Write Head Field Research Articles

Effect of Write Head Movement On Magnetic Spin Domain Reversal of Nanocube Co/Pd Alloy Material Using Micromagnetic Simulation

An analysis of the effect of the write head movement on the reversal time of the domain spin with magnetic Co/Pd on the magnetic recording layer has been carried out through micromagnetic simulation. The magnetic recording layer is modeled in the form of cubes (nanocubes) which consists of 5 domain spin. The write head, which is a transduser, moves along the domain spin to write data in the form of magnetic spins, which represent the bits on the magnetic recorder perpendicular. The results of this simulation are a profile of changes in the total magnetic field and reversal time of the domain spin when writing magnetic data for 6 nanoseconds. The calculation used in this study is an analytical calculation regarding the reversal time of the magnetic domain spin of the Co/Pd alloy material. The formulation for calculating the reversal time of domain-spin magnetization is a combination of graphical analysis and analytical calculations with visualization of the magnetic spin configuration that consisting of 5 domains spin. This simulation was carried out using the finite element method and obtained a saturation 5 field value of the magnetic alloy Co/Pd (Hs) material of 2.5 x 105 A/m and a write head (Hwh) field that 6 must be applied to the magnetic recording layer in order to reverse the uniform domain spin is 7.3 x 106 A/m. Each size of the domain spin requires a different write head, the smaller the nanocube size, the greater the write head field applied to the magnetic recording layer. Meanwhile, the effective write head 6 field amplitude that is suitable for the 20 nm domain spin is 8.3 x 106 A/m. A significant change in the total field occurs when the domain spin reverses 3 times in the first domain spin (n1), the third domain spin (n3) and the fifth domain spin (n5). The total field value when t=0.42 ns ( first domain spin reversal) is 73.69376 A/m, then the total field at t=0.42 ns (third domain spin reversal) is 3443.197 A/m and the current total field t=0.42 ns (fifth domain spin reversal) of 5480.696 A/m.

Write head field design for correcting transition curvature in heat assisted magnetic recording

In this paper, we present a method for correcting transition curvature in heat assisted magnetic recording. By appropriately varying the recording head field across the track width, straight transitions free of curvature can be created even with a circular thermal profile. Micromagnetic modeling study has been performed to show the consequent signal-to-noise ratio enhancement and corresponding area density gain. Effect of less-than-optimum head field has been analyzed.

Micromagnetic Model Analysis of Spin-Transfer Torque Oscillator and Write Heads for Microwave-Assisted Magnetic Recording

A micromagnetic model analysis was used to investigate spin-transfer torque oscillators (STOs) and magnetic write heads for microwave-assisted magnetic recording (MAMR). Our goal was to propose STOs, the Gilbert damping constant of the write head, and write head configurations that allow the STO element to operate at a low injected current density J. First, we derived the write head field and field generation layer rotations for various models with various main pole (MP) widths and MP throat heights. Second, we used the write head model and tried to reduce J by optimizing the Gilbert damping constant of the write head. Then, we used various STOs and write head structures and derived STO fields, clockwise and counterclockwise components, and FGL angles versus J. Finally, we performed media recording simulations and noted some challenges in the MAMR simulations.

Effects of Magnetic Properties of L1<sub>0</sub>-CoPt based Bit Patterned Media with Tilted-Easy Axis on Switching Field at Areal Density over 2 Tb/in<sup>2</sup>

Decrease of a switching field, Hsw, of the magnetic media with high magnetocrystalline anisotropy constant, Ku, can be handled by tilted-easy axis. Not only tilting easy direction of crystal but also optimum magnetic properties can improve writability. The effects of Ku and saturation magnetization, Ms, of L10-CoPt material of BPM with 45° tilted-easy axis are investigated in this article. The object oriented micromagnetic framework package based on Landau - Lifshitz - Gilbert equation has been used to analyze the magnetic properties of media in this paper. The results indicate that the Hsw decreases with decreasing Ku and increasing Ms. To achieve the Hsw lower than the maximum write head field, the Ku and Ms values should not be over 1.30 MJ/m3 and should exceed 0.30 MA/m, respectively.

