Perpendicular Media Research Articles

Achievement and expected issues in heat assisted magnetic recording

The main goals in magnetic recording are the continuous increase of the areal recording density and the highest possible signal-to-noise ratio (SNR). The conventional magnetic recording has approached its physical limits. Further growth of the areal density is limited by the superparamagnetic effect and by the limited possibilities to further improve write heads design and pole materials in order to enhance the writing field. The perpendicular magnetic recording (PMR) with pole head and perpendicular media is the best alternative, especially to defer the superparamagnetic limit. PMR is also better situated to face the challenging design trilemma of magnetic recording: To increase the areal recording density, smaller grain volumes are needed, but to ensure the thermal stability of recorded information, the anisotropy should be increased accordingly; or, the increased anisotropy asks for higher writing fields, which are unavailable with the saturation magnetization of the magnetic materials of the current heads. Obviously, an alternative technology is needed to overcome the physical limit of conventional perpendicular recording (CPR). The most promising successor of CPR is the heat–assisted magnetic recording (HAMR), which is a multidisciplinary technology, a combination of magnetic and optical recording technology that proved experimentally areal densities of more than 1 Tb/in2 . HAMR allows the use of very smallgrain media, required for recording at ultra-high densities, with a larger magnetic anisotropy at room temperature, thus assuring a good thermal stability, while the local heating leads to a temporary magnetic softening of the medium. The realization of HAMR involved some challenges from both optical design and media properties. In the optical design the use of near-field transducers (NFT) represents an important advance. Regarding the media, a thermal design was used to produce the heating and cooling of the media within a very short time, about 1 ns, in order to achieve the desired data rate and generate a large thermal gradient for sharp bit edge definition. The structure of a HAMR system is briefly discussed, as well as the processes characterizing the writing process. A special attention is paid to the requirements for the materials needed for this type of recording: the granular L10-ordered FePt:X alloy sputtered on glass disk, the FePt-C and FePtC-Ag granular films and the bitpatterned media. Some important challenges of the HAMR technology are also summarized.

Microwave-Assistance Effect on Magnetization Switching in Antiferromagnetically Coupled CoCrPt Granular Media

We examined microwave-assisted switching properties of an antiferromagnetically coupled (AFC) granular perpendicular medium consisting of CoCrPt–TiO2(5)/Co(0.7)/Ru(0.9)/Co(0.7)/CoCrPt–TiO2(3) (in nanometers). In a saturation remanent state, the magnetization of the lower CoCrPt–TiO2 + Co layer switched its direction to form AFC with that of the upper layer because of a thin-film thickness. We assessed the microwave-assistance effect on magnetization switching from the remanent state to the saturation state of the thick upper layer using an anomalous Hall effect. The reduction of a switching field was most enhanced by the application of microwaves of 10 GHz frequency. The switching field reduction by microwave assistance for the AFC medium, obtained by compensating a thermal agitation effect, was estimated as 26%.

CoPt Antidot Arrays Fabricated With Dry-Etching Using AAO Templates

We demonstrate a technique to fabricate large-area, highly ordered CoPt antidot arrays with perpendicular orientation. Using ultrathin anodic aluminum oxide (AAO) templates as a dry etching mask, the pattern of the pore array was transferred to the film without deteriorating the magnetic properties, in particular coercivity and preferential perpendicular orientation. The size, spacing, and density of the nonmagnetic nano-columns within the CoPt antidot arrays can be adjusted by changing the fabrication parameters of the AAO templates. The nano-columns can serve as pinning sites, impeding the magnetic domain wall motion, which can influence the magnetic properties of the film. The magnetic properties of the antidot arrays depend sensitively on the antidot morphology. The coercivity (H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ) of the CoPt antidot array with a pore density of 2.6×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sup> in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> fabricated using the AAO template anodized at 20 V is increased from 9.2 to 12.5 kOe due to the existence of such pinning sites.

Damping Dependence of Reversal Magnetic Field on Co-based Nano-Ferromagnetic with Thermal Activation

Currently, hard disk development has used HAMR technology that applies heat to perpendicular media until near Curie temperature, then cools it down to room temperature. The use of HAMR technology is significantly influence by Gilbert damping constants. Damping affects the magnetization reversal and coercivity field. Simulation is used to evaluate magnetization reversal by completing Landau-Lifshitz-Gilbert explicit equation. A strong ferromagnetic cobalt based material with size 50×50×20 nm 3 is used which parameters are anisotropy materials 3.51×10 6 erg/cm 3 , magnetic saturation 5697.5 G, exchange constant 1×10 -7 erg/cm, and various Gilbert damping from 0.09 to 0.5. To observe the thermal effect, two schemes are used which are Reduced Barrier Writing and Curie Point Writing. As a result, materials with high damping is able to reverse the magnetizations faster and reduce the energy barrier. Moreover, it can lower the minimum field to start the magnetizations reversal, threshold field, and probability rate. The heating near Curie temperature has succeeded in reducing the reversal field to 1/10 compared to writing process in absence of thermal field.

