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Read/Write Head Designs: Ferrite, Metal-In-Gap, And Thin-Film

Hard Drives 101: Magnetic Storage
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As disk drive technology has evolved, so has the design of the read/write head. The earliest heads were simple iron cores with coil windings (electromagnets). By today’s standards, the original head designs were enormous in physical size and operated at low recording densities. Over the years, head designs have evolved from the first simple ferrite core designs into the several types and technologies available today. This section discusses the various types of heads found in PC hard disk drives, including the applications and relative strengths and weaknesses of each.

Several types of heads have been used in hard disk drives over the years:

  • Ferrite
  • Metal-In-Gap (MIG)
  • Thin-film (TF)
  • Magneto-resistive (MR)
  • Giant magneto-resistive (GMR)
  • Perpendicular magnetic recording (PMR)


Note: By the end of 2005, hard drives based on PMR were being used in devices such as portable music players and laptop PCs. Desktop PC hard drives based on the technology became available in 2006.

PMR is covered in more detail at the end of this chapter.

Ferrite

Ferrite heads, the traditional type of magnetic-head design, evolved from the original IBM 30-30 Winchester drive. These heads have an iron-oxide core wrapped with electromagnetic coils. The drive produces a magnetic field by energizing the coils or passing a magnetic field near them. This gives the heads full read/write capability. Ferrite heads are larger and heavier than thin-film heads and therefore require a larger floating height to prevent contact with the disk while it is spinning.

Manufacturers have made many refinements to the original (monolithic) ferrite head design. One type of ferrite head, called a composite ferrite head, has a smaller ferrite core bonded with glass in a ceramic housing. This design permits a smaller head gap, which enables higher track densities. These heads are less susceptible to stray magnetic fields than the older monolithic design heads.

During the 1980s, composite ferrite heads were popular in many low-end drives, such as the Seagate ST-225. As density demands grew, the competing MIG and thin-film head designs came to be used in place of ferrite heads, which are virtually obsolete today. Ferrite heads can’t write to the higher coercivity media necessary for high-density disk designs and have poor frequency response with higher noise levels. The main advantage of ferrite heads is that they are the cheapest type available.

Metal-In-Gap

MIG heads are a specially enhanced version of the composite ferrite design. In MIG heads, a metal substance is applied to the head’s recording gap. Two versions of MIG heads are available: single sided and double sided. Single-sided MIG heads are designed with a layer of magnetic alloy placed along the trailing edge of the gap. Double-sided MIG designs apply the layer to both sides of the gap. The metal alloy is applied through a vacuum-deposition process called sputtering.

This magnetic alloy has twice the magnetization capability of raw ferrite and enables the head to write to the higher coercivity thin-film media needed at higher densities. MIG heads also produce a sharper gradient in the magnetic field for a better-defined magnetic pulse. Double-sided MIG heads offer even higher coercivity capability than the single-sided designs.

Because of these increases in capabilities through improved designs, MIG heads were for a time the most popular head design and were used in many hard disk drives in the late 1980s and early 1990s, and most recently in LS-120 (SuperDisk) drives.

Thin-Film

Thin-film heads are manufactured much the same way as a semiconductor chip—through a photolithographic process. This process creates many thousands of heads on a single circular wafer and produces a small, high-quality product.

TF heads have an extremely narrow and controlled head gap that is created by sputtering a hard aluminum material. Because this material completely encloses the gap, the area is well protected, minimizing the chance of damage from contact with the spinning disk. The core is a combination of iron and nickel alloy that has two to four times more magnetic power than a ferrite head core.

TF heads produce a sharply defined magnetic pulse that enables them to write at extremely high densities. Because they do not have a conventional coil, TF heads are more immune to variations in coil impedance. These small, lightweight heads can float at a much lower height than the ferrite and MIG heads; in some designs, the floating height is 2 micro-inches or less. Because the reduced height enables the heads to pick up and transmit a much stronger signal from the platters, the signal-to-noise ratio increases and improves accuracy. At the high track and linear densities of some drives, a standard ferrite head would not be capable of picking out the data signal from the background noise. Another advantage of TF heads is that their small size enables the platters to be stacked closer together, enabling more platters to fit into the same space.

Many of the drives in the 100 MB–2 GB range used TF heads, especially in the smaller form factors. TF heads displaced MIG heads as the most popular head design, but they have now themselves been ­displaced by newer magneto-resistive heads.

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  • 4 Hide
    SteelCity1981 , August 31, 2011 5:44 AM
    Wow very, very interesting article to say the least.
  • 8 Hide
    soccerdocks , August 31, 2011 6:30 AM
    "Density initially grew at a rate of about 25% per year (doubling every four years)"

    If density grows at 25% per year it would actually double in just barely over 3 years. At 4 years it would be 144% greater.
  • 1 Hide
    joytech22 , August 31, 2011 11:22 AM
    Quote:
    when passed over magnetic flux transitions.

    I somehow expected "Flux capacitors" instead.
  • -7 Hide
    johnners2981 , August 31, 2011 12:20 PM
    soccerdocks"Density initially grew at a rate of about 25% per year (doubling every four years)"If density grows at 25% per year it would actually double in just barely over 3 years. At 4 years it would be 144% greater.