Modeling interface exchange coupling: Effect on switching of granular FePt films

To raise the areal density of magnetic recording to ∼1 Tbit/in2, there has been much recent work on the use of FePt granular films, because their high perpendicular anisotropy allows small grains to be stable. However, their coercivity may be higher than available write-head fields. One approach to reduce the coercivity is to heat the grain (heat assisted magnetic recording). Another strategy is to add a soft capping layer to help nucleate switching via exchange coupling with the hard FePt grains. We have simulated a model of such a capped medium and have studied the effect of the strength of the interface exchange and thickness of hard layer and soft layer on the overall coercivity. Although the magnetization variation within such boundary layers may be complex, the net effect of the boundary can often be modeled as an infinitely thin interface characterized by an interface exchange energy density—we show how to do this consistently in a micromagnetic simulation. Although the switching behavior in the presence of exchange, magnetostatic, and external fields is quite complex, we show that by adding these fields one at a time, the main features of the M-H loop can be understood. In particular, we find that even without hard-soft interface exchange, magnetostatic coupling eliminates the zero-field kink in the loop, so that the absence of the kink does not (as has sometimes been assumed) imply exchange coupling. The computations have been done with a public-domain micromagnetics simulator that has been adapted to easily simulate arrays of grains.

Design and Micromagnetic Simulation of Fe/L10-FePt/Fe Trilayer for Exchange Coupled Composite Bit Patterned Media at Ultrahigh Areal Density

Exchange coupled composite bit patterned media (ECC-BPM) are one candidate to solve the trilemma issues, overcome superparamagnetic limitations, and obtain ultrahigh areal density. In this work, the ECC continuous media and ECC-BPM of Fe/L10-FePt/Fe trilayer schemes are proposed and investigated based on the Landau-Lifshitz-Gilbert equation. The switching field,Hsw, of the hard phase in the proposed continuous ECC trilayer media structure is reduced below the maximum write head field at interlayer exchange coupling between hard and soft phases,Aex, higher than 20 pJ/m and its value is lower than that for continuousL10-FePt single layer media andL10-FePt/Fe bilayer. Furthermore, theHswof the proposed ECC-BPM is lower than the maximum write head field with exchange coupling coefficient between neighboring dots of 5 pJ/m andAexover 10 pJ/m. Therefore, the proposed ECC-BPM trilayer has the highest potential and is suitable for ultrahigh areal density magnetic recording technology at ultrahigh areal density. The results of this work may be gainful idea for nanopatterning in magnetic media nanotechnology.

<i>L</i>1<sub>0</sub>-CoPt Bit Patterned Media with Tilted Easy Axis for Ultrahigh Areal Density over 2.5 Tb/in<sup>2</sup>

Ultrahigh areal density is the key target of hard disk drive technology. Hence, writing field strength from head and switching field, Hsw, of media should be improved. In this work, we propose the one of alternative method to increase data density and reduce Hsw of the media by using tilted easy axis technology for bit patterned media (BPM) at areal density beyond 2.5 Tb/in2. Moreover, transition noise and superparamagnetic limit have been eliminated owing to characteristics of BPM. The effect of exchange coupled between adjacent bits, Adot, of tilted easy axis of BPM is analyzed by micromagnetic simulation software - the object oriented micromagnetic framework based on Landau-Lifshitz-Gilbert equation. The BPM with tilted easy axis perform clearly the reduction of Hsw below perpendicular media and available writing head field. The Adot of BPM has no effect on decreasing Hsw. Anisotropy and Zeeman energy density of BPM with tilted easy axis are higher and lower than perpendicular BPM, respectively. Thereby, BPM with tilted easy axis have high potentiality to improve Hsw of media at ultrahigh data density.

The Effects of Magnetic Properties of <i>L</i>1<sub>0</sub>-FePt/Fe Based Exchange Coupled Composite Media on Switching Field

The L10-FePt/Fe based exchange coupled composite (ECC) bilayer media is one candidate to extend the areal density of magnetic recording system and solve writability issue in trilemma. L10-FePt is the great high magnetic anisotropy material. Therefore, the magnetic parameters of this material such as magnetocrystalline anisotropy constant, Ku, saturation magnetization, Ms, and exchange coupling between a soft/hard interface, are important on magnetic material properties. In this work, the effects of magnetic parameters on magnetic properties of L10-FePt/Fe based ECC bilayer media are simulated by the object oriented micromagnetic framework based on Landau-Lifshitz-Gilbert equation. The ECC bilayer media can reduce switching field, Hsw, of media lower than available writing head field. Hence, writability issues of high Ku media can be achieved. Reducing Hsw of ECC bilayer media obtains from lower Ku and higher Ms values. This work can achieve writing capability of a future magnetic recording system.

Investigation on Magnetic Properties of <i>L</i>1<sub>0</sub>-FePt/Fe Graded Media Multilayer

To improve writing capability of high magnetic anisotropy medium - L10-FePt, graded medium multilayer is one key candidate. Therefore, investigation of magnetic material properties of graded media multilayer is essential. In this work, we focus on magnetic properties of L10-FePt/Fe graded media multilayer such as hysteresis loop, magnetic energy and magnetic domain by the micromagnetic simulation the object oriented micromagnetic framework software based on the Landau - Lifshitz - Gilbert equation. The graded media multilayer can achieve reducing the switching field, Hsw, below available writing head field with high thermal stability. The anisotropy energy in graded media is higher than single layer and bilayer media that results in unstable magnetization in preferred direction. The proposed graded (f) and (g) structures are high grading magnetocrystalline anisotropy constant, Ku, that can reduce Hsw below single layer, bilayer and available writing head field. The hysteresis loop of low grading Ku structures has different steps; however, it disappear with high grading Ku structures of graded (e) - (g) structures. They have narrow loop compared with single layer. The results from this work lead to improve magnetic trilemma issue and increase data density.

