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A Different Kind of Current

We’ve seen a number of different ways in which magnetic interactions with electron current can be put to use thanks to the concept of spin. Those magnets are also conductors, so electrons are moving through materials having various (or no) magnetic polarization.

You might wonder why I went through the trouble to specify “electron” current. I mean, that’s what current is: a flow of electrons. Right? Well, it turns out there’s another more subtle current. Or perhaps better to say pseudo-current, since no actual object is moving. You can have a spin current too. Who knew!

This is really more of an “influencing” thing: the spin of one electron can be transferred to its neighbor, thence to another neighbor, and so forth. This is spin (or spin torque) transfer. So you’ve got this spin alignment thing going on, and it flows out from wherever it started. Think of it as spin going viral. So it acts like a current, even though it’s only the influence that’s moving; the electrons themselves aren’t.

Actually, the electrons can be moving while this happens; they’re just not moving in the same direction that the spin is. This came up in an article about work done at Japan’s Tohoku University to identify yet another type of spin interaction, which they call Spin Hall Magnetoresistance. While the previous types of magnetoresistance we’ve seen involve currents going through the magnets, this involves insulating magnets adjacent to an electron-current-carrying metal.

The fundamental idea is that spin from the current can transfer into the magnet even though the electrons themselves can’t move into the magnet. When that happens, it’s sort of like an energy leak from the wire, and it reduces the current in the metal, which, given a constant potential driving the wire, makes it feel like a higher resistance.

There are apparently two things going on that make this work. One is a polarization of spins in the wire, analogous to the charge polarization due to the normal Hall effect. In this case, instead of getting opposing charges on each side of the wire, you get opposing spins. So the side that abuts the magnet will have an accumulation of electrons with a particular spin.

The second piece has to do with what they call scattering, but fundamentally, they found that the spin at the metal/magnet boundary can only transfer into the magnet if the magnet is polarized perpendicular to the accumulated spin. In other words, you can, in theory, modulate the apparent resistance of the wire by changing the field direction of the adjacent insulating magnet.

Granted, it’s a small effect: the change is around 0.01%. But it’s yet another mechanism that might have promise for… something. And, fundamentally, it’s the first I’ve run up against this concept of a spin current. So it piqued my interest on that score too.

If it’s piqued yours, you can get more detail in this Physics article.

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