Magnetic Dot Arrays Research Articles

The Evolutionary Approach to the Optimization of Finite-Size Effects in Magnetic Dot Arrays

We considered simple 2D model of the magnetic dot array with uniaxial anisotropic dots. For this model we formulated the inverse magnetic problem, where desired static ground-state properties are attained by varying directions of the local anisotropy vectors. The problem is investigated numerically using a method based on evolutionary optimization.

Regular Array of Magnetic Nano-Dots Prepared by Nanochannel Glass Replica Masks

AbstractIn magnetic recording technology, barriers based on fundamental physical limits on the data density are being approached for the current longitudinal recording modes. However, demands for higher data storage density have escalated in recent years. Discrete perpendicular recording is a viable method to achieve 100 Gb per square inch and beyond. We report on the development of a novel technique to fabricate uniform arrays of nano-sized magnetic dots. Uniform arrays of nanometer-sized magnetic dots are obtained by magnetron sputtering deposition through a nanochannel glass replica mask. The platinum replica masks are fabricated using thin film deposition on etched nanochannel glass and contain uniform hexagonally patterned voids with diameters as small as 50 nanometers. The magnetic dot density can be as high as 1011 per square inch. Our method provides a simple yet effective way to create regularly arranged discrete magnetic media that can be used for perpendicular magnetic recording. The magnetic properties of the dots are studied with a vibrating sample magnetometer.

Simulation of Field Coupled Computing Architectures Based on Magnetic Dot Arrays

In this paper, we demonstrate that field-coupled nanomagnets can be used for digital information processing. The operation of logic devices is based on a QCA-like architecture, where information propagates by magnetostatic interaction between individual magnetic dots. Micromagnetic simulations indicate that simple logic gates function properly. Efficient design tools, based on the single-domain approximation are developed.

Vortex dynamics in superconductors with a lattice of magnetic dots

We report on molecular dynamics simulations of vortex patterns in the presence of a periodic array of ferromagnetic dots with out-of-plane magnetization. Dots and vortices show the interesting behaviour of having attractive or repulsive interaction depending on the mutual orientation of the flux of the vortices and the magnetic moment of the dots. This microscopic property strongly influences the simulated magnetization loops and critical force curves. Different configurations of the magnetization direction of the dot array have been systematically studied. For all dots magnetized in one direction the experimentally observed asymmetric magnetization loops are clearly reproduced. We also investigate random distributions of up and down magnetized dots. These simulations show that new vortex phases can be stabilized by the magnetic dot array.

Power-law relaxation decay in two-dimensional arrays of magnetic dots interacting by long-range dipole-dipole interactions

The magnetic relaxation of two-dimensional arrays of dipolar coupled magnetic dots has been measured and simulated. Arrays $(50\ifmmode\times\else\texttimes\fi{}50)$ with perpendicular magnetized Co dots $(2 \ensuremath{\mu}\mathrm{m}\ifmmode\times\else\texttimes\fi{}2 \ensuremath{\mu}\mathrm{m})$ were patterned using a high resolution ${\mathrm{Ga}}^{+}$ focused ion beam irradiation. Magnetic domain pattern and time relaxation of the dot arrays were investigated using Faraday magneto-optical microscopy. For arrays designed with high irradiation doses $(>~0.5 \mathrm{n}\mathrm{C}/\mathrm{c}\mathrm{m}),$ the magnetic relaxation of the array proceeds by the magnetization reversal of individual dots and follows a power-law time decay. The long-range character of the dipolar interaction is found to be responsible for magnetic relaxation with a power-law decay. Monte Carlo simulations, based on a modified Ising Hamiltonian, reproduce this time dependence, and show that the power law is not a consequence of either the finite size or the boundary of the arrays, and it is independent of the shape of dots as well.

Magnetoresistive measurement of switching behavior in nano-structured magnetic dot arrays

In the present study, geometrical and thermal effects in a mesoscopic magnetization reversal process have been studied on a novel nano-structure of magnetic relief dot with magnetoresistive measurements. Only the top layer of a substrate/CoPt(10 nm)/Cu(10 nm)/NiFe(6, 12 nm) film was structured into rectangular dots with various lengths ( L) and widths ( W) down to 0.2 μm. Coercive fields of NiFe relief dots ( W=0.2 μm) systematically decrease with the decrease of L/ W, as predicted from demagnetizing factors in single domain particle. About 50% reduction of H c due to a temperature rise, from 5 to 300 K, demonstrates considerable thermal activation in the magnetization reversal of nano-structured magnetic particles.