Micromagnetic Model Analysis of Planar-Type Write Head Field Response and Dependence on Pole Tip, Return Yoke, and Shield Structure

We used a micromagnetic model to investigate the high-frequency (1-5 GHz) response, peak recording field strength, recording field gradient, and adjacent track erasure (ATE) field for various main pole tip, shield, and return yoke structures in planar type heads. The effect of the pole tip area at the air-bearing surface (ABS) and the return yoke structure is also discussed. It was found that the pole tip and shield arrangement had a major effect on the peak recording field strength, recording field gradient, and ATE field, i.e., the trailing and side shields work to reduce the undesired field, however, they also slow the head field response due to the interaction between the main pole tip and the shield. The field responses are also dependent on the main pole tip-shield structures and pole tip structures. The return yoke thickness and area were important determinants of response speed and peak field.

Switching Phase Diagram of Two Frequencies MAMR for ECC Media

A two-particle model is utilized to study the switching phase diagram of ECC media in the microwave-assisted magnetic recording, by applying an in-plane AC magnetic field for microwave and a DC magnetic field for write head field. An interesting result is found that, by introducing two mixed linearly polarized AC fields when the ratio of frequencies f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> is in a certain range, the minimum switching DC field can be almost zero when H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ac</sub> is 20% of H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k1</sub> of hard magnetic layer. This phase diagram is also checked by a 3D micromagnetic model of ECC media with polycrystalline structure included; with two linearly polarized H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ac</sub> applied, the coercivity H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> can be reduced to almost zero when H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ac</sub> = 2.0 H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k1</sub> , and H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> still decreases from 10.5 kOe to 7.7 kOe when H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ac</sub> = 0.02 H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k1</sub> .

Influence of various underlayers on [001] texture and magnetic properties in FePt and FexNi1 − xPt thin films

Perpendicular magnetic recording thin films require proper crystallographic growth to ensure that the magnetic easy-axis is aligned normal to the growth plane. The texture and in situ growth stresses of (001) FePt and FexNi(1−x)Pt thin films on various underlayer combinations are studied. These were correlated with the magnetic properties allowing self consistent direct comparisons of the various parameters to be made. It was found that the (001) texture, L10 chemical ordering and perpendicular magnetic alignment was improved with the smaller lattice misfit that accompanied a combination of Pt/Cr multilayered and compositionally graded underlayers on a (001) MgO substrate. A Cr underlayer allowed the Pt film to deposit in a relatively strain-free lattice condition from which the FePt based films could grow in a similar strain-free epitaxial condition. In situ stress measurements confirmed that these underlayers reduced the intrinsic compressive growth stresses to approximately one-fifth the FePt stress value for being directly deposited onto the MgO surface. The incorporation of Ni to the (001) growth of FePt facilitated in-plane and out-of-plane c-variant growth for the L10 structure. Finally, a linear compositional gradient FexNi0.48−xPt0.52 (0<x<0.48) thin film was deposited, which exhibited good [001] orientation and perpendicular magnetic alignment. This has been proposed as a magnetic switching architecture for high density, thermally stable but switchable magnetic media grains using current write head fields.

A statistical model of write-errors in bit patterned media

In bit patterned media (BPM), a magnetic data storage medium is patterned into nanoscale magnetic islands each representing one binary digit (bit). The recording performance of BPM depends upon the variability of island position, geometry, and magnetic characteristics. To understand the impact of the distributions of these parameters on the performance of BPM a detailed statistical model of write-errors has been developed. The islands can either be single layer or two-layer exchange coupled composite (ECC) structures. The modeling of ECC islands was made possible by the development of a 2-spin model to calculate switching field, coercivity, and energy barrier as a function of applied field which enables medium design to be optimized for a non-uniform write head field. The statistical model predicts coercivities and error rates in good agreement with a micromagnetic model but at significantly lower implementation and computational cost and shows good agreement with experimental data from drag-test experiments at 500 Gbit/in2. The model enables the position tolerance of the write head to be determined from the magnetic characteristics of the write head and the storage medium and it is therefore a valuable system design tool.

