversal process of magnetic dot arrays

We investigate the magnetization reversal of a magnetic dot array by means of magneto-optical Kerr effect and magnetic force microscopy measurements as well as micromagnetic simulations. We find that the finite dimensions of the dot array introduce a global configurational anisotropy that promotes state transitions first in dots near the sample boundaries. From there, the reversal process expands towards the sample body by means of collective magnetization processes originating in the magnetostatic coupling between the dots. These processes are characterized by transition avalanches and the formation of magnetization chains. These findings are important in the development of applications that rely on a robust control of dot magnetization states in dot arrays.

Boundary waves in ferromagnetically ordered two-dimensional arrays of magnetic dots

The existence of surface modes localized at the boundary of magnetic-dot arrays with magnetization perpendicular to the plane of array is predicted. Numerical and analytic calculations show that the excitation energy of these boundary modes is above the continuum of modes propagating in the array.