MR Heads: The Next Step in Capacity and Performance
Since its advent in 1955, the magnetic recording industry has constantly and dramatically increased the performance and capacity of hard disc drives to meet the computer industry's insatiable demand for more and better storage. Not so long ago, a 40 Mbyte disc drive was a big deal. Today, it's a doorstop - and a 1 Gbyte drive is standard for most desktop computers. Applications like multimedia, real-time video and audio, and graphical user interfaces, along with ever-increasing program sizes, are driving the need for ever-greater storage capacity.
To meet these needs, the magnetic recording industry had been increasing the areal density storage capacity (measured in megabits-per-square-inches - Mbits/in2) of hard drives at a historic rate of roughly 27% per year. In recent years, the growth rate itself has increased to as much as 60% per year with the result that today's disc drives store information in the 600-700 Mbits/in2 range. By the year 2000, the areal density requirements are expected to reach 10 Gbits/in2. Sustaining this growth rate into the next century requires progressive advances in all technologies used to make a hard disc drive. The following discussion will focus on the part of a disc drive responsible for recording information on the disc: the read-write head.
The read-write head technology that has sustained the hard disc drive industry to date is based on the inductive voltage produced when a permanent magnet (i.e. the disc) moves past a wire-wrapped magnetic core (i.e. the head). Early recording heads were fabricated by wrapping wire around a laminated iron core analogous to the horseshoe-shaped electromagnets found in elementary school physics classes. Market acceptance of hard drives, coupled with increasing areal density requirements, fueled a steady progression of inductive recording head advances. This progression culminated in advanced thin-film inductive read-write heads that are fabricated using semiconductor-style processors in volumes large enough (>500 million heads/year) to meet the insatiable demands of the computer industry for data storage. Even though advances in inductive read-write head technology have been able to keep pace with increasing areal density requirements, the ability to cost-effectively manufacture these heads is nearing its natural limit. Hence, a new recording head technology is needed to fuel the disc drive industry's continued growth in capacity and performance. This new technology is the magnetoresistive (MR) read head (MRH).
A detailed analysis of a recording system using an inductive head shows that one of its critical limitations is that the recording head must alternately perform conflicting tasks of writing data on the disc as well as retrieving previously-written data. MRH technology circumvents this problem by separating the write and read function into two physically distinct heads. An inductive head, optimized for writing information, is integrated with an MR structure optimized for reading. This fundamental change in read-write technology will enable advances capable of carrying the disc drive industry well into the 21st century.
The MR Head
An MR read head consists of a read element located in the space between two highly-permeable magnetic shields. The shields help to focus the magnetic energy from the disc and reject stray fields. Using the design of a celestial telescope as an analogy clarifies the use of this technology. With the telescope, the tube serves as an optical shield, blocking stray light from reaching the optics. Thus, the telescope only sees the light that is directly in line with the optics. The MR head acts identically, except with magnetic fields instead of light. The magnetic shields serve as the tubing while the read element act as the optical sensor picking up the applicable track information.
The MR element is made from a ferromagnetic alloy whose resistance changes as a function of an applied magnetic field. This phenomenon was discovered by Lord Kelvin in 1857 and today is called the anisotropic magnetoresistance (AMR) effect.
Information is stored on the disc in the form of small, permanently magnetized regions written by the inductive write head. This information is then retrieved when the magnetic field from these permanently-magnetized regions modulates the resistance of the MR sensor which, in turn, is detected as a voltage change by the electronics.
The response of the MR sensor to a magnetic field is defined as the device transfer curve. The shape of the transfer curve is clearly not linear. In order to obtain a faithful reproduction of an applied field, it is necessary to simultaneously bias the magnetization of the MR element as well as limiting the magnitude of the applied field. The optimal bias angle of the MR element for the most linear response is found to be 45 degrees.
The most common method used today for biasing an MR sensor - i.e. linearizing the triangular curve - is to use the soft-adjacent-layer (SAL) method. A typical SAL MR sensor is made by stacking three metal layers together, with each layer performing a very specific function when the sense current flows through the MR sensor. A magnetic field is then generated, which magnetizes the adjacent soft layer. This magnetized soft layer gives rise to a magnetic field which, in turn, biases the magnetization in the MR element so that the angle of the MR element is 45 degrees. The role of the spacer layer is to magnetically separate the MR element from the soft film. Ideally, since all three layers are electronically in parallel, the resistivity of the soft film and spacer should be much greater than that of the MR element. This is readily achieved in practice. Although there exists many other methods of linearizing (i.e. biasing) the MR response, the SAL method has gained acceptance because it is very simple to process and is extendible to very high areal density/read-write configurations.
Advanced MR Head Technology: A Well-Known Friend to Seagate
Seagate Technology, a leader in the disc drive industry, has had a very active MR head program since 1982. This program was initiated in anticipation of the time when inductive thin-film head technology would no longer be able to meet the high areal density demands of the disc drive industry. During those years, Seagate scientists made many MR head contributions. Two of those contributions turned out to be very important to the overall development of MR head technology. The first was the design of the industry-standard MR read-write head configuration. Every MR read head used today in disc drives incorporates this structure. In addition, many MR tape heads utilize the same design configuration. The second advancement, called Boundary Control Stabilization, was used to eliminate the primary noise source (called Barkhausen noise) common to all MR heads. These and other developments have contributed to a very advanced, high performance MR head program within Seagate.
Seagate has announced and begun shipment of 2.5-inch, 3.5-inch, and 5.25-inch disc drives utilizing SAL MR head technology. By late 1997, it is anticipated that nearly all of Seagate's drives will be utilizing MR heads. Present SAL MR head technology will provide growth in areal densities to at least 3 Gbits/in2. Beyond that point will require advanced technologies, such as spin-valve or giant magnetoresistive heads. These advances will provide great sensitivity (amplitude) to the smaller magnetic regions that result from increased track and linear bit density.