A Study on Nonbinary LDPC Coding and Iterative Decoding System in BPM R/W Channel

In this paper, the iterative decoding system using the nonbinary low-density parity-check (LDPC) code is studied in the magnetic recording system using a bit-patterned media (BPM) R/W channel affected by the write head field gradient, the media switching field distribution (SFD), the demagnetization field from adjacent dots, and the dot position deviation in an areal recording density of 2 Tb/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The performance of iterative decoding system using the nonbinary LDPC code over Galois field of GF(2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sup> ) is evaluated by the computer simulation, and it is compared with the conventional iterative decoding system using the binary LDPC code. The results show that the nonbinary LDPC system provides better performance compared with the binary LDPC system.

Performance Evaluation of LDPC Coding and Iterative Decoding System in BPM R/W Channel Affected by Head Field Gradient, Media SFD and Demagnetization Field

We evaluate the performance of the write-margin for the low-density parity-check (LDPC) coding and iterative decoding system in the bit-patterned media (BPM) R/W channel affected by the write-head field gradient, the media switching field distribution (SFD), the demagnetization field from adjacent islands and the island position deviation. It is clarified that the LDPC coding and iterative decoding system in R/W channel using BPM at 3 Tbit/inch2 has a write-margin of about 20%.

High Field-Gradient Design of Single-Pole Write-Head With Planar Pole Structure

A planar single-pole head generating a large head field was investigated to improve head field distribution with a steep field gradient. Good writing performance is generally difficult for a narrow track writer. This work presents a novel design of the main pole shape, the three-step-slope, to achieve a high gradient and strong write head field for terabit recording. Furthermore, the medium design was investigated to increase head-to-medium interaction for achieving a high field gradient and narrow field distribution. A soft guard band in the recording layer and a cross-track groove on the SUL surface were effective for improving the field gradient.

Fabrication and Recording Properties of Shielded Planar Type Single Pole Write Head

We fabricated a shielded planar single-pole write head that combined a tapered main pole to obtain a strong write head field and a wraparound shield to obtain a steep head-field distribution. Anisotropy etching pits of Si were used as a template to fabricate the new head, and magnetic yoke material was embedded in the template to form the tapered main-pole tip. As a result, this new head could be fabricated using a novel technique that is different from that used for present recording heads. Our first test recording with the head was successful.

Modeling and Simulation of the Writing Process on Bit-Patterned Perpendicular Media

The recording physics of bit-patterned media is studied for areal densities of around 2 Tbits/inch2 . A model of the writing process, based on the head field gradient and switching field distribution, including various interference fields, is presented to extract the write-head and media parameters which are necessary to attain a wider write margin. Write head field distributions, calculated by the finite-element method, are presented for various head pole structures with side- and trailing-shields to improve head field gradient. Optimization of the pole configuration increased the gradient to over 500 Oe/nm for a head-to-soft magnetic under layer (SUL) spacing of 14 nm. Using the head field distribution, a recording performance analysis by micromagnetic simulation indicated that the write margin was improved. Position and size dispersions of the dots deteriorated the signal-to-noise ratio (SNR) and increased adjacent track erasure (ATE).

Experimental Study of Perpendicular Write Head Field Rise Time

In this paper, a simple technique that can determine the write head field rise time based on electrical measurements is presented. A model that relates the head electrical properties to the frequency dependent head efficiency epsiv(omega) and head field rise time tau is described in detail. This method only requires knowledge of the complex impedances of a magnetically saturated head and an unsaturated head. The results obtained are verified with recording measurements of the rise time for the same heads. Very good agreement was found for all heads. It was also found that the presence of a soft underlayer does not seem to significantly affect the measured head field rise time. Therefore, it is concluded that the head field rise time is determined by the yoke structure and not by the pole tip area

Patterned Media Based on Soft/Hard Composite Nanowire Array of Ni/CoPt

One of the drawbacks of using high-coercivity materials for magnetic data storage is the need for large write head fields for data writing. In attempts to solve this problem, we have fabricated an array of CoPt/Ni composite nanowires using anodized aluminum oxide (AAO) templates, electrochemical deposition, and an annealing treatment. The array consists of vertically aligned, ~25-nm diameter, composite nanowires with ~80-nm-long, magnetically soft Ni at the base and ~20-nm-long, magnetically hard, L1 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> phase CoPt at the top. To see the influence of the composite structure on the magnetic properties, separate samples with Ni and CoPt nanowires of the same height were prepared and annealed using identical conditions. M-H loops along the nanowire axis show that the Ni nanowires exhibit relatively soft magnetic coercivity of 242 Oe, while the CoPt nanowires show a very high coercivity of 10.97 kOe. The composite CoPt/Ni nanowires exhibit an intermediate coercivity of 1.96 kOe. The results show a reduction in coercivity versus the CoPt nanowires, potentially leading to easier data writing