Influence of blur on the visual evoked magnetic response to a pattern reversal stimulus

Blurring a pattern reversal stimulus increases the latency and decreases the amplitude of the visual evoked potential (VEP) P100 peak. Recording the visual evoked magnetic response (VEMR) is some subjects may therefore be difficult because their spectacles create excessive magnetic noise. Hence, the effect of varying degrees of blur (-5 to +5 D) on the VEMR was investigated in three subjects with 6/6 vision to determine whether refraction with non-magnetic frames and lenses was necessary before magnetic recording. Small (32') and larger (70') checks were studied since there is evidence that blurring small checks has a more significant effect on the VEP compared with large checks. The VEMR was recorded using a single channel dc-SQUID, second order gradiometer in an unshielded laboratory. The latency (ms) and amplitude (fT) of the most prominant positive peak within the first 130 ms (P100M) were measured. Blurring the 32' checks significantly increased latency aand reduced the amplitude of the P100M peak. The resulting response curves were parabolic with minimum latency and maximum amplitude recorded at 0 D. Blurring the 70' check had no significant effect on latency or amplitude. Hence, the magnetic P100M responds similarly to the electrical P100 in response to blur. It would be essential when recording the VEMR that vision is corrected with non-magnetic spectacles especially when small checks are used.

Focused ion beam manufacturing of magnetic nanostructures

A number of patterning methods including conventional photo-lithography and E-beam lithography have been employed to pattern devices with critical dimensions of submicrometer levels. The methods of device fabrication by lithography and multilevel processing are usually specific to the chemical and physical properties of the etchants and materials used, and require a number of processing steps. As an alternative, focused ion beam (FIB) lithography is a unique and straightforward tool to rapidly develop nanomagnetic prototyping devices. This feature of FIB is critical to conduct the basic study necessary to advance the state-of-the-art in magnetic recording. ^ The dissertation develops a specific design of nanodevices and demonstrates FIB-fabricated stable and reproducible magnetic nanostructures with a critical dimension of about 10 nm. The project included the fabrication of a patterned single and multilayer magnetic media with areal densities beyond 10 Terabit/in 2. Each block had perpendicular or longitudinal magnetic anisotropy and a single domain structure. The purpose was to demonstrate how the ability of FIB to directly etch nanoscale patterns allowed exploring (even in the academic environment) the true physics of various types of nanostructures. ^ Another goal of this study was the investigation of FIB patterned magnetic media with a set of characterization tools: e.g. Spinstand Guzik V2002, magnetic force microscopy, scanning electron microscopy with energy dispersive system and wavelength dispersive system. ^ In the course of this work, a unique prototype of a record high density patterned magnetic media device capable of 10 terabit/in 2 was built. The read/write testing was performed by a Guzik spinstand. The readback signals were recorded and analyzed by a digital oscilloscope. A number of different configurations for writing and reading information from a magnetic medium were explored. The prototype transducers for this work were fabricated via FIB trimming of different magnetic recording heads. ^

Step and flash imprint lithography: Fabrication of patterned media for extremely high density magnetic data storage devices