Promotion of Self-Assembly Patterning of FePt Nanoparticles by Tuning the Concentration of Oleylamine/Oleic Acid Surfactants in a Coating Solution

To provide a bit-patterned perpendicular magnetic recording medium consisting of an array of FePt magnetic nanoparticles, the effect of the total dispersant concentrations of oleylamine and oleic acid in a FePt-nanoparticle/toluene coating solution on a self-assembled nanoparticle array with a long-range periodicity on a flat SiO2/Si substrate and two patterned substrates, respectively, was examined. At a surfactant concentration greater than 0.8 vol%, FePt nanoparticles with an average size of 4.4 nm were arrayed in a two-dimensional and hexagonal-close-packed ordered array with 8.3-nm pitches on the flat surface. Self-assembled nanoparticles were arrayed on the bottom surface of the patterned trench, the array exhibited a hexagonal-close-packed structure. Furthermore the line defects in the arrays are caused by the waviness of the trench walls, and the surface roughness.

Magnetic Nanoparticle Arrays Self-Assembled on Perpendicular Magnetic Recording Media.

We study magnetic-field directed self-assembly of magnetic nanoparticles onto templates recorded on perpendicular magnetic recording media, and quantify feature width and height as a function of assembly time. Feature widths are determined from Scanning Electron Microscope (SEM) images, while heights are obtained with Atomic Force Microscopy (AFM). For short assembly times, widths were ~150 nm, while heights were ~14 nm, a single nanoparticle on average with a 10:1 aspect ratio. For long assembly times, widths approach 550 nm, while the average height grows to 3 nanoparticles, ~35 nm; a 16:1 aspect ratio. We perform magnetometry on these self-assembled structures and observe the slope of the magnetic moment vs. field curve increases with time. This increase suggests magnetic nanoparticle interactions evolve from nanoparticle–nanoparticle interactions to cluster–cluster interactions as opposed to feature–feature interactions. We suggest the aspect ratio increase occurs because the magnetic field gradients are strongest near the transitions between recorded regions in perpendicular media. If these gradients can be optimized for assembly, strong potential exists for using perpendicular recording templates to assemble complex heterogeneous materials.

Magnetic characteristics of CoPd and FePd antidot arrays on nanoperforated Al2O3 templates

Hard magnetic antidot arrays show promising results in context of designing of percolated perpendicular media. In this work the technology of magnetic FePd and CoPd antidot arrays fabrication is presented and correlation between surface morphology, structure and magnetic properties is discussed.CoPd and FePd antidot arrays were fabricated by deposition of Co/Pd and Fe/Pd multilayers (MLs) on porous anodic aluminum oxide templates with bowl-shape cell structure with inclined intercellular regions. FePd ordered L10 structure was obtained by successive vacuum annealing at elevated temperatures (530°C) and confirmed by XRD analysis. Systematic analysis of magnetization curves evidenced perpendicular magnetic anisotropy of CoPd antidot arrays, while FePd antidot arrays revealed isotropic magnetic anisotropy with increased out-of-plane magnetic contribution. MFM images of antidots showed more complicated contrast, with alternating magnetic dots oriented parallel and antiparallel to tip magnetization moment.

Determination of Transition Jitter Noise by Time Domain Analysis

Transition jitter noise can be caused by a variety of factors such as contributions by the medium, head, and drive electronics. In this work, transition jitter noise in magnetic recording is measured based on the time domain analysis. An example of transition jitter noise in perpendicular media is investigated using captured noisy replay signals compare with a reconstructed ideal signal. It is found that transition noise increased with increasing linear density and dropped sharply after the maximum point.

Skyrmion lattice in a magnetic film with spatially modulated material parameters

The problem of the skyrmion stability in the magnetic film with perpendicular anisotropy inside the area with the changed material parameters is considered. The solution describing the conditions of such stabilization in the absence of Dzyaloshinskii–Moriya interaction is obtained analytically. The easy method of nanomodification of ordinary magnetic perpendicular media such as Co,Fe/Pt,Pd,Ru superlattices allowing the formation of the dense enough (with the period less than 100nm) skyrmion lattices is suggested. By micromagnetic simulations it is shown that the skyrmion lattices can be initialized in the system by simple magnetization in the uniform external magnetic field.

