Future High-density Recording Research Articles

Fabrication of Long-Range Ordered Aluminum Oxide and Fe/Au Multilayered Nanowires for 3-D Magnetic Memory

Large-scale long-range ordered anodic aluminum oxide and multilayered nanowires (NWs) are attractive to 3-D nanostructured material applications, such as high-density 3-D magnetic memory. This article demonstrates long-range ordered aluminum oxide made by simple and inexpensive double imprinting with line-patterned stamp and uniform iron-gold multilayered NWs fabricated by galvanostatic electrochemical deposition with a single electrolyte bath. These two structural features show potential for future high-density recording systems that require long-range ordered devices separated from each other by insulation to eliminate crosstalk.

Nanoscale sub-100 picosecond all-optical magnetization switching in GdFeCo microstructures.

Ultrafast magnetization reversal driven by femtosecond laser pulses has been shown to be a promising way to write information. Seeking to improve the recording density has raised intriguing fundamental questions about the feasibility of combining ultrafast temporal resolution with sub-wavelength spatial resolution for magnetic recording. Here we report on the experimental demonstration of nanoscale sub-100 ps all-optical magnetization switching, providing a path to sub-wavelength magnetic recording. Using computational methods, we reveal the feasibility of nanoscale magnetic switching even for an unfocused laser pulse. This effect is achieved by structuring the sample such that the laser pulse, via both refraction and interference, focuses onto a localized region of the structure, the position of which can be controlled by the structural design. Time-resolved photo-emission electron microscopy studies reveal that nanoscale magnetic switching employing such focusing can be pushed to the sub-100 ps regime.

Influence of magnetic material composition of Fe100−xBx coated tip on the spatial resolution of magnetic force microscopy

Magnetic force microscope (MFM) tips are prepared by coating Si tips of 3 nm radius with 20-nm-thick Fe100−xBx (x = 0, 8, 18, and 34 at. %) alloys. As the B composition increases, the wettability of Fe-B alloy material to Si surface increases and a sharper and smoother MFM tip is obtained. The remanent magnetization of Fe-B alloy material decreases with increasing the B composition, which degrades the MFM signal detection sensitivity. The MFM resolution is influenced by both the tip sharpness and the remanent magnetization of coated Fe-B alloy material. Resolutions of 10.2, 9.2, 7.3, and 9.3 nm are obtained with tips coated with Fe, Fe92B8, Fe82B18, and Fe66B34 materials, respectively. Magnetic bits of 14.9 nm in length of a perpendicular magnetic medium recorded at 1700 kfci are distinguishable in the MFM image observed by using an Fe82B18-alloy coated tip. Fe-B coated tips are suitable to observe the magnetization structures of future high-density recording media.

A new DC-free multimode code design with error correction capability for optical storage systems

DC-free run-length limited codes have been the cornerstone of all three generations of optical recording, CD, DVD and BD. Research into efficient coding methods is paramount for the upcoming fourth generation. A Multimode coding technique is an efficient coding method that has been reported in the literatures. Under multimode coding rules, a user word is translated into a plurality of possible candidate words, and among the candidate words the encoder selects the codeword with the least low-frequency spectral content. In our paper, we will present a new coding technique for constructing error correcting high-rate DC-free multimode code using a low-density parity-check (LDPC) code for future high-density optical recording. The scheme achieves reliable DC-suppression and obtains considerable bit error rate (BER) performance gain through the iterative decoding (or turbo equalization) with the concatenated detector.

FePt-Based Exchange-Coupled Composite Perpendicular Recording Media

In order to demonstrate the strong and proper exchange coupling between bottom magnetic layer and capping magnetic layer in FePt-based exchange-coupled composite (ECC) perpendicular recording media, we have investigated the thermal stability, writability, degree of exchange coupling, and ECC gain factor of FePt-based ECC media as a function of the thickness of the capping FePt layer. With increasing the thickness of capping FePt layer, both thermal stability and media writability increase simultaneously as a result of the change from strong exchange coupling to proper exchange coupling. The proper exchange-coupled FePt ECC is a promising candidate for future high-density perpendicular recording.

Mechanical Performance Study of Pattern Media-Based Head-Disk Systems

Bit-pattern media is a promising approach for further increasing the recording density of future magnetic disk drives. Also, future high density recording requires further reduction of the magnetic spacing between head-slider and disk-media. As a result, the chance of slider-disk contact will be increased and the stress caused by such contact becomes an important issue for future disk drives. This paper reports authors' efforts in understanding the stress distribution due to asperity micro contacts. An elastic-plastic contact analysis for the HDI, based on a finite element model, and two representative pattern media cases are investigated and presented in this paper. Simulation results for the contact pressure at asperity micro contacts and subsurface von Mises equivalent stress are presented as well.