Currently the data storage industry is facing huge challenges with respect to the conventional method of recording data known as longitudinal magnetic recording. This technology is fast approaching a fundamental physical limit, known as the superparamagnetic limit. A unique way of deferring the superparamagnetic limit incorporates the patterning of magnetic media. This method exploits the use of lithography tools to predetermine the areal density. Various nanofabrication schemes are employed to pattern the magnetic material are Focus Ion Beam (FIB), E-beam Lithography (EBL), UV-Optical Lithography (UVL), Self-assembled Media Synthesis and Nanoimprint Lithography (NIL). Although there are many challenges to manufacturing patterned media, the large potential gains offered in terms of areal density make it one of the most promising new technologies on the horizon for future hard disk drives. Thus, this dissertation contributes to the development of future alternative data storage devices and deferring the superparamagnetic limit by designing and characterizing patterned magnetic media using a novel nanoimprint replication process called "Step and Flash Imprint lithography". As opposed to hot embossing and other high temperature-low pressure processes, SFIL can be performed at low pressure and room temperature. Initial experiments carried out, consisted of process flow design for the patterned structures on sputtered Ni-Fe thin films. The main one being the defectivity analysis for the SFIL process conducted by fabricating and testing devices of varying feature sizes (50 nm to 1 μm) and inspecting them optically as well as testing them electrically. Once the SFIL process was optimized, a number of Ni-Fe coated wafers were imprinted with a template having the patterned topography. A minimum feature size of 40 nm was obtained with varying pitch (1:1, 1:1.5, 1:2, and 1:3). The Characterization steps involved extensive SEM study at each processing step as well as Atomic Force Microscopy (AFM) and Magnetic Force Microscopy (MFM) analysis.

A nanoscratch method for measuring hardness of thin films

Thin solid films were extensively used in the making of solar cells, cutting tools, magnetic recording devices, etc. As a result, the accurate measurement of mechanical properties of the thin films, such as hardness and elastic modulus, was required. The thickness of thin films normally varies from tens of nanometers to several micrometers. It is thus challenging to measure their mechanical properties. In this study, a nanoscratch method was proposed for hardness measurement. A three-dimensional finite element method (3-D FEM) model was developed to validate the nanoscratch method and to understand the substrate effect during nanoscratch. Nanoindentation was also used for comparison. The nanoscratch method was demonstrated to be valuable for measuring hardness of thin solid films.

Spin-Valve-Like Magnetoresistance in Mn2NiGa at Room Temperature

Spin valves have revolutionized the field of magnetic recording and memory devices. Spin valves are generally realized in thin film heterostructures, where two ferromagnetic (FM) layers are separated by a nonmagnetic conducting layer. Here, we demonstrate spin-valve-like magnetoresistance at room temperature in a bulk ferrimagnetic material that exhibits a magnetic shape memory effect. The origin of this unexpected behavior in Mn2NiGa has been investigated by neutron diffraction, magnetization, and ab initio theoretical calculations. The refinement of the neutron diffraction pattern shows the presence of antisite disorder where about 13% of the Ga sites are occupied by Mn atoms. On the basis of the magnetic structure obtained from neutron diffraction and theoretical calculations, we establish that these antisite defects cause the formation of FM nanoclusters with parallel alignment of Mn spin moments in a Mn2NiGa bulk lattice that has antiparallel Mn spin moments. The direction of the Mn moments in the soft FM cluster reverses with the external magnetic field. This causes a rotation or tilt in the antiparallel Mn moments at the cluster-lattice interface resulting in the observed asymmetry in magnetoresistance.

A Communication-Theoretic Framework for 2-DMR Channel Modeling: Performance Evaluation of Coding and Signal Processing Methods

We develop a communication theoretic framework for modeling 2-D magnetic recording channels. Using the model, we define the signal-to-noise ratio (SNR) for the channel considering several physical parameters, such as the channel bit density, code rate, bit aspect ratio, and noise parameters. We analyze the problem of optimizing the bit aspect ratio for maximizing SNR. The read channel architecture comprises a novel 2-D joint self-iterating equalizer and detection system with noise prediction capability. We evaluate the system performance based on our channel model through simulations. The coded performance with the 2-D equalizer detector indicates similar to 5.5 dB of SNR gain over uncoded data.

Upper Bounds on the Size of Grain-Correcting Codes

In this paper, we revisit the combinatorial error model of Mazumdar et al. that models errors in high-density magnetic recording caused by lack of knowledge of grain boundaries in the recording medium. We present new upper bounds on the cardinality/rate of binary block codes that correct errors within this model. All our bounds, except for one, are obtained using combinatorial arguments based on hypergraph fractional coverings. The exception is a bound derived via an information-theoretic argument. Our bounds significantly improve upon existing bounds from the prior literature.