Sensitivity analysis of magnetic write head for shingled recording

Shingled magnetic recording (SMR) scheme is one of the promising techniques for high density magnetic recording. This paper presents a study of the sensitivity of the write field quality on the key design parameters of a perpendicular write head with wrap-around shield for SMR schemes. In the evaluation of the write head performance, the angle dependence of the switching field is considered to account for the characteristics of the perpendicular media. The field gradients of the head fields are examined at various critical locations of the write contour including the pole corner and the track edges. The dependence of the write performance on the variations of design parameters is investigated using a hybrid approach based on response surface methodology (RSM) and Taguchi's orthogonal arrays.

Magnetic force microscopy using tip magnetization modulated by ferromagnetic resonance

In magnetic force microscopy (MFM), the tip–sample distance should be reduced to analyze the microscopic magnetic domain structure with high spatial resolution. However, achieving a small tip–sample distance has been difficult because of superimposition of interaction forces such as van der Waals and electrostatic forces induced by the sample surface. In this study, we propose a new method of MFM using ferromagnetic resonance (FMR) to extract only the magnetic field near the sample surface. In this method, the magnetization of a magnetic cantilever is modulated by FMR to separate the magnetic field and topographic structure. We demonstrate the modulation of the magnetization of the cantilever and the identification of the polarities of a perpendicular magnetic medium.

Modification of the structural and magnetic properties of granular FePt films by seed layer conditioning

The steadily increasing amount of digital information necessitates the availability of reliable high capacity magnetic data storage. Here, future hard disk drives with extended areal storage densities beyond 1.0 Tb/in2 are envisioned by using high anisotropy granular and chemically L10-ordered FePt (002) perpendicular media within a heat-assisted magnetic recording scheme. Perpendicular texturing of the [001] easy axes of the individual grains can be achieved by using MgO seed layers. It is therefore investigated, if and how an Ar+ ion irradiation of the MgO seed layer prior to the deposition of the magnetic material influences the MgO surface properties and hereby the FePt [001] texture. Structural investigations reveal a flattening of the seed layer surface accompanied by a change in the morphology of the FePt grains. Moreover, the fraction of small second layer particles and the degree of coalescence of the primarily deposited FePt grains strongly increases. As for the magnetic performance, this results in a reduced coercivity along the magnetic easy axis (out of plane) and in enhanced hard axis (in-plane) remanence values. The irradiation induced changes in the magnetic properties of the granular FePt-C films are traced back to the accordingly modified atomic structure of the FePt-MgO interface region.

Effective damping factor for CoPtCr-SiO&lt;sub&gt;2&lt;/sub&gt; perpendicular magnetic recording medium with partially switched magnetic domains

Effective damping factor for CoPtCr-SiO&lt;sub&gt;2&lt;/sub&gt; perpendicular magnetic recording medium with partially switched magnetic domains

Damping Dependence of Reversal Magnetic Field on Co-based Nano-Ferromagnetic with Thermal Activation

Currently, hard disk development has used HAMR technology that applies heat to perpendicular media until near Curie temperature, then cools it down to room temperature. The use of HAMR technology is significantly influence by Gilbert damping constants. Damping affects the magnetization reversal and coercivity field. Simulation is used to evaluate magnetization reversal by completing Landau-Lifshitz-Gilbert explicit equation. A strong ferromagnetic cobalt based material with size 50×50×20 nm3 is used which parameters are anisotropy materials 3.51×106 erg/cm3, magnetic saturation 5697.5 G, exchange constant 1×10−7 erg/cm, and various Gilbert damping from 0.09 to 0.5. To observe the thermal effect, two schemes are used which are Reduced Barrier Writing and Curie Point Writing. As a result, materials with high damping is able to reverse the magnetizations faster and reduce the energy barrier. Moreover, it can lower the minimum field to start the magnetizations reversal, threshold field, and probability rate. The heating near Curie temperature has succeeded in reducing the reversal field to 1/10 compared to writing process in absence of thermal field.

Magnetic Switching Behavior of Granular Perpendicular Magnetic Media With Different Lateral and Vertical Exchange Coupling

The magnetic switching behavior was measured for granular oxide media containing an exchange control layer (ECL). Investigations of the switching behavior show that the initial minor loop slope represents the degree of incoherent switching in the film. Multilayered granular oxide media with different inter-granular coupling in both lateral and vertical directions, is prepared by varying the oxide grain isolation and ECL thickness. The switching behavior of well isolated oxide grains strongly depends on the thermal stability of the switching segments through the ECL layer. The oxide grains with higher thermal stability vertically decouple at thinner ECL, resulting in a stronger incoherency of switching. In addition, the increase of lateral exchange coupling between oxide grains reduces the vertical decoupling at given ECL condition. For media with a smaller cluster size, the ECL magnetization and thickness has a more pronounced effect on the switching, indicating that fabrication of well isolated grains is important to optimize the media switching mode.