Formation of a Cobalt Magnetic Dot Array via Block Copolymer Lithography

Single-domain cobalt dot arrayswith high magnetic particle density, patterned over large areas (e.g., 10 cm diameter wafers) are fabricated by self-assembled block copolymer lithography, using a polystyrene-poly(ferrocenyldimethylsilane) copolymer as a template. By varying the copolymer type and etching conditions the magnetic properties can be tuned. The Figure shows a typical array of Co dots with tungsten caps obtained via this procedure.

Fabrication and properties of CoPt patterned media with perpendicular magnetic anisotropy

Magnetic dot-arrays with dot diameters of 80 to 1000 nm have been fabricated by patterning CoPt alloy continuous films. All of the fabricated dots had their magnetic easy axes perpendicular to the plane. From MFM observations, dots with diameters of 160 nm and larger had multi domain structures. For the dot-array with 80 nm dot diameter, every dot was observed as a single domain. CoPt dot-array with 80 nm dot diameter showed excellent thermal stability in comparison with conventional longitudinal media and perpendicular media.

Spin wave quantization in laterally confined magnetic structures (invited)

An overview of the current status of the study of spin wave excitations in arrays of magnetic dots and wires is given. We describe both the status of theory and recent inelastic light scattering experiments addressing the most important issues; the quantization of localized spin waves due to the in-plane confinement of spin waves in elements, dipolar coupling between the quantized modes, and the localization of the modes within rectangular elements due to an inhomogeneous demagnetizing field.

Neural network approach to magnetic dot arrays modeling

Two-dimensional square lattice model of the magnetic dot array is presented. The intradot self-energy is predicted using the neural network approachand interdot magnetostatic coupling is approximated by the several dipolar terms. The model is applied to disk-shaped cluster involving 193 ultrathin square dots with 772 interaction centers. Among the intradot magnetic structures retrieved by neural networks the important role play single-vortex intradot magnetostatic modes. Several aspects of the model have been understood by the numerical study using the simulated annealing method.

Variable-Range Hopping in the Array of Magnetic Quantum Dots

ABSTRACTWe analyzed the spin-dependent conductivity in the system of paramagnetic quantum dots embedded in semi-insulating matrix, which is due to bound magnetic polaron (BMP) inter-dot hopping. If such a system is characterized by wide distributions of the “bare” electron energies and BMP shifts, variable-range and variable-polaron-barrier hopping can be observed at low temperaturesT. It results in the giant magnetoresistance,ρ(H, T ) governed by a non-activation law, lnρ /α [T0(H)/T ]p, whereT0(H) drops with magnetic field,H. Depending on the conditions, parameters of the material, and the dimensionality of the system, the exponent 0.25 < p < 0.75. This type ofT -dependence has been observed in GaMnAs and MnGe magnetic semiconductors.

Fluxon Pinning by Artificial Magnetic Arrays

The field dependent transport properties of superconducting niobium films, which are modulated by a regular array of non-magnetic and magnetic normal-metal inclusions (dots), have been investigated. Strong peaks in the critical current are seen for fields at which the vortex density in the superconductor is some rational or sub-rational multiple of the dot density. This commensurate peak effect is enhanced for the magnetic dot arrays when the dots are magnetized in a direction parallel to the applied magnetic field and suppressed when they are anti-aligned. Qualitative information on the strength of this dot-vortex interaction is inferred from the commensurate peaks present and missing for different regular lattices.

Reorientational magnetic transition in high-density arrays of single-domain dots

A theoretical study on the reorientational transition from in-plane to out-of-plane magnetized state is performed for two-dimensional magnetic dot arrays coupled by magnetostatic interaction. The square lattice of nanoscale cylindrical dots is considered with the assumptions that the dots are magnetically soft and they have uniform magnetization. The present study predicts that the interdot magnetostatic coupling determines the reorientation transition for close-packed arrays of such magnetic dots. Recent experimental results on the nanometer-scale single-domain dot arrays are discussed in light of the present calculation.

Fabrication of patterned media for high density magnetic storage

Arrays of discrete, lithographically-patterned magnetic elements have been proposed as a new generation of ultra-high density patterned magnetic storage media. Interferometric lithography has been used to make prototype arrays over large areas with periods of 100 – 200 nm. Arrays of magnetic pillars, pyramids, and dots have been made by electrodeposition, evaporation and liftoff, and etching processes, and the magnetic properties of the particles and their mutual interactions have been measured. This paper was invited from the 43 rd International Conference on Electron, Ion and Photon Beam Technology and Nanofabrication (EIPBN 99). The full text of this paper can be found in the Journal of Vacuum Science and Technology B, Nov/Dec. 1999.

