Improvement of recording performances through exchange coupling in triple-layered PMR media

Improved recording performances with exchange coupling between bottom magnetic layer and medium magnetic layer in triple-type perpendicular magnetic recording (PMR) media are presented. Fundamentally, triple-type PMR media with exchange coupling between magnetic layers reduces DC erase noise and improves writability. Furthermore, triple-type PMR media could be expected clear magnetic cluster as well as relatively sharp transition pattern. Consequently, better spectral integral signal-to-noise ratio (SpiSNR), improved OW and about 1.5–2.0 order better bit error rate (BER) performances in triple-type PMR media with exchange coupling between magnetic layers were attributed to the reduction of DC erase noise and sharp transition as well as improved writability. In addition, work of micro-magnetic modeling on SNR profiles could help to verify the understanding on the role of exchange coupling between magnetic layers in triple-type PMR media.     

Recording media research for future hard disk drives

Perpendicular magnetic recording (PMR) technology has recently been introduced in hard disk drives (HDDs) by almost all the HDD manufacturers. It is expected that because of PMR technology, the superparamagnetic limit can be circumvented until around 2012. At this stage, it is necessary to seek out alternatives to extend the current PMR technology, or to invent new alternatives to move beyond the PMR technology. There are several research opportunities becoming available, as we try to extend the competitive advantage of magnetic recording. This article is an attempt to provide a brief review of the research that has been carried out, interspersed with an analysis of possible new directions.     

Novel approaches to high-density perpendicular recording media

Two different approaches to reduce the intermediate layer (IL) thickness in perpendicular recording media are proposed. Such a reduction in IL thickness could lead to improvement in writability and recording performance. The first approach involved the introduction of a magnetic intermediate layer (MIL), to obtain C-axis growth. Media with CoCr alloy layer as the MIL were studied. Reasonably good C-axis growth with a Δθ₅₀ of about 3.8° could be obtained for MIL thickness of about 10 nm. Noise could be controlled by introducing exchange-breaking layers. The other approach involved the use of crystalline soft underlayers (SUL) to obtain suitable growth conditions for the recording layers. For this purpose, CoFeTaCr alloys with a FCC(1 1 1) texture were prepared. A good C-axis dispersion in the recording layer with a Δθ₅₀ of about 3.1° could be obtained for IL thickness of about 5 nm. The present study indicates that the recording medium deposited on crystalline SUL is relatively noisier than that deposited on amorphous SUL. Reducing the noise of the crystalline SUL is a way towards higher areal densities.     

A high-performance, flexible substrate for thin-film media

Advances in magnetic recording tape and flexible disk systems are limited to a large extent by the properties of the substrate, particularly for the case of evaporated or sputtered thin-film media. In this paper we report on the magnetic properties of thin-film media deposited onto a new high-performance polymeric film with improved tensile strength, which can withstand temperatures as high as 600°C without any adverse effects. Good magnetic properties were obtained by oblique incidence evaporation of cobalt films onto the heated substrates.     

Substrates for flexible magnetic recording media: The role of base films for modern performance requirements

The trend in magnetic recording media is towards higher frequencies and larger storage capacities. Base film technology has developed in a manner analogous to corresponding demands on particulate and thin-film media, i.e. in the direction to reduced thickness, smoother surfaces, and very high uniformity. Key elements for the success of polyester films as substrates for all kinds of flexible media are new concepts for pigmentation and surface design. Future digital video recording systems and thin-film media will require new substrates with higher mechanical strength and thermal stability. Trends in base film development including dual-surface films and alternative polymer substrates are discussed.     

Micro magnetic structure of magnetic clusters in CoCrTaPtB recording media

Micro magnetic structure in CoCrTaPtB recording media with H_c>3 kOe was studied by magnetic force microscope and LLG simulation. Two types of the characteristic magnetic structure were observed. One is the convergent-type magnetic structure, which is observed as localized magnetic cluster on bit. The other is the vortex structure, which is responsible for the recording bit peroration. These micro magnetic structures cause the medium noise in high-density recording and must be suppressed by adjusting coercive force H_c and inter-granular interaction to explore high density recording media.     

Magnetic properties of soft layer/FePt--MgO exchange coupled composite perpendicular recording media

The magnetic properties of exchange coupled composite （ECC） media that are composed of perpendicular magnetic recording media FePt MgO and two kinds of soft layers have been studied by using an x-ray diffractometer, a polar Kerr magneto-optical system （PMOKE） and a vibrating sample magnetometer （VSM）. The results show that ECC media can reduce the coercivities of perpendicular magnetic recording media FePt-MgO. The ECC media with granular-type soft layers have weaker exchange couplings between magnetic grains and the magnetization process, for ECC media of this kind mainly follow the Stoner Wohlfarth model.     

