# How to Build a Computer 25: The Magnetoresistive Effect

Our story starts with Lord Kelvin, one of the great old school physicists. You can read about his career from anonymous’s old Saturday Night Science. Actually, at the point he enters this story I don’t think Kelvin had made lord yet; he was just some bloke named Thompson. This Thompson fellow was playing around with magnets and electricity and that sort of thing. What he discovered is that you can change the resistance of a wire with a magnetic field. And furthermore that that change in resistance depends on the angle between the wire and the magnetic field.

Let’s take that a little more slowly. Change in resistance when you’re in a magnetic field? Okay, I can buy that; there’s all this nonsense about wires and magnets and whatnot that I’ve been blathering about up until this point. Angle? The resistance in your wire will vary a great deal whether it’s parallel or antiparallel to the magnetic field on your disk. (Antiparallel means parallel, but facing the other direction. The northbound lane on a highway is antiparallel to the southbound lane.) If your wire is running current right-to-left and your magnetic field is pointed left-to-right then your wire’s resistance is at it’s highest because of your antiparallel configuration.

Modern hard drives will actually stack a column of these heads together to increase the data transfer rate. Here, this is what your write head looks like:

For write heads, I recommend Muenster cheese.

Actually, that’s not that different from the kielbasa configuration; you’ve got the coil and you’ve got the gap. This is what your read head looks like:

Smoked Colby. No joke here, I’m just enjoying cheese.

The wire going through the middle (and connecting the two pencil-wires coming in from the top) is where you get your magnetoresistance to measure. We don’t use straight magnetoresistance these days; you can get it a lot more sensitive if you take advantage of some quantum effects. In my college physics days I actually sat through a colloquium on the subject, but between the presenter’s thick Indian accent and my unfamiliarity with the subject I got almost nothing out of it. Except for the cookies.

You can get ‘giant’ magnetoresistance (that’s a technical term mind you) by layering materials. Start with an electrode. Then put down a permanent magnet facing one direction. Put a thin layer of non-magnetic material on top of that. Then put your platter surface where you’re going to be writing stuff on top of that. The magnetic layer below reinforces the magnetoresistance effect in the top layer (something to do with quantum spin coupling; I don’t understand it either.)

You can actually do one better; if you epitaxially (uh, I should have explained that word before now, and probably will in the future. No spoilers!) epitaxially add a layer of magnesium oxide in between magnetic layers you get a quantum tunneling interaction between your magnets. This really boosts the signal, from like +/- 5% with normal magnetoresistance to about +/- 110% with Tunneling Magnetoresistance (commonly abbreviated TMR). All modern hard disk drives use TMR.

Since TMR boosts your signal so much you can start with a smaller signal. That is, you can make your spots smaller, get a higher areal density, and store more gigabytes of cat pictures. As with the older heads, it’s still a good idea to boost your signal in other ways. Current hard drives float the heads about three nanometers above the surface of the platter, using a cushion of air to keep it from scraping. (Don’t get dust on your hard drive platter; bad things happen if you do.)

How small is it theoretically possible to make your spaces? A discrete unit of these consists of something like eighteen grains. If you make ’em much smaller than that the magnetism can randomly flip directions (you’d be small enough that quantum effects come into play), which obviously messes with your ability to accurately store and retrieve data.

We’ve covered the topic of reading and writing information to your disk, and we’ve covered it pretty thoroughly. I’ll be backing out a bit to focus on the mechanical aspects of an HDD, how you’re spinning the disk and whatnot. Join us next week for “A Closer Look at what’s Heaping on your Platter” or “Rust or Bust!”

This is part twenty-five of my ongoing series on building a computer, the New Model Army way. You may find previous parts under the tag How to Build a Computer. This week’s post has been brought to you by Ricochet Silent Radio! Join us next week for a thrilling story of future as we follow the unlikely career of Judge Mental: Lord Protector of the United States! Remember; three chimes mean good times on Ricochet Silent Radio!

1. Member
Arahant
@Arahant

Mmm, smoked Colby.

2. Member
Judge Mental
@JudgeMental

Hank Rhody, Acting on Emotion: (Antiparallel means parallel, but facing the other direction. The northbound lane on a highway is antiparallel to the southbound lane.)