Micromagnetics and interaction effects in the lattice of magnetic dots

This work reports micromagnetic calculation of the distribution of stray fields in a regular planar array of cylindrical magnetic dots uniformly magnetized in the plane of the array. Explicit analytical expressions for distribution of stray fields are given.

Spin-wave excitations in cylindrical magnetic dot arrays with in-plane magnetization

Discrete spin-wave spectrum of tangentially magnetized cylindrical magnetic dots is calculated taking into account both intradot dipolar and exchange interactions. The case when the dot radius is much larger than the dot height is considered. The quantization effect of the spin-wave frequencies appears due to the finite dot radius. The spectrum of the discrete spin-wave modes is calculated in a practically important case of square array of permalloy (FeNi) cylindrical dots.

Flux pinning by regular arrays of ferromagnetic dots

The pinning of flux lines by two different types of regular arrays of submicron magnetic dots is studied in superconducting Pb films; rectangular Co dots with in-plane magnetization are used as pinning centers to investigate the influence of the magnetic stray field of the dots on the pinning phenomena, whereas multilayered Co/Pt dots with out-of-plane magnetization are used to study the magnetic interaction between the flux lines and the magnetic moment of the dots. For both types of pinning arrays, matching anomalies are observed in the magnetization curves versus perpendicular applied field at integer and rational multiples of the first matching field, which correspond to stable flux configurations in the artificially created pinning potential. By varying the magnetic domain structure of the Co dots with in-plane magnetization, a clear influence of the stray field of the dots on the pinning efficiency is found. For the Co/Pt dots with out-of-plane magnetization, a pronounced field asymmetry is observed in the magnetization curves when the dots are magnetized in a perpendicular field prior to the measurement. This asymmetry can be attributed to the interaction of the out-of-plane magnetic moment of the Co/Pt dots with the local field of the flux lines and indicates that flux pinning is stronger when the magnetic moment of the dot and the field of the flux line have the same polarity.

Flux pinning in a superconducting film by a regular array of magnetic dots

We have studied flux pinning in thin superconducting Pb films covering a triangular array of submicron magnetic Au/Co/Au dots with in-plane magnetization. The rectangular dots can have two possible in-plane magnetic states: a single-domain state where the magnetization lies parallel to the long axis of the dot and a multi-domain state. In this paper, we report on the measurements of the field dependence of the critical current I c( B) of a superconducting Pb film evaporated on top of this magnetic array. We observe a clear influence of the magnetic state of the dots on the pinning properties of the magnetic array, showing that there is an important pinning contribution due to the stray fields of the magnetic dots.

Structural characterization of periodic arrays of magnetic dots

We have investigated the structure and regularity of macroscopically large arrays of submicron magnetic Au/Co/Au dots, which were prepared by combining electron beam lithography, molecular beam epitaxy, and lift-off techniques. Atomic force microscopy was used to characterize the surface roughness of the dots and to show the reproducibility of dot size and periodicity over large areas; low-angle x-ray diffuse scattering experiments were employed to construct a mapping of the reciprocal space in the vicinity of the (000) reflection of the dots. The reciprocal space mapping thus provides information about the internal structure of the dots, as well as about the lateral periodicity of the dot array. This work underlines the important complementarity of scanning probe and x-ray diffraction techniques for the detailed characterization of patterned structures. We observed that the x-ray diffraction patterns are extremely sensitive to the orientation of the dot lattice with respect to the incoming x-ray beam, allowing a quantitative analysis of the lateral periodicity of the dot array and the size of the individual dots.

Spin-waves in cylindrical magnetic dot arrays with in-plane magnetization

The discrete spectrum of dipole-exchange spin-wave modes of a tangentially magnetized cylindrical magnetic dot is calculated from the solution of the Landau–Lifshitz equation and the magnetostatic Maxwell equations in a cylindrical geometry. The general surface spin-pinning conditions at the radial dot boundary are considered. The main simplifying assumptions are: (i) the dot radius is much larger than the dot height; (ii) the distribution of the variable magnetization along the dot height is uniform. The approximate dispersion equation for spin-wave modes in a dot is obtained in a simple analytical form similar to the form of the dispersion equation in an infinite film. The quantization effect of the spin-wave frequencies appears due to the finite dot radius and is essential for submicron magnetic dots. The discrete spin-wave frequencies are calculated in a practically important case of the square array of permalloy cylindrical dots. The relative intensities of spin-wave modes, when observed by Brillouin light scattering, are considered. The role of interdot dipole–dipole coupling is discussed.