EDGE EFFECTS AND SUBMICRON TRACKS IN MAGNETIC TAPE RECORDING Graduation Committee Prof. dr. ir. J. van Amerongen Univ. Twente (chairman) Prof. dr. J.C. Lodder Univ. Twente (promotor) Dr. ir. J.P.J. Groenland Univ. Twente (assistant promotor) Prof. dr. ir. P.P.L. Regtien Univ. Twente Dr. ir. L. Abelmann Univ. Twente Prof. dr. P.R. Bissell Univ. Central Lancashire, UK Prof. dr. J.-P. Nozi`eres Spintec, CEA/CNRS Grenoble, FR Prof. dr. S.B. Luitjens Philips Research Laboratories The research described in this thesis was funded by the Dutch Technology Foundation STW, project TTF.5041 “High-density recording in magnetic tape”. The work was performed in the Systems and Materials for Information storage group (SMI) of the MESA+ Institute for Nanotechnology, University of Twente. Printed by W¨ohrmann Print Service, Zutphen Copyright c 2005 by Adrian Hozoi ISBN 90-365-2166-1 EDGE EFFECTS AND SUBMICRON TRACKS IN MAGNETIC TAPE RECORDING DISSERTATION to obtain the doctor’s degree at the University of Twente, on the authority of the rector magnificus, prof. dr. W.H.M. Zijm, on account of the decision of the graduation committee, to be publicly defended on Thursday 17 March 2005 at 15.00 by Adrian Hozoi born on 18 February 1977 in Ia¸si,Romania This dissertation is approved by promotor: Prof. dr. J.C. Lodder assistant promotor: Dr. ir. J.P.J. Groenland Contents 1 Introduction 1 1.1 Brief Story of Magnetic Tape Storage . 1 1.2 Magnetic Tape Storage Nowadays . 5 1.2.1 A Multitude of Formats . 5 1.2.2 Areal Density Trends . 6 1.2.3 Technology Status . 10 1.2.4 Toward Higher Track Densities . 11 1.3 This Thesis . 13 2 High Density Magnetic Tape Recording 15 2.1 Digital Magnetic Recording . 15 2.2 Medium Magnetization . 17 2.3 Write Process . 18 2.3.1 Williams-Comstock Model . 18 2.3.2 Thick Tape Media . 22 2.4 Read Process . 24 2.4.1 Reciprocity Principle and Readback from Single Transition 24 2.4.2 Spectrum of Square Wave Recording . 26 2.5 Narrow Track Recording and Edge Effects . 29 2.5.1 Side Write and Erase . 29 2.5.2 Side Read . 31 2.6 Conclusion . 32 3 Magnetic Tapes and Heads 35 3.1 Tape Recording Heads . 35 3.1.1 Helical Scan Silicon Heads . 37 vi CONTENTS 3.1.2 MIG Ferrite Heads . 40 3.2 Magnetic Tapes for High Density Recording . 42 3.2.1 Particulate Tapes . 42 3.2.2 Thin-Film Tapes . 45 3.3 Conclusion . 50 4 Test Equipment for Tape Recording Experiments 51 4.1 Testers for Linear and Helical Scan Recording . 52 4.2 Considerations for High Precision Tape-Drum Testers . 53 4.2.1 Tape-Drum Construction . 55 4.2.2 Air Bearing Spindle . 58 4.2.3 Head Positioning Mechanism . 61 4.2.4 Read/Write Instrumentation . 67 4.3 Redesign and Optimization of an Existing Tester . 72 4.3.1 Construction of the Head Positioning Mechanism . 74 4.3.2 Development of the Read/Write Module . 76 4.3.3 General Measurement Conditions . 79 4.4 Conclusion . 80 5 Measurements and Interpretation of Track Profiles 81 5.1 Track Profiles versus Magnetic Imaging . 82 5.2 Full-Track and Microtrack Profiles . 84 5.2.1 Profiling Bases . 84 5.2.2 Track Profiles of HSS Heads . 86 5.3 Imbalanced Overwrite Method . 89 5.3.1 Overwrite Procedure . 89 5.3.2 Analysis of Side Write Asymmetry . 91 5.4 Conclusion . 99 6 Novel Interpretation Model of Triple-Track Profiles 101 6.1 Model Description . 102 6.2 Experimental Procedure . 104 6.3 Analysis of Triple-Track Profile Measurements . 105 6.3.1 Well Aligned HSS Head . 105 6.3.2 Ferrite MIG Head . 108 6.3.3 Discussion on Measured Erase Bands . 111 6.3.4 Accuracy of the Triple-Track Model . 111 6.4 Edge Effects of a 3.5 µm HSS Head . 113 6.5 Track Profiles and MFM Imaging . 118 6.5.1 Triple-Track and Erase Profiles . 119 6.5.2 MFM Study of Erase Bands . 123 CONTENTS vii 6.6 Conclusion . 127 7 Side Write Effects on Microtracks 129 7.1 Experimental Considerations . 129 7.2 Effect of Poles Alignment on Recording Spectra . 131 7.3 Side Write Dependence on the Trailing Pole . 135 7.4 Conclusion . 139 8 Conclusion 141 8.1 Edge Effects . 141 8.2 Test and Investigation Methods . 142 8.3 Recommendations . 143 Bibliography 145 Samenvatting 161 List of Publications 167 Thanks 169 Chapter 1 Introduction Since the early stages of computing, magnetic tape has been the medium of choice for information storage due to its high volumetric capacity and compet- itive pricing. Steady advances in drive, media and automation robotics helped it to remain the most cost-effective and reliable solution for data backup and archiving. Most advanced tape technologies offer nowadays capacities between 200 and 500 GB per cartridge, and multi-terabyte recording is already envis- aged. They will continue to evolve to meet the increasing demands for capacity, performance and functionality of data storage applications. 