Perpendicular Magnetic Recording Media Research Articles

Synthesis of Inorganic Nanocomposites by Selective Introduction of Metal Complexes into a Self‐Assembled Block Copolymer Template

Inorganic nanocomposites have characteristic structures that feature expanded interfaces, quantum effects, and resistance to crack propagation. These structures are promising for the improvement of many materials including thermoelectric materials, photocatalysts, and structural materials. Precise control of the inorganic nanocomposites’ morphology, size, and chemical composition is very important for these applications. Here, we present a novel fabrication method to control the structures of inorganic nanocomposites by means of a self‐assembled block copolymer template. Different metal complexes were selectively introduced into specific polymer blocks of the block copolymer, and subsequent removal of the block copolymer template by oxygen plasma treatment produced hexagonally packed porous structures. In contrast, calcination removal of the block copolymer template yielded nanocomposites consisting of metallic spheres in a matrix of a metal oxide. These results demonstrate that different nanostructures can be created by selective use of processes to remove the block copolymer templates. The simple process of first mixing block copolymers and magnetic nanomaterial precursors and then subsequently removing the block copolymer template enables structural control of magnetic nanomaterials, which will facilitate their applicability in patterned media, including next‐generation perpendicular magnetic recording media.

Direct Measurement of the Magnetic Cluster Size Using High-Resolution Magnetic Force Microscopy

High-resolution magnetic force microscopy was used to image high linear density tracks and measure the magnetic cluster size of the ac-erased state of perpendicular magnetic recording media. The full-width at half-maximum (FWHM) of the auto-correlation function corresponding to an ac-erased region determines the correlation length, i.e., the magnetic cluster size. This is demonstrated by comparing the calculated FWHM of auto-correlation function obtained on simulated Voronoi granular structures with the expected value for magnetically decoupled grains. The cluster size we obtained using the auto-correlation procedure is about four grain sizes. The impact of the finite cluster size on the recording performance was assessed by following the trend of tracks' auto-correlation function with increasing the linear density.

Method for Measurement of Switching-Field Distribution of Heat-Assisted Magnetic Recording Media

Due to increased emphasis on heat-assisted magnetic recording (HAMR), the characterization of the intrinsic switching-field distribution (iSFD) at elevated media temperatures has become important. In this paper, we introduce an efficient method to measure iSFD based on optical heating and magnetooptic Kerr effect measurement. The major and minor hysteresis loops of perpendicular magnetic recording media and FePt granular media were acquired at different optical heating powers. The optical heating power (or temperature) dependent iSFD of FePt granular media was measured. We observe iSFD broadening of HAMR media at elevated temperatures, which is consistent with theoretical predictions.

Magnetic interactions in CoCrPt-oxide based perpendicular magnetic recording media

First order reversal curves (FORC) method has been reported to be an efficient tool to study interaction between grains and layers of magnetic materials. Although a few studies have been carried out on perpendicular recording media in the past, a study on the effect of systematic variation of exchange interaction in granular perpendicular magnetic recording media on FORC contours has not been carried out in detail. Such a study will help to understand the use of FORC better. In this paper, we have made a systematic set of samples in order to study the variation in exchange coupling and its effect on FORC contours. The pressure during the deposition of the second ruthenium layer and the magnetic layer was varied to alter the separation between the grains and hence the exchange interaction between the grains in the CoCrPt-oxide recording layer. In addition, the thickness of Co-alloy cap layer was used as an additional tool to control the exchange interaction between the magnetic grains. The results indicated that the interaction field obtained from the FORC does not vary in a significant manner when the changes in exchange interaction are small. In comparison, the peak intensity of the FORC shows a clear trend as the exchange coupling is varied, making it a more suitable parameter to study the exchange and magnetostatic interactions in systems such as magnetic recording media.

ChemInform Abstract: L10 FePt‐Based Thin Films for Future Perpendicular Magnetic Recording Media

AbstractReview: 79 refs.