L1_0-FePt合金单层磁性薄膜的微磁学模拟

具有四方结构的L10-FePt合金因其具有高磁晶各向异性和良好的化学稳定性而成为超高密度薄膜磁记录介质的最佳选择.对实验制备得到的磁性能良好的垂直取向L10-FePt合金单层膜进行了微磁学分析.在传统微磁学模型的基础上,根据晶体的对称性,引入了四角磁晶各向异性能密度的唯象表达形式;又依据薄膜生长过程中晶格对称性的破坏,考虑了薄膜面内的应力,并引入了磁弹性能.以四角磁晶各向异性能和磁弹性能为重点,对L10-FePt合金单层膜的磁滞回线进行了详细的分析,并且用微磁学方法确定了薄膜面内应力的大小.     

Heat Assisted Magnetic Recording with Matching Media and Recording Head Field

The dynamic performance of heat assisted magnetic recording （HAMR） on different media is investigated. Signal and signal-to-noise ratio enhancement are achieved in high coereivity perpendicular media with the aid of laser heating. Linear recording density is increased while saturation write current is lowered. Trailing field partial erasure is observed in lower coercivity media with a ring head, which causes signal reduction with increasing write current or application of a laser. Precautions should be taken against partial erasure in overall recording system optimization of HAMR in order to achieve ultrahigh recording density.     

Narrow-track perpendicular write heads

Narrow-track perpendicular write heads are reviewed. Because of the strong magnetic interaction between the write head and double-layered medium in perpendicular recording, various types of media are also considered. Current technology is discussed to illustrate design issues; then, for areal densities beyond 1 Tb/in², future technological requirements, including single-pole-type (SPT) heads for discrete track and bit-patterned media, are examined based on numerical simulations.     

交换弹性磁性材料的研究进展

对近年来交换弹性磁性材料（包括高性能永磁和磁存储介质）等方面的最新研究进展作了一些简单的综述，总结了存在的一些问题并指出了用以解决这些问题的可能途径，为进一步的深入研究提供参考。     

Fabrication challenges for patterned recording media

Lithographically patterned recording media are one of the approaches to achieving Tb/in² and beyond recording densities. This will require fabrication of sub-10 nm discrete magnetic islands covering a full disk with tight spacing and size distributions and a narrow switching field distribution. To become an economically successful technology, this will need to be achieved with high throughput and low cost. The technology to fabricate such patterned media will need to be developed, and may require innovative solutions such as self-assembly and nanoimprinting, along with improved magnetic thin films for achieving high anisotropy and narrow switching field distributions.     

Micromagnetic analysis of transition noise for high-density perpendicular recording

In this paper, specific issues related to high-density perpendicular magnetic recording processes, such as transition noise properties and cross-track correlation lengths, were investigated with the help of micromagnetic analysis. The effects of media parameters were taken into consideration, including intergranular exchange coupling, and exchange distribution, irregular grain shapes, magnetization saturation distribution, and anisotropy distribution. The micromagnetic simulation results showed that the effect of anisotropy distribution on transition noise is more significant than magnetization saturation distribution, and it is crucial to reduce this effect to achieve a high signal-to-noise ratio. Additionally, a new method to further estimate the partial erasure threshold was proposed to approximate the partial erasure effects, and the relation between the microtrack jitter and total track jitter was investigated.     

Observation of high density recording states using magnetic fine particles made by sputtering method

High density recording patterns were visualized by sputter-deposition of magnetic fine particles on recording media. Very clear patterns were formed when the magnetic particles were deposited under a weak magnetic field onto the sample surface covered with a thin oil layer. By using this process, the recorded patterns at a density of 100 k FCI (bit length 0.25 μm) were very clearly observed by a scanning electron microscope. This method gives very effective measures to investigate the recording states of magnetic and magneto-optical media.     