Most of us call that ‘backwards’.

3. Contributor
Hank Rhody, Acting on Emotion
@HankRhody

Hank Rhody, Acting on Emotion: (Antiparallel means parallel, but facing the other direction. The northbound lane on a highway is antiparallel to the southbound lane.)

Most of us call that ‘backwards’.

You’ll never get the sweet, sweet grant money with that kind of talk.

4. Member
Nanda Panjandrum
@

WooHoo, @hankrhody! You’ve got a sponsor…Is this a series of educational podcasts for R>SRN?  Way cool!

5. Member
Judge Mental
@JudgeMental

Hank Rhody, Acting on Emotion: (Antiparallel means parallel, but facing the other direction. The northbound lane on a highway is antiparallel to the southbound lane.)

Most of us call that ‘backwards’.

You’ll never get the sweet, sweet grant money with that kind of talk.

Clearly not.

6. Contributor
Gary McVey
@GaryMcVey

Thanks to Hank Rhody’s generous notice of next week’s pseudoradio pseudodrama, normal brains and positronic brains have come together to solve The Mystery of the Inspired Crossover Cross-Plugs.

“It’s a solid germanium block. But it’s been doped, man! It’s not just NPN anymore. Sometimes, it’ll be PNP. Featuring Hank’s Mu Meson Scatterers with their rocking hit, “Time Dilation Experiment”.

7. Member
RightAngles
@RightAngles

Cheese!

8. Member
Judge Mental
@JudgeMental

Thanks to Hank Rhody’s generous notice of next week’s pseudoradio pseudodrama, normal brains and positronic brains have come together to solve The Mystery of the Inspired Crossover Cross-Plugs.

“It’s a solid germanium block. But it’s been doped, man! It’s not just NPN anymore. Sometimes, it’ll be PNP. Featuring Hank’s Mu Meson Scatterers with their rocking hit, “Time Dilation Experiment”.

Whatever you do, don’t try playing it anti-parallel.

9. Contributor
Gary McVey
@GaryMcVey

I have to admit I feel a lot more woke now that I realize that it’s not that I’m heterosexual, it’s that I’m antiparalell to homosexual.

Sounds more inclusive, right?

10. Coolidge
dnewlander
@dnewlander

Hank, do you know anything about “Bernoulli Boxes“?

Back when hard drives used to physically crash fairly often, these used the Bernoulli Principle to draw the physical magnetic media close to the read/write head by blowing air past them both, which meant that crashes couldn’t happen. (Back when a “hard drive crash” meant “the head physically hit the magnetic media and destroyed it” and not “my drive stopped working”.

My dad’s office had one (1) that they used to back up important, classified stuff when I spent a summer working in the lab building test heat shields for the Midgetman Missile. Spoiler: we used differing layers of Kevlar, carbon fiber, and fibreglas! Double spoiler: the resin we used to vacuum seal them together could only be removed with acetone. Which is cold! Triple spoiler: one of the best tests ever devised for these things was the “screwdriver test”. Which actually meant trying to put a real phillips-head screwdriver through a sample.

Anyway, the Bernoulli Box they had was really interesting.

And fraking loud.

11. Coolidge
dnewlander
@dnewlander

Hank, do you know anything about “Bernoulli Boxes“?

Back when hard drives used to physically crash fairly often, these used the Bernoulli Principle to draw the physical magnetic media close to the read/write head by blowing air past them both, which meant that crashes couldn’t happen. (Back when a “hard drive crash” meant “the head physically hit the magnetic media and destroyed it” and not “my drive stopped working”.

My dad’s office had one (1) that they used to back up important, classified stuff when I spent a summer working in the lab building test heat shields for the Midgetman Missile. Spoiler: we used differing layers of Kevlar, carbon fiber, and fibreglas! Double spoiler: the resin we used to vacuum seal them together could only be removed with acetone. Which is cold! Triple spoiler: one of the best tests ever devised for these things was the “screwdriver test”. Which actually meant trying to put a real phillips-head screwdriver through a sample.

Anyway, the Bernoulli Box they had was really interesting.

And fraking loud.