Magnetization Reversal in Exchange Spring Bilayer System Under Circularly Polarized Microwave Field

Microwave assisted magnetization reversal are studied in the bulk bilayer exchange coupled system. We investigate the nonlinear magnetization reversal dynamics in a perpendicular exchange spring media using Landau-Lifshitz equation. In the limit of the infinite thickness of the system, the propagation field leads the reversal of the system. The reduction of the switching field and the magnetization profile in the extended system are studied numerically. The possibility to study the dynamics analytically is discussed and an approximation where two P-modes are coupled by an interaction field is presented. The ansatz used for the interaction field is validated by comparison with the numerical results. This approach is shown to be equivalent to two exchange coupled macrospins.

PORE STRUCTURE RECONSTRUCTION AND MOISTURE MIGRATION IN POROUS MEDIA

Three kinds of porous media (isotropic, perpendicular anisotropic and parallel anisotropic porous media) with the same porosity, different pore size distributions and fractal spectral dimensions were reconstructed by random growth method. It was aimed to theoretically study the impact of microscopic pore structure on water vapor diffusion process in porous media. The results show that pore size distribution can only denote the static characteristics of porous media but cannot effectively reflect the dynamic transport characteristics of porous media. Fractal spectral dimension can effectively analyze and reflect pores connectivity and moisture dynamic transport properties of porous media from the microscopic perspective. The pores connectivity and water vapor diffusion performance in pores of porous media get better with the increase of fractal spectral dimension of porous media. Fractal spectral dimension of parallel anisotropic porous media is more than that of perpendicular anisotropic porous media. Fractal spectral dimension of isotropic porous media is between parallel anisotropic porous media and perpendicular anisotropic porous media. Other macroscopic parameters such as equilibrium diffusion coefficient of water vapor, water vapor concentration variation at right boundary in equilibrium, the time when water vapor diffusion process reaches a stable state also can characterize the pores connectivity and water vapor diffusion properties of porous media.

A study of perpendicular magnetic recording media using polarized SANS

The study of thin film magnetic systems that are structured on the nanoscale is an area of intense interest. Small-angle neutron scattering is an extremely powerful probe of nanomagnetism in the bulk, but in thin-film systems the experiments are challenging due both to the small scattering volume available and also to scattering from other sources such as the substrate and sample environment. We have demonstrated that such experiments are however possible in magnetic films as thin as 10 nm. A good example to illustrate this is the case of perpendicular magnetic recording media. These materials are found in all modern magnetic hard drives, the data storage technology that continues to be of tremendous commercial and technological importance. These media are advanced functional multilayered materials, containing an active recording layer of only around 10 nm in thickness. This recording layer is compositionally segregated into 8 nm-sized grains of a magnetic CoCrPt alloy separated by a thin oxide shell, typically SiO2. These media have their magnetic moments oriented perpendicular to the plane of the film. Determining the local magnetic structure and reversal behavior is key to understanding the performance of perpendicular media in recording devices. Polarised SANS has proved to be a very effective tool to measure these materials at a sub-10nm length scales. The signal of interest must however also be distinguished from the scattering from other layers in the structure, some of which are also magnetic. We will present a summary of some recent results on recording media, including measurements of the grain-sized dependent switching with and without the presence of an exchange spring. We will also briefly mention experiments that demonstrate the viability of extending this approach to measurement for lithographically defined structures similar to those for application in bit-patterned media, including 2d artificial spin-ice and structurally glassy arrays.

Investigations of stacking fault density in perpendicular recording media

In magnetic recording media, the grains or clusters reverse their magnetization over a range of reversal field, resulting in a switching field distribution. In order to achieve high areal densities, it is desirable to understand and minimize such a distribution. Clusters of grains which contain stacking faults (SF) or fcc phase have lower anisotropy, an order lower than those without them. It is believed that such low anisotropy regions reverse their magnetization at a much lower reversal field than the rest of the material with a larger anisotropy. Such clusters/grains cause recording performance deterioration, such as adjacent track erasure and dc noise. Therefore, the observation of clusters that reverse at very low reversal fields (nucleation sites, NS) could give information on the noise and the adjacent track erasure. Potentially, the observed clusters could also provide information on the SF. In this paper, we study the reversal of nucleation sites in granular perpendicular media based on a magnetic force microscope (MFM) methodology and validate the observations with high resolution cross-section transmission electron microscopy (HRTEM) measurements. Samples, wherein a high anisotropy CoPt layer was introduced to control the NS or SF in a systematic way, were evaluated by MFM, TEM, and magnetometry. The magnetic properties indicated that the thickness of the CoPt layer results in an increase of nucleation sites. TEM measurements indicated a correlation between the thickness of CoPt layer and the stacking fault density. A clear correlation was also observed between the MFM results, TEM observations, and the coercivity and nucleation field of the samples, validating the effectiveness of the proposed method in evaluating the nucleation sites which potentially arise from stacking faults.

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

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.