    No you're wrong, how embarrassing :) . You're using compound interest. Quit trying to be a smartass
  • 4 Hide
    Device Unknown , August 31, 2011 12:53 PM
    johnners2981No you're wrong, how embarrassing . You're using compound interest. Quit trying to be a smartass


    I'm no math guy, in fact i suck at it, but I see his point, why wouldn't it be compound? and even at compound interest is 144 still accurate? please enplane
  • 9 Hide
    soo-nah-mee , August 31, 2011 3:05 PM
    I believe soccerdocks is right - example...
    Beginning value: 10
    After one year: 12.5
    After two years: 15.625
    After three years: 19.531 (Almost double)
    After four years: 24.41
  • -8 Hide
    johnners2981 , August 31, 2011 3:08 PM
    Device UnknownI'm no math guy, in fact i suck at it, but I see his point, why wouldn't it be compound? and even at compound interest is 144 still accurate? please enplane


    Please enplane??? Compound interest is used to calculate interest and not things like density.
    They were right in saying "doubling every four years" and he was trying to correct them when there was no need so showed him who's boss, oh yeah
  • -7 Hide
    johnners2981 , August 31, 2011 3:10 PM
    soo-nah-meeI believe soccerdocks is right - example...Beginning value: 10After one year: 12.5After two years: 15.625After three years: 19.531 (Almost double)After four years: 24.41


    His calculation is right not the application, why is he using compound interest to calculate the percentage increase in density? It doesn't make sense.
  • 6 Hide
    soo-nah-mee , August 31, 2011 3:13 PM
    johnners2981His calculation is right not the application, why is he using compound interest to calculate the percentage increase in density? It doesn't make sense.
    It's not compound "interest", but it is compounding. If you say something increases 25% each year, you can't just keep adding 25% of the original value! Silly.
  • 2 Hide
    striker410 , August 31, 2011 3:24 PM
    soo-nah-meeIt's not compound "interest", but it is compounding. If you say something increases 25% each year, you can't just keep adding 25% of the original value! Silly.

    I agree with the others on this one. Since it's adding 25% each year, it is compound. You are thinking of it from the wrong angle.
  • 2 Hide
    asymetriccircle , August 31, 2011 4:17 PM
    ok i dont know all this interest shenanigans, but that first diagram, shouldn't the magnetic field be going the other way given the direction of current they show? i was just thinking right hand rule.
  • -8 Hide
    johnners2981 , August 31, 2011 4:22 PM
    soo-nah-meeIt's not compound "interest", but it is compounding. If you say something increases 25% each year, you can't just keep adding 25% of the original value! Silly.


    How's it silly? If you're going compound annually then why not compound it monthly, daily, hourly then???
  • 7 Hide
    soo-nah-mee , August 31, 2011 4:47 PM
    johnners2981How's it silly? If you're going compound annually then why not compound it monthly, daily, hourly then???
    You HAVE to compound it at some interval, and since the author said 25% PER YEAR...
  • -6 Hide
    johnners2981 , August 31, 2011 4:55 PM
    soo-nah-meeYou HAVE to compound it at some interval, and since the author said 25% PER YEAR...


    You don't have to compound it, the author also says it doubles in 4 years which obviously involves NO COMPOUNDING
  • 8 Hide
    bobfrys , August 31, 2011 5:04 PM
    Either way the author made a mistake. If you say density increases 25% a year that means 25% larger THAN THE PREVIOUS YEAR.

    If they extrapolated that 25% number from doubling every 4 years then they did their math wrong. It all depends which number is the real number, and which number did they incorrectly pulled out of their ass.
  • 3 Hide
    jamie_1318 , August 31, 2011 5:36 PM
    You have to compound it with the same length as the growth rate is factored in. That's the only way the metrics make sense. It means that each year is 25% more storage than the last year.
  • -2 Hide
    PhoneyVirus , September 1, 2011 1:15 AM
    Getting this for Christmas from the girlfriend I have 18th edition witch was the best version for the FSB X48 last of its kind the good old days, the 19th Edition felt like there was to much information took out. I'm hoping 20th Packs the info about Core 5/7 and some AMD, I found older versions didn't talk about ADM much so again I hope its packed. Looking forward to reading the book this winter well after Christmas then Windows 7 inside out deluxe edition here I come.
  • 0 Hide
    Nnymrod , September 1, 2011 3:00 AM
    Hard drives are really cool, and while SSDs are definitely better in almost every way, and are most definitely the future... I still like hearing platters spin up :)  And they will be the best option for backup drives for awhile.
  • 2 Hide
    Haserath , September 1, 2011 7:36 AM
    asymetriccircleok i dont know all this interest shenanigans, but that first diagram, shouldn't the magnetic field be going the other way given the direction of current they show? i was just thinking right hand rule.

    Actually he is showing the electron flow with the arrows in the first diagram, but the current is the 'positive flow' which is the opposite of the electron flow.
  • 0 Hide
    fstrthnu , September 1, 2011 7:48 AM
    Well Tom's Hardware certainly did their homework here
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