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Handbook of Magnetic Materials
North-Holland
Copyright © 2011 Elsevier B.V.
All right reserved. ISBN: 978-0-444-53781-2
Chapter One
Magnetic Recording Heads J. Heidmann and A.M. Taratorin
Contents
1. Part I: Overview of Magnetic Recording System 1 1.1. Evolution of magnetic recording systems 1 1.2. Magnetic media 3 1.3. Recording density, disk drive and channel electronics 5 1.4. Read/write process 8 1.5. Read head considerations 13 2. Part II: Magnetic Write Heads 14 2.1. Perpendicular recording heads 14 2.2. Energy-assisted write heads 29 2.3. Dynamic properties 37 2.4. Characterization of write heads 45 3. Part III: Magnetic Read Sensors 53 3.1. Overview 53 3.2. The AMR effect 54 3.3. The GMR effect 66 3.4. Tunnel junctions 76 3.5. Noise in magnetic sensors 82 3.6. Sensor characterization 91 References 98
1. Part I: Overview of Magnetic Recording System
1.1. Evolution of magnetic recording systems
The areal density (AD) of magnetic recording systems has dramatically increased over the last several decades. The area used to store 1 bit of information in 1970 stores more than 5 million bits in 2010. The historical evolution of AD is depicted in Fig. 1.1 and was driven by several factors: shrinking recording head dimensions, increasing sensitivity of read sensors, developing low noise, high resolution recording media, advanced signal detection channels and decreasing head-medium spacing. Development of nanometer-scale recording head geometries and highly sensitive magnetoresistive (MR) read sensors had a major impact on recording density evolution.
The recording head is an essential element of a magnetic recording system, performing write and read functions. A very simplified structure of a perpendicular recording head is shown in Fig. 1.2. The recording head is an integrated structure, built into the body of the head slider flying over recording media at small separation (typically several nanometres). The media recording layer is closest to the head surface. Data recording is performed during write operation. Write current of alternating polarity is supplied to the write coil. The flux generated in the soft magnetic write head yoke is concentrated in the main pole (or recording pole), generating a high magnetic field (1.2–2 T) in the small area adjacent to the recording medium (typical pole dimensions are 100 × 200 nm). The perpendicular magnetic field creates an area of perpendicular magnetization (transitions) in the hard recording layer. A soft underlayer (SUL) provides a write flux closure path to the return pole of the write head. The read head is adjacent to the write head structure, and it consists of a sensitive multilayer read element [usually tunnelling magnetoresistive (TMR) or giantmagnetoresistive (GMR) sensor] placed between soft magnetic shields. During head read operation, the read element is exposed to the local media magnetic field of the transition. The changes of sensor resistance are transformed into playback voltage by read channel electronics; this results in data detection.
In order to understand the basic principles and requirements for recording head operation we shortly review magnetic recording principles.
1.2. Magnetic media
The recording of magnetic information on the medium is based on specific magnetic grain properties. These magnetic particles have a preferred direction of magnetization (easy axis) in the absence of the external magnetic field. Modern recording media are based on perpendicular magnetic recording, that is the easy axis is oriented perpendicular to the disk surface. This is different from older longitudinal recording media which had the grain easy axis orientation in the disk plane (Guarisco et al., 2006; Richter, 2007a; Wang and Taratorin, 1999).
When a strong magnetic field is applied in the direction, opposite to the easy axis, the direction of grain magnetization changes (grain switching) and remains opposite to the initial direction after the field is removed. The binary switching behaviour of individual grains holds for large media volumes and statistically results in magnetic hysteresis, or M–H loop of a hard magnetic material.
The hysteresis curve is a dependence of media magnetization M on the applied magnetic field H. The main property of the hard magnetic material is that after an external magnetic field, produced by a magnetic head, exceeds some critical value Hc, called the coercivity of the medium, the magnetization of the medium switches its direction. When the field is higher than the coercivity, the magnetization reaches its saturation level Ms. In terms of individual grains the saturation magnetization means that all grains are aligned. After the external field is removed, media magnetization returns to the level of remanence magnetization Mr, which is close to the saturation magnetization Ms. Some collective relaxation processes are taking place in the grain ensemble, with slight rotation of magnetization. Therefore, after an external field exceeding the coercivity of the medium has been applied, the medium magnetization indefinitely stays (or "freezes") at Mr.
Therefore, if the recording head field significantly exceeds the Hc value, it becomes possible to create regions of magnetic medium with a given magnetization. By changing the direction of the external magnetic field, regions of changing magnetization are created. Each magnetization direction corresponds to a bit of information (either 0 or 1, depending on the magnetization direction). Depending on the particular recording scheme, the changes of magnetization (magnetic transitions) or the magnetization direction are detected by the read sensor.
