Magnetic Nanodot Arrays Research Articles

Coherent magnetic semiconductor nanodot arrays

In searching appropriate candidates of magnetic semiconductors compatible with mainstream Si technology for future spintronic devices, extensive attention has been focused on Mn-doped Ge magnetic semiconductors. Up to now, lack of reliable methods to obtain high-quality MnGe nanostructures with a desired shape and a good controllability has been a barrier to make these materials practically applicable for spintronic devices. Here, we report, for the first time, an innovative growth approach to produce self-assembled and coherent magnetic MnGe nanodot arrays with an excellent reproducibility. Magnetotransport experiments reveal that the nanodot arrays possess giant magneto-resistance associated with geometrical effects. The discovery of the MnGe nanodot arrays paves the way towards next-generation high-density magnetic memories and spintronic devices with low-power dissipation.

Controllable manipulation of superconductivity using magnetic vortices

The magneto-transport of a superconducting/ferromagnetic hybrid structure,consisting of a superconducting thin film in contact with an array of magneticnanodots in the so-called ‘magnetic vortex state’, exhibits interesting properties. Forcertain magnetic states, the stray magnetic field from the vortex array is intenseenough to drive the superconducting film into the normal state. In this fashion, thenormal-to-superconducting phase transition can be controlled by the magnetic history.The strong coupling between superconducting and magnetic subsystems allowscharacteristically ferromagnetic properties, such as hysteresis and remanence, to bedramatically transferred into the transport properties of the superconductor.

Magnetic Properties of Fe55Pd45 Films Deposited on Si (100) Nano-meter Wide Pillar

Magnetic nano-dot arrays with (tilted) perpendicular anisotropy are useful for the high-density magnetic recording. In this study, we deposited two kinds of ferromagnetic sample: Fe55Pd45/Si (100)-plane and Fe55Pd45/Si (100)-pillar. Each sample underwent a rapid thermal annealing (RTA) treatment; with a heating rate of 40oC/sec up to 500oC, being annealed there for 30 minutes, and then quenched to room temperature. After fabrication, X-ray diffraction (XRD) indicated that after RTA the FePd alloy transformed from the fcc to the fct phase with lattice constants: a = 0.380 nm and c = 0.378 nm. The Si(100) pillars are 500 nm in length, 65nm in diameter, and with a density of about 1012 cm-2. On top of each Si(100) pillar there sat a prolate ellipsoid shape Fe55Pd45 particle: length L ≅ 108 nm and short-axis diameter d ≅ 67 to 83 nm. Magnetic domain (MD) pattern of the FePd nano-dot array was studied by magnetic force microscopy (MFM). The overall magnetic domain size (D) is from 285 to 452 nm. The squareness ratio (SQR) of the magnetic hysteresis loop reaches as: (SQR)z = 0.65 > (SQR)y = 0.45 >(SQR)x = 0.40 for the FePd/Si(100)-pillar film. From the inplane rotation angle (φ) and the out-of-plane tilting angle (θ) dependencies of the coercivity (HC), we find that the former exhibits the characteristics of the curling-mode-like switch, while the latter exhibits the Stoner-Wohlfarth-like switch.

Recent Patents on Self-Organised Magnetic Nanodot Arrays

As the continuous advance in the process of device miniaturisation reaches down to the nanometre range, fabrication techniques based on self-organisation, i.e., the spontaneous formation of ordered patterns on surfaces, are becoming increasingly attractive as potential, highly efficient alternatives to lithographic methods. In this article we review some of the methodologies that have been developed recently to produce ordered arrays of nanomagnets using self-organised surface templates, and we list the patents that have been filed recently to protect those fabrication procedures. We describe the underlying phenomena giving rise to the appearance of the ordered structures, and discuss their characteristics and the controllable parameters.

Fabrication of Magnetic Nanodot Array Using Nanoimprint Lithography: In Combination with Electrochemical Deposition or Ion Irradiation

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Fabrication of magnetic nanodot array using electrochemical deposition processes

We investigated fabrication processes of magnetic nanodot arrays for the ultra-high density magnetic recording media by using an electrodeposition. A CoZrNb underlayer was sputter-deposited on a glass disk substrate as a soft magnetic underlayer (SUL). Nano-patterns were formed on the substrate by UV-nanoimprint lithography (UV-NIL) and CoPt was electrodeposited into the nano-patterns. For obtaining uniform CoPt nanodot arrays with high perpendicular coercivities, we applied thin Cu intermediate layer on CoZrNb SUL and minimized its thickness. As a result, we obtained CoPt nanodot arrays with 150-nm diameter, 300-nm pitches, and 20-nm heights, which have uniform structures, on the substrates with the construction of Cu (1–2 nm)/CoZrNb (100 nm)/Cr (5 nm)/glass disk. The perpendicular coercivity of the CoPt nanodot arrays was as high as 5.4 kOe. From these results, we showed that the Cu intermediate layer with even 1–2 nm thick considerably improved the deposition condition on the substrates with CoZrNb SUL to successfully fabricate CoPt nanodot arrays with the diameter and pitches of 80 nm and 160 nm with sufficient uniformity.

