Sunday, August 14, 2005

Magnetic RAM (MRAM) and Spintronics

The latest RAM (Random Access Memory) chips are based on the spinning of electrons, not on the mere presence of electrons.

Many of you know, electrons are negative particles, circling around a positively charged atomic nucleus. Modern physists have problems with that image, as the electron is not just a particle, it's also a wave. And, on top of that, not only does an electron circle around a nucleus, it also spins, circles around itself.
It either spins clockwise, or anti-clockwise (you could also say: it always spins in the same direction, but some electrons stand on their head). This could just as well represent a binary '0' or '1' as (electrical) tension, or current. And that is what MRAM is all about.

As you were told in highschool, electric current and magnetic fields always go hand-in-hand. Move a magnetic field, and you will get an electric current; an electric current will generate a magnetic field.
Transformers are built on this principle: you send (alternating) current through a wire, thus generating an alternating magnetic field. You place a coil in that magnetic field, and a current will result from that. As the magnetic field itself "moves" (it alternates), your coil will keep picking it up. Another rule was the corkscrew rule, defining the relation between force, direction of current and magnetic field direction.

Now, remember those pickup coils on the magnetic drums called computer storage? Those coils actually had to pick up the magnetic field, causing a minute change in electric current (one percent would be normal). Higher density disks, with higher throughput, call for those magnetic fields to become smaller, but also for pick up mechanisms to become smaller. All this causes more 'noise', thus making it extra hard to distinguish the noise from the signal.
Until... Giant Magnetoresistance (GMR) was discovered by Albert Fert and Peter Grünberg, back in 1988. This phenomena, best compared to an optical filter like Polaroid glass, causes a change in magnetic fields to be amplified so that the signal-to-noise ratio improves tenfold. Changes in currents would be 10 percent or more (as opposed to one).

A pickup head would be made out of two parts: a fixed magnetic component combined with a variable magnetic component, glued together with a conductive layer. The variable magnetic component would alter with the passing of a magnetic field (from the platter), and just like light with the rotating of two Polaroid glass sheets, current can either pass easily, or not at all. Well, harder, as due to miniaturization, there is a lot of leaking, but as said, ten percent change in current would be doable.
Disk manufacturers, like IBM, have been using the spinning of electrons on disk platters since some time now (there's a beautiful animated explanation on the IBM website), and the size of the magnetic structures has gone back to some 30 by 200 nm (1 nm = 10^-9 m), some 30 nm apart. All this races underneath a pickup head that flies a couple of nanometers above the platters, which has to be able to distinguish billions of magnetic field changes every second.

The next step.

Some years later, magnetic tunneling was discovered. The first thesis was published in 1995 by Jagadeesh Moodera, and the main difference between the pickup element above, which comprised of three (electrically) conductors, the two magnetic layers are no longer separated by a conductive layer, but by an insulator.
Making this layer thin enough (we're talking atoms here; three to five atoms thickness), electrons can tunnel their way through. Now, we're on quantummechanics territory here, not my forte, but the mechanism is much like the electrical tunneling as used in diodes, e.g. I'll leave it with that. And with the remark, that this tunneling effect makes the difference between passing of electrons increase to 50 or 60%.

Last fall, the first prototype of an MRAM memory chip was presented by Motorola and Cypress Semiconductor. According to some, it's not just a prototype, but a working product, available to companies that want to incorporate it in their products. It's capacity (256 kilobit) is not large enough for any serious use (even your phone uses several Megabit!), but it's a serious start.

By now companies like IBM, Motorola and not since long, Intel, are planning commercial introduction of MRAM, so not much details are revealed. It must be clear, that all has to become smaller, and faster. If all that comes true, picture this:
cellular phones and MP3 players which need recharging only when used. PC's starting up in seconds or less, because the Operating System is loaded off MRAM.
Watch your local store this autumn.

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