Magnetic Bit Patterned Media Fabrication Using Block Copolymer Directed Assembly

Perpendicular magnetic recording media in excess of 1Tbit/in2 faces thermal stability and/or writeability challenges that are currently driving intense research in alternative technologies. Bit patterned media (BPM), in which magnetic bits are lithographically patterned as individual islands, stands as a promising technology for thermally stable, writable media. One of the main challenges of BPM lies in its lithographic specifications which push dimensions beyond those established for the conventional semiconductor industry roadmap. The most promising solution to the lithographic challenge can be found in directed self assembly of block copolymer films which has recently evolved as a viable technique to achieve sub-20nm lithography in time for BPM technology. The periodic features naturally formed in block copolymer films display superior size uniformity at ultra-high densities, making them ideal lithographic masks to define the highly periodic data bits in the data sectors of hard disk drives for bit patterned media (BPM) technology at densities beyond 1Tbit/in2. Nanofabrication challenges towards bit patterned media, however, reach far beyond pattern formation at small length scales. The challenges faced in the fabrication of templates for BPM have spurred a set of important innovations in nanofabrication techniques such as the formation of geometries that deviate from the typical shapes naturally formed by block copolymers, as well as advances in high-fidelity pattern transfer and progress in scalability towards higher feature densities. In this presentation, we discuss some of these innovations. We demonstrate a process that combines directed self assembly with nanoimprint lithography to achieve rectangular patterns mounted on circular tracks. We will also discuss the impact of the nanofabrication process on the magnetic performance of the magnetic islands and the outlook for BPM technology. We will also explore additional innovations in nanofabrication techniques made possible by block copolymer lithography including opportunities in electroplating using self assembled masks.

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.

Head and Media Challenges for 3 Tb/in<inline-formula> <tex-math notation="TeX">\(^{\boldsymbol {2}}\) </tex-math></inline-formula> Microwave-Assisted Magnetic Recording

A specific design for microwave assisted magnetic recording (MAMR) at about 3 Tb/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> (0.47 Tb/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> or 4.7 Pb/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) is discussed in detail to highlight the challenges of MAMR and to contrast its requirements with conventional perpendicular magnetic recording (PMR). In particular, it has been determined that MAMR-optimized media should have: higher damping than today's PMR media upon which ferromagnetic resonance measurements are reported, very low intergranular exchange coupling, and somewhat stronger layer to layer exchange coupling. It was found that with exchange-coupled composite type media (i.e., graded anisotropy with controlled exchange), a spin torque oscillator in the write gap of a wider shielded pole cannot adequately define the written track at high track density. Adequate lateral field gradient, however, is achieved by modifying the pole tip geometry. Other details of this conceptual design are also discussed.

L10 FePt-based thin films for future perpendicular magnetic recording media

Current magnetic recording media using perpendicular CoCrPt-Oxide granular films are reaching their physical limit (approx 750Gbit/in2 density) due to thermal fluctuations that hinder a further reduction of grain size (<6–7nm) needed to scale down the bit size. L10-FePt alloy is currently considered the most promising candidate for future recording media with areal densities above 1Tbit/in2 thanks to its high magneto-crystalline anisotropy (K=6–10MJ/m3), which enables it to be thermally stable even at grain sizes down to 3nm. However, its huge anisotropy implies an increase of the switching field, which cannot be afforded by current available write heads. To simultaneously address the writability and thermal stability requirements, exchange coupled composite media, combining two or multiphase hard and soft materials, where the hard phase provides thermal stability and the soft phase reduces the switching field, have been recently proposed. This paper briefly reviews the fundamental aspects as well as both experimental approaches and magnetic properties of L10 FePt-based single phase films and exchange coupled systems for future perpendicular magnetic recording media.