Quantitative Observation of Magnetic Flux Distribution in New Magnetic Films for Future High Density Recording Media

Off-axis electron holography was used to observe and quantify the magnetic microstructure of a perpendicular magnetic anisotropic (PMA) recording media. Thin foils of PMA materials exhibit an interesting up and down domain configuration. These domains are found to be very stable and were observed at the same time with their stray field, closing magnetic flux in the vacuum. The magnetic moment can thus be determined locally in a volume as small as few tens of cubic nanometers().

Study of slider-defect interaction at ultralow flying height by dynamic flying height control

In this study, the interaction of the magnetic head slider and magnetic disk defects were studied as the function of the head flying height controlled by the dynamic flying height (DFH) control method. In the conventional glide test, the flying height is set as a function of the rotational velocity. However, in the glide test using the DFH control method, the flying height could be set as a function of DFH power on a constant rotational velocity. A magnetic disk with two types of defects—a crater and a 5nm bump—was prepared. The acoustic emission (AE) signal at the slider-defect contacts from the conventional glide test and the DFH glide test were then compared. The results of the experiment indicated that the DFH glide test could detect these defects at a few nanometers flying height, but the conventional glide test could not detect them. This was because the AE signal burst below a flying height of 3nm. The AE signal output of the DFH glide test was higher than that of the conventional glide test, and the AE signal of the defect contact increased at a greater rotational velocity. It was concluded that the DFH control glide testing was one of the most important techniques for the future high-density recording disks.

CPP-GMR technology for magnetic read heads of future high-density recording systems

This paper introduces CPP-GMR technology, its features, routes to output enhancement, problems to be solved and possibilities as a recording head. For instance, use of high spin-dependent bulk scattering, high resistivity, or half-metallic magnetic materials for free and reference magnetic layers were shown as ways to improve the output of CPP-GMR. A current state of CPP-GMR head development was also mentioned in view points of sensor downsizing, magnetic head noise and high-density recording demonstration. CPP-GMR still has some points to be improved, however it is believed that the CPP-GMR will actualize a next high-performance magnetic read head in no distant future.

Microstructure and magnetic properties of FePt/MgO multilayers deposited by RF magnetron sputtering

FePt (50 nm) and [FePt( a nm)/MgO( b nm)] 5/glass ( a=1, 2, 3; b=1, 2, 3) films were prepared by radio frequency (RF) magnetron sputtering technique, and then were annealed at 600°C for 30 min. The effect of MgO layer thickness on the structures and magnetic properties of the FePt/MgO multilayers was investigated. The coercivities and inter-grain interactions of the FePt/MgO films were decreased, yet the degree of (001) texturing drastically increased with the increase in MgO layer thickness when the FePt layer thickness was fixed. Thus, the FePt/MgO films with appropriate coercivities, high perpendicular anisotropy, and weak intergrain interactions were obtained by controlling the MgO layer thickness. Overall, these results indicate that the FePt/MgO nanostructured films are promising candidates for future high-density perpendicular recording media.

A MAP-Based Algorithm for Joint Estimation of Transition Jitter and Timing Error

To improve the robustness to timing errors in future high-density perpendicular recording channels where the dominant noise source is transition jitter, this paper proposes a scheme that jointly estimates transition jitter and timing error along with the data recovery. This estimation is based on the intuition that timing error is independent from transition jitter, while both can be quantized and characterized in an extended trellis together with the bit sequence. Simulation results demonstrate significant performance improvement using this scheme, where the cycle-slip dominant bit error rate (BER) is lowered to the same order of magnitude as the detection performance in the absence of timing errors

CPP-GMR films with a current-confined-path nano-oxide layer (CCP-NOL)

We investigated the film performance and nanostructure of current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) spin-valve film with a current-confined-path nano-oxide layer (CCP-NOL). By applying ion-assisted oxidation (IAO) for the CCP-NOL formation, we enhanced the MR ratio to 5.4% at a small RA value of 500 mΩ µm2 for conventional Co90Fe10 layers. Furthermore, the use of bcc-Fe50Co50 also increased the MR ratio to 8.2% at a small RA value of 580 mΩ µm2. A modified Valet–Fert model for the CCP-NOL showed that the MR enhancement by the IAO is due to the improvement in resistivity of the CCP, and that by Fe50Co50 is due to a larger spin-dependent interface scattering effect. Analysis by cross-sectional TEM and three-dimensional atom probe confirmed the formation of the CCP-NOL structure. A reliability test for test element devices showed almost no change even under acceleration stress. The CPP-GMR spin-valve film with the CCP-NOL is extendable to future high-density recording heads due to its potential for a higher MR ratio at a small value of RA.