Self-organized growth of high density magnetic Co nanodot arrays on a Moiré template

We report the self-organized growth of cobalt nanodot arrays using a Gd-Au Moiré superlattice as a template. After analyzing the influence of the Co flux and the substrate temperature, we obtain the suitable parameters to maximize nanodot density, homogeneity, and individual size. Depending on the growth conditions an areal density of up to 54 Teradots/inch2 can be achieved. Below the limit of lateral coalescence, independent nanodots made of ∼1000 Co atoms exhibit room temperature remanent magnetization.

Probing of Faraday Effect with Micron Laser Spot for Patterned Array of Magnetic Nanodots

A Faraday-effect system to measure small patterned areas with about a 6-μm sized laser spot was constructed. We found that highly accurate Faraday measurements were possible for a patterned array of nanodots and also obtained the dependence of the Faraday effect on the angle of the sample. Two lasers with wavelengths of 633 nm and 690 nm were close to the local surface plasmon resonance (LSPR) wavelengths of the noble material of Au. A nanodot array, consisting of magnetic materials made up of a Co/Pt multilayer, sandwiched by Au, that was not more than 100 nm in diameter, was fabricated, and the optical and magneto-optical properties were investigated.

Magnetization Reversal in Arrays of Magnetic Nanoperforations

Nanoperforated ZrO2 membranes, produced via organic/inorganic self-assembly using block copolymer micelles, serve as template for a Co/Pt multilayer stack with out-of-plane magnetic anisotropy. The deposition of the magnetic film results in an array of magnetic nanodots surrounded by a continuous magnetic film. These magnetic dots are found to be in a single-domain state and appear as magnetically exchange isolated from the surrounding film. The latter observation can be related to the specific morphology of the nanoperforations. In addition, strong domain-wall pinning of the continuous magnetic film at the locations of the nanoperforations was observed. Therefore, this approach is promising to create a percolated magnetic medium, which is considered as future recording media.

Preparation and properties of perpendicular CoPt magnetic nanodot arrays patterned by nanosphere lithography

Perpendicular anisotropy Co80Pt20 films were patterned into nanopillars of 40–90nm using nanosphere lithography. A monolayer of polystyrene spheres was coated onto the sputtered films and size tailored by reactive ion etching in oxygen. These reduced spheres were used as masks for ion milling of the magnetic films. Increased coercivity and squareness resulted in decreasing nanopillar size. A significant increase in intrinsic coercivity was demonstrated with the decrease in pillar size. The reversal mechanism appears similar to reported results of nucleation of a small reversed volume followed by rapid domain wall motion.

Micromagnetic numerical analysis of magnetization processes in patterned ferromagnetic films

Magnetization processes in planar arrays of magnetic nanodots are modelled, starting from the space-time discretization of the Landau-Lifshitz equation. The magnetostatic interactions between particles is investigated by varying interdot distance, material properties, and dot shape and size.

Nonlithographic fabrication of 25 nm magnetic nanodot arrays with perpendicular anisotropy over a large area

A simple method is demonstrated to fabricate 25 nm magnetic nanodot arrays with perpendicular anisotropy over 10 cm2 coverage area. The nanodot arrays are fabricated by depositing Co/Pt multilayers (MLs) onto the SiO2 dot arrays formed on a Si wafer. At first, arrays of the SiO2 dots are fabricated on a Si wafer by anodizing a thin Al film deposited on it. The SiO2 dots are formed at the base of the anodized alumina (AAO) pores due to the selective oxidation of the Si through the AAO pores during over anodization of the Al film. The average diameter, periodicity, and height of the SiO2 dots are about 24, 43, and 17 nm, respectively. Then {Co(0.4 nm)/Pt(0.08 nm)}8 MLs with a 3 nm Pt buffer layer is deposited onto the SiO2 dot arrays by sputtering. The average diameter and periodicity of the Co/Pt nanodot arrays are 25.4 and 43 nm, respectively, with narrow distribution. The nanodot arrays exhibit strong perpendicular anisotropy with a squareness ratio of unity and negative nucleation fields. The coercivity of the nanodot arrays is about one order higher than that of the continuous film, i.e., the same structure deposited on the SiO2 substrate. The magnetization reversal of the continuous film is governed by domain-wall motion, while the magnetization reversal of the nanodot arrays is dominated by the Stoner–Wohlfarth-like rotation. These results indicate that the fabricated structure can be considered as an isolated nanodot array.