1.1 Brief Story of Magnetic Tape Storage The basis of magnetic recording dates back to 1878 when Oberlin Smith es- tablished the principle to record electrical signals from a telephone onto a steel wire. He described his idea in the journal Electrical World of Sep. 8, 1888. It was in 1898 that the Danish engineer Valdemar Poulsen built the first working magnetic recorder, called the Telegraphone. The device used an electromag- netic head to record telephone messages on a steel wire wrapped around a drum (Fig. 1.1). Over the next few years Poulsen produced improved sound recorders employing steel wire or steel tape. During the 1920s and 1930s, important ad- vances in magnetic recording were achieved in Germany with the introduction of electronic amplification and the transition to coated tapes. The steel wires and tapes were first replaced by paper tapes covered with iron powder. Then BASF developed the plastic tape coated with a magnetic oxide intended for an advanced recorder designed by AEG, called the Magnetophon. They were pre- sented to the public during the 1935 Radio Fair in Berlin. The Magnetophon became a machine of excellent quality with the advent of high frequency bias- 2 CHAPTER 1 Introduction Figure 1.1: Early version of Poulsen’s Telegraphone. ing. After 1947, first American tape recorders were developed based on German expertise and designs. Originally intended for government use, they quickly spread onto the commercial market. The data was recorded in parallel tracks from the beginning of the tape to the end, method also known as linear scan. However, its performances were insufficient for video recording, and Ampex in- troduced in 1956 a professional video recorder using rotary heads. The record- ing heads, mounted onto a cylinder rotating at high speed, were scanning tracks perpendicular to the tape travel direction. This implementation was favorable to increase the relative head to tape velocity and reach higher track density. Toshiba introduced a superior rotary head configuration in 1959, known as helical scan recording. In this case the tracks were recorded diagonally to the tape, at a relatively small angle to its travel direction. Due to high performance and capacity, rotary heads systems became the standard for professional video applications. The operating principles of linear and helical scan recording are schematically described in Fig. 1.2. In the late 40s IBM understood that punched cards would not be enough to solve the increasing need of data storage and processing, and was searching for alternative technologies. Magnetic tape was just coming into audio systems and offered great potential to become the primary storage medium for computers. Eckert and Mauchly were probably the firsts to use magnetic tape to record data with their Binary Automatic Computer (BINAC), completed in 1949. The unit used steel tape, and an improved version was integrated with the Universal Automatic Computer (UNIVAC), released in 1951. The first IBM magnetic tape storage system, the IBM 726, was announced in 1952 together with the IBM 701 Defense Calculator. It was a reel-to-reel drive using half-inch 1.1 BRIEF STORY OF MAGNETIC TAPE STORAGE 3 magnetic tape, with a plastic substrate coated with iron oxide. The diameter of the reel was 270 mm, and the data was recorded in 7 parallel tracks at 4 b/mm (100 b/in) linear density. The IBM 726 reached a capacity over 0.25 MB at a data rate of 6.6 kB/s, and became the industry standard. Throughout the 60s and the 70s more advanced media formulations became available and the open-reel systems evolved by mainly increasing the bit density and the tape speed. The Quarter-Inch Cartridge (QIC) was introduced in 1972 by 3M Corporation as a storage format for telecommunications and data acquisition applications. QIC data cartridges stored the tape onto two reels driven by a built-in belt, and were available with capacities of few megabytes. The QIC minicartridge appeared in 1975, and many QIC-based recording formats started to develop in the following years. Capacity gains were achieved by upgrading the media, increasing bit densities, and adding more parallel tracks. Due to its relative low cost, QIC became in the 80s a popular data storage system for entry-level applications. The latest IBM open-reels tape unit by early 80s, the IBM 3420, could store 180 MB of data in a 9-track configuration. In 1984 IBM introduced a completely new system, the 3480, featuring a closed cartridge including a single reel with half-inch CrO2 tape.