Grain size refining and microstructure of FePt/MgO nanogranular thin films

A MgO (1∼8 nm) single layer was inserted into Fe/Pt multilayers to introduce controllable defects on the interfaces between the MgO and Fe/Pt multilayers. The insertion of the MgO layer effectively reduced the average grain size and the size of the magnetic domains. The out-of-plane coercivity of the FePt films decreased with increasing thickness of the MgO insert layer. Transmission electron microscopy observation revealed that the average grain size of FePt/MgO films is controlled to be around 9 nm and thus suitable for application in future high-density recording media.

Fabrication and Recording Study of All-Metal Dual-Spin-Valve CPP Read Heads

All-metallic giant magnetoresistive (GMR) sensors in a current-perpendicular-to-plane (CPP) geometry are attractive candidates for future high-density recording sensors due to their low intrinsic resistance. With increasing recording density the cross-sectional area of CPP sensors (such as TMR or GMR sensors) must also decrease, resulting in a rapid increase in sensor impedance unless the RA product of the sensor stack also decreases. Thus, as recording densities increase above 300 Gb/in2, and sensor sizes decrease below 50nm, low-resistance CPP-GMR sensors become of greater interest to minimize impedance, reduce noise and improve the bandwidth characteristics of the read head. Here we present results on the magnetotransport properties, fabrication, and performance of all-metal CPP-GMR shielded read heads with ultra-narrow physical widths between 40 and 60 nm.

Recording performance and system integration of perpendicular magnetic recording

Perpendicular recording has been actively developed for future high-density recording system. We studied the integration of the perpendicular recording system to the hard disk drive (HDD). Double layer perpendicular media and single-pole type perpendicular head with GMR reader were employed in 2.5″ HDD. As a result of the integration test, it was confirmed that perpendicular recording 2.5″ test HDDs functioned well at the capacity of 50GB/platter. Through the drive integration, the features of the perpendicular recording were thoroughly studied. The complementary features between perpendicular recording and longitudinal recording are also discussed.

New statistical method for characterization of structured recording media magnetization processes

In this paper one presents a method for statistical characterization of the magnetization processes in two-dimensional networks of single-domain ferromagnetic particles. These systems are similar with the structured materials that are seen as future high-density recording media. The method allows very interesting developments in the characterization of the stability of the magnetic states which is an important factor for a recording medium.

Electrical Performance and Reliability of Tunnel Magnetoresistance Heads for 100->tex<$hboxGb/in^2$>/tex<Application

Tunnel magnetoresistance (TuMR) heads are attractive candidates for future high-density recording. To achieve the high areal density, it is necessary for TuMR heads to get lower resistance and higher delta R/R film. A low resistance and high delta R/R tunneling junction film has been developed and used for this study. The resistance area product and delta R/R are 3 /spl Omega//spl middot//spl mu/m/sup 2/ and 18%, respectively. Reliability that includes lifetime was also studied. We have found TuMR heads can be a promising candidate for 100 Gb/in/sup 2/ application.

Nanopatterning of Magnetic Disks by Single‐Step Ar+ Ion Projection

Large‐area Ar+ projection has been used to generate planar magnetic nanostructures on a 1″‐format hard disk in a single step (see Figure). The recording pattern was transferred to a Co/Pt multilayer without resist processes or any other contact to the delicate media surface. It is conceivable that magnetic nanostructures can be replicated from a single stencil mask on large quantities of disks for future high‐density recording.

Coupled granular/continuous medium for thermally stable perpendicular magnetic recording

We studied coupled granular/continuous (CGC) perpendicular media consisting of a continuous multilayer structure and a granular layer. The addition of Co/Pt multilayers decreased the nucleation field from 200 to −1800 Oe and increased the squareness from 0.9 to 1.0. The moment decay at room temperature was significantly reduced from −4.8% to −0.05% per decade. At elevated temperatures, strong exchange coupling between a granular layer and a continuous layer is needed for thermal stability. The exchange-coupled continuous layer reduces thermal demagnetization as it effectively increases the grain size, tightens the grain distribution, and prevents the reversal of individual grains. Magnetic Force Microscope image showed a larger magnetic cluster size for the CGC structure. Compared to the CoCr 18Pt 12 medium, the CGC medium had 2.3 dB higher output. However, the noise for the CGC medium increased with the recording density, while the noise for the CoCr 18Pt 12 medium remained constant from 4 to 15 kfc/mm. Further optimization and noise reduction are still required for future high density recording.

Microstructure study of CoCrPt/Ti/NiAl perpendicular media

Perpendicular medium with an average grain size of 8.2 nm with 20 nm thickness has been produced by using a-CoCrPt/Ti/NiAl tri-layer structure. The signal-to-noise ratio improves with the addition of a NiAl seed layer. From the TEM image analysis, the introduction of the NiAl seed layer may enhance the separation between the CoCrPt grains. The ultra-small grain size will improve the performance of perpendicular media for future high-density recording applications.