Investigation of the Local Temperature Increase for Heat Assisted Magnetic Recording (HAMR)

Heat assisted magnetic recording (HAMR) has been proposed for the next generation hard disk drive due to its potentially higher storage areal density. In HAMR systems, a laser is focused onto the disk to heat the magnetic storage layer to the Curie temperature. In this paper, a novel method is proposed to study the temperature increase in a perpendicular magnetic recording media. The disk was prerecorded with a uniform single tone magnetic pattern by flying a magnetic head above it and using a magnetic read/write device. The decay of the magnetic pattern resulting from laser heating under different laser power levels was evaluated quantitatively using a magnetic force microscopy. The temperature increase was estimated by use of an Arrhenius-Neel model of thermally activated magnetization. In addition, a 3-D numerical model was created to obtain the disk's thermal response under the laser heating. The thermally erased area was calculated from the temperature distribution based on the numerical results. The calculated erased area was compared with the experimental results from the magnetic images. A good agreement was observed between the experimental and the simulation results. The method should be reliable when used with HAMR media also whenever it becomes available.

Quantitative relationship between contact stress and magnetic signal strength in perpendicular recording media

A series of nanoscratch experiments is conducted using constant loading scratch profiles to apply mechanical contact stress on perpendicular magnetic recording (PMR) media to cause its magnetic signal strength decay, which is characterized by the magnetic force microscope. The dependence of the magnetic signal strength on the applied normal load is quantitatively investigated. The results indicate that an increase of the applied normal load leads to a decrease of the magnetic signal strength. In addition, in order to obtain a more complete understanding of the results, a 3D finite element model is created to calculate the stress under different normal loads. Finally, the quantitative relationship between residual shear stress and magnetic signal strength is identified.

Equiatomic CoPt thin films with extremely high coercivity

In this paper, magnetic and structural properties of near-equiatomic CoPt thin films, which exhibited a high coercivity in the film-normal direction—suitable for perpendicular magnetic recording media applications—are reported. The films exhibited a larger coercivity of about 6.5 kOe at 8 nm. The coercivity showed a monotonous decrease as the film thickness was increased. The transmission electron microscopy images indicated that the as fabricated CoPt film generally consists of a stack of magnetically hard hexagonal-close-packed phase, followed by stacking faults and face-centred-cubic phase. The thickness dependent magnetic properties are explained on the basis of exchange-coupled composite media. Epitaxial growth on Ru layers is a possible factor leading to the unusual observation of magnetically hard hcp-phase at high concentrations of Pt.

A HAMR Media Technology Roadmap to an Areal Density of 4 Tb/in$^2$

This paper discusses heat-assisted magnetic recording (HAMR) media requirements and challenges for areal densities (AD) beyond 1 Tb/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . Based on recent roadmap discussions the focus is primarily on granular chemically ordered L10 FePtX-Y-perpendicular media with reduced average grain size down to 〈D〉 = 3-5 nm relative to current CoCrPt based perpendicular magnetic recording (PMR) media with average grain size 〈D〉 = 7-9 nm. In HAMR media the combination of thermal conductivity and Curie temperature TC determines the required laser power during recording. Key challenges are sigma variations of D and TC which need to be reduced to σD/D ~ 10-15% and σTC/TC ~ 2%. In addition AD is limited by switching field distribution (SFD) and thermal spot size. The key goal going forward is to optimize heads, media, head-media-spacing (HMS) and read-back channel technologies to extend AD to 4 Tb/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and beyond.

Readout method from antiferromagnetically coupled perpendicular magnetic recording media using ferromagnetic resonance

We fabricate perpendicular magnetic recording media comprising two antiferromagnetically coupled Co/Pt multilayers and investigate its magnetic properties by ferromagnetic resonance (FMR) measurement. In such media, the magnetizations of the two perpendicular magnets are designed to compensate each other in the remanent state in order to reduce the dipolar field, which is a limiting factor in high-density magnetic recording devices. We measure FMR absorption spectra of the media and estimate the magnetic anisotropy and interlayer exchange coupling. We also demonstrate that FMR measurement can be employed to read out the magnetization direction. The principles behind this readout method are different from those behind the conventional method of detecting the stray field from the media by means of a magnetoresistive sensor; therefore, the proposed readout method is applicable to magnetic recording media having zero remanent magnetization. We expand this readout scheme to three-dimensional magnetic recording with several vertically stacked recording layers. By providing each recording layer with a different FMR frequency, we experimentally confirm that layer selective readout is possible.

