magnetoresistance
/ mag-NEE-toh-ree-ZIS-tance /
Imagine driving down a straight road, then someone switches on a crosswind that keeps shoving your car off-line. You have to weave, your path gets longer, and you arrive later. Electrons carrying current feel a magnetic field much like that crosswind — it bends their paths, makes them travel farther between bumps, and so makes the material harder to push current through. That change in resistance is magnetoresistance.
Concretely, magnetoresistance is how much a material's electrical resistance shifts when you apply a magnetic field. In most ordinary metals the field curves the electrons' trajectories, lengthening their effective journey and nudging the resistance up by a small amount that grows with the field. But in specially built layered materials the change can be enormous — turning the field on and off can swing the resistance by tens of percent or more, because the field reorients tiny magnets inside the layers and switches the easy flow of electrons on or off.
Magnetoresistance matters because it turns a magnetic field into an electrical signal you can read. The giant version, discovered in thin magnetic sandwiches, is what let hard-disk read-heads detect the faint fields of densely packed data and made cheap, huge storage possible — a discovery honored with a Nobel Prize. An honest caveat: there are many distinct kinds, ordinary, giant, colossal, and they arise from quite different mechanisms, so the single word covers a whole family of effects rather than one tidy law.
The read-head in older hard drives is a microscopic stack of magnetic and non-magnetic films. As it skims over a recorded bit, the bit's tiny field flips one layer's magnetism, and the stack's resistance jumps — giant magnetoresistance turning a magnetic pattern into a stream of ones and zeros.
Giant magnetoresistance let read-heads sense single bits, and is why a terabyte of storage costs so little.
Magnetoresistance is about a magnetic field changing the resistance you measure with current; the Hall effect is about a sideways voltage. They are cousins, often measured in the same experiment, but they answer different questions.