Ferromagnetic Resonance Investigation of Macroscopic Arrays of Magnetic Nanoelements Fabricated Using Polysterene Nanosphere Lithographic Mask Technique

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> A dense plane periodical array of cylindrical magnetic nanodots has been fabricated using a lithographic mask formed by self-organization of polystyrene nanospheres. In this paper, we study collective static and dynamic magnetic behavior of this array. We find that this technique produces samples with reasonably small dispersion of magnetic parameters of individual dots. This is evidenced by magnetometry and well-resolved discrete frequencies of standing spin waves measured with cavity and coplanar-waveguide ferromagnetic resonance. The standing spin wave resonances could be reliably observed in a large range of frequencies (4–15 GHz). However the measured linewidth of resonances is about ten times larger than for unpatterned Permalloy. This may be due to some variation in shape and magnetic parameters from dot to dot resulting in inhomogeneous broadening of the resonance lines. </para>

Magnetic dipolar interaction in NiFe nanodot arrays formed on vertical carbonnanotubes

A new and simple method for the fabrication of densely packed magnetic nanodot arrayswas developed using conventional sputtering deposition at room temperature. An anodizedalumina template was employed for the formation of nanodot assemblies, consisting ofcarbon nanotubes (CNTs) and magnetic nanodot arrays. Each nanodot was formed exactlyon top of a CNT and was arranged with a well-ordered structure in a wide range of area. Itwas also found that the size of dots and the distance between dots can be tailored bychanging the length of CNTs, inducing a change of strength of dipolar interaction betweennanodots.

Fabrication of CoPt magnetic nanodot arrays by electrodeposition process

We attempted to fabricate patterned media using the electrochemical deposition process along with nanopatterned substrates prepared by the electron beam lithography (EBL), UV nanoimprint lithography (UV-NIL), and spin-on-glass nanoimprint lithography (SOG-NIL) approaches. CoPt was electrodeposited into the nanopatterned substrates and chemical mechanical polishing was carried out to planarize the surface. It was clarified that CoPt nanodot arrays were successfully deposited into the patterned nanopores fabricated by UV-NIL and SOG-NIL as well as by EBL with high area selectivity and uniformity. The density of the CoPt nanodot arrays deposited into the nanopores fabricated by EBL was equal up to an areal recording density of 250 Gbit/in 2.

Magnetic properties of GaMnAs nanodot arrays fabricated using porous alumina templates

Ordered arrays of GaMnAs magnetic semiconductor nanodots have been fabricated using anodic porous alumina templates as etch masks. The magnetic behavior is studied for prepared arrays with 40 nm dot diameter, 15 nm dot thickness, and 80 nm periodicity. The disklike nanodots exhibit an easy axis for fields applied in the radial direction and a hard axis in the smaller direction. In the radial direction superparamagnetism is observed with a blocking temperature of 30 K. The fabrication technique is convenient for preparing nanodot arrays of compound semiconductors that cannot be formed by self-assembly techniques.

Switchable collective pinning of flux quanta using magnetic vortex arrays: Experiments on square arrays of Co dots on thin superconducting films

We constructed a superconducting/ferromagnetic hybrid system in which the ordering of the pinning potential landscape for flux quanta can be manipulated. Flux pinning is induced by an array of magnetic nanodots in the magnetic vortex state, and controlled by the magnetic history. This allows switching on and off the collective pinning of the flux-lattice. In addition, we observed field-induced superconductivity that originates from the annihilation of flux quanta induced by the stray fields from the magnetic vortices.

Fabrication of Magnetic Nanodot Arrays for Patterned Magnetic Recording Media

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Patterning of high density magnetic nanodot arrays by nanoimprint lithography

In this work the authors present results from thermal nanoimprint lithography patterning of high density arrays of circular holes with 100nm diameters at 300nm pitch, or with 50nm diameters at 100nm pitch, throughout an area of 1cm2. They demonstrate the fabrication of dense gigabit magnetic nanodot arrays using polymethyl methacrylate and polymethyl glutarimide resist bilayers and a lift-off process. The magnetic properties of these nanodot arrays are shown to be readily characterized by alternating gradient magnetometry. Several issues are addressed which arise during patterning and characterization, such as template damage induced by Au metallization, metal residue control, and nanodot diameter tuning by oxygen plasma etching.

Multilevel magnetic nanodot arrays with out of plane anisotropy: the role of intra-dot magnetostatic coupling

Intra-dot inter-layer magnetostatic coupling have been investigated in trilayers which consist of two [Co/Pt] multilayers with out-of-plane anisotropy and different coercivities separated by a thick Pt layer. For single domain nanodots, the strong intra-dot inter-layer magnetostatic interactions manifest as a shift of the soft layer hysteresis loop. The amplitude of this shift decreases when the size of the dots is increased and/or when the temperature is increased. Satisfactory agreement is found between calculations and experimental results.