By the way, the Bernoulli Principle is why when you drive past a semi at speed on the highway, you have to turn your wheel in the direction opposite the semi, because otherwise you get sucked towards 80 feet of death.

Physics, everyone!

12. Member
Matt Balzer, Straw Bootlegger
@MattBalzer

Hank Rhody, Acting on Emotion: This really boosts the signal, from like +/- 5% with normal magnetoresistance to about +/- 110% with Tunneling Magnetoresistance (commonly abbreviated TMR). All modern hard disk drives use TMR.

I hate that process. Always with the tunneling.

13. Member
RightAngles
@RightAngles

… the Bernoulli Principle is why when you drive past a semi at speed on the highway, you have to turn your wheel in the direction opposite the semi, because otherwise you get sucked towards 80 feet of death.

Physics, everyone!

14. Contributor
Gary McVey
@GaryMcVey

I feel bad not sticking more closely to the OP, which as always is wonderfully written, witty, and instructive. Hey, anybody got any rhymes for “quantum spin coupling”?

15. Member
Nanda Panjandrum
@

I feel bad not sticking more closely to the OP, which as always is wonderfully written, witty, and instructive. Hey, anybody got any rhymes for “quantum spin coupling”?

“tantrums ‘n’ snuffling”?…

16. Member
Saxonburg
@Saxonburg

Fun description, Hank.   Separating the reader from the writer also allowed us to use fewer strands of spaghetti in the writer.  Fewer strands of spaghetti allows us to write at higher frequencies — about 50 times faster than before.   Designers were happy to cut the cheese, too.

17. Contributor
Hank Rhody, Acting on Emotion
@HankRhody

dnewlander (View Comment):
Hank, do you know anything about “Bernoulli Boxes“?

Can’t say I’ve ever heard of these things; they sound pretty cool. I’ve got to figure that they’re working something like that in the modern HDDs; you get a layer of air pushed along by the spinning of the disk (a slower HDD these days runs at 5400 RPM, which is still a heck of a lot of Rs every M). They rely on the moving air to keep the heads from hitting the disk (the infamous ‘click of death’ is much more rare these days than it has any right to be). I’ve got to figure they’re using Bernoulli’s Principle to pull the thing closer. But that’s me speculating, not speaking from knowledge.

dnewlander (View Comment):
Triple spoiler: one of the best tests ever devised for these things was the “screwdriver test”. Which actually meant trying to put a real phillips-head screwdriver through a sample.

The best tests are usually the simplest.

18. Contributor
Hank Rhody, Acting on Emotion
@HankRhody

Saxonburg (View Comment):
Designers were happy to cut the cheese, too.

19. Contributor
Hank Rhody, Acting on Emotion
@HankRhody

Cheese!

Oh, right. Everyone.

20. Reagan
GLDIII
@GLDIII

We are being told that this week the objective it to enhance the magnetoresistance, with discussions of layering a quantum tunneling and quanta spin coupling…..Sound like this computer building is becoming and exercise in “who moved my cheese”….

NYTimes Review

Join the Magneto Resistance!

21. Inactive
blank generation member
@blankgenerationmember

I feel bad not sticking more closely to the OP, which as always is wonderfully written, witty, and instructive. Hey, anybody got any rhymes for “quantum spin coupling”?

No, but renowned physicist Gwyneth Paltrow did come up with “conscious uncoupling”.  Perhaps her ground breaking materials science research led her to this insight.

22. Member
Reese
@Reese

dnewlander (View Comment):
Hank, do you know anything about “Bernoulli Boxes“?

Can’t say I’ve ever heard of these things; they sound pretty cool.

They were quite the leap in technology.  On a 5.25 ” disk we jumped from, what? 720KB to 10 MB, then 20 MB, then… eventually 230 MB, always about \$100 per disk and \$1000 for the readers.  Then came “Jaz” drives, 3.5″ version going up to 2GB, last I saw.  We used them where I work in the way @dnewlander describes through the early Oughts, until 640 MB CDs, then 4GB DVDs became dependably writeable.  Now our HDs are multi TB and 64 GB Micro SDs go for thirty or fifty bucks.

23. Contributor
Gary McVey
@GaryMcVey

dnewlander (View Comment):
Hank, do you know anything about “Bernoulli Boxes“?