Generalized Partial Response Equalization and Data-Dependent Noise Predictive Signal Detection Over Media Models for TDMR

Two-dimensional magnetic recording (2-D TDMR) is an emerging technology that aims to achieve areal densities as high as 10 Tb/in(2) using sophisticated 2-D signal-processing algorithms. High areal densities are achieved by reducing the size of a bit to the order of the size of magnetic grains, resulting in severe 2-D intersymbol interference (ISI). Jitter noise due to irregular grain positions on the magnetic medium is more pronounced at these areal densities. Therefore, a viable read-channel architecture for TDMR requires 2-D signal-detection algorithms that can mitigate 2-D ISI and combat noise comprising jitter and electronic components. Partial response maximum likelihood (PRML) detection scheme allows controlled ISI as seen by the detector. With the controlled and reduced span of 2-D ISI, the PRML scheme overcomes practical difficulties such as Nyquist rate signaling required for full response 2-D equalization. As in the case of 1-D magnetic recording, jitter noise can be handled using a data-dependent noise-prediction (DDNP) filter bank within a 2-D signal-detection engine. The contributions of this paper are threefold: 1) we empirically study the jitter noise characteristics in TDMR as a function of grain density using a Voronoi-based granular media model; 2) we develop a 2-D DDNP algorithm to handle the media noise seen in TDMR; and 3) we also develop techniques to design 2-D separable and nonseparable targets for generalized partial response equalization for TDMR. This can be used along with a 2-D signal-detection algorithm. The DDNP algorithm is observed to give a 2.5 dB gain in SNR over uncoded data compared with the noise predictive maximum likelihood detection for the same choice of channel model parameters to achieve a channel bit density of 1.3 Tb/in(2) with media grain center-to-center distance of 10 nm. The DDNP algorithm is observed to give similar to 10% gain in areal density near 5 grains/bit. The proposed signal-processing framework can broadly scale to various TDMR realizations and areal density points.

Explosive consolidation of amorphous cobalt-based alloys

This paper addresses the explosive consolidation of amorphous cobalt-based alloys. Using the experimental setup introduced in the present paper, specimens with high compact density, excellent magnetic properties and great wearability have been made. In comparison with permalloy and ferrite, the present specimens exhibit superior magnetic properties. Therefore, the compact is deemed as being a promising material for magnetic recording heads.

新型磁存储介质L10有序FePt薄膜研究综述

L10有序FePt合金薄膜有大的各向异性能、矫顽力和饱和磁化强度，而且根据制备工艺条件的不同，其易磁化轴可以平行或垂直于膜面，因此极有可能成为下一代超高密度磁存储的介质，近年来引起了广泛的关注，详细介绍了FePt薄膜近年来的研究结果，分析了其大矫顽力的机制、降低有序化温度、控制易磁化轴取向、降低粒子间相互作用的方法等对磁存储至关重要的问题，并对其在磁存储中的应用前景作了分析。

Development of pure and doped gamma ferric oxide

Optimum conditions and experimental details for the formation of v-Fe203 from goethite have been worked out. In another method, a cheap complexing medium of starch was employed for precipitating acicular ferrous oxalate, which on decomposition in nitrogen and subsequent oxidation yielded acicular y-Fe203. On the basis of thermal decomposition in dry and moist nitrogen, DTA, XRD, GC and thermodynamic arguments, the mechanism of decomposition was elucidated. New materials obtained by doping ~'-Fe203 with 1-16 atomic percent magnesium, cobalt, nickel and copper, were synthesised and characterized