Ultrahigh-density storage media prepared by artificially assisted self-assembling methods

Two types of recording media possessing nanodot structures were investigated. The media were prepared by an artificially assisted self-assembling (AASA) method, which includes simple nanopatterning using a nanoimprint and fine nanopatterning using self-assembling organic molecules. One type of recording media is circumferential magnetic patterned media prepared on a 2.5-in.-diam glass plate. A Ni master disk possessing spiral patterns with 60-250 nm width lands and a 400 nm width groove was pressed to a resist film on a CoCrPt film to transfer the spiral patterns. A diblock copolymer solution was cast into the obtained grooves, and then annealed to prepare self-assembling dot structures aligned along the grooves. According to the dot patterns, the lower magnetic films were patterned by ion milling to yield patterned media with 40 nm diameter. We have also prepared FePt dot media with high magnetic anisotropy for near-field and magnetic-field hybrid recording aiming at more than 1 Tbin.2 density. A Ni stamp disk with aligned dot structures was also prepared by the AASA method to produce patterned media at the lowest cost. The other type of media was organic patterned media for X-Y type near-field optical storage. Bulky dye molecules were evaporated in vacuum to produce self-assembling amorphous nanodots. The dots were arranged by the AASA method, i.e., according to the polymethylmethacrylate film hole arrays or chemically patterned surface.     

Extensions of perpendicular recording

Gains in storage density in magnetic recording have fundamentally been achieved by scaling—reduced geometrical dimensions under the assumption that the recording physics does not change if all dimensions are scaled appropriately. It is becoming clear that evidence of the breakdown of scaling is now seen. We will here discuss ways to break the constraints on magnetic recording set by scaling. In particular, we will discuss energy-assisted recording, domain-wall-assisted recording, and bit-patterned media, with some emphasis on recording system considerations.     

Optical and Thermal Simulator for Laser-assisted Magnetic Recording

“Laser-assisted magnetic recording”, in which a recording media is heated by a laser beam while writing data, is attracting attention as a technology that enables a recording density of 1 Tb/in.². There exists another technology for media in which the recording layer is constructed with many small projections that enable high magnetic coercivity. This is called “patterned media”. For developing hard disk drives using these methods, we developed a simulator that analyzes the optical intensity distribution from the optical head for laser-assisted recording and the temperature profile on the patterned media. The simulator calculates the optical model using the finite-difference time-domain (FDTD) method. The thermal analysis of the three-dimensional model allows fast calculations using the alternating direction implicit (ADI) method. The heat source distribution data for thermal analysis is calculated in order to use the results of optical analysis. The optical and thermal analyses of the laser-assisted recording model were investigated with the simulator.     

Designing magnetics of capped perpendicular media with minor-loop analysis

A novel method of analyzing the switching field distribution (SFD) and magnetic correlation length of perpendicular magnetic recording media that uses major and minor magnetization loops is proposed. By applying the analysis to a series of capped perpendicular media, we found that a thick capping layer with a low saturation magnetization effectively reduced SFD without rapidly increasing the magnetic correlation length. Transmission electron microscope observation suggests that the SFD is narrowed by the increased uniformity of intergranular exchange coupling via the thick capping layer. Evaluations of recording characteristics demonstrated a close correlation between narrower SFDs and improved recording performance. Reducing exchange coupling dispersion is a clear solution for improving the performance of recording media.     

Research and development of metal powder for magnetic recording

Since its first application within the compact cassette in 1978, magnetic recording media using metal powder have been improved at a rate of roughly 1 dB per year as a result of advances in such fields as tape materials, tape-making technologies, etc. Today, metal tapes have a widely expanded application area, including video and data-information applications. Dowa Mining Co., Ltd., began its research regarding magnetic powder for recording media in 1978. The Company successfully developed metal powder for 8 mm video tape. Since then, Dowa has endeavored to improve the magnetic properties and reduce the particle size of metal powder. It accomplishes these goals through the full use of the Company's unique Al doping method. Especially, during the past several years, significant improvements of the magnetic properties of metal powder have been achieved. These improvements have resulted from the introduction of new technologies, including Fe–Co alloying, sintering prevention, new reaction processes, and many other new techniques. Currently, Dowa Mining is supplying a new type of metal powder for the most technologically advanced high-density recording media. Dowa's new metal powder has an axis length of 0.1 μm, H_c of 2400 Oe, and σ_s of 155 emu/g.     

Bit-array alignment effect of perpendicular SOMA media

One effective way to overcome the superparamagnetic limit of magnetic recording system is to reduce the grain number per bit at given signal-to-noise ratio (SNR) level by using uniformed media grains. The self organized magnetic array (SOMA) is designed to have uniform grains with perfect grain array structure. It is believed that high enough SNR with small number of grains per bit can be acheived. But in the engineering application, the recorded bit on SOMA media may align with the regular array at different locations and angles due to non-grain synchronized writing, skew angle, and circular track. This induces the bit-array alignment effect and degrades system performance of SOMA media. In this paper, the micromagnetic simulation results show that the bit array alignment effect causes large level SNR fluctuation on the same media. The SOMA media is not preferred to be used in the conventional recording configuration. It is only suitable for the configuration of patterned media.