Co–Pt–Cr–CoSi–CoO Sintered Target for Low Ar-gas-pressure Deposition of ${\hbox{CoPtCr-SiO}}_{2}$ Granular Film with Stoichiometric $\hbox{SiO}_{2}$ Phase

A new type of composite target is proposed for low Ar-gas-pressure (PAr) deposition of CoPtCr-SiO2 granular perpendicular magnetic recording media. The proposed target is a Co-Pt-Cr-CoSi-enriched CoO sintered target, consisting of CoSi and CoO powders instead of SiO2 powder. Systematic analysis of film composition, microstructure, and magnetic properties of the granular films revealed that 1) the Co74Pt16Cr10-8 mol%SiO2 granular film with O/Si ratio of 2 in the film is realized by using a CoO/CoSi ratio of 3 in the target for our sputtering system, and 2) media with S=1, Hc=5.0 kOe, and α = 2.9 was realized even under a low PAr of 0.6 Pa. This means that just the oxidization of Si into SiO2 is important for magnetic decoupling.

The effect of high deposition energy of carbon overcoats on perpendicular magnetic recording media

High-energy carbon deposition techniques provide thin overcoats with high corrosion and wear protection for magnetic recording media applications. The effect of high-energy (0–300 V substrate bias) deposition on the implantation induced changes in magnetic and structural properties of granular perpendicular magnetic recording media is studied. To observe subtle changes in a thin region of recording media, antiferromagnetically coupled layer structure was used. Clear changes in the magnetic properties, observed as a function of the carbon deposition energy, correlate with other measurements such as X-ray photoelectron spectroscopy, indicating the need to consider such effects when designing media and overcoat.

Magnetic design evolution in perpendicular magnetic recording media as revealed by resonant small angle x-ray scattering

We analyze the magnetic design for different generations of perpendicular magnetic recording (PMR) media using resonant soft x-ray small angle x-ray scattering. This technique allows us to simultaneously extract in a single experiment the key structural and magnetic parameters, i.e., lateral structural grain and magnetic cluster sizes as well as their distributions. We find that earlier PMR media generations relied on an initial reduction in the magnetic cluster size down to the grain level of the high anisotropy granular base layer, while very recent media designs introduce more exchange decoupling also within the softer laterally continuous cap layer. We highlight that this recent development allows optimizing magnetic cluster size and magnetic cluster size distribution within the composite media system for maximum achievable area density, while keeping the structural grain size roughly constant.

Effects of Substrate Bias With Recording Layer on the Magnetic Properties and Microstructure of Perpendicular Magnetic Recording Media

A direct current (dc) bias was added on the substrate during the sputtering deposition of the recording layer of perpendicular magnetic recording (PMR) media. It has been found that the bias has significant effects on the magnetic properties, such as coercivity, nucleation field, coercivity squareness, domain size, and microstructure, such as grain size, grain segregation. It has been also observed that the surface roughness of the media is reduced by using the bias. The mechanism of the sputtering deposition with dc bias is discussed.

Development of Thin Film Technology for High-Density Magnetic Recording Media

The paper briefly reviews the development of thin film technology of longitudinal and perpendicular magnetic recording media. Based on experimental results on current high-end perpendicular media, future possibilities are discussed. Limitations associated with granular-type media are reduced by introducing a new-type media, bit patterned media (BPM) with which the thermal stability, the resolution, and the signal to noise ratio of recorded bit information can be more easily maintained. The shift from granular media to BPM will take place at any areal density ranging between 1 - 6 Tb/in2. When a magnetic material with very high Ku is applied to BPM, an areal density of more than 60 Tb/in2 seems feasible.

High resolution switching magnetization magnetic force microscopy

We introduce switching magnetization magnetic force microscopy based on two-pass scanning atomic force microscopy with reversed tip magnetization between the scans. Within this approach the sum of the scanned data with reversed tip magnetization depicts local van der Waals forces, while their differences map the local magnetic forces. Here we implement this method by fabricating low-momentum magnetic probes that exhibit magnetic single domain state, which can be easily reversed in low external field during the scanning. Measurements on high-density parallel and perpendicular magnetic recording media show enhanced spatial resolution of magnetization.