Carbon-decorated FePt nanoparticles

FePt magnetic nanoparticles are an important candidate material for many future magnetic applications. FePt exists as two main phases, that is, a disordered face-centered cubic (fcc) structure, which is generally prepared by chemical methods at low temperatures, and the high-temperature chemically ordered face-centered tetragonal (fct) structure. The fee FePt, with low coercivity but associated with superparamagnetic properties, may find applications as a magnetic fluid or as a nanoscale carrier for chemical or biochemical species in biomedical areas, while fct FePt is proposed for use in ultrahigh-density magnetic recording applications. However, for both of these applications an enhancement of the intrinsically weak magnetic properties, the avoidance of magnetic interferences from neighbor particles, and the improved stability of the small magnetic body remain key practical issues. We report a simple synthetic method for producing FePt nanoparticles that involves hydrothermal treatment of Fe and Pt precursors in glucose followed by calcination at 900 degrees C. This new method produces thermally stable spheroidal graphite nanoparticles (large and fullerene-like) that encapsulate or decorate FePt particles of ca. 5 nm with no severe macroscopic particle coalescence. Also, a low coercivity of the material is recorded; indicative of small magnetic interference from neighboring carbon-coated particles. Thus, this simple synthetic method involves the use of a more environmentally acceptable glucose/aqueous phase to offer a protective coating for FePt nanoparticles. It is also believed that such a synthetic protocol can be readily extended to the preparation of other graphite-coated magnetic iron alloys of controlled size, stoichiometry, and physical properties.

Synthesis and fabrication of a thin film containing silica-encapsulated face-centered tetragonal FePt nanoparticles

Self-assembly of monodisperse, silica-encapsulated, face-centered tetragonal FePt nanoparticles forms closely packed 2D arrays (see figure). Placing monodisperse FePt nanoparticles in silica nanocapsules allows the transition from a disordered face-centered cubic phase to a ferromagnetic crystalline face-centered tetragonal structure at elevated temperature without severe sintering. These materials are potential candidates for the generation of ultrahigh-density magnetic recording media.

Perpendicularly self-oriented and shape-controlled L1(0)-FePt nanorods directly synthesized by a temperature-modulated process

The synthesis and self-assembly of tetragonal phase-containing L1(0)-Fe(55)Pt(45) nanorods with high coercive field is described. The experimental procedure resulted in a tetragonal/cubic phase ratio close to 1:1 for the as-synthesized nanoparticles. Using different surfactant/solvent proportions in the process allowed control of particle morphology from nanospheres to nanowires. Monodisperse nanorods with lengths of 60 +/- 5 nm and diameters of 2-3 nm were self-assembled in a perpendicular oriented array onto a substrate surface using hexadecylamine as organic spacer. Magnetic alignment and properties assigned, respectively, to the shape anisotropy and the tetragonal phase suggest that the self-assembled materials are a strong candidate to solve the problem of random magnetic alignment observed in FePt nanospheres leading to applications in ultrahigh magnetic recording (UHMR) systems capable of achieving a performance of the order of terabits/in(2).

Formation Mechanism via a Heterocoagulation Approach of FePt Nanoparticles Using the Modified Polyol Process

Herein, we report a new approach of an FePt nanoparticle formation mechanism studying the evolution of particle size and composition during the synthesis using the modified polyol process. One of the factors limiting their application in ultra-high-density magnetic storage media is the particle-to-particle composition, which affects the A1-to-L1(0) transformation as well as their magnetic properties. There are many controversies in the literature concerning the mechanism of the FePt formation, which seems to be the key to understanding the compositional chemical distribution. Our results convincingly show that, initially, Pt nuclei are formed due to reduction of Pt(acac)(2) by the diol, followed by heterocoagulation of Fe cluster species formed from Fe(acac)(3) thermal decomposition onto the Pt nuclei. Complete reduction of heterocoagulated iron species seems to involve a CO-spillover process, in which the Pt nuclei surface acts as a heterogeneous catalyst, leading to the improvement of the single-particle composition control and allowing a much narrower compositional distribution. Our results show significant decreases in the particle-to-particle composition range, improving the A1-to-L1(0) phase transformation and, consequently, the magnetic properties when compared with other reported methods.