Among many requirements for magnetic media, the most basic one is the ability to retain the magnetization direction for a long time. Every media grain can be potentially flipped by a thermal fluctuation. The grain stability is determined by the product of grain anisotropy Ku and volume V, the stability factor is given by the ratio of the energy barrier to thermal energy, given by exp(-KuV/kT), where k is the Boltzmann constant and T is absolute temperature (Richter, 2007a; Taratorin et al., 2005). Therefore, thermal stability requires high anisotropy and sufficient volume of recording grains. These requirements are difficult to achieve due to a number of reasons. A high anisotropy medium has extremely high coercivity; however, the highest field generated by a recording head is limited by material properties and do not exceed 2.4 T (Shukh, 2004). Large volume grains result in high media noise and are not acceptable for high recording density applications. Careful tuning of media properties is therefore required for acceptable performance. New recording schemes are proposed in order to overcome these limitations: heat-assisted recording and microwave-assisted recording (Kryder et al., 2008; Seigler et al., 2008a; Zhu et al., 2008a). These schemes will be shortly described in this chapter and are described in more detail in later sections.
The basic requirements for a magnetic recording medium can be summarized as follows:
• High coercivity Hc (typically higher than 4000–5000 Oe).
This is required in order to make media stable and to accommodate sharp magnetic transitions in order to sustain high recording density.
• Sufficiently high media remanent magnetization and small thickness.
This is required in order to achieve a sufficient level of magnetic fields emanating from the disk and correspondingly, to maximize the read-back signal, while keeping media as close to the head as possible.
• Uniform, small, closely spaced and magnetically isolated magnetic grains in order to minimize media noise.
The grain size is, however, limited by the super-paramagnetic limit, that is small magnetic grains become unstable and may spontaneously change their magnetization due to thermal fluctuations.
The magnetic disk is a complicated multilayer structure. It consists of a substrate (typically glass or aluminium), underlayer. These numerous substrates and undercoats are required in order to get well-defined, uniform magnetic properties of the main magnetic recording layer (or several layers), typically having a thickness of 10–30 nm. The magnetic recording layer is covered by a carbon layer and a lubricant to provide a smooth surface. Multiple additional magnetic and non-magnetic layers are required for optimal disk performance. For example, advanced recording layer structures, such as exchange-spring structures, are composed of several layers with different magnetic moment and coercivity. This is done for optimized switching and sharpness of recorded magnetic transition. A perpendicular recording medium also has a soft magnetic underlayer (SUL) which provides a path for the magnetic flux closure during the recording process. A special interface layer is required to decouple recording and SUL layers and to provide optimal granular structure of the recording layer.
The typical schematic representation of modern perpendicular medium cross-sectional structure is shown in Fig. 1.3 (not to scale). Typical thicknesses of protective layer are 1–2 nm, recording layer 10–20 nm, SUL 40–100 nm. The SUL in most designs consists of two antiferromagnetically coupled (AFC) layers in order to reduce domain formation and associated noise. Note that recording layer might also have a complicated multilayer structure, such as "exchange-spring" consisting of several hard magnetic layers with different moment (Gao et al., 2008; Richter, 2007a).
1.3. Recording density, disk drive and channel electronics
The circular disk is spinning at high velocity [typical rotational speed of a modern disk drives is 3000–15,000 rotations per minute (RPM)], depending on the particular application. The recording head is flying over a particular disk region, writing and reading information from/to the circular tracks of data. The typical dimensions of magnetic bits in the down-track direction are in the range of 20 nm. This corresponds to linear densities of about 1000 kfci (1000 flux changes per inch). Typical cross-track (track-width) dimensions are in the order of 100 nm. As the quest for higher recording density continues, both down-track and cross-track dimensions are shrinking. Tracks are usually separated by a small additional distance (guard band) in order to avoid cross-track interference and adjacent track erasure. The AD of magnetic recording is the product of down-track and cross-track densities. Typical recording densities of high-end commercial disk drives are around 500 Gbit/in.2 and approaching 1 Tb (terabit)/in.2 (Guarisco et al., 2006; Klaassen et al., 2004a; Wang and Taratorin, 1999).
The magnetic track is split into sectors, so that the recording and reading of information are taking place in a given sector on a selected track. Special track and servo information is recorded in the disk area between the data sectors in order to be able to determine physical location on the disk surface.
The write and read head dimensions correspond to the typical track-widths (100–150 nm). The magnetic recording head is fabricated on a special air-bearing slider and is flying over the magnetic medium. Today's products have very low flying height, usually less than several nanometres. In order to achieve tight flying height control, a special heater element is incorporated in the head structure. Increased temperature results in thermal expansion (protrusion) of the head towards the disk surface. Typically, the heater power is adjusted in order to achieve a flying height of 2–3 nm over the disk surface. The head is attached to a suspension with proper electrical interconnects for write and read heads. The suspension is mounted on a rotary actuator, capable of moving the head assembly over the magnetic medium (Fig. 1.4).
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