ABSTRACT

This chapter reviews nanomagnetic recording, mainly on hard disk drive (HDD), with regard to head-and media-related technologies. For the next-generation HDD, we have to overcome the arising balance issue of superparamagnetic limit and writability for system design. The chapter discusses promising technologies of perpendicular exchange-coupled composite (ECC), heat-or microwave-assisted magnetic recording (MAMR), and patterned media, indicating a tendency of hybrid recording system with combination of several techniques for extremely high-density magnetic storage. 14.1 Overview of Magnetic Recording and Hard

Disk DriveMagnetic recording meets the requirement of high performance, low cost, and nonvolatile characteristics for information storage

and has been most widely used during the past several tens of years. It includes magnetic tape recording, magnetic floppy drive, magnetic HDD, and even magneto-optical (MO) recording. Although they have been partly replaced by the flash memory nowadays, the HDD is still one of the most common storage devices, especially for the vast capacity situation. The HDD is the main device for magnetic recording, usually used in our computers and data centers. More and more people are using electronic copies instead of papers. With the increasing development of network, backing up data on the Internet has become a reality. Large amounts of data in various applications with mass amount of information can be stored somewhere in data centers, which need a huge number of HDDs.Magnetic recording is non-volatile; because of this, digital information is stored in a stable manner with two distinctive states of magnetization at a localized site. The two states can be read by a magnetic sensor (read head) and altered by an inducer driven by a current (write head). The fundamental concept of magnetic recording in HDD uses an inductive magnetic head, which normally consists of coils wound around a magnetic soft pole, to write down the information on the medium (magnetically hard material). Figure 14.1 illustrates the fundamental principle of magnetic recording in the case of magnetization along the film plane. The inset enlarges a transition area between two opposite magnetizations with some magnetic grains.During the write process, a current passes through the coils, generating a magnetic field above the magnetic medium. We can change the write current directions and consequently reverse the magnetization directions of the bits on the medium, which represents “1” and “0” states. The data is read by measuring the stray magnetic field from the transitions. Finally, a signal-processing unit transforms the analog read-back signal into a stream of data bits.Figure 14.2 shows a hard disk drive, which consists of several parts: magnetic disks (or platters) with double-side recording media, a head for each disk surface locating at the edge of a slide on suspension (actuator arm), a mechanical servo system including spindle, actuator, etc., which ensure the high-speed rotation of the disks and moving of the heads, control chips, and interfaces.