Can’t say I’ve ever heard of these things; they sound pretty cool.

They were quite the leap in technology. On a 5.25 ” disk we jumped from, what? 720KB to 10 MB, then 20 MB, then… eventually 230 MB, always about \$100 per disk and \$1000 for the readers. Then came “Jaz” drives, 3.5″ version going up to 2GB, last I saw. We used them where I work in the way @dnewlander describes through the early Oughts, until 640 MB CDs, then 4GB DVDs became dependably writeable. Now our HDs are multi TB and 64 GB Micro SDs go for thirty or fifty bucks.

When I was a kid, digital storage went for about \$1 per bit. So in theory, that Micro SD would have cost a modest \$64 billion.

24. Contributor
Gary McVey
@GaryMcVey

I feel bad not sticking more closely to the OP, which as always is wonderfully written, witty, and instructive. Hey, anybody got any rhymes for “quantum spin coupling”?

No, but renowned physicist Gwyneth Paltrow did come up with “conscious uncoupling”. Perhaps her ground breaking materials science research led her to this insight.

Pioneering metallurgist Rosie O’ Donnell had some interesting theories regarding the melting point of steel.

25. Member
Reese
@Reese

Gary McVey (View Comment):
When I was a kid, digital storage went for about \$1 per bit. So in theory, that Micro SD would have cost a modest \$64 billion.

And is that in “when you were a kid” money?  (Will guess your age:  Were standard candy bars a nickel or a dime when you were a kid?)

Hmm.  “Byte” or “bit?”  You got another factor of eight there?

26. Contributor
Gary McVey
@GaryMcVey

1952, baby, born under the sign of Truman. Actually, I don’t even know the answer to the most basic level of the joke, “byte” or “bit”, but I figured the billion-fold expansion of value sneaked the 8 question.

27. Member
Reese
@Reese

Gary McVey (View Comment):
1952, baby, born under the sign of Truman. Actually, I don’t even know the answer to the most basic level of the joke, “byte” or “bit”, but I figured the billion-fold expansion of value sneaked the 8 question.

Oh.  Okay.  No joke.  Mr. Rhody will likely get to this, but a byte, usually abbreviated with a big “B” is eight bits of memory, bit being “be or not be,” the smallest “bit” of information possible according to my viewing of the movie “Tron.”  It’s his magnet pointing one way or the other (or not pointing at all, depending on how you set up your system).  And a byte is eight of those.  This gives you 2^8 possible combinations of binary information, or 256 possible “letters” or “characters” or– “bytes.”

Bytes are just a bigger chunk of info than bits.  So when I say 64GB, that’s 64*8 = 512 gigabits on a chip the width, height and thickness of my pinkynail.

28. Contributor
Hank Rhody, Acting on Emotion
@HankRhody

Reese (View Comment):
bit being “be or not be,” the smallest “bit” of information possible according to my viewing of the movie “Tron.”

Fun thing about that; they actually gave the bit four possible pieces of information. It would say “Yes” and “No”, but then it’d also go “NoNoNoNoNo” when something bad happened.

29. Member
Reese
@Reese

Hank Rhody, Acting on Emotion (View Comment):
Fun thing about that; they actually gave the bit four possible pieces of information. It would say “Yes” and “No”, but then it’d also go “NoNoNoNoNo” when something bad happened.

Yeah. What about all the cognition needed to understand and respond to Jeff Bridges’ questions?  (Oh we’re analyzing a cartoon character– cut it out.)

30. Contributor
Gary McVey
@GaryMcVey

There’s a bit of a bait and switch going on here and you know it, I know it, Hank knows it, Bob Dole knows it (okay, a diversion to 1996 SNL). Normal ol’ recording heads put a change in a magnetic domain, so when a coil of wire is held over a track it generates a tiny amount of current because of the motion of the disc. OK, you had me there.

Now you’re telling me that more sophisticated hard drives, presumably the kind who look like Audrey Hepburn in Breakfast at Tiffany’s with those long silk gloves, use variable resistance as a signal? Or worse, quantum tunneling effects? These How to Build a Computer posts are valuable but sometimes ego deflating.